CA2777025A1 - Method and system for energy dissipation in steel structures - Google Patents

Abstract

Steel concentric brace frame structures have been a common approach to lateral load resistance of structures for over 60 years. However, evolving building codes require that many such structures are either retro-fitted or demolished and replaced when major work or change of use is undertaken. Seismic fuses according to embodiments of the invention are introduced into steel CBF structures using inline seismic fuses singly or in combination within individual braces which are either retro-fitted to existing braces or integrated into new braces. Additional embodiments provide for seismic fuses for multi-brace designs as well as discrete brace elements. Through correlation of experimental measurements to finite element modes seismic fuses may be designed for the desired characteristics of elastic and non-elastic behavior over different seismic stimuli, for performance and cost tradeoffs within the different building code zones, as well as for variations in jurisdictional requirements of steel CBF
structures with seismic fuses.

Description

METHOD AND SYSTEM FOR ENERGY DISSIPATION IN STEEL STRUCTURES
FIELD OF THE INVENTION
[001] The present invention relates to concentrically braced frame structures and more specifically to increasing their resistance to seismic activity both in retrofit and new construction applications.
BACKGROUND OF THE INVENTION

[002] Earthquakes, also known as a quake, tremor or temblor, are the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time. Globally the US Geological Survey estimates that there are 1.3 million earthquakes of magnitude 2.0-2.9 and that for each increasing order of magnitude the number of earthquakes similarly reduces by an order of magnitude such that at magnitudes between 6.0 and 6.9 there are an estimated 130 events per annum, between 7.0 and 7.9 an estimated 15 per annum and an average 1 per annum at magnitudes 8.0 to 8.9. Table 1 depicts the occurrences within the United States over the past 10 years recorded by the US Geological Survey.
Magnitude US Events Detected 8.0-8.9 0 7.0-7.9 8 6.0-6.9 68 5.0-5.9 687 4.0-4.9 4,699 3.0-3.9 18,185 Table 1: Earthquakes 2000-2011 in United States [003] Magnitude 3 or lower earthquakes are mostly almost imperceptible and magnitude 7 and over potentially cause serious damage over large areas, depending on their depth. The largest earthquakes in historic times have been of magnitude slightly over 9, although there is no limit to the possible magnitude. The most recent large earthquake of 9.0 or larger was a 9.0 magnitude earthquake in Japan in 2011. The shallower an earthquake, the more damage to structures it causes, all else being equal. At the Earth's surface, earthquakes manifest themselves by shaking and displacement of the ground. As evident in Figures lA for North and Central America the areas of highest risk are the western Pacific Coast from Alaska down through British Columbia to California, southern Mexico, Guatemala and El Salvador. On the eastern side Cuba, Haiti, and localized regions of Ontario-Quebec in Canada and eastern states of the United States including Arkansas, Tennessee, Georgia, Alabama and the Carolinas. As can be seen from Figure 1B these regions, except Alaska, also correspond to areas of high populations.

[004] In these areas an earthquake is a significant threat to American and Canadian safety and security for substantial portions of the population. However, it was not until the mid 1970s that it was understood how inappropriate existing regulations were leading to revamping and issuance of new seismic codes and the construction of new buildings and infrastructure designed to specific defined requirements. A map of current seismic code assignments for the United States is shown in Figure 2 wherein regions are denoted by codes 1, 2A, 2B, 3, and 4 wherein the overlap of these code regions can be seen to be that of the regions of high seismic activity in Figure 1A. Similar code regions exist in Canada. However, this creates the situation that a large portion of built infrastructure therefore has unknown earthquake performance in all these regions of seismic activity having been built prior to the standards or to standards that are now requiring increased performance. Within this built infrastructure steel concentrically braced frame structures (CBFs) make up an important portion of this infrastructure overall having been a popular steel framing choice for engineers for over a hundred years.

[005] Accordingly over time the design guidelines and performance of CBFs has evolved to meet evolving building code regulations. For example during the 1940s and 1950s tension only steel CBFs were typically formed using rivets for joining structural members together. By the 1960s and 1970s such tension only steel CBFs typically employed bolts for joining the elements.
Subsequently steel CBFs have evolved into different categories including:

= special concentrically braced frames (SCBFs) wherein these are designed such that diagonal braces buckle and dissipate energy resulting from the design earthquake;
= ordinary concentrically braced frames (OCBFs) which have highly restricted applications in high seismic design categories due to their limited expected ductility; and = eccentrically braced frames (ECBFs) where are designed with links designed to yield whilst the diagonal braces are intended to essentially elastic under the design earthquake.

[006] Equivalent Canadian categories to the US code are:
= Convention Construction ¨
= Type LD (Low Ductility); and = Type MD (Moderate Ductility).

[007] A design earthquake being the magnitude of the earthquake that a steel CBF structure is designed to survive wherein elements of the CBF structure yield to accommodate the stress and strain induced within the structure in an inelastic manner but the structure must still be capable of resisting gravity loads and of withstanding aftershocks without collapse. Referring to Figure 3 there are depicted in CBF set 300A images of different common CBF
configurations as employed within the prior art comprising first to eighth structures 310A
through 310H wherein the braces 330 are depicted relative to the vertical and horizontal beams of the overall structure.
Accordingly it can seen from building schematic 300B how seventh structure 310G repeats vertically within an overall building comprising an array of 4 x 4 vertical columns 340 with 8 horizontal floor sections 350 wherein the brace pairs 360 repeat every 2 floors. It would be evident in respect of the other first to sixth structures 310A through 310F
and eighth structure 310H that these bracing patterns repeat every floor.

[008] Accordingly steel CBF structures built prior to the 1980s can pose a significant risk due to their anticipated inability to dissipate seismic energy without compromising the structure.
It would therefore be evident that it would be beneficial to provide a means of increasing the ability of such historic steel CBF structures to absorb the compressive and tensile forces placed on them during seismic activity. It would therefore be beneficial for there to be a means of retro-fitting such existing CBF structures in a manner allowing their performance to be improved at low Cost. According to embodiments of the invention seismic fuses may be retro-fitted to installed braces within such existing steel CBF structures.

[009]
However, whilst such retro-fitting addresses historic steel structures the remaining population of legacy steel CBF structures from the mid-1970s to today would still benefit from a design solution that allows for a similar retro-fitting of a seismic fuse to improve the seismic performance of the steel CBF structure and increases the performance of the steel CBF structure under seismic stimulus such that compliance to updated building codes may be achieved thereby allowing owners of legacy steel CBF structures to increase return on investment wherein compliant structures allow extended operational lifetime of their structures and increased rental per square foot. Accordingly seismic fuses according to embodiments of the invention may introduced into steel CBF structures using inline seismic fuses singly or in combination within individual braces which are either retro-fitted to existing braces or integrated into new braces, such as for example braces within first to fourth structures 310A through 310D, sixth structure 310F, and eighth structure 310H. Other embodiments of the invention employ multi-brace seismic fuses may be similarly retro-fitted to existing braces or integrated into new braces where braces intersect between horizontal and vertical beams such as fifth and seventh structures 310E
and 310G respectively. Variants of these multi-brace seismic fuses may also be employed in brace nodes such as within third structure 310C.

[0010] According to embodiments of the invention seismic fuses may be designed to provide the desired characteristics of elastic and non-elastic behavior over different seismic stimuli allowing the seismic fuses to be designed for performance and cost tradeoffs within the different building code zones as well as variations in jurisdictional requirements of steel CBF structures with seismic fuses. These design tradeoffs may also be employed to adjust the design of seismic fuses within a single steel CBF structure allowing different seismic fuse performance for example to be provided at the lower levels of a steel CBF structure to those at the upper levels or vice-versa. It would be further beneficial if seismic fuses provided an extra degree of freedom for the structural engineer allowing them to adjust the load ¨ deformation profile of the braced frames and apply CBFs in higher seismic design codes.

[0011] Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
SUMMARY OF THE INVENTION

[0012] It is an object of the present invention to concentrically braced frame structures and more specifically to increasing their resistance to seismic activity both in retrofit and new construction applications.

[0013] In accordance with an embodiment of the invention there is provided a brace for a structure comprising a first end for attaching the brace to the structure at a first predetermined location, a second distal end for attaching the brace to the structure at a second predetermined location, a brace member of predetermined geometry disposed between the first end and the second distal end, and a seismic fuse of a plurality of seismic fuses, each seismic fuse disposed at a predetermined location along the brace member.

[0014] In accordance with an embodiment of the invention there is provided a seismic member comprising a first central member comprising a plate having an outer circular geometry and an inner geometry and an integer N tabs integrally formed with the first central member, each tab having a predetermined width disposed at a predetermined position around the periphery of the first central member.

[0015] In accordance with an embodiment of the invention there is provided a method comprising:
establishing a seismic loading for a structure in dependence upon at least the location of the structure;
determining a maximum lateral capacity for the structure under the seismic loading;
determining a minimum elastic loading required to sustain wind loads without plastic actions;
calculating with a microprocessor a maximum drift value for a predetermined portion of a frame of the structure;

generating with the microprocessor a specification for a seismic fuse to form a predetermined portion of a bracing structure to be attached to the predetermined portion of the frame of the structure; and generating with the microprocessor a numerical control file for the seismic fuse.

[0016] Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein:

[0018] Figures lA and 1B depict seismic activity and population density maps of North America;

[0019] Figure 2 depicts a map of the United States indicating seismic building codes zones;

[0020] Figure 3 depicts steel CBF structures according to the prior art;

[0021] Figure 4A depicts the deformation of a steel CBF structure according to the prior art under seismic motion;

[0022] Figure 4B depicts a steel CBF structure fitted with multi-brace seismic fuses according to an embodiment of the invention;

[0023] Figure 5 depicts a recovered brace from a historical steel CBF
structure;

[0024] Figure 6 depicts load ¨ deformation hysteresis results for recovered braces recovered from historical steel CBF structures;

[0025] Figure 7A depicts load ¨ deformation hysteresis results for recovered braces recovered from historical steel CBF structures with varying bolt standard;

[0026] Figure 7B depicts bolts from recovered braces that failed during testing;

[0027] Figures 8A through 8D depict fuses according to the prior art of Timler and Tremblay;

[0028] Figures 9A through 9G depict load limiting structures according to the prior art;

[0029] Figure 10A depicts an ideal load ¨ deformation curve for a seismic fuse;

[0030] Figure 10B depicts engineering drawings for in-line seismic fuse variants 1C and 1D
according to embodiments of the invention;

[0031] Figures 11A and 11B depict optical micrographs of in-line seismic fuse variants 1C
and 1D according to embodiments of the invention under varying load;

[0032] Figure 12 depicts the load ¨ deformation curves for in-line seismic fuse variants 1C
and ID according to embodiments of the invention;

[0033] Figure 13 depicts load - deformation curves for a discrete brace and in-line seismic fuse variants according to embodiments of the invention;

[0034] Figure 14 depicts an in-line line seismic fuse according to an embodiment of the invention and it's assembly with an historical brace for testing;

[0035] Figure 15 depicts load ¨ deformation results for an original historical brace and retro-fitted brace according to an embodiment of the invention;

[0036] Figure 16 depicts optical micrographs of the braces of Figure 15 after buckling;

[0037]
Figure 17 depicts load ¨ deformation hysteresis results for an original historical brace and retro-fitted braces fitted with in-line seismic fuses 4C and 4D according to an embodiment of the invention;

[0038] Figure 18 depicts schematics of cross-braces of legacy braces and braces with seismic fuses according to embodiments of the invention;

[0039] Figure 19A depicts a pair of in-line seismic fuses according to an embodiment of the invention fitted to a steel CBF structure;

[0040] Figure 19B depicts an optical micrograph for a steel CBF structure with conventional prior art braces after deformation testing;

[0041] Figure 20 depicts load ¨ deformation hysteresis results for the steel CBF structure with conventional prior art braces;

[0042] Figures 21 and 22 depict optical micrographs of the steel CBF structure of Figure 18 with in-line seismic fuses according to an embodiment of the invention before and after deformation testing;

[0043] Figure 23 depicts load ¨ deformation hysteresis results for the steel CBF structure of Figure 18 with in-line seismic fuses according to an embodiment of the invention;

[0044] Figure 24 depicts examples of single and dual inline seismic fuses according to embodiments of the invention;

[0045] Figure 25 depicts a design for a multi-brace seismic fuse according to an embodiment of the invention;

[0046] Figure 26 depicts optical micrographs of a multi-brace seismic fuse according to an embodiment of the invention during deformation testing;

[0047] Figure 27 depicts load ¨ deformation hysteresis results for a steel CBF
structure employing a multi-brace seismic fuse according to an embodiment of the invention;

[0048] Figure 28 depicts load ¨ deformation hysteresis results in the positive force displacement quadrant for steel CBF structures with and without seismic fuses according to embodiments of the invention; and [0049] Figure 29 depict multi-brace seismic fuses for asymmetric brace frames according to embodiments of the invention.
DETAILED DESCRIPTION

[0050] The present invention is directed to concentrically braced frame structures and more specifically to increasing their resistance to seismic activity both in retrofit and new construction applications.

[0051] The ensuing description provides exemplary embodiment(s) only, and is not intended to limit the scope, applicability or configuration of the disclosure. Rather, the ensuing description of the exemplary embodiment(s) will provide those skilled in the art with an enabling description for implementing an exemplary embodiment. It being understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope as set forth in the appended claims.

[0052] A "braced frame" as used herein and throughout this disclosure, refers to a structural system which is designed primarily to resist lateral forces. This includes structures, but is not limited to, structures wherein members in a braced frame are designed to work in tension and compression, similar to a truss. Typically such braced frames employ steel members but other materials may be employed according to the application.

[0053] Braced frames as used herein and throughout this disclosure may include Concentrically Braced Frames (CBFs), such as Ordinary (OCBF), Special (SCBF), Convention Construction, Type LD (Low Ductility), Type MD (Moderate Ductility); as well as Eccentrically Braced Frames (EBFs) and Buckling Restrained Brace Frames (BRBFs). Whilst the disclosure has been presented and described with respect to CBFs these other types of braced frames may exploit seismic fuses according to embodiments of the invention. CBFs are braced frames wherein members intersect at a node, the centroid of each member passing through the same point.

[0054] Typically OCBFs do not have extensive requirements regarding members or connections, and accordingly tend to be used in areas of low seismic risk.
SCBFs have extensive design requirements, and accordingly may be used in areas of high seismic risk. The primary goal of the SCBF design requirements being to ensure adequate ductility, i.e.
to stretch without breaking suddenly. ECBFs are intentionally designed to accommodate eccentricity in the connections between members and are generally highly ductile and permitted in areas of high seismic hazard. BRBFs are typically a special case of concentrically braced frames wherein the CBF, steel core, is surrounded by a hollow steel section, coated with a low-friction material, and then grouted with a specialized mortar. The encasing and mortar limit the CBF
buckling when in compression, while the coating prevents axial load from being transferred to the encasement.
Whilst BRBFs are highly effective for energy dissipation they have been shown to be vulnerable under large deformations and drift of the structure stories, see for example Fahnestock et al "Experimental Evaluation of a Large-Scale Buckling-Restrained Braced Frame"
(J. Struct. Eng., Vol. 133, pp1205-).

[0055] Referring to Figure 4A there is shows a simplified elevation of a CBF
structure in first mode 400A under normal stationary conditions and second mode 400B after deformation due to lateral load V resulting in a displacement of the CBF structure laterally by an amount A . In first mode 400A each brace 410 is under neither significant tension nor compression although some tension or compression may be inherent from the overall assembly, settling, etc. of the CBF
structure. In a tension only CBF structure only one member 420 of the cross brace comprising braces 410 is considered active under deformation as the other member 430 is under compression. Typically within a tension only brace system the compression brace will take some compression, but the basic idea is that the compression brace is not considered in design and so buckling is not important As will be described below in respect of embodiments of the invention a structure 400C comprising vertical beams 460, horizontal beams 440 and flooring 480 employs seismic fuses 470 which are deployed within the braces 450 that form part of a fused CBF structure. As will be evident from embodiments described below multiple seismic fuse configurations are feasible according to the design requirements, cost, installation requirements (e.g. retro-fit or new installation) etc. Such options include in-line seismic fuses installed within each brace 450 and multi-brace seismic fuses linking multiple braces 450 together.

[0056]
Initially the inventors addressed physical testing and modeling for retro-fitting considerations on existing structures from the 1960s and 1970s although earlier legacy and historical structures are expected to have similar performance and design concerns as reflected in the development of standards governing load determination and design.
Accordingly initial expectations of legacy CBF structures prior to seismic code compliant buildings employing CBF
structures was based on existing design aids, original drawings, personal interviews and prior art academic papers. These evaluations were augmented using brace samples removed from structures being demolished in order to run laboratory tests which indicated a number of areas of unanticipated failures which will be discussed below. These led to the inventors establishing recommendations on the analysis of seismic performance of existing legacy structures.

[0057] A. Expected Performance of Le2acv CBF Structure: The expected performance of existing structures is based largely on loading and design guidelines that were present in the design codes of the period. For example structural steel design in Canada is governed by the National Building Code of Canada (NBCC), wherein the current standard was published in 2010 with previous releases in 2005, 1995, and approximately every 5 years previously to the first federal code released in 1941. These now reference the parallel Canadian Standards Association (CSA) S16 Standard. In the United States the equivalent standard is the International Building Code (IBC) which is a model building code developed by the International Code Council (ICC) and originally issued in 1997 to provide a code with no regional boundaries and thereby superseding the three regional model code groups established since the early 1900s. The fifth edition of the International Building Code being published in 2012.

[0058] Considering Canadian requirements then the CSA S16 Standard provides the minimum design requirement for steel structures but early editions do not contain many seismic design requirements. For example, the 1965 CSA S16 Standard only provides a clause on increasing allowable stress under certain seismic and wind combinations otherwise the base shear must be sustained in the lateral loading system and brace requirements state that a structure must be adequately braced, but no more. The capacity design approach, where designers ensure the most ductile members are weaker than the rest of the system, and the dynamic nature of seismic effects were not considered or developed at this point. At the same time slenderness requirements did exist, and as expected often controlled the sizing of braces.
Large scale changes to the seismic provisions of the CSA S16 Standard began in the 1980s and continue up to and including the 2009 CSA S16 Standard wherein Clause 27 is dedicated to seismic design.
Multiple structural types are discussed and the various limits, requirements and checks required for each are provided, along with general design information.

[0059] Research into the ductility of steel CBF structures and the importance of connection detailing began in greater detail in the 1980s, when among other findings it was determined that the gusset plates must be able to bend without interference in order to allow buckling and avoid brittle failure, see for example Astaneh-Asl et al. "Cyclic Out-of-Plane Buckling of Double-Angle Bracing" (J. Struct. Eng., Vol. 115(5), pp1135-1153) and "Earthquake Resistant Design of Double-Angle Bracings" (Eng. J. Am. Inst. Steel Constr., 4th Qtrõ pp133-147, 1986).
Accordingly, legacy structures should be assessed on a case by case basis with close attention to multiple aspects of the CBF structure including, but not limited to, the brace connections, bolts, gusset plate design, and gusset plate to beam or column connections in order to determine expected seismic performance.

[0060] B. Legacy CBF Structures Tests: Working with a demolition company in Montreal approximately two dozen braced connection test specimens were collected from buildings during demolition which were built in the 1960s through to late 1980s. All specimens included the gusset plate and a section of the brace, while some of them included a piece of the beam or column that the gusset plate was connected to. Due to removal and transportation constraints it was not possible to collect the full length of the brace; the specimens retrieved were similar to the sample shown in Figure 4. All samples retrieved were from industrial sites and either single or double angle braces.
[00611 As a result of recovering partial sections of braces it was not possible to perform full reversed cyclic tests of the specimen as buckling in compression would not occur properly.
Accordingly, the samples were tested in tension only cycles according to Federal Emergency Management Agency (FEMA) standard 461 entitled "Interim Testing Protocols for Determining the Seismic Performance Characteristics of Structural and Nonstructural Components" as published in 2007. This testing protocol suits specimens for which displacement has a key role, see for example Krawinkler in "Loading Histories for cyclic Tests in Support of Performance Assessment of Structural Components" (3rd Int. Conf. Adv. Experimental Structural Engineering, 2009), as according to embodiments of the intention the aim is to improve ductility of the CBF
structures.
[0062] Using expected material properties by year and calculations established upon CSA
S16 standard brace resistances were calculated, from which a predicted failure mode was identified for each specimen obtained. Accordingly bolt shear failure and brace corner block tear out were the most common predicted failure modes, in common with Hartley "Performance and Retrofit of Seismically Deficient Existing Braced Steel Frame Structures:
Testing of Brace Connections from Existing Concentrically Braced Steel Frames" (McGill University, M.Sc Thesis, 2011, pp. 104). The tests were performed with a 12MN test frame with 10001th hydraulic grips and load cell where the exact test setup varied depending on how to best connect to the removed brace system. For some tests where minimal damage was incurred by the specimens at the brace and gusset during bolt shear failure these were re-tested but with the typical A307 bolts or fully threaded A325 bolts of the original installations replaced with new A325 or A490 bolts.
[0063] Whilst bolt shear was the most brittle failure mode, the other failures seen, namely brace net section failure, block tear out, and punching shear failure of the column web, these specimens were only slightly more ductile. Referring to Figure 6 there are depicted examples of system load versus deformation data obtained for a subset of the reclaimed brace samples tested.
It should be noted that there was some slip in the testing and only a section of brace was involved, not full length. Coupons were then taken from each brace member and tested to determine the material properties where the measured yield stress and ultimate strength were found in all cases to be above those specified by the "Handbook of Steel Construction"
(Canadian Institute of Steel Construction, 2006). Where the structure could be assigned an age (between 1962 and 1987) yield stress indicated use of 350 MPa steel, with one undated steel strength indicating 300 MPa steel. The handbook indicates steel yield stress of 230MPa for structures built from 1964 to 1973 (CISC, 2006), this appears to be overly conservative. The ultimate strength found in coupon tests did match the CSA S16 Standard specified ranges.
[0064] C. Unanticipated Failures: Using the CSA S16 Standard and similar codes to determine expected performance of the brace connections whilst providing a good base for calculations it did show that some issues identified during physical testing should be afforded special attention. For example, the common use of non-high strength bolts was surprising, as was the fact that some A325 bolts employed in these legacy CBF structures were fully threaded which was in contrast to the expectations based on bolt markings and concerns existed relating to connection of the gusset plate to the beam or column within the original buildings as a result.
[0065] On some of the brace members that had been exposed to environmental elements during their life the bolt heads were highly worn and it was not initially realized that many of the bolts used in the 1960s structural members tested were not the expected high strength bolts, but cheaper unhardened ordinary carbon steel A307 bolts. The A307 bolts (an ASTM
designation) were originally referred to as unfinished bolts with expected ultimate shear stress of 69 MPa for those produced in the 1960s and 1970s. This can be compared to the then standard high strength A325 bolt that had an ultimate expected shear stress of 152MPa in the 1960s and 1970s, see Brockenbrough "AISC Rehabilitation and Retrofit Guide: A Reference for Historic Shapes and Specifications" (American Institute of Steel Construction, 2002). Accordingly the bolts failed in shear at less than half of the expected load.
[0066] Accordingly, the capacity in these specimens could be increased by replacing the original bolts with high strength bolts, thereby avoiding bolt shear and caused failure to occur elsewhere. Referring to Figure 7A the test results for specimen 10 first with A307 bolts (dotted line 710) which lead to bolt shear failure are shown together the results (dashed line 720) where the bolts employed were very high strength A490 bolts wherein specimen 10 failed with weld tear-out on the column.
[0067] The second major unanticipated issue observed related to the threaded length of the bolts. Almost all high strength bolts used in modern designs have a shank, or un-threaded, section of bolt that presumably would intersect the shear plane to increase shear resistance. If the shear plane intersects the threaded portion of the bolt the shear resistance decreases by approximately 30%, see CSA S16 Standard. All A307 bolts were fully threaded as would normally be expected. However, a portion of the bolts clearly labeled A325 heavy hex were also without shank or had a much shorter shank length compared to what is commonly assumed today, as shown in Figure 7B. Further research determined that there were two types of A325 heavy hex bolts which carried the same bolt head marking - A 325 Heavy Hex, and A 325 Heavy Hex Structural. Only the specified structural bolts contained the shank, for which it was a standard feature, see Hartley. Accordingly for existing legacy steel CBF
structures the inventors consider it prudent to assume that the A325 bolts used in the 1960s and 1970s are fully threaded, and if markings are unclear be treated as ordinary carbon steel (A307) bolts.
[0068] Accordingly the inventors have demonstrated that the evaluation of existing legacy and historical CBF steel structures require attention to details, material properties and bolts as an evaluation of the system through calculation of members is not sufficient and special attention must be paid to connection details. Accordingly, it is important as to how the connection is laid out, including where and how the gusset plate connects to the framing system.
Further an evaluation of the flexibility of the gusset plate to undergo deformations in buckling should also be done using engineering judgment in cases where spacing does not meet layout requirements of current design codes.
[0069] Similarly it should be assumed that the bolts have not been pre-tensioned and unless an evaluation of such legacy steel CBF structures includes the removal and/or replacement of the bolts it must considered the potential performance limit of the structure is determined by the installed bolts and that these are common steel (A307) and fully threaded thereby giving a low failure threshold. In many cases a cost effective solution to enhanced seismic performance for legacy steel CBF structures is the replacement of the bolts throughout such structures.
[0070] D. Seismic Fuses and Prior Art: In many legacy steel CBF structures the retrofit upgrades to bolts, knowledge of material properties, inspection etc. are insufficient to provide for a ductile response of the structure under the design level seismic ground motion. Accordingly the inventors have addressed this through the consideration of integrating a structural fuse to the steel CBF structural system in accordance with the design requirements of the steel CBF
structure with options for in-line seismic fuses for discrete brace frame elements and multi-brace seismic fuses for use with multiple brace frame elements simultaneously.
Within the prior art a general study of options was published by Rezai et al. "Seismic Performance of Brace Fuse Elements for Concentrically Steel Braced Frames" (Conf. on Behaviour of Steel Structures in Seismic Areas, STESSA 2000, pp.39-46) as well as more specific concepts as presented by Tremblay et al "Overview of Ductile Seismic Brace Fuse Systems in Canada"
(Proc.
EUROSTEEL 2011, Session BF 12, Paper 1) and as presented by Timler et al in "Ductile Fuses for HSS Seismic Bracing of Low-rise Buildings"
(http://seabc.ca/documents/seminars/ductile_fuses.pdf).
[0071] A seismic fuse is designed to dissipate seismic energy and reduce force demands on the steel CBF structure and the overall structure within which these CBF
systems are deployed.
Within the prior art of Rezai, Tremblay, and Timler the seismic fuses were designed for integration with 4" (102mm) square cross-section brace members with flat plate connections to the mounting elements. As evident from Figures 8A through 8D these designs are quite complex and varied in approach. The overall summary of limier was that whilst ductile brace fuses for steel CBFs were an effective means of reducing the demand on non-ductile elements that more research was required to improve the proposed systems and develop a model to predict the failure of such brace fuse systems. Accordingly, these prior art fuse designs do not provide a reproducible design for commercial deployment, do not allow a system to be tailored to a particular seismic requirement for either legacy or new building requirements, are costly and complex in their design, and prone to installation variances.
[0072] Another approach within the prior art is that of the Pall friction damper, see for example US Patent 4,409,765, wherein friction pads dissipate energy as heat when they slide against each other. The friction dampers are installed at the junctions of the steel CBF structure and under normal conditions, the friction pads in the dampers do not slip, and the cross-braces act as ordinary cross-braces to stiffen the building against lateral loads such as wind and minor earthquakes. Such cross-brace friction dampers are designed to operate in tension only whereas in-line friction dampers mounted between the structure and brace may in tension and compression. When a major earthquake occurs, the action of the building creates sufficient forces in the braces for the friction pads to slip converting kinetic energy into heat. The Pall friction dampers fall within the four major groups of seismic dampers currently used in North America which are fluid viscous dampers (FVDs), viscoelastic dampers (VEDs), friction dampers (FDs) and metallic yielding dampers (MYDs). However, such systems are expensive and complex as evident from their exploitation to date in large commercial retro-fits rather than general commercial exploitation.
[0073] Also within the prior art are other approaches to load limiting such as depicted within Figures 9A through 90 respectively. Figures 9A and 9B according to L.D.
Reaveley et al in US
Patent 8,037,647 entitled "Perforated plate seismic damper" depict a seismic damper which is intended to substantially eliminate non-linear displacement in an attached support structure. As taught by Reaveley the seismic damper includes a plurality of nodes which have a reduced area wherein the shear, compressive, and/or tensile forces acting on the plate are focused at the nodes.
Similarly Figure 9C according to L.D. Reaveley et al in US Patent 8,099,914 entitled "Perforated plate seismic damper" wherein the damper operates in a similar manner by absorbing energy that is focused at the nodes 910 formed in the portions of the damper that narrow between the cut-outs and perimeter apertures.
[0074] Figures 9D and 9E according to A. Gjelsvik in US Patent Application 2006/0,059,796 entitled "Energy absorber and method of forming the same" similarly depict a damper wherein ductile members in each deform under shear motion of the frame and according to Gjelsvik are designed to keep all brace members under tension during the shear motion induced by a seismic event. Similarly in Figure 9F according to A. Capra in US Patent Application 2002/0,011,037 entitled "Device for limiting the relative movement of two elements of a civil engineering structure and structure including said device" the braces attached to the damper are designed to traverse predetermined arcs as the multiple damper elements between the vertical joining elements deform under the shear force applied to the structure. Figure 9G
depicts a damper according to G.R. Thomas in US Patent Application 2006/0,150,538 entitled "Load Limiting Device" wherein as depicted the damper deforms by hinging at the corners which are connected to the brace members.
[0075] E. In-Line Seismic Fuse: Referring to Figure 10A there is depicted a load versus deformation response for an ideal seismic fuse showing the idealized tensile response which embodiments of the invention emulate to varying degrees. The initial ring reaction would be purely elastic, as shown by the initial region "A". The steel CBF structure will often be under low lateral loading from the structure it forms part, such as wind loads, and must not permanently deform under these actions. When the structure is placed under lateral loads during strong ground motion from a seismic event exceeding the design threshold of the seismic fuse then the intent is for the seismic fuse to enter into an inelastic range of behaviour to dissipate energy and to survive the seismic event, e.g. earthquake, after having been permanently deformed. This being shown by the section labeled "B" representing the region in which the structure, i.e. building, will sustain damages, and is where the ring will undergo plastic deformation. It is important that over this energy dissipation region the slope be greater than zero, as some stiffness is required to stabilize the structure. The actual slope of this "B" region would be determined in dependence upon factors including, but not limited to, the allowable inter-storey drift and lowest expected failure mode of the structure.
[0076] Many structures possess a reserve capacity, which is represented by the section of line labeled "C". Within this region the seismic fuse would strain harden and increases in load carrying capacity. Typically the ultimate source of failure within the steel CBF structure incorporating a seismic fuse according to an embodiment of the invention at larger structure drifts would not be located in the seismic fuse. Accordingly it would be evident that the seismic fuse will provide ductility into a stiff system, allowing the structural engineer to make a simple adjustment and avoid the brittle failure of the system. Development of this seismic fuse concept according to embodiments of the invention was based upon addressing requirements of existing legacy structures with brittle failure concerns at the brace connections. This seismic fuse allows control of the system performance without causing overloading conditions in other areas sometimes seen after strengthening procedures are undertaken. The seismic fuse concept aims to take advantage of the geometric changes and steel stiffening to provide ideal seismic fuse performance.
[0077] Initial calculations of seismic fuse designs and performance were done using common stress strain relationships based upon Pilkey's stress strain equations; see Pilkey "Formulas for Stress Strain and Structural Matrices" (J. Wiley & Sons, ISBN 978-0471527466).
These calculations were checked against finite element models, which produced a large range of yield strengths depending on how the meshing was set. Table 1 shows some of the results for a ring seismic fuse compared to actual results.

Pilkey Finite Element Test Load at Yield 2441(N 2501(N to 5001(N 2501(N
Load at Ultimate 3671(N NA 780kN
Table 1: Comparison of Seismic Fuse Simulations with Test Results for Initial Designs [0078] Accordingly, it was apparent that physical testing was required to provide calibration information on the performance of a seismic fuse formed from a thick steel plate. Accordingly as depicted in Figure 10B seismic fuse designs 1C and 1D were fabricated and tested for calculation calibration. Both seismic fuse designs contain tabs which allow them to be connected to existing braces in retrofit applications and were used by the inventors to connect the seismic fuse designs into the test frame. The outside diameter of the two seismic fuse designs is 400 mm in each case whilst the inner diameter is 250mm (10 inch) in first seismic fuse designs 1C and 300mm (12 inch) in the second seismic fuse 1D. Both seismic fuse designs use 95mm (32 inch) wide tabs with connecting radius of 100mm and have a thickness of 19mm 3 inch). Each seismi ( ¨ c fuse being cut from the thick steel plate using computer guided plasma cutters thereby yielding smooth curves and surfaces on the delivered product.
The steel along the end of the cuts had been hardened due to the heat when cutting.
[0079] Testing was performed using the same 12MN actuator with hydraulic grips and 10001(N load cell as employed above in testing the legacy braces described in respect of Figures 6 and 7A. Each seismic fuse was gripped by the tabs and displacement based loading applied monotonically. Each seismic fuse was also coated with a lime based white wash, which flakes off of the steel locally at approximate yield loading. Data was collected at a rate of two records per second together with load cell readings, global internal linear variable differential transformer (LVDT) and six local LVDTs recording movements, and three strain gauges. Visual data was collected using a camera with timed photographs.

[0080] The deformations of the two seismic fuses 1C and 1D during the tests are shown in Figures 11A and 11B respectively. As can be seen, the seismic fuses begin to yield in six locations, either side of the tab at the top and bottom, and at the centre locations on either side.
These yield regions spread around the areas as the test progresses and geometry changes, progressively yielding locations at higher strengths. Seismic ring 1C began to fold inwards around the load axis at higher load levels, which is not shown well in Figure 11A. The resulting tensile resistance versus axial deformation of the two seismic rings tested is presented in Figure 12. Seismic ring 1C had higher yield and ultimate strength, while seismic ring 1D yielded much earlier, provided an initial post yield slope providing over 50mm deformations, before stiffening increasingly to failure.
[0081] Based upon the data extracted from the initial seismic fuse trials calibration of the finite element model was possible. Subsequent simulations were performed on different seismic fuse designs with the intent of iterating towards a design having a response similar to that shown in Figure 10A. Referring to Figure 13 axial load versus deformation responses for first to third seismic fuse designs 310 to 330 respectively are presented together with design sketches. The inventors investigated ten different geometries during these simulation and measurement activities. As evident from Figure 13 a seismic fuse such as described above in respect of Figure 10B as well as a design consisting of a circular outer geometry with rounded square inner edge profile present responses that mimic the desired profile depicted in Figure 10A. Accordingly design variations of each allow structural engineers to tune the final geometry for an inline seismic fuse for a single brace.
[0082] F. Ductility Improvements: A seismic fuse according to the third seismic fuse 1330 was employed in cyclic tension tests where the testing procedure matched that used with the previous seismic fuses and legacy brace specimens. A set of legacy braces removed from one of the existing structures constructed in 1962 were employed for retrofitting with the third seismic fuse design 1330. Compression tests were again not possible as the full length of brace was not available. The third seismic fuse design 1330 and assembly to the test legacy brace is depicted in Figure 14 wherein the reclaimed braces included a piece of column, and so the gusset to column connection was also included in the test.
[0083] These legacy braces comprised eight similar samples retrieved from a structure, four of which were tested to determine likely problems in existing structures, see Hartley, and their basic properties and effect of bolt changes, while the remaining four were used to test the inline seismic fuse based upon the FEMA 461 protocol. This protocol uses quasi static cyclic testing to investigate seismic effects but it was not possible to use the compression aspects of the cyclic loading as only a small portion of the brace was available and buckling would not be representative. Therefore the testing was designed to run in tension only cycles. In the original testing these brace members had initial failure mode of bolt shear failure in the A307 common steel bolts employed in constructing the steel CBF structure when initially installed. When these bolts were changed for high strength bolts, failure mode changed to the weld ripping out of the attached column web. In these tests a first specimen was left as original, containing A307 bolts, while the other three specimens had column to gusset connections strengthened and bolts changed to A425 bolts.
[0084] These results are presented in Figure 15 wherein the axial load versus system deformation of the brace member tested in the as-received state, and a similar member retrofitted with an inline seismic fuse are presented. Whilst both tests failed due to bolt shear; the specimen without an inline seismic fuse failed before the first tension cycle was completed whereas the inline seismic fuse system provided a stable deformation which allowed for a substantial increase in the energy dissipated compared with the original brace. Whilst the seismic fuse underwent large deformation it did not fracture.
[0085] At this point reduced scale testing of brace system was performed in order to allow enable investigation of the effects of reversed cyclic loading and the buckling that was anticipated to occur in the brace member and inline seismic fuse. Based on a full size structure the brace system was scaled to approximately half full size and was designed to fail at the brace connection by brace net section failure. Structures were tested with the brace member only, a single inline seismic fuse, and a dual inline seismic fuse. A dual inline seismic fuse was included based upon its potential to allow greater ductility over a narrower band of load between maximum wind loading and system failure; as well as providing a new connection surface at either end. The resulting buckled shapes of these tests are shown in Figure 16.
[0086] No low cycle fatigue fracture was seen in any of the tests; however, due to the stroke limit of the actuator it was not possible to buckle the two inline seismic fuse specimens to the same displacement as the tension tests, nor was it possible to fail the two inline seismic fuse specimens. The single inline seismic fuse specimen had a clean gradual bend develop in the ring during compression cycles, which alleviated potential concerns with possible kinking and low cycle fatigue failure. Very little compressive force could be taken in the system; however, this was expected given that the intent was for the braces to act as tension only members. The dual inline brace system had a more interesting buckled shape. No signs of fatigue were apparent after investigating the seismic fuse surfaces post-test.
[0087] Review of the axial load ¨ deformation data for all three systems confirms the original design hypothesis that the inline seismic fuse increase energy dissipation.
Referring to Figure 17 the reduced scale test results are plotted, the base sample of a discrete brace member is shown by first plot 1710, the single inline seismic fuse system by second plot 1720, and the dual inline seismic fuse by third plot 1730. Both inline seismic fuse systems provided increased energy dissipation than the initial discrete brace member wherein the final failure occurring in the discrete brace sample and single inline seismic fuse were statistically the same resistance level.
The specimen using the dual inline seismic fuses was stronger due to the pair of seismic fuses being connected to the gusset plates instead of the brace angles. Net section failure of the brace member no longer governed for this specimen. As evident from third plot 1730 for the dual inline seismic fuse assembly the overall axial load ¨ deformation profile now demonstrates the desired characteristics of the ideal structure.
[0088] G: Inline Seismic Fuse and Historic Structures: The inline seismic fuse discussed supra in respect of Figures 10B through 17 provides a low cost retro-fit solution for steel CBF
structures considered as modern heritage. Further inline seismic fuse can easily be inserted into structures that do not meet seismic requirements and through complete removal of the original brace and its storage, this approach is also fully reversible should that be desired.
[0089] Through increasing the ductility of rigid existing steel CBF structures it is possible to improve the ability of the structure to dissipate seismic energy. Further through the establishment of finite element models for the inline seismic fuse with good correlation to experimental results it is possible to produce a design guideline to assist structural engineers in selecting and using these inline seismic ring fuses which may be integrally formed within replacement brace members and cut according to computer controlled cutting processes for controllability of seismic fuse geometry. Accordingly, a process for retro-fitting an existing legacy structure might proceed by a process flow such as described below:
= Analysis of structure, determining worse case non-seismic and seismic loadings;
= Analysis of structure, determining maximum lateral capacity;
= Determine if structure requires any retro-fitting, beyond replacement of bolts;
= Calculation of maximum inter-storey drift values and corresponding seismic fuse deformation limits;
= Selection of seismic fuse based on factors including, but not limited to, maximum non-seismic loading, maximum lateral capacity, and maximum deformation limits; and = Recalculation of seismic loading and performance for structure with increased ductility.
[0090] Potentially selection of the seismic fuse may be based upon the structural engineer searching a database of standard inline seismic fuses from manufacturers or through entry of design parameters into a computer software application wherein an inline seismic fuse design is provided in a format such a computer assisted manufacturing file format compatible with laser cutting and/ or milling systems for example.
[0091] H: Inline Seismic Fuses within Cross Brace CBF Elements: Based upon the tension only tests at reduced scale full size full cyclic tests were carried out.
These full size full cyclic tests used the same L 127x76x9.5 angles and the same seven, three quarter inch, bolt connections as in the full size tests of discrete inline seismic fuses and legacy braces.
In these full size tests the angle failed in net section with ultimate strength occurring at 6501(1=1 and 38mm axial displacement closely matching the design predictions. These frame tests were carried out in a 7.5m long by 4.8m high frame using two 10001N actuators acting along the top beam in the system. Schematics of the control cross brace sample, i.e. a legacy installation, a cross brace with in-line seismic fuses according to an embodiment of the invention in each brace, and a cross brace with a multi-brace seismic fuse according to an embodiment of the invention are shown in Figure 18 as first to third structures 1810 through 1830 respectively. As described below in respect of Figures 24 through 27 the multi-brace seismic brace depicted in third schematic 1830 is connected to both cross-braces whereas in second schematic 1820 two cross-braces each with an inline seismic fuse according to embodiments of the invention described above in respect of Figures 10A through 17.
[0092] The pin connected frame tests were designed to include design of the seismic fuse to maximize the ductility while still causing failure in the original brace system prior to four percent inter-storey drift. Design guidelines of steel structures indicate two percent as the maximum allowable inter-storey drift; however, a ductile structure can sustain up to four percent drift prior to reaching collapse prevention performance level, see "NEHRP
Commentary on the Guidelines for the Seismic Rehabilitation of Buildings" (Federal Emergency Management Agency, FEMA 274, 1997). During initial development of the seismic fuse were primarily considered only for the purpose of life safety through collapse prevention.
Brace performance was estimated using design calculations with safety factors removed and actual material data, the full size cyclic tests provided further confirmation failure expectations; and seismic fuses were designed based on these numbers.
[0093] The expected results for the control cross brace sample were failure before 50mm lateral displacement of the frame, or prior to 1.3% inter-storey drift, with less than 690kN lateral frame resistance. The single inline seismic fuse in each brace system was expected to fail closer but prior to 162mm lateral displacement (4% inter-storey drift), with the same lateral force as the control sample. As with previous testing the protocol was adapted from FEMA
461 for quasi static cyclic testing. All three structures were testing using the same testing protocol, in order to enable easier comparison of results. Instead of the initial cycle being force determined, an initial lateral displacement was selected as 4mm for the first 6 cycles, after which the cycle was increased by 40% over the previous cycle size every two cycles with completion of the test at 162mm. The test time per cycle was not constant, nor was the rate, but instead these were adjusted to slowly load the structure during the first cycle of any given displacement, while being fast enough to finish the entire testing procedure within three hours.
[0094] The braces were bolted into place in the test frame with brace to gusset bolts snug tight and the gusset to frame bolts pre-tensioned using turn of the nut method. Each test recorded 35 channels of data, including frame monitors, the actuator load cells, lateral control string pots, out of plane movements, and strain gauges. All specimen were coated with a white lime wash that would flake off locally at approximate metal yielding to indicate where yielding is occurring in the specimen. In come cases this coating did not adhere well to the surface due to oil spills or space for strain gauges. Referring to Figure 19A the cross-brace with inline seismic fuses is depicted mounted to the frame prior to testing.
[0095] HI. Control Brace Performance in Cross-Brace: The standard cross brace test showed ideal performance for structural design. Figure 19B shows the bucked shape of the tension only brace system during the test. The brace member performed much better than expected, yielding in gross area prior to net section failure as is ideal, sustaining much higher lateral resistance than expected, and reaching 4% inter-storey drift prior to failure. Figure 20 shows the lateral force ¨ lateral displacement hysteresis performance of the control cross brace system. Post-test investigation showed that cracking associated with net section failure had begun in the brace samples and would likely have cause failure to occur not far beyond where the test was stopped. Following the unexpected ductile behavior of the brace coupons were cut from all three test specimen to compare with previously tested coupons from the same batch of steel angles.
[0096] H2. Single Inline Seismic Fuses in Cross Brace: The single ring fuse in each cross brace had much lower compressive resistance and greater pinching effects than the control sample as was expected from the longer buckling length of the brace members.
As shown in Figure 21, the buckling of each brace did not interfere with performance of the brace in tension.

Even if the buckling brace had buckled toward the brace in tension instead of away (which is unlikely due to eccentricities inherent in angle braces) it would likely have improved compressive resistance and therefore system performance. Figure 22 shows the progressive deformation of the inline seismic fuses. The lateral load ¨ displacement plot for the cross brace with inline seismic fuses is depicted in Figure 23 illustrating stable deformation and gradual stiffening of the brace system as desired. Failure is not reached during testing, as had been expected; due to higher than expected brace strengths. Other than the occasional bolt slip occurring, the hysteretic performance can be defined as an elastic region and post yield region.
[00971 I: Multi-Brace Seismic Fuse: As noted above in respect of Figures 18, 19A, and 21-23 a cross-brace can be formed from two braces wherein each of these braces may be provided with increased seismic resistance through the addition of one or more inline seismic braces such as depicted in respect of Figure 24 wherein examples of inline seismic braces with single central seismic fuse in first brace 2410 and dual seismic fuses at each end in second brace 2420. It would be apparent to one skilled in the art that multiple seismic fuses may be integrated into such a brace. Optionally different fuses may be integrated with different load versus displacement characteristics.
[0098] It would be evident that in some seismic retro-fit upgrades and new installations it may be beneficial to provide a seismic fuse that couples multiple braces together. Accordingly the inventors have designed a multi-brace seismic fuse that may be retro-fitted to a steel CBF
structure or integrated into a single piece cross brace with or without inline seismic fuses. Such a multi-brace seismic fuse according to an embodiment of the invention is depicted in Figure 25 wherein a circular seismic fuse 6D, similar to inline seismic fuse 1D in Figure 10B, comprises the central ring together with two pairs of diametrically opposed tabs. Each pair of diametrically opposed tabs are intended to mount to brace members or alternatively are part of continuous brace members wherein the multi-brace seismic fuse is integrated with the brace members in a single element. Referring to Figure 30 another multi-brace seismic fuse according to an embodiment of the invention is presented for use within an asymmetric brace frame such as depicted within third structure 310C in Figure 3 wherein three brace elements meet.

[0099] Referring to Figure 26 a multi-brace seismic fuse according to an embodiment of the invention as depicted in Figure 25 is shown during testing wherein the multi-brace seismic fuse bends back and forth as the direction of force changes, adding a stiffening effect to the system when the steel must again be bent back to "original" shape prior to going in the other direction.
Referring to Figure 27 the hysteretic performance of such a multi-brace seismic fuse is presented. It would also be apparent to one skilled in the art that the multi-brace seismic fuse is also self-centering and does not require maintenance once installed unlike prior art damper solutions. As depicted in Figure 27 the load versus deformation trace for the multi-brace seismic fuse shows the increased stiffening occurring each cycle can be seen. This increases the area under each curve, and therefore also the energy dissipated per cycle.
[00100] J: Materials Analysis: The pin connected tests raise some questions, including why were the braces more ductile and stronger than expected? One important variable for quantitative comparison of the brace systems is the dissipated energy. How do the brace system performances compare at key points based on the energy dissipated by each?
[00101] J1. Material Variation: As noted the test legacy braces were more ductile and stronger than expected and their results did not match the coupon tests and analytical expectations. Further they did not match the failure mode the angles had been designed to have, and they did not match the performance of earlier angles which should have been from the same batch with the same connection details. Review of the test setup did not provide any initial ideas as to why this could have occurred, leaving questions regarding the material properties. From each of the tested braces areas exposed to the least stresses were removed and turned into coupons for further material testing - in the standard cross brace the only un-yielded region was on the short leg of the angle above the bolt connection. The coupon test procedure followed the ASTM A307 standard (2007) wherein the coupon material results, and expected failure type, are summarized in Table 2. This table indicates two important results, firstly, there was significant variation between the results for the short and long legs of the angle which is unexpected and suggests the batch material properties were not as well controlled initially thought. Second, with only one exception, the original coupons have lower ultimate stresses than those tested.

Sample Angle Leg F F Expected Failure Type Original Coupon Long 341 490 Net Section Short 383 506 Net/Gross Control Brace Short 392.5 527.3 Net Section Short 383.8 516.2 Net Section Multi-Brace Short 408.1 542.9 Net Section Seismic Fuse Long 349.3 520.1 Gross Section Single Inline Short 410.5 529.8 Net Section Seismic Fuse Long 362.0 479.7 Net Section Table 2: Material Analysis Results for Coupons of Legacy Braces [00102] While discussion regarding material variation falls outside the considerations of this specification it is important to note that greater material variation was present than expected. In most castes it would not be possible for the design engineer or structural engineer to know this, and they would have to design for the worse case. However, we look at what could have happened if the braces had failed at expected loads predicted in calculations and previous tests.
Accordingly Figure 28 shows the hysteretic performance of the three tests with failure lines added for 650IN and 6901th lateral force, outlining the expected failure range. As can be seen, the control sample and the sample containing one ring in each brace would both have reached failure loads during the test under these scenarios.
[00103] J2. Energy Dissipation: As each test was exposed to identical test protocols it is possible to compare dissipated energies of each system during the test up to the final cycle, when only the control sample completed both cycles. Dissipated energy was calculated through single integration of hysteretic performance results. The resulting calculated dissipated energies are given below in Table 3 wherein the energies were calculated for:
- at the expected peak strength of the control sample (38mm);
- the dissipated energy at the peak of the cross brace system containing one ring fuse for each sample (123mm);and - the energy dissipated at the completion of the test (123mm).

[00104] If the energy dissipated by each system at their expected peaks are compared, then the control system would dissipate 64kJ; the single inline seismic fuses 211kJ, or 330% of control;
and the multi-brace seismic fuse 480kJ, or 750% of control and 227% of the single inline seismic fuses. At the lower displacements the single inline seismic fuses system provides much less dissipated energy, only 53% of the control sample at expected control peak.
While the multi-brace seismic fuse yields at a lower force than the control system, though the self centering effects it has similar dissipated energy to the control at 38mm. If the three systems are compared for actual performance at 123mm displacement, with all samples having had identical displacement histories, and noting the unexpected ideal performance of the control system, the single inline seismic fuses continue to dissipate less energy, only 56% of control, while the multi-brace seismic fuse dissipated over 123% of the energy the control sample did. This suggests that the multi-brace seismic fuse could not only provide benefit to existing structures in danger of brittle seismic force resisting system failure, but also provide benefit to new structures where ductility is not a concern. As the multi-brace seismic fuse failed at a lower load than expected, it is also evident that it is possible to increase the size of the multi-brace seismic fuse for the given system, which may provide even larger seismic advantages.
Control Cross Dual Inline Multi-Brace Brace Seismic Fuses Seismic Fuse Fuse Design 5F 6D
Control Expected Peak (650kN at 38mm) 63.9kJ 34.0kJ 62.3kJ
Expected Peak (6501th at 123mm) 378.8kJ 211.0kJ 467.3kJ
Max Drift (162mm) 449.7k1 258.6kJ 480.1kJ
Table 3: Energy Dissipation Calculations [00105] It would also be evident to one skilled in the art that alternative configurations of the multi-brace seismic brace may be implemented without departing from the scope of the invention to accommodate alternate bracing structures including, but not limited to, those depicted in Figure 3 by first to eighth structures 310A through 310H
respectively. Accordingly in Figure 29 first to fourth seismic fuses 2910 through 2930 respectively are depicted wherein first seismic fuse 2910 is compatible with third structure 310C, second seismic fuse 2920 is compatible with fourth structure 300D, third seismic fuse 2930 is compatible with first structure 300A, and fourth seismic fuse 2940 is compatible with fifth structure 300E but where the braces cross away from the center of the frame.
[00106] Within the descriptions above in respect of Figures 10A through 29 the disclosure has been primarily from the viewpoint of steel CBF structures. Accordingly in respect of such embodiments of the invention as described within these Figures and within the associated specification it might be assumed that the inline and multi-brace seismic fuses may be formed from steel. However, it would be evident to one skilled in the art that steel may be one of a plurality of grades as well as special steels according to one or more factors including but not limited to the requirements of the structure within which the seismic fuses are being installed, the materials of existing braces to which the seismic fuses are being retro-fitted, the building code requirements, seismic activity of the area, and cost.
[00107] Accordingly steels which may be employed include carbon steel, high strength low alloy steels having small additions (typically < 2% by weight) of other elements, typically manganese, to provide additional strength for a modest price increase. Others may include low alloy steels, alloyed with other elements, such as molybdenum, manganese, chromium, or nickel, in amounts of up to 10% by weight to improve the hardenability of thick sections; stainless steels, containing chromium and often combined with nickel; fenitic stainless steels which are magnetic; and austenitic stainless steels which are nonmagnetic. Other high strength alloys may include dual-phase steel, which is heat treated to contain both a ferritic and artensitic microstructure; Transformation Induced Plasticity (TRIP) steel; Maraging steel with nickel and other elements but low or no carbon; galvanized steel, and Twinning Induced Plasticity (TWIP) steel.
[00108] Examples of standard structural steels include A36 and A529 carbon steels; A441, A572 and A270 high strength low alloy steels; and A514 quenched and tempered alloy steel as specified by ASTM International. It would also be evident that other materials may be employed in different applications including but not limited plastics, ceramics, non-steel alloys, and other metals without departing from the scope of the invention. It would be evident that where the seismic fuse is provided with tabs and is attached to the brace members by methods including, but not limited to, riveting, welding, and bolting that the seismic fuse and brace members may be different materials unlike the instance that the seismic fuse and brace members are all formed integrally from a single initial plate of material.
[00109] Within the embodiments of the invention manufacturing of seismic fuses has been described with respect to computer guided plasma cutting but it would be evident that other manufacturing techniques to form the seismic fuses with or without integral brace members may be employed including milling, laser cutting, oxy-acetylene, etc. including CNC based machines.
Wherein manufacturing techniques such as CNC based milling are employed the thickness of the seismic fuse may be varied with respect to that of the brace members integrally formed with it or with the tabs integrally formed with it for connecting to the brace members.
In some embodiments of the invention the seismic fuse may be thinner or thicker than the brace members and in others equal. Wherein multiple seismic fuses are provided within a single brace member or brace assembly these may likewise be of different designs in terms of plate thickness, cut out geometry, etc. as well as tab width, tab thickness, etc.
[00110] Specific details are given in the above description to provide a thorough understanding of the embodiments. However, it is understood that the embodiments may be practiced without these specific details. For example, circuits may be shown in block diagrams in order not to obscure the embodiments in unnecessary detail. In other instances, well-known circuits, processes, algorithms, structures, and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments. The foregoing disclosure of the exemplary embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure.
The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents.

[00111] Further, in describing representative embodiments of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention.

Claims (16)

1. A brace for a structure comprising:
a first end for attaching the brace to the structure at a first predetermined location;
a second distal end for attaching the brace to the structure at a second predetermined location;
a brace member of predetermined geometry disposed between the first end and the second distal end; and a seismic fuse of a plurality of seismic fuses, each seismic fuse disposed at a predetermined location along the brace member.

2. The brace according to claim 1 wherein, the seismic fuse comprises a circular plate with a circular cut-out in the middle thereby defining a ring of constant width.

3. The brace according to claim 1 wherein, the seismic fuse comprises a circular plate with a cut-out comprising a square with rounded corners.

4. The brace according to claim 1 wherein, the seismic fuse is disposed at least one of: in the middle of the brace, towards the first end of the brace, and disposed towards the first end of the brace with a second seismic brace disposed towards the second distal end of the brace.

5. The brace according to claim 1 wherein, the portions of the brace member connecting the seismic fuse to the first end and second ends are at least one of:

disposed diametrically opposite one another;
integrally formed with the seismic fuse; and attached to the seismic fuse.

6. The brace according to claim 1 wherein, a first predetermined subset of the plurality of seismic fuses are designed with a first predetermined load versus deformation characteristic and are disposed at a first series of predetermined locations along the brace member; and a second predetermined subset of the plurality of seismic fuses are designed with a second predetermined load versus deformation characteristic and are disposed at a second series of predetermined locations along the brace member.

7. A seismic fuse comprising:
a first central member comprising a plate having an outer circular geometry and an inner geometry;
an integer N tabs integrally formed with the first central member, each tab having a predetermined width disposed at a predetermined position around the periphery of the first central member.

8. The seismic fuse according to claim 7 wherein;
when N is two the predetermined positions are diametrically opposite one another.

9. The seismic fuse according to claim 7 wherein;
when N is four they are disposed symmetrically with respect to two axes of symmetry orientated at right angles to one another and running through the centre of the circle defining the outer periphery of the seismic fuse.

10. The seismic fuse according to claim 7 wherein, the tabs are of sufficient length to allow the seismic fuse to be attached to a brace frame structure.

11. The seismic fuse according to claim 7 wherein, the inner geometry comprises at least one of a circle and a square with rounded corners.

12. A method comprising:
establishing a seismic loading for a structure in dependence upon at least the location of the structure;
determining a maximum lateral capacity for the structure under the seismic loading;
determining a minimum lateral capacity for which the structure should remain elastic;
calculating with a microprocessor a maximum drift value for a predetermined portion of a frame of the structure;
generating with the microprocessor a specification for a seismic fuse to form a predetermined portion of a bracing structure to be attached to the predetermined portion of the frame of the structure; and generating with the microprocessor a numerical control file for the seismic fuse.

13. The method according to claim 12 wherein, the maximum drift value relates to an inter-storey drift.

14. The method according to claim 12 wherein, generating the specification comprises entering parameters to an analysis tool employing at least one of finite element techniques and seismic simulations, the parameters relating to at least one of the maximum non-seismic loading, the maximum lateral capacity of the bracing structure, the maximum deformation limits of the bracing structure, and the material properties of the material from which the seismic fuse will be formed.

15. The method according to claim 12 wherein, generating the numerical file further comprises providing in addition to the seismic fuse data relating to at least one of:
an integer N tabs to be integrally formed with the seismic fuse; and an integer M brace members to be integrally formed with the seismic fuse forming a predetermined portion of the brace structure.

16. The method according to claim 12 wherein the seismic fuse comprises a circular plate with a cut-out comprising at least one of a circle and a square with rounded corners.