US20120180423A1

US20120180423A1 - Yielding Rod to Counter Seismic Activity

Info

Publication number: US20120180423A1
Application number: US13/348,557
Authority: US
Inventors: Francisco Avila
Original assignee: Seismic Design Toolbox Inc
Current assignee: Seismic Design Toolbox Inc
Priority date: 2011-01-19
Filing date: 2012-01-11
Publication date: 2012-07-19

Abstract

A yielding rod to counteract seismic activity having a generally cylindrical shape with a narrow portion having a reduced cross-sectional diameter relative to the remaining portions of the yielding rod. The narrow portion spans a length that is approximately 8 times its diameter to approximately 16 times its diameter. A transition area connects the narrow portion of the rod to the remainder of the rod. The transition area spans a length of approximately 0.25 inch to approximately 1.5 inch and has a preferred slope of 30 degrees or less relative to a longitudinal axis of the rod.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This patent application claims the benefit of U.S. Provisional Patent Application No. 61/461,495 entitled “Threaded Rod with a Reduced Cross-Section,” filed Jan. 19, 2011, which application is incorporated in its entirety here by this reference.

TECHNICAL FIELD

This invention relates to anchoring rods that counter seismic activity.

BACKGROUND

In the structural design of building structures located in areas of high seismic risk, it is desirable to control the behavior of the structure and thus limit the damage. One method of controlling behavior is by designing certain structural components to fail in a controlled and desirable manner
Building codes are now requiring that structures that are expected to experience seismic events, behave in a ductile manner That is, a structure must be able to undergo large inelastic deformations without losing its strength. One particularly problematic issue is creating a ductile connection at the base of a structure where it is anchored to its concrete foundation. Current building codes require that the failure of anchorage connections be governed by the “steel strength of a ductile steel element” (ACI 318-08, section D.3.3.4), as opposed to the brittle behavior associated with failure of concrete or epoxies. This design goal is currently extremely difficult and expensive to achieve due to the fact that steel is extremely strong in tension relative to the strength of concrete and epoxy. It is especially expensive, both in terms of costs associated with labor and materials, to achieve this goal in existing structures. This is due to the fact that the geometry of the existing foundation is already set and cannot be changed with relative ease. It is important to solve this problem, in order to build more dependable, better performing and economical structures.
Structures are generally anchored to a concrete foundation with steel anchor bolts that prevent the structure from lifting away from the foundation or moving laterally. The types of bolts that are most commonly used in new construction are J-bolt, L-bolt, hex head bolt and threaded rods. For anchor bolts placed in existing hardened concrete, such as would occur in a remodel or retrofit of the building, a commonly used method of anchoring of building is to drill a hole in the concrete foundation, fill the hole with an adhesive (commonly referred to as epoxy), and then insert a threaded rod into the hole. For buildings located in areas of seismic risk current building codes require that failure of anchor bolt connections be limited to the failure of a ductile steel element and not the concrete or epoxy. While this requirement is relatively easy to fulfill in cases of new construction and were anchor bolts are placed far away from the edges of the supporting concrete, the same is not true for anchor bolt installations near the edges of supporting concrete elements.
The most common solution for new construction involves increasing the amount of concrete and reinforcement around the anchor bolts. For existing construction, the most common solution to ensure ductile failure involves drilling bolts through the foundations, which may be several feet deep, and digging such that a nut and washer can be placed at the end of the anchor bolt. The problem with these solutions is that they require an increased amount of labor, material, and time to build.
Other fields also utilize anchors designed to withstand or compensate for lateral impact. However, these anchors are insufficient to use for anchoring a building to a foundation to withstand seismic activity. This is because these anchors have reduced cross-sectional areas that are designed to snap off upon impact rather than yielding.
For the foregoing reasons there is a need for a method and a device that will withstand seismic activity while being economical and efficient to install and replace.

SUMMARY

The yielding rod is an anchor device to counteract seismic activity encountered by a building, that also reduces the amount of labor, material, and time required for concrete anchorage connections of the building to its foundation. The yielding rod comprises a cylindrical bar having a specifically designed reduced cross-sectional area, such that it counteracts seismic activity by yielding itself rather than through failure of the concrete or epoxy. Due to the reduced cross-sectional area, seismic activity is less likely to damage the foundation of building because the reduced cross-sectional area dissipates energy through yielding in tension during a large earthquake. Even if the earthquake was so powerful as to cause damage, the damage would be to the reduced cross-sectional area of the yielding rod as opposed to the concrete foundation. However, replacing or repairing the yielding rod will result in a tremendous amount of savings in labor, material, and time because of the ease with which these anchors can be easily replaced or repaired without modifying or reinforcing the concrete foundation.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an elevation view of an embodiment of the present invention;

FIG. 2 shows an elevation view of an embodiment of the present invention in use; and

FIG. 3 shows an elevation view of an embodiment of the present invention in use to repair a damaged anchor; and

FIGS. 4A-4C show the stress on a yielding rod at different stages of tension applied to the yielding rod.

DETAILED DESCRIPTION OF THE INVENTION

The detailed description set forth below in connection with the appended drawings is intended as a description of presently-preferred embodiments of the invention and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description sets forth the functions and the sequence of steps for constructing and operating the invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and sequences may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention.
The present invention is directed towards a yielding rod 100 for anchoring a building or house to a foundation 20, wherein the rod 100 is configured to yield under stress, such as that created by an earthquake. The rod 100 is generally cylindrical in shape having a first portion 102 terminating at a first end, a second portion 104 terminating at a second end opposite the first end, the rod 100 defining a longitudinal axis A that goes through the first and second portions 102, 104. Portions of the rod 100 in between the first and second ends taper radially inward to a narrow portion 106 having a reduced cross-sectional diameter D₁relative to the cross-sectional diameter D₂of the first and second ends 102, 104. Thus, the tapering portions define a first transition area 108 connecting the first portion 102 to the narrow portion 106 and a second transition area 110 connecting the second portion 104 to the narrow portion 106.
The ductility of the yielding rod 100 is realized by the precise relationship of the dimensions of the diameter D₁of the narrow portion 106 relative to the length L₁of the narrow portion 106, and the length of the transition areas L₂, L₃. In other applications using rods with a reduced cross-sectional diameter, the length of the cross-sectional diameter area is much smaller than the diameter of the reduced cross-sectional area, which allows the anchor to snap or break at the reduced cross-sectional diameter area with the application of force. However, the ductility of steel increases with an increase in its length. Therefore, elongating the length L₁of a narrow portion 106 and the lengths L₂, L₃of the transition areas 108, 110 could improve its ductility and reduce its propensity to snap under lateral forces.
Simply elongating the length L₁of a narrow portion 106 would not necessarily result in an effective yielding rod. If the dimension of the diameter D₁of the narrow portion 106 is not proper, then the yielding rod 100 may not properly perform its function.
For example, the narrow portion 106 of the yielding rod 100 must have a static strength in tension that is less than the static breakout strength in tension and the static pullout strength in tension of a concrete foundation 20 in which the yielding rod 100 has been embedded. This will assure that the weakest point of the foundation is the narrow portion 106 of the yielding rod 100 and not the concrete 20. Therefore, during an earthquake, the lateral forces imparted on the foundation by the building 10 will cause the yielding rod 100 to absorb the tension rather than causing the yielding rod 100 to break out or pull out from the concrete foundation 20, thereby avoiding destruction of the concrete 20. With the concrete 20 still intact then, only the yielding rod 100 needs to be replaced or repaired.
The static concrete breakout strength in tension and the static pullout strength in tension of the concrete foundation can be determined based on calculations established by the International Code Council and the American Concrete Institute. The lesser of the two values will be designated as the minimum concrete strength (MC). The diameter D₁of the narrow portion 106 of the yielding rod 100 is then calculated by the following equation:
D ₁=Square root of [(MC)/((pi/4)×(T))], Eq. 1:
where MC is the minimum concrete strength and T is the upper limit of the tensile strength of the yielding rod 100 material (e.g. steel).
With the requisite diameter D₁of the narrow portion 106 known, the dimensional ranges for the transition lengths L₂, L₃can be determined by performing a non-linear finite element analysis of a wide range of geometries. The transition lengths L₂, L₃are the distance along the longitudinal axis A covered by a transition area 108, 110. The transition lengths L₂, L₃must be long enough such that it helps remove any concentrated stresses, but short enough so that it does not take up too much space. In developing the models, several long slender rods of steel were tested in order to determine the mechanical properties of the steel. In a first analysis, a steel rod with transition lengths L₂, L₃of 1.5 inch was used. The mechanical properties were plotted in a graph depicting the stress versus strain with the stress representing the force imparted on the rod along its longitudinal axis and the strain representing the length of elongation of the narrow portion 106. The data points from the stress-strain curve were then input into the computer model in order to realistically depict the behavior of the invention.
A second analysis was then run with each transition length L₂, L₃of 0.5 inch and a length L₁for the narrowed portion 106 at 8 times its diameter D₁, which established the upper bound of angles for the transition areas 108, 110.
A third analysis was run with a 0.5 inch transition lengths, L₂, L₃and variable lengths L₁for the narrowed portion 106 to study the effects of lengthening the narrowed portion 106.
Tables 1-3 shows some results of the experiment correlating the yielding characteristics of a steel yielding rod 100 to different parameters of the length L₁of the narrow portion 106, the diameter D₁of the narrow portion 106, and the transition lengths L₂, L₃. All dimensions are shown in inches. Table 1 shows the result of a steel rod having fixed transition lengths L₂, L₃of 1.5 inch. The diameter D₁of the narrow portion 106 ranged from 0.25 inch to 0.4375 inch. The length L₁of the narrow portion 106 was always started at 8 times the diameter D₁. The percent strain (or elongation of the yielding rod) was calculated as follows:
% strain=(LE/L ₁)×100, Eq. 2:
where LE is the length of elongation of the narrow portion 106 and L₁is the length of the narrow portion 106 prior to elongation.

TABLE 1

D₁	L₂, L₃	L₁(8 × D₁₎	Elongation (LE)	% strain

0.25	1.5	2	0.2	10
0.3125	1.5	2.5	0.23	9.2
0.375	1.5	3	0.29	9.666667
0.4375	1.5	3.5	0.33	9.428571

Table 2 shows the result of a steel rod having transition lengths L₂, L₃of 0.5 inch. The diameter D₁of the narrow portion 106 ranged from 0.25 inch to 0.4375 inch. The length L₁of the narrow portion 106 was always 8 times the diameter D₁.

TABLE 2

D₁	L₂, L₃	L₁(8 × D₁)	Elongation (LE)	% strain

0.25	0.5	2	0.17	8.5
0.3125	0.5	2.5	0.22	8.8
0.375	0.5	3	0.26	8.666667
0.4375	0.5	3.5	0.31	8.857143

Table 3 shows the result of a steel rod having fixed transition lengths L₂, L₃of 0.5 inch. The diameter D₁of the narrow portion 106 ranged from 0.25 inch to 0.4375 inch. The length L₁of the narrow portion 106 varied from 4 inches to 5.25 inches as shown in Table 3.

TABLE 3

D₁	L₂, L₃	L₁	Elongation (LE)	% strain

0.25	0.5	4	0.35	8.75
0.3125	0.5	4.6875	0.42	8.96
0.375	0.5	5.25	0.46	8.761905
0.4375	0.5	5.25	0.47	8.952381

FIGS. 4A through 4C show the Mises stress (ksi) of one rod from the finite element analysis. FIGS. 4A through 4C show the different amounts of stress on different portions of the rod 100 with increasing force in the longitudinal direction. As shown in FIG. 4A, the stress is greatest at the narrow portion 106 and the junction 112. The amount of stress gradually decreases moving up the transition area 108 towards the first portion 102 with the least amount of stress at the first portion. Similar results occurred in all rods 100 tested.
Based on the extensive experimentation, it was determined that in the preferred embodiment, the length L₁of the narrowed portion 106 should be approximately eight to approximately sixteen times the diameter D₁of the narrowed portion 106. However, the length L₁did not have as much effect on the maximum strains as did the lengths L₂, L₃of the transition areas 108, 110. For example, a rod 100 with a reduced diameter of 0.25 inch and a length of 2 inches (8 times its diameter) achieved an elongation of 0.17 inch, which translated into an 8.5% strain. A rod 100 with a reduced diameter D₁of 0.25 inches and a length L₁of 4 inches (16 times its diameter) achieved an elongation of 0.35 inches, which translated into an 8.75% strain. Therefore, the length L₁of narrow portion 106 should have a minimum length of 8 times its diameter D₁and can be adjusted to greater lengths based on the desired elongation for the connection.
Surprisingly, it was the transition lengths L₂, L₃that appeared to have a significant impact on the maximum strain on the yielding rod 100. Each tapering portion or transition area occurs over a transition length L₂, L₃of approximately one-quarter (0.25) inch to approximately two (2) inches. After extensive experimentation, it was determined that the transition lengths L₂, L₃are half (0.5) inch to approximately one and a half (1.5) inch were preferable. Most preferably, transition lengths L₂, L₃are approximately three-quarters (0.75) inch.
The transition areas 108, 110 ensure that the axial stresses are smoothly transferred to the narrowed portion L₁and avoid premature fracture due to concentrated stresses at the junctions 112, 114 where the transition areas 108, 110 meet the narrow portion 106. The junctions 112, 114 also serve as a point at which the onset of yielding begins. After these junctions 112, 114 yield, it will experience strain-hardening and allow the entirety of the narrowed portion 106 to yield and exhibit plastic behavior. The junctions 112, 114 may be sharp or abrupt transitions from the respective transitional areas 108, 110 to the narrowed portion 106 or they may be smooth, curved transitions from the transitional areas 108, 110 to the narrowed portion 106.
In order to determine the range of angles for the transitional areas 108, 110 relative to the longitudinal axis A, a first analysis was conducted using a 1.5 inch for each transition length L₂, L₃and different reduced diameters D₁with a length L₁of the narrowed portion at 8 times its diameter D₁. This established the lower bounds of the ranges of angles.
It was determined from the analysis that the maximum angle for the slope S (defined as the angle between the surface of the transitional area and the longitudinal axis) of the transitional area 108, 110 should not exceed approximately 30 degrees. It was determined that any angle greater than approximately 30 degrees will begin to significantly reduce the ductility of the invention due to increased concentrated stresses at the transition area by increasing the stress concentrations at the junction 112, 114 where the transition areas 108, 110 meet the narrow portion 106. Therefore, in the preferred embodiment, the slope of the transition area relative to the longitudinal axis is preferably approximately 30 degrees or less. However, having transition area, 108, 110 is better than not having transition areas; therefore, the yielding rod 100 can have a transition area 108, 110 with a slope S of less than 90 degrees, which will still work better than a rod without a transition area.
In the preferred embodiment, the first portion 102 and the second portion 104 may be partially or completely threaded. The threading allows the rod to receive a coupler 120 for securement and attachment purposes. Threading also allows the yielding rod 100 to be quickly and easily replaced or repaired. For example, if during an earthquake the yielding rod 100 brakes at the narrowed portion 106 (since it is the weakest portion of the rod and designed to be weaker than the breakout or pullout strength of the concrete) the yielding rod 100 can be repaired by cutting out the damaged or broken narrowed portion 106 and the transition areas 108, 112 and replacing it with a new truncated yielding rods 101 as shown in FIG. 3. The new yielding rod 101 also having threaded ends can be secured to the old yielding rod 100 still embedded in the foundation at one portion 104 with a coupler 120 and secured to the yielding rod 100 still embedded in the building 10 at the opposite portion 102 with a second coupler 121.
The yielding rod 100 is preferably made out of material that exhibit ductile behavior as defined by the American Concrete Institute®. The preferred material should have a tensile test elongation of at least approximately 14 percent and reduction area of at least approximately 30 percent. Currently for threaded rods the following materials qualify as ductile steels: ASTM A307, ASTM F1554, ASTM A193; and for deformed reinforcement: ASTM A615, ASTM A706 and ASTM A955. It is also desirable that the material used have a narrow range of tensile strengths. For example, ASTM F1554, Gr. 36, material has an allowable tensile strength range of 58,000 psi to 80,000 psi. For optimal design and predictable behavior, the range should be kept within 20%. For example, if ASTM F1554, Gr. 36, material is used, the manufacturer should ensure that the upper limit of the tensile strength should be 69,600 psi, instead of 80,000 psi.
To manufacture a proper yielding rod for anchors used in existing foundations, based on the existing foundation information, the maximum breakout strength of the concrete and the maximum pullout strength of the anchor in tension must be calculated.
The lesser of the breakout strength of the concrete and the pullout strength of the anchor in tension is used to calculate the required diameter D₁of the narrowed portion 106. In calculating the diameter D₁of the narrowed portion 106, the maximum probable strength of the material is used. For example, if the material has a specified tensile strength ranging between 58,000 psi and 70,000 psi, use 70,000 psi. This will ensure that the strength of the steel will still govern if the steel provided has tensile strength at the upper limit of its allowable range.
Based on the reduced diameter calculated above, calculate the design strength of a single anchor or group of anchors by multiplying the nominal strength of a single anchor or group of anchors in tension, Nsa, with the strength reduction factor for ductile steel elements in tension, F, as governed by the steel strength. F×Nsa should be calculated using the lower value of the specified tensile strength of the material. For example, if the material has a specified tensile strength ranging between 58,000 psi and 70,000 psi, use 58,000 psi. This value will be the limiting strength of the connection and is to be used in checking against the factored tensile force applied to an anchor or group of anchors, Nua, determined from analysis of the structure.
The length L₂, L₃of the transitions areas 108, 110 and the narrow portion 106 based on physical constraints and desired performance for its intended use can be determined For example, if there is not much space, use a shorter length L₁for the narrow portion 106. If there is plenty of space available and it is desirable to get more elongation capacity out of the anchor, a longer length L₁for the narrow portion 106 can be used.
For anchors used in new foundations, the required diameter of the narrowed portion 106 is calculated based on the factored tensile force, Nua, determined from analysis of the structure.
By way of example only, to use the yielding rod in an existing construction, a hole is drilled into the existing foundation, the hole having a diameter that is prescribed by the epoxy manufacturer's report. The hole as is cleaned as prescribed by the epoxy manufacturer's report. The hole is filled with epoxy adhesive that complies with building code regulations. One end 104 of the yielding rod 100 is inserted into the epoxy, such that the one transition area 110, the narrow portion 106, and a portion of one end 104 are all projecting out of the concrete 20. The other end 102 can be secured to an anchorage device 30 which attaches to the structure 10.
In situations where a yielding rod 100 is being repaired, or a generic thread rod is being replaced, a threaded portion of the rod 104 projecting out from the concrete foundation 210 can be cut and filed if necessary. One end 104 of the new yielding rod 101 can be connected to the existing threaded rod 100 with a coupler 120. The coupler 120 may be bolt having internal threads to screw onto the threaded rod 100 at one end and the yielding rod 101 at the other end. This process can be repeated at the opposite end of the yielding rod 101 connected to a threaded rod 102 projecting from the building 10 or house. In some embodiments, the coupler 120 may have two different diameters at the opposite ends to accommodate an existing threaded rod 100 and a yielding rod 101 that may have two different diameters D₂.
By way of example only, to use the yielding rod in a new construction, place the rod 100 with a template at the location where the bolt will anchor down the structure, such that the one end 104 will be embedded in the concrete 20. Provide a nut and plate washer at the bottom of the first end. Ensure that the rod 100 is placed as vertical as possible and ensure that the other end 102, the transitional areas 108, 110, and the narrowed portion 106 will all project outside of the concrete. Pour the concrete and secure the yielding rod 100 to the building 10.
The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention not be limited by this detailed description, but by the claims and the equivalents to the claims appended hereto.

Claims

1. A yielding rod defining a longitudinal axis, comprising:

a. a first portion terminating at a first end having a first cross-sectional diameter;

b. a second portion opposite the first portion, the second portion terminating at a second end and having a second cross-sectional diameter;

c. a narrow portion in between the first and second portions, the narrow portion have a third cross-sectional diameter and a length;

d. a first transition area in between and connecting the first portion and the narrow portion; and

e. a second transition area in between and connecting the second portion and the narrow portion,

f. wherein the third cross-sectional diameter is smaller than the first and second cross-sectional diameters,

g. wherein the first transition area and the second transition area each have a taper that gradually narrows towards the narrow portion, and

h. wherein the length of the narrow portion is greater than the third cross-sectional diameter,

i. wherein the first and second transition areas each have a transition length of approximately 0.25 inch to approximately 2 inches,

j. wherein the first and second transition areas each have a slope relative to the longitudinal axis greater than zero degrees and less than approximately 90 degrees,

k. wherein the length of the narrow portion is at least approximately 8 times the diameter of the third cross-sectional diameter, and

l. wherein the narrow portion has a static strength in tension that is less than a static concrete breakout strength in tension and a static pullout strength in tension of a concrete foundation in which the yielding rod is embedded.

2. The rod of claim 1, wherein the first and second transition areas each have a transition length of approximately 0.5 inch to approximately 1.5 inch.

3. The rod of claim 1, wherein the first and second transition areas have a slope relative to the longitudinal axis of approximately 30 degrees or less.

4. The rod of claim 2, wherein the length of the narrowed portion is between about 8 times and about 16 times the diameter of the third cross-sectional diameter.

5. A rod defining a longitudinal axis, comprising:

d. a transition area in between and connecting the first portion and the narrow portion,

e. wherein the third cross-sectional diameter is smaller than the first and second cross-sectional diameters,

f. wherein the transition area has a taper that gradually narrows towards the narrow portion, and

g. wherein the length of the narrow portion is greater than the third cross-sectional diameter.

6. The rod of claim 5, wherein the transition area has a transition length of approximately 0.25 inch to approximately 2 inches,

7. The rod of claim 5, wherein the transition area has a transition length of approximately 0.5 inch to approximately 1.5 inches.

8. The rod of claim 5, wherein the transition area has a slope relative to the longitudinal axis greater than zero degrees and less than 90 degrees.

9. The rod of claim 5, wherein the first and second transition areas have a slope relative to the longitudinal axis of approximately 30 degrees or less.

10. The rod of claim 5, wherein the length of the narrow portion is at least approximately 8 times the diameter of the third cross-sectional diameter.

11. The rod of claim 5, wherein the length of the narrow portion is between about 8 times and about 16 times the diameter of the third cross-sectional diameter.

12. The rod of claim 5, wherein the narrow portion has a static strength in tension that is less than a static concrete breakout strength in tension and a static pullout strength in tension of a concrete foundation in which the yielding rod is embedded.

13. A rod defining a longitudinal axis, comprising:

a. a first portion having a first end and a second end, and a first cross-sectional diameter;

b. a narrow portion having a second cross-sectional diameter and a length;

c. a transition area operatively connecting the first portion to the second portion, the transition area having a taper,

d. wherein the second cross-sectional diameter is smaller than the first cross-sectional diameters, and

e. wherein the taper of the transition area gradually narrows towards the narrow portion forming a slope of approximately 30 degrees or less relative to the longitudinal axis.

14. The rod of claim 13, wherein the transition area has a transition length of approximately 0.25 inch to approximately 2 inches,

15. The rod of claim 13, wherein the transition area has a transition length of approximately 0.5 inch to approximately 1.5 inches.

16. The rod of claim 13, wherein the length of the narrow portion is at least approximately 8 times the diameter of the second cross-sectional diameter.

17. The rod of claim 13, wherein the length of the narrow portion is between about 8 times and about 16 times the diameter of the second cross-sectional diameter.

18. The rod of claim 13, wherein the narrow portion has a static strength in tension that is less than a static concrete breakout strength in tension and a static pullout strength in tension of a concrete foundation in which the yielding rod is embedded.