US3327028A - Method of making composite metal and concrete structures - Google Patents

Description

June 20, 1967 1. H. ROSENBLATT 3,327,028

METHOD OF MAKING COMPOSITE METAL AND CONCRETE STRUCTURES BY 51M www ATTORNEY June 20, 1967 J. H. RosENBLATT 3,327,028

METHOD OF MAKING COMPOSITE METAL AND CONCRETE STRUCTURES Filed Oct. 19, 1964 2 Sheets-Sheet 2 ATTORNEY United States Patent O 3,327,028 METHOD F MAKING CMPSETE METAL AND CONCRETE STRUCTURES .loel H. Rosenblatt, 11619 Le Baron Terrace, Silver Spring, Md. 20902 Filed Uct. 19, 1964, Ser. No. 404,651 Claims. (Cl. 264-34) This invention relates to improvements in composite metal and concrete structures suitable for use in the construction of bridges, buildings, and the like.

More particularly, the invention is concerned with improvements in constructions of the above-mentioned type, wherein concrete slabs are supported by structural metal flexural members and serve as the deck or floor of the composite structure.

In conventional construction of this type, utilizing a continuous flexural metal structural member, the concrete applied in the region of negative bending moment is not considered as participating in the structural resistance to the stresses of applied loads. The reasons for this, as will be apparent to those skilled in the art, are essentially that the flexural stresses occurring in the region of negative bending moment are tensile in direction; and further, that ordinary concrete cannot be relied upon to withstand any substantial amount of tension. Accordingly, the strucural value of the concrete in those regions is ignored, so far as tensile resistance is concerned, and the actual design of the structure, in practice, is predicated upon the assumption that the concrete in those regions is cracked.

The principal object of the invention is to provide a method of constructing such composite structures, whereby the concrete applied in zones of negative bending moment of a continuous metal structural member will participate compositely with the flexural structural member in supporting the loads for which the structure is designed. Thus, by the construction of the invention, it becomes possible to utilize, for any given load design, flexural metal components of reduced cross-sectional thickness and hence of lighter weight and lower cost.

In accordance with the invention, stated briefly, the continuous flexural member and the concrete slab are so combined with one another as to impart to the composite structure the characteristics of a variable moment Vof inertia beam insofar as concerns the stress distribution fromsubsequent loading. By thus combining the concrete slab with the flexural structural member, more of the structure that may otherwise be structurally Wasted is made useful for supporting the loads.

The invention will be more fully understood from the detailed description below and from the accompanying drawing:

FIGS. 1, 2, 3 and 4 are views illustrating several successive steps utilized in the practise of the invention;

FIG. 5 is a fragmentary View on an enlarged scale, partly in cross-section, showing a composite structure of metal and concrete embodying the invention; and

FIG. 6 is a view in perspective, illustrating the structure as it appears in the step illustrated in FIG. 3.

In the drawing, the invention is illustrated as applied to a three-span continuous beam, such as might be used in building a bridge.

The beam, indicated by reference numeral 10, may be of any suitable shape, such as the Wide flange I-shape beam shown in FIGS. 5 and 6.

The arrows indicated by numerals 11 and 12, respectively, represent the end supports and the interior supports for the beam.

The top flange of the beam is provided with suitably spaced shear connectors or transfer elements indicated at 15. These elements 15 may be of any of the forms of shear connectors commonly employed for establishing a bonding connection between a metal flexural member and a concrete slab supported thereby. Thus, these elements 15 may be in the form of angles, channels, headed bolts or pins, or spirals, Welded to the top surface or the beam for engagement with the concrete of the slab. Desirably, the shear-transfer elements are spaced more closely together in the zones of the supports than in mid-span. As will be understood, n these elements 15 have suilicient strength and are welded securely enough to mechanically bond the slab to the beam. These shear connector elements 15 may be welded to the flange of the beam either at the factory or on the job site, depending upon the particular form of the shear connectors used.

FIG. l depicts diagrammatically the beam and the shear connectors attached thereto in place on its supports indicated by arrows 11, 12.

As depicted in FIG. 2, the slab is poured in sections, the sections first poured, indicated at 20, extending over the interior supports 12 in the regions of normal negative bending moments. As will be understood by those skilled in the art, during the addition of the dead load of Wet concrete in the step of the process depicted in FIG. 2, the elastic distribution of bending moments and dead load stresses thereby induced is the same as the distribution encountered in conventional construction. The

maximum bending moments for a series of uniformlyl loaded equal spans Will occur as negative bending moments over the interior supports, as indicated by the portions 26 of the dotted line curve representing the dellection pattern of the three-span continuous beam, FIG. 2. The intervening regions of positive bending moment are indicated by the portions 28 of curved line 27.

In accordance with the invention, pre-stressing tendons 21 comprising wires or rods of high tensile yield and provided with suitable end fittings 22, are embedded in the concrete of the sections 20.

After the concrete of the slab sections 20 has hardened sufficiently and before pouring the concrete for the remaining or intervening sections of the slab, the slab sections 20 are pre-stressed by exerting tension in any conventional manner on the pre-stressing tendons 21.

By pre-stressing the slab sections 20 at this stage of the construction, so that they `behave in flexure as elastic material, the concrete is rendered elastically usable at this stage and the structure :thereby transformed, in effect, into that of a continuous Ibea-m with a variable moment of inertia. The numerous advantageous characteristics of a variable moment of inertia type of beam have long been recognized |by engineers. In order to dbtain these characteristics, it has been necessary in prior practice to build up the metal member in the vicinity of the interior supports, eil-ther by attaching additional steel plates to the flanges in that area or by fabricating the member so that it becomes geometrically deeper in this area, or by a combination of both. In contrast thereto, in accordance with Ithe invention, the effect is achieved by utilizing material, i.e., the concrete in the zones of negative Ibendingrrnoment, most of which, has heretofore 'been ignored for design purposes and its utility thereby wasted.

By thus imparting to the beam 10 the properties of a variable moment of inertia member, further loading of the structure, as by pouring the remaining sections 24 of the slab, Will result in transferring more of the load from the simple lsteel support areas over to the composite section areas which 'have lbecome proportionally stiffer, due to the increased moment of inertial of the now composite lsection in the region of negative bending moment and which consequently Will carry a greater share of the subsequent loading.

Moreover, by reason of the fact that the sla-b sections 20 in the regions 26 of normal negative bending moments,

are pre-stressed after having been interlocked with the `steel member by means of the shear connectors 15 along the interface between the Steel and the concrete, not only is the slab itself pre-stressed, but the metal members 10 themselves are thereby also pre-stressed. Furthermore, the pro-stressing force thereby induced in the metal member not only will oppose and thereby reduce the stresses `developed when loads are applied to the finished structure, but this pre-stressing iniiuence is not confined to the portions of the metal member in the negative bending zone in the vicinity of the interior supports, 'but extends throughout the length of the metal member in the negative bending zone in the vicinity of the interior supports, but extends throughout the length of the metal member. The resultant eect upon the regions of positive bending moment of the beam 10 is `similar to that which may 'be derived if xed end moments were applied to the positive bending portions of the beam considered as a free body, and is essentially similar to that of a variable moment of inertia beam, in that some of the mid-span positive bending is transferred out of the mid-span Zone of the beam whereby to reduce the positive ben-ding at mid-span.

As a result of the composite action established between the metal member and the concrete, the pre-stressing of the concrete in the zones of negative bending induces locked-in stresses throughout the steel section in directions opposite to the direction of the stresses induced by the loads the structure is intended to sustain. The algebraic sum of these two stresses will be lower than the stresses in the absence of the lpre-stressing force, leaving more capassity for carrying additional loads within a given limit of total stress.

After the slab sections 2@ have been pre-stressed to the -desired degree, and the tensioning mechanism has been disconnected from the littings 22, the concrete for the -remaining slab 4sections 24 is then poured to complete the structure.

Instead of pre-stressing the slab sections in the manner described above, known as post-tensioning, i.e., wherein the prestressing takes place after the concrete in these negative bending areas has hardened, it may be desirable in some instances to utilize procedure wherein the metal member 10 is pre-tensioned in these areas before the concrete of these slab sections is poured. In accordance with such procedure, an end block holding device may be attached to the top flange of the beam just outside of the area of the pour to be pre-stressed and a suitable size pre-stressing tendon attached thereto and tensioned before the concrete is poured. Thus, at this stage, the naked steel Iof the `beam alone is actually pre-stressed. When the irst pour of concrete is then placed, the de-ad load stresses induced thereby are combined with the -prestressed condition of the steel to establish the stresses at this stage. After the concrete of these slab sections has hardened, the end holding devices are released, or the tendons may be cut between the edge of the slab and the end blocks, enabling the stresses to be transferred to the concrete by Way of the shear connectors 15. The end blocks used as temporary anchorages for the pre-stressing tendons may then be removed, whereupon completion of the structure may proceed by placing of the concrete for the slab sections 24.

Although I have hereinabove referred to the stressing of the slab sections20 (either pre-tensioning or post-tensioning) by exerting tension on wire or :rod tendons 21 embedded in the concrete of sections 20, it is to be understood that in lieu of thus stressing these sections, lsubstantially the same effects as hereinabove set forth may be obtained by utilizing any known type of concrete composition that inherently exerts an axial compressive force on the lconcrete of slab sections 20, or that inherently exerts an axial expansive force `from sections 24 against the concrete of the adjacent slab sections 20.

I claim:

1. A method of combining a concrete slab with a continuous flexural member, which comprises iirst applying concrete along transversely spaced areas constituting sections of the slab in negative bending areas of the continuous exural member to effect a shear-transferring bond between said sections of the slab and said flexural member, pre-stressing the said sections of the slab and together therewith the compositely joined iiexural member, whereby to impart the properties of 4a variable moment of inertia beam to the continuous composite structure, and then applying the concrete to the remaining `sections of the slab.

2. A method of constructing a continuous composite concrete and structural flexural member which comprises placing a metal exural member on suitably spaced supports, said yilexural member `having shear transfer devices aixed to the surface thereof which is to constitute the interface between the concrete and said member, :applying concrete in the zones of 'said exural member constituting the zones or negative bending moment, imposing stresses in the concrete applied to said zones, whereby to impart the properties of a variable moment of inertia beam to the composite structure, Iand then applying concrete to the remaining zones of said flexural member.

3. A method of constructing a composite metal `and concrete structure, which `comprises providing a plurality of shear transfer elements along the surface of the metal member which is to constitute the interface between the concrete and the metal member, applying concrete in the regions of negative bending moment of said metal member, imposing stresses in the concrete of said regions to induce stresses in the metal member by shear transfer rfrom the concrete along said interface and applying concrete to the lremaining zones of `said member.

44. The method Iof claim 1 wherein said pre-stressing of said first-named slab sections atnd the compositely joined flexural member is carried out by tensioning of tendons incorporated in the concrete of said first-named slab sections.

5. The method of claim 3, wherein 'said imposed stresses of the first-named concrete sections and the compositely joined iiexural member is carried out by tensioning of tendons in said negative bending areas before the placing yof the concrete for said first-named sections.

References Cited UNITED STATES PATENTS 2,413,990 l/ 1947 tMuntz 52--223 2,510,958 6/1950 Coff 52-223 2,917,901 12/'1959 Lackner 52--223 XR FOREIGN PATENTS 164,961 9/ 1955 Australia.

ROBERT F. WHITE, Prim-ary Examiner.

I. A. FINLAYSON, Assistant Examiner,

Claims (1)

1. A METHOD OF COMBINING A CONCRETE SLAB WITH A CONTINUOUS FLEXURAL MEMBER, WHICH COMPRISES FIRST APPLYING CONCRETE ALONG TRANSVERSELY SPACED AREAS CONSTITUTING SECTIONS OF THE SLAB IN NEGATIVE BENDING AREAS OF THE CONTINUOUS FLEXURAL MEMBER TO EFFECT A SHEAR-TRANSFERRING BOND BETWEEN SAID SECTIONS OF THE SLAB AND SAID FLEXURAL MEMBER, PRE-STRESSING THE SAID SECTIONS OF THE SLAB AND TOGETHER THEREWITH THE COMPOSITELY JOINED FLEXURAL MEMBER, WHEREBY TO IMPART THE PROPERTIES OF A VARIABLE MOMENT OF INERTIA BEAM TO THE CONTINOUS COMPOSITE STRUCTURE, AND THEN APPLYING THE CONCRETE TO THE REMAINING SECTIONS OF THE SLAB.

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