CA2028868C - Composite concrete-beam structure - Google Patents

Abstract

A composite beam and concrete structure utilizes a shear transfer bar above the beam. Floor panels may be cast in concrete between the beams rising bowed plywood panels.

Description

2o~s~~s Title: Composite Concrete-Beam Structure Backqround to the Invention It has been appreciated for some time that the load bearing efficiency of a combination or "composite" concrete and steel beam can be enhanced by coupling the upper surface of the steel beam to the lower surface of the concrete through shear-force transfer devices. Such devices prevent separation from occurring between the beam and concrete when loads are applied to the composite structure, and allow the neutral axis to be displaced downwardly within the concrete to the degree necessary to ensure that no portion of the concrete is exposed to tensile stress beyond its tensile limit.
U.S. Patent 3,401,497 to Gregory depicts a beam with an upper surface that is covered by an array of protuberances known in the industry as "Nelson Studs". These studs, welded in arrays of varying density, are used to transfer shear stress from the lower region of a concrete beam, or slab portion, to the steel beam lying beneath.
Another form of shear transverse device is depicted in U.S. patent 2,479,476 to Cueni. In this patent a continuous upper zig-zag shaped strip is welded intermittently to the upper surface of a steel beam. While this patent recognizes that it is customary to provide such shear transverse devices in the form of spirals or other sinuous shapes, no suggestion is made as to the possibility of providing a stress transfer element which is straight and is aligned with the upper surface of the beam.

2 In U. S . patent 3 , 177 , 619 to Benj amin, it is proposed to form a composite concrete floor slab on a steel beam by welding corrugated steel sheeting transversely along the side flanges of the upper surface of the steel beam. Welded on top of the transversely running corrugated sheeting in this patent disclosure are a series of reinforcing bars. Also welded to the upper surface of the corrugated sheet at places overlying the side flanges of the steel beam are a series of "S"-shaped connectors which serve as shear transverse devices. The "S"-shape of these connectors is a variation on the vertical-plate type of shear transfer device proposed as an alternative to nelson Studs. Examples of such vertical-plate devices are shown in Cueni (item 16, 17, 19) and in U.S. patent 3,010,257 to Naillon (item 55). An example of an inclined plate of similar intended effect is shown in U.S, patent 1,597,298 to Kahn (item 3).
All of these references rely on discrete, erect elements, extending upwards from the upper surface of the steel beam into the body of the overlying concrete in order to transfer the shear stress within the concrete to the beam.
Figure 4 in Benjamin appears to depict such an arrangement, but as shown in Figure 5 of the same patent the longitudinally-oriented reinforcing bars (items 14) are welded across the tops of the upper peaks of the corrugated sheeting, outboard from the region above the steel beam. As conceived by

- 3 -Benjamin, such reinforcing bars (14) do not participate in the transfer of shear stress. This is apparent from the fact that separate "S"-shaped sheet metal strips (16) are proposed to serve this function.
It has been known to align concrete-imbedded reinforcing bars in a spaced parallel position above the upper surface of a supporting steel beam. Two examples of such an arrangement are shown in U.S. patent 1,688,723 to Lathrop and in U.S. patent 3,010,257 to Naillon. In both of these patents, however, no provision is made to transfer shear stress between the reinforcing bar and the steel beam.
In Lathrop bolts (9) with curved hooks engage the reinforcing bar (6a) to draw it down towards the upper flange of the beam (1). By its very nature, such a curved hook, is incapable of transferring shear stress between the bar (6a) and the beam (1).
In Naillon, Figure 7 shows a reinforcing rod (43) mounted on a series of standoff connectors (49) above the upper surface of a beam 37. In this figure, the beam is loaded as a cantilever and the reinforcing rod is pretensioned in order to improve the strength of the beam-rod combination.
This rod serves solely as a tension bearing element and is incapable of transmitting shear stress from the rod to the beam. This is apparent from column 8, lines 42-45 of this patent which states:

4 "A plurality of cable connectors 49 is optionally fixed to the upper flange; these engage the rod 43 with a close sliding fit to prevent lateral movement between the rod and girder".
Since a sliding fit is provided, no shear stress can be transferred. To transfer shear stress the rod would have to be rigidly attached to each connector. In all events, the configurations of Naillon does not contemplate the formation of a composite structure of both concrete and steel.
From the foregoing analysis, it is apparent that the prior art has contemplated two distinct mechanisms by which shear stress may be transferred from an overlying body of concrete to an underlying and supporting beam;
(1) by a series of discrete connectors in the form of studs or upright plates; and (2) by a continuous, sinuous or helical shear-transferring rod or strip.
Neither of these schemes have, however, achieved the efficiency of the invention herein in transferring shear stress from concrete to the supporting beam. This is because no one in the prior art has recognized the advantages of collecting the shear stress continuously along a shear collecting rod which is mounted above the supporting beam and is aligned with the direction of the shear forces within the concrete.

- 5 -Summary of the Invention According to the invention, therefore, a beam for use in a composite structure comprising concrete overlying said beam, is provided above the upper surface of such beam with a continuous shear force-collecting means that is aligned with the direction of the shear forces that will accumulate in said concrete once the predetermined design load is applied to the composite structure, said shear force-collecting means being further rigidly attached at intervals to the upper surface of said beam by shear transfer connectors. Customarily the shear force-collecting means will be a steel reinforcing rod with a texturized outer surface, and the direction of alignment of the rod will be parallel to the upper surface of the beam.
It it not necessary for the shear force-collecting means to be deeply imbedded into the concrete. It is usually sufficient for it to be positioned above the beam at a distance which is between an eighth and one-half of the depth of the supporting beam. Such a positioning will generally place the neutral axis of the composite concrete beam structure, when subjected to the predetermined design load within the concrete and between the beam and the shear force-collecting means. Preferably the rod will be positioned just above the upper surface of the beam. The actual preferred design criteria for the placement of the shear force-collecting means is to provide that the neutral axis is ~~~8~~8

- 6 -displaced to such a degree as to ensure that none of the concrete, under its design load, will be stressed beyond its tensile limit.
The shear transfer connectors may be any form of coupling which assures that the shear force-collecting means is held in a rigid, fixed relationship with the upper surface of the beam at the points of connection with the beam. This may be achieved by welding, for example, reinforcing rod to steel spacing blocks or plates of the requisite height, which blocks or plates are, in turn, welded to the upper surface of the supporting beam. Alternately, a flanged beam may be provided with bolt-holes through the upper flange, and clamping devices of the type known in the art may fasten the reinforcing rod to bolts that are seated in and extend upwardly through the bolt-holes in the beam.
The beam may be of any customary form, but a "C"
cross-section steel beam has been found particularly suited to this application. Such a beam can be provided with pre-punched bolt-holes in the upper flange that will allow for on-site assembly of the beam with the reinforcing bar.
The shear force-collecting means is preferably chosen from reinforcing bars that have a texturized outer surface.
Such a surface is customarily produced by rolling annular ridges onto the outer surface of the reinforcing bar, but any means of effecting a high frictional coupling between the shear force-collecting means and the concrete may be utilized.

A high efficiency coupling between the concrete and the shear force-collecting means is desirable in order to ensure that a minimum of creep occurs between the concrete and the bar which is being grasped therein. Otherwise, the maximum efficiency of collecting shear stress from within the concrete will be lost.
While the presence of a continuous shear force-collecting means within the lower region of the overlying concrete has the salutary effect of collecting shear stress in a smooth and regular manner in the region adjacent to the shear collecting bar itself, not all of the shear forces in concrete remote from the bar will thereby be collected. To supplement the transfer of residual shear forces within the concrete directly above the beam to the beam itself, traditional upright shear transfer plates may also be installed across the upper surface of the beam. Conveniently, these may be combined with, or placed adjacent to, the shear transfer connectors, relying on the positive attachment of such connectors to the upper surface of the beam to provide a secure anchoring of the traditional upright plates to the beam.
The ancillary use of discrete shear transfer devices in combination with the continuous shear transfer effect achieved by the invention will reduce the demands being placed on the interface between the concrete and the continuous shear force-collecting means. At the same time, the presence of the ~~~ss~s _8_ continuous system will reduce the need for a high density of discrete shear transfer devices that might otherwise be required to transfer a specific load.
A further advantage of the invention arises from the avoidance of the necessity to use a high density array of discrete shear transfer devices. It is preferable that such devices be in the form of bolts that pass through holes in the upper flange of the beam. However, a high density array of bolt-holes will weaken a beam, leaving fabricators with the less desireable option of welding shear transfer devices to the beam. By reason of the lower density of shear tranfer connectors that is made possible by the use of a continuous shear force-collecting system, the option of utilizing bolt-holes is made available in cases where such option could not otherwise be chosen.
In the foregoing description, the shear collecting means has been described as being continuous. It is sufficient for such means to be continuous only between the shear transfer points established by the shear transfer connectors. While it is highly convenient to run a single piece of reinforcing bar along the length of a beam in order to provide a continuous shear collecting means, the invention would still be present where the bar is interrupted and supported in segments down a portion of the length of the beam.

_ g _ The beam referred to in the above description has customarily been described as being made of steel. The beam may also be made of aluminum or other equivalent material of sufficient strength and rigidity to carry the weight of the concrete and absorb the shear stress.
Summary of the Figures In drawings which illustrate embodiments of the invention:
Figure 1 is a cross section normal to the span of the cold formed elements;
Figure 2 is an enlarged cross section of an arrangement at a cold formed section;
Figure 3 is an elevation view of a cold formed section with attached bond bar;
Figure 4 is a large scale elevation view of the welded shear transfer device;
Figure 5 is an elevational view of a bolted shear transfer device using a spring clip;
Figure 6 is a cross section view of a bolted shear transfer device using steel castings:

~~2~~~~

Figure 10 is a shear force transfer device to be used at the end extremity of the bond bar;
Figure 11 is an elevational view of the end bearing arrangement of the cold formed section;
Figure 12 is a cross section view of the same;
Figure 13 is a small scale plan view of the forming system;
Figure 14 is a large scale cross section showing detail for the support of the soffit form;
Figure 15 is a cross section view showing the support of the forming at the edge of a floor area;
Figure 16 is a cross view of a floor in which the concrete slab has an increased thickness in the proximity of the cold formed section:
Figure 17 is a cross section view of the soffit form used in the thickened slab arrangement;
Figure 18 is a cross section view of the soffit form support system;
Figure 19 is a section view showing the end of the cold formed section embedded in a cast in place concrete beam; and Figure 20 is a large scale cross section showing the end bearing reinforcement of the cold formed section.

~Q2~t~~~8 Description of Embodiments Figure 1 illustrates the overall arrangement of a typical cross section. In this 1 is the concrete slab, and 2 is the wire mesh which functions as the transverse reinforcing and as longitudinal shrinkage reinforcing. The cold formed section 3 functions as the tension flange in conjunction with the compression flange comprised by the concrete slab 1. The necessary shear flow is gathered by the bond bar 4 and transferred intermittently to the top flange of the cold formed section 3. The soffit form 5 is a standard width waferboard or plywood sheet. This soffit form supports the freshly poured concrete by bearing upon the soffit form support pieces 6. These soffit support pieces consist in inverted top hat cold formed sections. The wire mesh 2 is draped over the bond bar 4 and then allowed to sag to the bottom part of the slab 1 mid span. In the cross section of Figure 2 the arrangement for the support of the soffit form is shown. The soffit form support piece 6 bearings upon the cold formed saddle 7 which is placed loose on top of the cold formed beam section. In this view, holes 8 in the top flange permit bolting of the necessary shear transfer device (not shown).
Holes 9 in the web permit the passage of electric wiring and domestic water piping. The theoretical composite action of the concrete slab in compression and the cold formed steel channel section acting in tension is achieved by the transfer of the horizontal shear flow between the concrete slab 1 and the cold formed section 3. This transfer starts with the transfer of incremental compression force from the slab 1 to the bond bar 4 throughout its length and its transfer at intermittent points from the bond bar to the cold formed section. Figure 3 is an elevation view of a cold formed section 3 with bond bar 4 and intermediate shear transfer device 10 and end shear transfer device 11. The cold formed channel is supported by end reactions 12. The bond bar 4 consists in a suitable size ordinary deformed reinforcing bar. The number if interior shear transfer devices 10 is dependent upon the total shear flow to be transferred, which is in turn dependent on the load and span.
Typical interior shear transfer devices 10 may take a variety of forms. Figure 4 illustrates solid steel block in which the bond bar 4 is fastened by weld 11 to the shear/block 10 and this in turn is fastened by weld 12 to the top flange of the cold formed channel 3. In this arrangement the bond bar 4 accumulates incremental compression on the side towards the center of the span and incremental tension on the side away from the center of the span. The total accumulated force from the length tributary to each transfer device is then transferred to the cold formed channel 3. In addition the shear transfer device 10 acts directly as a stress compression block to transfer compressive force from the concrete to the cold formed channel 3. Figure 5 illustrates an alternate shear transfer device consisting in a spring steel clip 13 2~2$$~$

equal in width to the cold formed channel flange width. Said steel clip has eccentric holes 14 through which the bond bar 4 passes. The steel clip also has a vertical eccentric hole through its high central point through which the bolt 15 is passed. Tightening of this bolt deforms the spring. steel clip 14 so that it bears at numerous points on the bond bar 4 to effectively anchor it.
Another alternate shear transfer device is illustrated in Figure 6 which consists in a lower cast steel piece 16 and an upper cast steel piece 17. The bond bar 4 is gripped by the serrated surfaces of the semi-circular grooves 18, when the bolt is tightened. Said clamp action is enhanced by the prying action resulting from the eccentric location of the bolt 15. The faces of the upper and lower pieces 16 and 17 and the bolt head provide a direct compression block for the concrete to the cold formed section.

Figure 7 illustrates an end shear transfer device to be located at the end of the bond bar. This end shear transfer device, 11 in Figure 3, consists in a steel casting 29 with closed end hole 30 to receive the bond bar 4. The weld 31 affixes the device to the cold formed section 3.
Alternate arrangements using bolts, comparable to the various forms of bolt affixed intermediate shear transfer devices, could also be used for end shear transfer devices.
Figure 8 is an elevation view of the open side of a cold formed section at its end bearing. In the case illustrated the end bearing support consists in a steel beam 32. The end stiffener 33 is welded into the web space, this end stiffener 33 is of rectangular shape with corners cut off and is seen in elevation in Figure 9. This end stiffener transfers the shear reaction of the C section to the bottom flange bearing. It also serves as a bulkhead in cases where the ends of the beam are embedded in concrete. Also in Figure 11 are the end clips 34 and 35. These are bolted to the end stiffener 33 with the bolt 36. In the case illustrated clips 34 are needed to stabilize the beam 32. In the case of the floor system being continuous at both sides of the beam 32 clips 35 only would be required.
Figure 10 is a plan view to a small scale illustrating the arrangement of the form system prior to the placing of concrete. In Figure 10 the cold formed sections 3 are supported on a poured concrete or concrete block wall 37 and a steel beam 32 for example. The cold formed section 3 are spaced apart to have a clear inside dimension slightly greater than the width of a standard waferboard or plywood sheet 5. Said plywood sheet 5 serves as the soffit form for 5 the concrete slab. Spaces less than the standard board size are made up from cut sheets 39. The soffit form is in turn supported by the soffit support pieces 6. In the case of a space 38 between cold formed sections being less than the standard sheet width the soffit form support pieces are 10 echeloned to suit.
The soffit support pieces 6 are supported at their ends as shown in Figure 11. Cold formed steel top hat section

7 rests loose on the channel 3. The soffit form 5 is carried by soffit support piece 6 which consists in an inverted top 15 hat section. The dimensions of the hanger piece 7 are such that the concrete extends slightly below the top of the cold formed section 3. To strip the soffit forms the support pieces 6 are knocked out from the saddle 7 and this permits the removal of the soffit from 5. The support of the freshly poured concrete at an edge parallel to the cold formed section 3 is achieved by a modified saddle 40, C.F. Figure 12. On its inner edge it carries a typical support piece. Its outer edge is bent to support lumber form pieces 41 to serve as the bottom and side forms for the edge of the slab.
The alternate arrangement for the whole of the floor system is depicted in Figure 13. This features a slab 1 that is of increased depth 42 adjacent to and above the cold formed section 3. This increased the shear capacity of the slab in its transverse action thereby increasing the distributed load strength. It also greatly increases the flexural strength of the composite section in longitudinal action, thus permitting the use of longer spans. Also illustrated in Figure 13 is a continuous chair 43 which is used to retain the wire mesh in its desired location. The wire mesh passes above and is tied to the bond bar 4 and then is supported and tied down to the chair 43 which is in turn tied to the soffit form. The curved soffit of Figure 13 is formed by the use of waferboard panels as illustrated in Figure 14. In this arrangement the waferboard or plywood sheet 44 is arched into the desired shape by the cold formed steel tie 45. The plywood is retained at its edges by end pieces 46. A continuous center block 47 is used to stabilize the arched soffit form under during construction loads.
The support of the curved soffit form is illustrated in Figure 15. The cold formed saddle 48 rests loosely on the channel section 3. The lower ends of the saddle are bent into 180 degrees reversal to receive the form support anchors 49 and 50. These support anchors are fabricated with an end fillet to permit their removal by hammering. The support piece 49 is used on the web side of the channel 3 and the support of the curved soffit 44 and tie 45 is illustrated.
Soffit form support piece 50 is for use on the open side of the channel 3 and at one end it has a stabilizing pin 51 welded to it. Upon the concrete achieving its required strength, the form support pieces 49 and 59 are removed by hammering and the soffit forms 44 removed.
During pouring and curing of the concrete slab 1 the cold formed channels 3 are supported on temporary shoring at their mid-span point. This permits the floor to be built without other shoring and also reduces the dead load deflection prior to composite action being achieved.
In an alternate arrangement to which all of the foregoing is applicable, the cold formed section 3 may be replaced by a cold formed ZEE section.
In another alternate arrangement two parallel but spaced bond bars may be used.
In a further alternate arrangement, the described flooring system may be used in a concrete frame building in which the major beams and girders are either precast or cast in place. Such an arrangement is shown in Figure 16. The cold formed channels 3 would have their ends 52 set into the side forms (not shown) of the beam 53. The cold formed channels 3 would have an internal stiffener 54 which would also serve as a bulkhead to contain the freshly poured concrete. A splice bar 55 would lap with the bond bars 4 to provide negative moment capacity over the beam 53. In this arrangement the concrete slab 1 would be monolithic with the concrete of the beam 53.

Figure 17 illustrates an arrangement in which the combination end stiffener and bulkhead 54 is installed in the end of the cold formed channel 3. The bond bar 4 and end shear transfer piece 11 are also shown. The cold formed section 56 is shaped to be tightly internally fitting to the cold formed channel 3. An extension of its web is bent to serve as the combination stiffener bulkhead 54. The cold formed section piece 56 is inserted in the cold formed channel 3 so that the open side of the piece 56 is adjacent of the web side of the cold formed channel 3.
In an alternate arrangement in a concrete building the cold form channels may be placed on the top surface of a precast beam and the upper part of said beam may be poured in situ to embed the ends of the cold formed channels, and to be monolithic with the floor slab.

Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A beam with two ends and a longitudinal extent with a central portion located there between for use in a composite support structure wherein concrete will overlie said beam and carry a design load, said beam having an upper beam surface and being provided with a continuous shear force-collecting means with a circumferential concrete-engaging surface that is:
(a) mounted above the upper surface of said beam in a spaced relationship thereto, overlying said upper beam surface and extending continuously along the longitudinal extent of the beam (b) aligned substantially in the direction of the longitudinal extent of the beam;
(c) rigidly attached at intervals to the upper surface of said beam by shear transfer connectors fastened to both said shear force-collecting means and to the upper surface of said beam, wherein said shear force collection means has a substantially unobstructed space between its circumferential surface and the upper beam surface intermediate the shear transfer connectors whereby concrete may fully envelop the shear force collecting means and occupy such substantially unobstructed space, and wherein the shear transfer connectors transfer shear forces arising in the shear force collecting means from concrete, to the beam to provide, when concrete is applied over the beam, a composite action between the beam and the concrete.

2. A beam as in claim 1 wherein said shear force-collecting means is mounted in parallel alignment with the upper surface of said beam.

3. A beam as in claim 2 wherein the neutral axis is located above the upper surface of said beam.

4. A beam as in claim 2 wherein said shear force-collecting means is spaced above said beam by an amount which will cause the neutral axis within said composite structure to be located, under design load, within the concrete between the beam and the shear force-collecting means.

5. A beam as in claim 1, 2, 3 or 4 wherein said shear force-collecting means is mounted above said beam at a distance which is between one eighth and one half of the depth of said beam.

6. A beam as in claim 1, 2, 3, 4 or 5 wherein said shear transfer connectors comprise bolts which are fastened to said beam through bolt-holes formed in the supper surface of said beam.

7. A beam as in claims 1, 2, 3, 4, 5, or 6 wherein said shear force-collecting means comprises a steel rod with a texturized outer surface.

8. A beam as in claim 7 wherein said beam is provided with supplemental discrete shear transfer devices in the form of upright plates fastened transversely above the upper surface of said beam at a positioned adjacent to the point of fastening of shear force-transfer connectors to the upper surface of said beam.

9. A beam as in claims 1, 2, 3, 4, 5, 6, 7 or 8 in combination with a concrete layer that:
(a) overlies the upper beam surface;
(b) envelopes the shear force collecting means; and (c) occupies said substantially unobstructed space.

10. A beam as in claims 1, 2, 3, 4, 5, 6, 7, 8 or 9 wherein said beam is "C"-shaped in cross-section.