COMPOSITE, PRE-STRESSED, STRUCTURAL MEMBER

Abstract: A method of making a composite prestressed structural member in the inverted position, comprising a support member being built up of two vertical joined I-beams (11,13) lying above, and a mould (25,27) for the concrete slab (41) lying beneath. The mould (25,27) is being deflected. The support member has a flange at or near the neutral axis, with respect to this load conditions.

Description: This invention relates to composite, pre-stressed structural members and methods for making such structural members. In the field of construction composite, pre-stressed structural members, many methods of pre-stressing are available. A particularly desirable method of pre-stressing such composite structural members is shown in U.S. Patent No. 4,493,177. Here the pre-stressing is achieved by form­ ing the composite structure upside down. The upside down forming includes connecting the steel beams of the composite member to the upper side of a mould so that shear connectors extend downwardly into the mould. The steel beams and the mould are joined and supported so that deflection of the mould causes a parallel deflection of the steel beams. As the mould is filled with concrete, the steel beams and mould deflect downwardly from the weight of the beams, mould and concrete, thus pre-stressing the beams. The top flange of the inverted beams (bottom flange when upright) receives a compression pre-stress. After the concrete hardens, the mould is removed and the connected beams and concrete slab are inverted so that the composite structure is upright. In the upright position the bottom flange of the beams receives a tension stress which is reduced by the compression pre-stress achieved by the inverted moulding. The concrete, of course, receives a compression stress. This type of pre-stressing produces an improved pre-stress resulting from the pouring of the concrete itself. No separate pre-stress activity is required. In addition, because the uppermost or surface concrete is the concrete formed at the bottom of the mould, the concrete surface is less permeable and harder than concrete structures which are not inverted. Still further, this type of pre-stressing results in a pre-stress relationship based upon the weight distribution of the concrete and beam combination. This pre-stress relationship is much improved compared to pre-stressing resulting from jacks which concentreates mmore on the pre-stressing at a single point. The composite structural member of the present invention provides improved strength and resistance to bending with less cost. According to the present invention, there is provided a method of making a composite prestressed structural member, said method comprising forming a mould, providing a support member, filling the mould with a mouldable material which is hardened to form a portion of the structural member, which, in use, is supported by said support member, connecting the support member to the upper side of the mould so that deflection of the mould causes deflection of the support member, providing support member connector means to extend downwardly in the mould, mounting the mould and support member so that deflection of the mould and support member can occur, then filling the mould with a mouldable material which hardens to form a composite structural member with said support member and deflecting the mould prior to completion of the hardening of the mould material, such that the support member is placed in a stress condition to form a composite, prestressed structural member upon hardening of the mouldable material, wherein the support member has a flange at or near the neutral axis with respect to a vertical deflection of the inverted support member and away from the neutral axis with respect to a vertical deflection of the upright composite structure, thereby to increase the resistance to bending of the upright composite structure. The invention also provides a composite prestressed structural member comprising a moulded, upper concrete slab and a lower metal support member, extending beneath and connected by connection members, said metal support member being joined with said slab and prestressed by connecting the support member to the upper side of a mould, so that deflection of said mould causes an approximately parallel deflection of said support member with the mould and support member being supported so that deflection of the mould and support member can occur, and the concrete slab having been formed by filling the mould with concrete to flex the mould and the support member so that the support member is prestressed by the deflection, wherein the support member has a flange at or near the neutral axis with respect to the vertical deflection of the inverted support member and away from the neutral axis with respect to a vertical deflection of the upright composite structure, thereby to increase the resistance to bending of the upright composite structure. A particularly desirable lower support member includes first and second beams which have first and second flanges, respectively, which together form the flange near the neutral axis of the inverted support member. For example, two I-beams can be stacked and their flanges welded together to form the support member. Often the cost per unit of weight of the smaller beams is less than the cost per unit of weight of the larger beams reducing the cost even further than simply the savings produced by reducing the amount of steel. In order that the invention may more readily be understood, the following description is given, merely by way of example, reference being made to the accompanying drawings, in which:- Figure 1 is a perspective view of a portion of two stacked and joined beams used in the method of the present invention; Figure 2 is a cross-sectional view of a composite, pre-stressed structural member being formed in accordance with the method of the present invention; Figure 3 is a schematic side elevational view of the structural member of the present invention during one of the formation steps; Figure 4 is a schematic side elevational view of a structural member of the present invention ready for use; and Figure 5 is an end view of a structural member constructed in accordance with the present invention. The method of the present invention is especially suited for use in connection with the method described in U.S. Patent No. 4,493,177. For a further understanding of this invention, reference should be made to the description of this patent, which description is hereby incorporated by reference herein. Referring now to Figure 1, the present invention provides a support for a composite, pre-stressed structural member which comprises stacked steel I-beams 11 and 13. The upper beam 11 is welded at its lower flange 15 to the upper flange 17 of the lower beam 13. If, as shown in Figure 1, the I-beams 11 and 13 are of sufficiently different size, a welding surface 19 is provided on the larger flange. A continuous weld 21 (or spotwelds at regular intervals) along the welding surface 19 is necessary in order to completely secure the I-beams 11 and 13 with respect to each other. Referring now to Figure 2, once the stacked beams 11 and 13 have been joined, they are inverted and placed in a moulding apparatus 23. The moulding apparatus includes a mould bottom 25 and mould sides 27 which form the mould into which the concrete is to be poured. Spacers 29 support the beams 11 and 13 at the ends of the mould so that the beams have a proper height with respect to the bottom surface 25 of the mould. The spacers are also part of the end support system. Shear connectors 47 extend downwardly into the mould from flange 30 of the beam 11. A connection assembly including upper cross beams 31 and lower cross beams 33 joined by connection rods 35 connect the beams 11 and 13 to the mould. The connection assemblies are spaced along the beams 11 and 13 and the mould so that deflection of the mould causes a parallel deflection of the beams 11 and 13. Nuts 37 are threaded to opposite ends of the rods 35 to adjustably join the upper cross beam 31 to the lower cross beam 33. The entire connected mould and cross beams are supported at opposite ends by end supports 39. Referring now to Figure 3, following the preparation of the connected mould and beams, concrete is poured into the mould causing the beams 11 and 13 and the mould to deflect downwardly between the supports 39. As the beams 11 and 13 deflect downwardly due to the weight of the beam,the mould and the wet concrete, the neutral axis A-A of the inverted deflected beams is at or near the joined middle flanges 15 and 17. After the concrete has been poured into the mould causing deflection of the beams and mould, the concrete is allowed to harden into a concrete slab 41. The concrete slab 41 is fixed to the beam 11 and 13 by the shear connectors 47 which extend from the flange 30 of beam 11 into the concrete slab 41. Following hardening of the concrete slab 41,the mould is removed from the concrete and the composite slab and beams are turned upright as shown in Figure 4. When in use, this composite structural member will be supported at its ends 42 and 43. Considering the composite structure supported at its ends, the bending moment of live and dead loads on the composite member causes a downward deflection of the composite member. The neutral axis B-B of the composite structure with respect to a vertical deflection is at or near the upper flange 30 of beam 11. With the neutral axis B-B near the flange 30, the flanges 15 and 17 are sufficiently below the neutral axis greatly to increase the section modulus of the composite structure compared to a composite structure supported by appropriately designed single I-beams. This provides a much improved resistance to bending of the composite, prestressed structural member. The advantage of the stacked beams 11 and 13 in the method and structural member described herein is that a high section modulus in the combined structural member is obtained while retaining a low section modulus in the beams 11 and 13 as the concrete is poured to form slab 41. This allows less steel to be used while obtaining the same or a higher section modulus. Further, because the cost of the combined, smaller beams is often less than the cost of a single beam of the same weight, the cost reduction is even more than the savings in steel. Referring now to Figure 5, an end view of the composite structure is shown including haunches 45 in the concrete slab 41 providing a neutral axis of the composite member farther from the flanges 15 and 17 of the beams 11 and 13. The haunches 45 can be formed by pouring the concrete in two steps. First, the concrete is poured to a desired slab level in the mould and allowed to sufficiently harden so as to support a second pour. New forms are placed on either side of the shear connectors 47 to form the mould space for the haunches 45. The haunches 45 are then poured up to the height of the flange 30 of beam 11. The shear connectors 47 extend into the first pour through the haunches 45. While the above embodiments show stacked and welded I-beams, many beams or combinations of beams having a flange near the neutral axis of the beam or beams can achieve the desired result of a low section modulus as the beams are pre-stressed and a high section modulus in the composite structure. For example, T-shaped beams could be welded to a middle plate (the neutral axis flange) to achieve a custom-designed ratio of beam section modulus to composite structure section modulus. Examples The following calculations detail the design of the two composite structures having a 18.29 m span with a slab 3.25 m wide and 0.178 m thick. Example 1 is supported by two single cover plated I-beams (W24x55) and Example 2 is supported by two stacked I-beams (W14x22, top and W18x35, bottom). The two structures are pre-stressed and formed as described above, except Example 1 uses single beams without flanges at the neutral axis. Both of the above designs are acceptable resulting in very similar final stresses. However, the stacked beam example is clearly superior because it uses less steel, requires no added pre-stress moment, has a lower concrete stress, and will deflect less. One way of determining the superiority of the stacked beam example versus the cover plated rolled beam (I-beam) example is to compare the ratio of composite to non-composite section moduli. The Example 1 section modulus ratio is while the Example 2 section modulus ratio is

Claims: 1. A method of making a composite prestressed structural member, said method comprising forming a mould (23,25,27), providing a support member (11,13), filling the mould (23,25,27) with a mouldable material with is hardened to form a portion of the structural member, which, in use, is supported by said support member, connecting the support member (11,13) to the upper side of the mould (23,25,27) so that deflection of the mould (23,25,27) causes deflection of the support member (11,13), providing support member connection means (47) to extend downwardly in the mould, mounting the mould and support member so that deflection of the mould and support member can occur, then filling the mould with a mouldable material (41) which hardens to form a composite structural member with said support member and deflecting the mould prior to completion of the hardening of the mould material, such that the support member is placed in a stress condition to form a composite, prestressed structural member upon hardening of the mouldable material, characterised in that the support member (11,13) has a flange (15,17) at or near the neutral axis (A-A) with respect to a vertical deflection of the inverted support member and away from the neutral axis (B-B) with respect to a vertical deflection of the upright composite structure, thereby to increase the resistance to bending of the upright composite structure. 2. A method according to claim 1, characterised in that said mouldable material is concrete. 3. A method according to claim 1 or 2, characterised in that said lower support member comprises a steel beam. 4. A method according to claim 3, characterised in that said support member comprises first and second steel beams (11, 13) joined at said flange (15,17). 5. A method according to claim 4, characterised in that said first and second steel beams (11,13) have first and second flanges (15,17), respectively, which are joined to form said flange. 6. A composite prestressed structural member comprising a moulded, upper concrete slab (41) and a lower metal support member (13,17), extending beneath and connected by connection members (47), said metal support member (11,13) being joined with said slab (41) and prestressed by connecting the support member to the upper side of a mould (23,25,27), so that deflection of said mould causes an approximately parallel deflection of said support member with the mould and support member being supported so that deflection of the mould and support member can occur, and the concrete slab (41) having been formed by filling the mould (23,25,27) with concrete to flex the mould and the support member so that the support member is prestressed by the deflection, characterised in that the support member (11,13) has a flange (15,17) at or near the neutral axis (A-A) with respect to the vertical deflection of the inverted support member and away from the neutral axis (B-B) with respect to a vertical deflection of the upright composite structure, thereby to increase the resistance to bending of the upright composite structure. 7. A composite structure as claimed in claim 6, characterised in that said support member comprises first and second beams (11,13) stacked and joined at said flange (15,17). 8. A composite structure claimed in claim 7, characterised in that said first and second beams (11,13) have first and second flanges (15,17) respectively, which together form said flange. 9. A composite structure as claimed in claim 8, characterised in that said first and second flanges (15,17) are welded together to form said flange.

Inventors: GROSSMAN, STANLEY JOSEPH
Applicants: KEITH, GUY NELSON & GROSSMANN, STANLEY JOSEPH TRADING AS KEITH & GROSSMAN LEASING COMPANY
Application year: 1986
Main classification: B28B23
IPC codes: E04B5, E01D2, E01D1, B28B23, E04C2, E04C3, B28B19, E01D101