Composite structure of metal and concrete
US 2675695 A
Description (OCR text may contain errors)
April 20, 1954 L. cor-'F 2,675,695
COMPOSITE STRUCTURE OF METAL AND CONCRETE Filed June lO, 1950 2 Sheets-Sheet l INVENTOR. [0 COff' www April 20, 1954 1 QOFF 2,675,695
COMPOSITE STRUCTURE OF' METAL AND CONCRETE Filed June 10, 1950 2 Sheets-Sheet 2 INVENToR. LEO COFF Patented Apr. Z0, 1954 UNITED STATES PATENT OFFICE COMPOSITE STRUCTURE 0F METAL AND CGNCRETE 3 Claims.
The present invention relates to composite structures of metal and concrete, the application being a continuation-in-part of my co-pending application Ser. No. 649,010, filed February 2G, 1946, now Patent No. 2,510,958, issued June 13, 1950.
In that application I have disclosed means in the `form of a temporary or permanent reaction member for absorbing the stresses of a tensioned cable or other prestressing member, arranged to impart an upward camber to a metal shape, during the time necessary for the hardening of a concrete layer which is supported on the metal shape and to which the stress of the cable is subsequently transferred, the concrete and the metal thereafter co-operating as a unitary7 structure in the support of the live load applied thereto.
By way of extending the principles described in said application, and of advantageously utilizing certain features suggested but not claimed therein, it is now proposed to employ a permanent reaction member for the purpose of substantially entirely absorbing the stresses due to dead load, transmission of these stresses to the metal shape being prevented by providing for relative movement between the reaction member and the metal shape until after the concrete has been applied, whereupon the concrete layer, the metal shape and the reaction member are effectively bonded together to form the unitary structure desired. Thus, my present invention has for its principal object the provision of a composite metal and concrete structure whose deiiection under dead load will be substantially nil yet which can be erected without excessive longitudinal compression of its metal shape, whereby any danger oi buckling of this shape will be obviated.
A still further extension of the principles referred to leads to a structure wherein the' concrete slab itself, or some portion thereof, serves as the reaction member, the essential requirement again being that effective consolidation oi the slab and the metal shape be deferred until after the prestressing for dead load has been completed.
The invention will be described with reference to the accompanying drawing in which:
Fig. 1 is a sectional View, on the line l-i of Fig. 2, or a structure similar to that shown in Fig. e is a view similar to Fig. 2 but with the concrete layer omitted, the structure being similar to that of Fig. 3 but comprising a reaction member of modied form;
Fig. 5 is a fragmentary top plan View of a structure representing another embodiment of the invention;
Fig. 6 is a section on the line 6-6 of Fig. 5;
Fig. 7 is a View similar to Fig. 1, showing a modified structure resembling that of Figs. 5 and 8; and
Fig. S is a section similar' to Fig. 2, showing yet a further modification of the structures illustrated in Figs. 5, 6 and 7.
Referring iirst to Figs. 1 and 2, there is shown an arched layer or slab I i supported by a plurality of parallel, longitudinally extending steel beams i, the ends of these beams resting on spaced supports S (only one shown) The layer i i is partially broken away in Fig. l to expose a reaction member in the form of a row of pre-cast blocks 2li extending along the beam or girder l. Side plates 25, welded or otherwise secured to the underside of beam l, are provided at spaced locations to form anchorages for pins 6 which are underslung by a pair oi cables 1, the threaded terminals l5 of these cables being engaged by nuts l1 which bear upon anchor plates l5. Ther anchor plates l5 bear, in turn, upon the end faces of the outermost blocks 24 and the tensioning oi the cables l will thus keep the row of blocks in compression. It will be seen that these outermost blocks are suitably bored or channeled, as indicated at i 9, to enable the unhindered passage ci' cables 1 which also traverse the upper ilange of the beam l by way of suitable slots (not shown). It will be readily understood that these slots, as well as the channels I9, must afford sufficient clearance around the cables 'l to enable their displacement in planes parallel to the axis of beam l without the development of objectionable shear stresses in these cables.
Before the slab H is poured, the only positive connection between any of the blocks 24 and the metal structure is provided by the engagement of cables i with pins 6 and by the pins 26. The latter pins pass through certain of the blocks 24 and engage slots 27 provided in the side plates Iii. The slots are sufhciently elongated to enable relative movement between the blocks and the steel beam i when these blocks are placed in compression by tensioning of the cables 1 preparatory to the application of the dead load, as well as after the slab I! has `been poured but beforeV it has hardened. No substantial compression will,
therefore, be communicated from the blocks to the steel so that any danger of buckling of the shape I will be obviated.
The blocks 2d are preferably scored or indented, e. g. as indicated at 22, to aiTord interlocking engagement between the precast and the poured concrete section and to enlarge the contact surface therebetween over which bonding is to take The poured concrete also envelops the side plates 25 and penetrates the slots 2'I thereof so that, once the layer II has hardened, relative movement between the row of blocks 24 and the steel shape I will no longer be possible. Accordingly, any further tensioning of the cables 'I (due, for example, to the application of live load to the structure) will be borne jointly by the metal and the concrete, the structure then functioning substantially as a monolithic whole.
If it is desirable to delay tensioning of the cables I not only for live'load but also for dead load until after the slab has hardened, e. g. in order to prevent buckling of the row of blocks 2.4i, then the imparting of any substantial compression to the steel shape I at the time of prestressing for dead load can be avoided by providing cavities 28 about the side plates 25, these cavities being lled with grout after completion of such prestressing.
Fig. 3 shows a steel beam Ia supporting a flat slab IIa. The upper flange of the beam has bolted thereto a plurality of brackets Sta slidably supporting a pipe 2da which represents the reaction member. in the finished structure the reaction member is imbedded in the concrete. Bearing endwise upon the pipe 24a are anchor plates Ita to which are secured the threaded terminals I6a of cables la by means of nuts I'Ia. The pipe 24a is shown recessed at 22a to insure intimateand interlocking contact with the concrete subsequently poured thereover.
The reaction member (pipe 2da) is again free to move relative to the shape Ia until the slab IIa is in place and has hardened around the brackets 35a and pipe Ella. The hardening of the slab thus immobilizes the pipe with respect to the concrete and the steel. As before, tensioning of the cables prior to and after application of the dead load (but before hardening of the concrete) places only the pipe in compression. Buckling of the pipe is prevented by the brackets 35a. After the concrete has hardened, the compressive strength of the pipe contributes only to a minor extent to the support of the load.
lf the weight of the dead load is such that the combined steel section ic, 2da is in danger of buckling before the hardening of the slab IIa, it may be necessary to employ temporary supports at intermediate-points of the structure, to be removed after the concrete has set, and to delay until such time the tensioning of the cables la. In this event, too, the steel of the shape Ia should be subjected to little if any compression in the absence of live load, hence it will be necessary to allow for relative movement between the slab and the shape and to provide means for only thereafter solidifying the structure by rigidly coupling the two sections together. This may be done in simple manner by providing slots 27a in the upper ange of the beam ic through which pass the bolts 26a holding the brackets 35a onto the flange. After the slab ila has become strong enough to take compression, the cables 'la are tensioned with the nuts of bolts 25a loosened to enable longitudinal displacement of the brackets 35a, pipe 24a and slab Ila relative to the beam la. Subsequently, the nuts of the bolts 26a. are tightened to produce a condition identical with that previously described. It may be desirable to defer tightening of these nuts until after shrinkage and plastic flow have reduced the layer or slab I Ia to its ultimate size and after the cables 'la have been retensioned to compensate for this reduction in size. It will, of course, be necessary to provide suitable means for preventing the entrance of concrete into the slots 27a in order to maintain relative mobility between the slab iid and the shape Ia; this can be done, for example, by the formation of cavities around these slots substantially in the manner described in connection with Figs. 1 and 2.
A variety of elongated metal reaction members may replace the pipe 24a shown in Fig. 3. Fig. 4 shows an I-beam Zlib supported on and extending longitudinally of the main beam or girder ib, the two shapes being joined together by means of bolts 26h which, however, permit longitudinal relative movement thereof by passing through longitudinal slots in the flange of either or both beams, in the manner illustrated at 27a in Fig. S. By providing portions 6b of inverted U-shape in the pins 5b, lateral abutments are provided for the cables Tb which, as explained in my cri-pending application Ser. No. 789,704, filed December fi, 1947, now Patent No. 2,590,685, issued March 25, 1952, prevents buckling of the shape ib due to tensioning of the cables, the arrangement being similar to that shown in Fig. 2 of my (zo-pending application Ser. No. 57,179, iiled October 29, 1948. By reason of the presence of the bolts 2th the buckling strength or" the main beam Ib will be eectively added to that of the auxiliary shape 24h, so that the danger of buckling due to prostressing for dead load may be readily averted by suitably choosing the number and spacing of the pins 6b.
Figs. 5 and `o show how the slab itself, rather than a reaction member imbedded therein, may be made to absorb the stresses due to dead. load without communicating any material portion ci these stresses to the supporting shape or shapes, this method being applicable wherever the dead weight of the slab is so low as not unduly to deflect the beams before the cables are tensioned, or where it is convenient to provide temporary intermediate shoring means. Furthermore, the invention in such a case enables the slab to be composed, in its entirety or for its major part, of precast elements which by the action of the Vdead-load stresses themselves are constrained to act as a unit. These precast elements are shown at IIc and are of generally rectangular configuration, each such elements spanning a pair of adjacent longitudinal beams ic only two of which are shown. The elements lic abut one another in longitudinal direction while being spaced in transverse direction, thereby forming longitudinal channels above the webs of the beams ic which channels are ultimately filled with grout as indicated at 25e. Thus the finished slab consists of the elements I ic and of the groutings 2do.'
Projecting sideways from each of the elements I I c are the ends of reinforcing rods ric are bent around longitudinally'extending rods 2de, each of the latter rods being thereby supported in one ofthe aforementioned channels in alignment with the web of the respective beam ic and spaced from the upper flange thereof. Stirrups 35o straddle the rods 2&0 and their threaded extremities, engaged by nuts Zc, pass through longitudinal slots 21e provided in the upper ange of beam Ic. Thus it will be seen that the slab, completed when the grouting 28o in the longitudinal channels has been hardened, will retain a certain mobility relative to the beams Ic until after the nuts 25o have been tightened, this operation being deferred until after the cables 1c have been stressed against the slab to balance the structure for dead load. The tension of these cables operates at the same time to stress the longitudinally adjacent elements I Ic together for concerted action, an effect which may be enhanced by form-tting the adjoining edges together, e. g. in the interlocking manner indicated at I Ic.
The arrangement shown in Fig. 7 diiers from that of Figs. 5 and 6 by the use of hollow precast elements I Id in lieu of the solid slab portions I Ic. These hollow elements I Id have the shape of inverted pans, forming cavities bounded by side walls IId and end walls Ild, which, by virtue of their reduced weight, may be higher than the slab portions I Ic, this in turn enabling the anchor plates d to be raised above the level of their counterparts |50, thereby increasing the upwardly acting component of the tension in the cables 1d.
Fig. 8 shows how, in a structure comprising two longitudinal beams Ie, each pre-cast slab portion IIe may laterally extend, cantilever fashion, beyond these beams while being of double T-bearn configuration by virtue of having two longitudinally extending ribs I Ie' respectively aligned with these beams, a central cavity and a pair of lateral recesses being dened by these ribs and by end walls I Ie". Stirrups 35e engage longitudinal reinforcing rods 24e extending within the ribs IIe', the entire arrangement being structurally similar and functionally equivalent to that of Figs. 5, 6 and 7 (after the hardening of the grouting separating laterally adjacent elements IIc or I Id).
It may be mentioned that some means is preferably provided for breaking the bond between the vertical legs of the stirrups 35o (Figs. 5, 6), 35d (Fig. 7) or 35e (Fig. 8) and the surrounding slabs, as by asphalting or otherwise coating these legs, so that tensioning thereof by the tightening of the associated nuts will be communicated to the rods 24e, 24d or 24e, respectively, and thereby to the entire slab, rather than just to the concrete in the immediate neighborhood of these stirrups. This Will force the slab into frictional contact with the supporting flanges along practically the whole area of the latter, so that the stirrups will be relieved from bearing the entire shear due to the tendency of the slab to shift under live load relative to the steel shapes.
The embodiments specifically described hereinabove have been given merely by way of illustration and not as a limitation upon the scope of the invention. Thus it will be understood that a single elongated, precast member may be used in lieu of the aligned blocks 2li, and that a large variety of means well known per se may be used for anchoring this or some other reaction member to the composite structure at the opportune moment. It may also be mentioned that relative mobility between the metal shape, on the one hand, and the reaction member and/or the concrete layer, on the other hand, may be enhanced through the interposition of an anti-friction layer, e. g. of the character set forth in my aforementioned Patent No. 2,590,685.
1. A composite structure comprising an elongated metal shape having a substantially flat, horizontal bearing surface, a concrete layer resting on said surface, an elongated, compressionresistant element imbedded in said layer and extending along said surface, at least one iiexible, elastic tie member anchored to the ends of said element and extending underneath said surface, tension means maintaining said tie member under stress, bracing means supporting at least one intermediate point of said shape on said tie member, connecting means securing said element to said shape, and means including said layer preventing relative displacement between said element and said shape.
2. A structure according to claim 1 wherein said element comprises a body of precast concrete.
3. A structure according to claim 1 wherein said element comprises a metallic shape.
References Cited in the file 0f this patent UNITED STATES PATENTS Number Name Date 9,172X Witty Oct. 14, 1835 273,922 White Mar. 13, 1883 1,000,088 Haas Aug. 8, 1911 1,594,505 Frye Aug. 3, 1926 2,101,538 Faber Dec. 7, 1937 2,378,584 Schorer June 19, 1945 2,382,139 Cueni Aug. 14, 1945 2,449,276 Chalos Sept. 14, 1948 2,510,958 Coif June 13', 1950 FOREIGN PATENTS Number Country Date 117,344 Great Britain July 18, 1918 464,361 Great Britain Apr. 16, 1937