US3465484A - Prestressed concrete beam - Google Patents

Description

P- 9, 1969 o. ZALDASTANI ET AL 3,465,484

PRESTRESSED CONCRETE BEAM Original Filed Feb. 20. 1967 INVENTORS OTHAR ZALDASTANI MICHAB. J.A.-H. JOLLIFFE XF V ATTOR N EYS United States Patent 3,465,484 PRESTRESSED CONCRETE BEAM Othar Zaldastani and Michael J. A. H. Jolliife, both Nichols, Norton, and Zaldastani, Inc., 131 Clarendon St., Boston, Mass. 02116 Continuation of application Ser. No. 617,181, Feb. 20, 1967. This application Oct. 22, 1968, Ser. No. 772,466 Int. Cl. 1304c 3/26; EtMb 5/04 US. Cl. 52-127 3 Claims ABSTRACT OF THE DISCLOSURE This application is a continuation of Ser. No. 617,181 filed Feb. 20, 1967 and now abandoned.

BACKGROUND OF THE INVENTION The field to which the invention pertains comprises precast reinforced concrete beams intended for support of or integration with a variety of deck materials including concrete, whether precast or poured in place, lumber and metallic deck systems.

The prior art includes two well-known precast structural systems using reinforced concrete load-bearing members, namely, slab units and integral beam-and-deck units. In any such structure the load induces tensile stresses in the lower portions and compressive stresses in the upper portions of load-bearing members. Since concrete is relatively weak in tension, the tensile stresses are borne almost entirely by longitudinal reinforcing wires around which the concrete is cast.

In any practical slab structure, inefliciency in the use of concrete tends to increase greatly with an increase in the span and load. Much of the concrete in the portions under tensile loading has little utility. The integral beamand-deck units overcome this limitation to a degree, but have other defects. They are relatively costly to produce and lack flexibility in installation and use under different loadings within a building. Also, they are often too costly where deck materials other than concrete could be used. In many instances the structures rest upon transverse girders which limit the overhead clearance.

Some work has been done on precast, prestressed concrete beams, that is, beams cast around reinforcing wires that are held deflected and under tension in the mold until after the concrete has set. After severance of such wires from the mold and removal of the beams therefrom, the beams have a slight curvature inverse to that of the intended load, such curvature being termed the camber. Undesirable variations occur in the camber of beams previously in use, largely due to differences in thickness of concrete at different parts of the cross section. Also, lateral structural instability exists in some cases, necessitating cross bracing for longer spans.

Other disadvantages exist in one or more of the foregoing structures, including difliculty in storing the precast units and in erecting them on the site, and also in making satisfactory and stable connections to supporting members.

3,465,484 Patented Sept. 9, 1969 ice SUMMARY OF THE INVENTION This invention avoids the foregoing disadvantages by providing an easily precast, prestressed beam of novel configuration and design, and one that is readily removed from the mold. In cross section the beam may be considered as comprising a combination of three elements, namely, two vertically tapered and oriented, parallel reinforced web portions, and a vertically tapered transverse and continuous, reinforced concrete flange portion or diaphragm connecting and integrally joined with the web portions. These three portions act compositely to form the beam unit. Such beams may also act compositely with other structural or deck elements which they support, depending on the nature of such elements and the mode of connection or support. The diaphragm or flange provides lateral stability to the beam. Also, it may be extended longitudinally so as to act as an end bracket for support by other structural elements such as girders. Further, the composite beam defines a space that may be usefully employed for acoustical absorption or for lighting fixtures, pipe and ductwork of any desired description.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary longitudinal elevation in section showing a projecting end of a preferred embodiment of the beam.

FIG. 2 is a cross section of the beam showing details of the end reinforcing bracket.

FIG. 3 is an elevation in section of a midportion of the beam illustrating its use in conjunction with telescoping forms for casting a deck, and also showing the use of perforated panels closing the space defined by the beam and thus forming a resonator which absorbs sound by the Helmholtz effect.

FIG. 4 is an elevation in section of an end of the beam and the deck supported thereby, both resting upon a notched concrete girder.

FIG. 5 is a plan view corresponding to FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 3 shows a typical cross section near the middle of the beam, and FIG. 2 shows a cross section near one end. The beam is designated generally at 12 and comprises an integrally cast, reinforced structure including two web portions 14 and a flange or diaphragm portion 16. The beam is cast in a mold of the appropriate length for a span to be supported, for example from 40 feet to feet. The side walls of the mold are preferably tapered so as to produce a beam having outer sides 18 sloping inwardly toward the bottom, and inner sides 20 having the reverse slope preferably with the same angle to the vertical as the sides 18. The taper of the sides 18 and 20 is preferably about one part in sixteen measured in relation to the vertical.

The beam preferably has continuous, longitudinal, tapered notches 22 of uniform cross section extending along its uppermost shoulders or edges, such notches being adapted for support of retractable or telescoping forms 24, such forms being placed on the beams after the latter have been erected in the structure to act as temporary supports for casting a continuous deck 26 over the beams.

The distance s between central vertical lines of symmetry through the web portions 14, shown in FIG. 2, is preferably sixteen inches, which is a modular building dimension. The thickness of the web portions at their lowest extremities is preferably about five inches which is sufficient to allow room for two spaced vertical rows of reinforcing wires. For simplicity of illustration, however, the described embodiment employs a single row of reinforcing wires 28 in each web, such rows being located within the web portions 14 where tensile stresses exist under load conditions, and along the aforementioned lines of symmetry. It will be understood that in manufacture, the wires 28 are correctly located within the casting form and held under tension between the ends of the form, ordinarily with a point or points between the ends restrained in positions closer to the bottom than the points of restraint at the ends. This is in accord with conventional construction methods employed in the manufacture of prestressed concrete beams.

End reinforcing brackets designated generally at 30 are also placed in the ends of the mold form. These brackets may be formed in various ways, the preferred form for the illustrated single row of wires comprising a pair of steel plates 32 each welded to a plate 33, a pair of relatively thick rods 34 welded to each plate 32 in such manner as to embrace the row of longtiudinal wires 28, wire extensions 35 of the plates 32, and a network of wires or stirrups 36 each of U-shape, welded to the plates 32 and wires 35. Also, U-shaped stirrups 37 are ac curately located in spaced relationship along the entire length of the form between the end reinforcing brackets.

With the foregoing reinforcements in place, the concrete is cast into the form with the stirrups 36 and 37 projecting above the surface of the concrete, and allowed to harden for about 16 to 24 hours with the prestressing wires 28 held in tension as stated above, during which time the reinforcing wires and brackets become firmly bonded to the concrete and after which the beam may be stripped from the mold. This merely requires cutting the ends of the wires 28. When the beam has been removed from the mold it has a measurable upward bow in the center, referred to as camber. It has been found that this camber is very uniform for beams of the same dimension, and this fact is extremely helpful both in storing the beams and in erecting them on the site. The uniformity of camber is attributable largely to the illustrated cross section configuration which comprises the two web portions 14 and the connecting flange portion 16.

A graded family of beam sections may be produced in the foregoing manner for different loads and spans. Preferably, the modular distance s is the same in all members, but the inside vertical clearance depth d and the external overall depth D are variable, the former preferably from about 12 to about 36 inches, and the latter from about 18 to about 48 inches in two-inch intervals, thereby accommodating any span from about 40 to about 120 feet with typical deck construction and loading, as in a parking garage. Throughout this family of sizes the taper of the sides '18 and 20 of the webs is also preferably a constant. Thus for deeper webs than those illustrated in the drawing the flange is of greater lateral width as well as greater depth. The dimensions of the flange portion are a vital structural consideration, since this portion bears the compressive load on the beam and provides its lateral stability.

The foregoing configuration comprises a very efiicient section, namely one having a large area for distribution of compressive stress balancing the tensile stress on the wires 28. Also, the proportion of the various dimensions discussed above is such as to reduce the possibility of cracks forming in the beam, including those caused by shrinkage as well as by loading.

Beams formed in the manner described above are brought to the site and erected between walls or girders 38 (FIGS. 4 and 5), the beams being spaced on 6- to 15- foot centers. To this end each beam preferably has an endwise projection formed by an extension 16a of the flange portion 16, the projection resting within a notch 40 in the girder. To prevent stress concentration the beam has a chamfer 42 at the juncture of the extension 16a with the ends of the web portions 14. This chamber opposes a corresponding chamfer on the lip of the notch 40. A clearance space 44, preferably of about inch, is provided between all surfaces of the beam and the girder, the beam resting on a neoprene pad 46 of approximately the same thickness as the desired clearance space. This space is to accommodate dimensional tolerances and movements of the type normally encountered in a building structure, such as those attributed to temperature changes and plastic flow.

As shown in FIG. 4, a deck 26 is cast upon the top surface of the beam over the stirrups 36 and 37, and extends to the end of the extension 16a. The deck may be cast in place using the retractable forms 24 as previously described; or a pre-formed deck may be applied in the form of sheets or planks, either of concrete or of any other suitable deck material.

One variation of the structure may be accomplished by eliminating the projecting stirrups 36 and 37, in which case the resulting structure does not become a composite one in which the deck 26 assumes a part of the compressive load with the flange portion 16 as previously described.

As previously stated, the cross section of the beam provides a longitudinal space, designated as 48, which may have any one or more of several useful functions. Inserts 50 may be cast into the beam for attachment of hanger wires 52, at spaced intervals, such wires supporting means such as Z-brackets 54 in which perforated sound absorption panels 56 may be suspended. The panels 56 thus enclose the space 48 as an acoustical sound absorption space. In this case, sound absorption takes all of the forms described in Patent No. 2,933,146 issued Apr. 19, 1960, to Zaldastani and Junger, namely, the Helmholtz resonator effect, the black body effect and an organ pipe effect.

The tapered inner walls 20 contribute to the black body effect by causing dispersion of sound energy by reflection within the space 48.

The Helmholtz effect is expressed by the relationship between the volume of the chamber, the total area of openings 58 provided in the panels 56 and the thickness of the panels, in accordance with the formula given in said patent. These values may be varied in relation to one another to tune for suppression of any frequency between about 250 and 1,000 cycles per second.

The organ pipe effect is a function of the depth d of the space 48, which is one-fourth of the wavelength of the frequency suppressed.

Also, sound absorptive material can be placed inside the cavity formed by the perforated panels and the beam, and thus provide additional sound absorptive properties.

It will be obvious that the space 48 may also be employed for enclosing electrical wiring, conduits, ductwork, pipes and the like. This may be done without sacrifice to the head space within the structure, or its appearance. At the same time, all such enclosed objects remain easily accessible for maintenance or other purposes.

It will be apparent that in place of notches 40 as shown in FIG. 5, the girders 38 may have continuous ledges of the cross section shown in FIG. 4. Also, it is possible to eliminate the end extensions 16a of the beam, resting its web portions 14 either upon the top surfaces of the supporting members such as walls or girders, or upon ledges or in notches as described above. Other structural variations may be employed consistently with the features of this invention, and without departing from its spirit or scope.

We claim:

1. An integrated structural deck system having, in combination,

a pair of spaced supports,

a plurality of cast, generally parallel, mutually spaced, prestressed concrete beams each supported at its ends by said supports and having a pair of spaced, parallel, longitudinal web portions of substantially uniform trapezoidal cross section and height, uniformly tapered side walls and decreasing width toward the bottom, said web portions containing wire prestressing strands, and a flange portion joining the web portions and forming a smooth continuation of the noncontiguous side Walls thereof, said flange portion having a depth sufiicient to distribute the compressive stresses corresponding to the tensile stresses in the web portions under load, the height of the beams being at least the distance between the center lines of their web portions, the beams being spaced on centers a distance greater than their height, and a deck extending over the beams and the spaces be tween the beams and being supported thereby.- 2. A deck system according to claim 1, in which said flange portion has continuous notched shoulders defining an upper surface, said shoulders being adapted for support of retractable forms, and said flange portion also has metal reinforcing members partially imbedded in its upper surface between said shoulders, and the deck is cast upon the top surfaces of the forms and beams and over said reinforcing members, the deck providing clearance for subsequent retraction of the forms.

3. A deck system according to claim 2 in which a beam has suspension means within the space defined by the Web and flange portions, and perforated panel means supported by said suspension means in position to enclose said space to define a tuned resonant sound-absorption chamber therein.

References Cited UNITED STATES PATENTS 1,891,763 12/1932 Henderson 52602 X 2,007,374 7/1935 Kuehne 52145 2,130,285 9/1938 Marqua 52283 X 2,428,304 9/1947 Abeles et al. 52-723 X FOREIGN PATENTS 675,943 12/ 1963 Canada. 1,166,294 6/1958 France.

821,409 11/1951 Germany.

5/1944 Great Britain. 7/1964 Switzerland.

ALFRED C. PERHAM, Primary Examiner US. Cl. X.R.

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