US 3084481 A
Description (OCR text may contain errors)
2 Sheets-Sheet 1 INVENTORS.
W. J. SILBERKUHL ETAL PRESTRESSED CONCRETE BODIES April 9, 1963 Filed Deo. 18, 1959 Wllhelm Johannes Silberkuhl Uwe Kosl ErnsrHoeussler w. J. slLBERKUHL ETAL 3,084,481
PEEsTREssED CONCRETE BODIES April 9, 1963 2 Sheets-Sheet 2 Filed Dec. 18, 1959 N .EN m N m Wilhelm Johannes Silberkuhl Uwe Kastl Ernsm` Hoessler IN VEN T OR-f.
E AG1/EN T.
3,084,481 PRESTRESSED CNCRETE BOBIES Wilhelm Johannes Silberknhl, Uwe Kastl, and Ernst Haenssler, all of Brunuenstrasse 29, Essen (Ruhr),
Germany Filed Dec. 1S, 1959, Ser. No. 860,579 Claims priority, application Germany Dec. 19, 1953 4 Claims. (Cl. Sti-129) Our present invention relates to prestressed elongated concrete bodies, such as girders or slabs, adapted to be employed in load-bearing structures (eg. bridges, overpasses, elevated highways and the like).
It is an object of our invention to provide a light-weight, structurally rigid prestressed concrete body of exceptional strength.
Another object of our invention is to provide a hollow load-supporting concrete slab adapted to span sizable distances and having a relatively large strength-to-weight ratio.
A further object of the invention is to provide a concrete span adapted to support a large load while obviating the disadvantages of great bulk and high cost encountered in conventional bridge and road construction.
A known type of prestressed concrete members used for single-span or continuous structures are lin the form of hollow girders of generally rectangular cross-section, which may be extended sideways at the level of their upper flanges to form a road bed,the lateral webs of these girders being usually of considerable width to accommodate sheaves of prestressing cables or rods extending generally horizontally therewithin while following a wavy line in `the region of maximum tensile stress. Thus, each prestressing element rises and falls in a given vertical plane, reaching its zenith above the supports and its nadir substantially midway therebetween. Since the wall thickness of the aforementioned webs greatly exceeds Athat required by static considerations, it is a more particular object of the instant invention to provide an improved hollow girder of this general type in which this Wall thickness, and therefore the overall weight of the structure, is considerably reduced for a predetermined load-bearing capacity.
In accordance with the present invention we imbed a sheaf of prestressing elements in the shell of a hollow concrete girder in such manner that at least the sloping portions of these elements, extending between the high and low points thereof, are relatively staggered in axial direction of the girder so as to be substantially equidistant from the girder axis. These sloping portions, accordingly, define a sheet Whose thickness dimension generally coincides with that of the girder webs so that the latter may be made much narrower than heretofore. Preferably, we provide two such sheaves extending around the girder axis in opposite directions so as to intersect at their high and low points in the upper and ythe lower Wall portion of the girder, respectively.
Advantageously, each sheaf of prestressing elements denes a sheet wrapped with a long lay about the girder axis, its pitch corresponding to the spacing of adjacent span supports. The girder prole is, for this purpose, preferably rounded into broadly elliptical shape, with the major axis of the ellipse or ovoid extending horizontally, so that each prestressing element describes an elliptically deformed helicoidal line. The pitch of this deformed helicoidal line may be varied throughout the length of the girder so ythat the prestressing element follows more or less closely the curvature of a line whose vertical position in the structure is in accordance with the magnitude of the moments along the girder, and which therefore may be referred to as a moment line and the aver-age direction of all of these elements extends gen- Patented Apr. 9, 1953 ECC erally along a fiber subjected to maximum tensile stress in the unprestressed structure. rIhus, the pitch may be greatly lengthened in the unsupported intermediate region of the span, where a major part of the moment line passes underneath the girder axis, so that the prestressing element extends nearly horizontally and axially in that region.
We have found that a concrete girder of annular crosssection, provided in this manner with imbedded sheaves of oppositely wound, generally helically extending prestressing elements (eg. steel wires), is under sufficient axial and peripheral compression to withstand large and varying live loads without requiring additional, unprestressed reinforcements; this results in a further saving of weight and cost.
The above and other objects, .features and advantages of the invention will become more readily apparent from the following description, reference being made to the accompanying `drawing in which:
FIG. 1 is a side-elevational Iview of a bridge according `to our invention;
FIG. 2 is a fragmentary sectional top view of the structure of FIG. l, showing the disposition of the reinforcing elements therein, taken generally along line Il-II of FIG. 1;
FIG. 3A is a cross-sectional View taken along line IIIA-IIIA of FIG. l;
FIG. 3B is a cross-sectional View taken along line IlIB-IIIB of FIG. 1; and
FIG. 3C is a cross-sectional view taken along line IIIC-IIIC of FIG. 1.
In the ydrawing we show a continuous, prestressed concrete bridge comprising a tubular girder 2 supported, via anti-friction bea-rings 10, by longitudinally spaced pairs of pillars 1, 1. The girder 2 has its upper wall portion integrally extended laterally at -both sides to form a roadway-supporting slab 3- bounded by curbstones 3". The inner cavity 7 of the hollow girder structure 2 is of roughly elliptical cross-section (best seen in FIGS. 3A- 3C) and extends longitudinally along the bridge axis. The girder 2 is provided with prestressing rods which are arranged in two sheaves 4a, 4b imbedded in its shell. As lbest shown in FIG. 2, the sheaves 4a, 4b define fiat sheets which are helically wound about the girder axis and whose major faces are substantially parallel to the wall of cavity 7 throughout their length. It will be apparent that, as viewed in the direction of the section plane denoted by line IIIA-IIIA in FIG. 1, shaft 4a is Wound in a clockwise direction about the axis while sheaf 4b is wound in the opposite sense. The rods forming `the sheaves 4a, 4b are individually anchored under suitable tension to the concrete at the extremities of the structure, at the piers (not shown) forming the outermost supports, in the usual manner.
The two sheaves. are crossed within the lower girder wall at a single intermediate location 8 and within the upper wall portion, above the pillars 1 and 1', at 9. In their lower reaches the sheaves 4a, 4b are wound with a relatively long lay so as to extend substantially horizontally and more nearly parallel to the axis of the girder 2. Between the crossings 8 and 9, the prestressing elements of sheaves slope up and down in axially staggered relationship, as `shown at 5 and `6. As best illustrated in FIGS. 3A-3C, the wall of girder 2 is thickened somewhat in the region above the pillars 1, 1 to absorb the added compressive forces there. Since at all locations each of the sheaves 4a, 4b defines a curved sheet hugging the cavity 7, the entire girder shell may be of a minimum thickness while manifesting full load-bearing strength. The curved configuration of the girder, whose cross-sectional outline rises arcuately upwardly from the median zone 8 (FIG. 3A) along an approximately elliptical are at least in the middle region between supports 1 and 1', is maintained over the major part of the length `of the girder between these supports, as will be apparent from FIGS. 3A and 3B, and in particular will -be seen to exist beyond the points of intersection of the prestressing elements 4a, 4b with the plane of FIG. 2 which approximately corresponds to the horizontal mid-plane of the girder; it will be noted that these points vof intersection lie closer to the piers 1, 1 than to the intervening midpoint 8.
In the erection of the structure, the rods 4a, 4b may be post-tensioned in the usual manner against the hardened concrete after having `been imbedded therein with suitable bond-breaking means known per se, such as sheathing or asphalting.
Although the invention has been specifically described with reference to a bridge structure, it will be readily apparent that many modifications and variations may be made without departing from the spirit and scope of our invention except as further limited by the appended claims.
1. A load-bearing structure comprising at least two horizontally spaced-apart supports, a tubular concrete girder with an upper wall portion forming a roadway-support ing slab, said girder resting substantially horizontally on said Isupports and being provided with a lower wall portion of reduced wall thickness whose outer surface rises arcuately upwardly from a median zone at least over the major part of its length between said supports, said girder having lan interior surface generally paralleling said outer surface over said major part of the length of said' girder, and two sheaves of elongated prestressing ele- -ments extending under tension along lgenerally helicoidal ,lines of opposite pitch within the rgrder wall, said sheaves intersecting at two spaced locations in said upper wall portion above said supports, respectively, and at a single intermediate location in said lower wall portion substantially midway between said supports, said sheaves descending from said two spaced locations to said intermediate location substantially along a line whose vertical position in the structure is in accordance with the magnitude of the bending moments along the length of said fgirder.
2. A structure according to claim 1 wherein said lower wall portion has a cross-section whose outline substantially extends along an arc of an ellipse with horizontal major axis at least in the region of said intermediate location.
3. A structure according to claim 1 wherein said elements are wound with relatively short pitch in the upper part of said girder and with relatively long pitch in the lower part thereof, thereby traversing the horizontal midplane of said girder at points closer to said supports than to the mid-point therebetween.
4. A structure according to claim 1 wherein said girder is provided with roadway-supporting lateral extensions integral and level with said upper flange, said girder being of lgreater lateral -wallthickness above said supports than at said intermediate location, said girder extending beyond said supports, said prestressing elements continuing substantially helicoidally beyond said supports after in- Vtersecting in said spaced locations.
References Cited in the tile of this patent UNITED STATES PATENTS 1,060,922 Luten May 6, 1913 1,596,669 Ligonnet Aug. 17I 1926 2,303,394 Schorer Dec. 1, 1942 2,414,011 Billner Ian. 7, 1947 FOREGN PATENTS 386,108 Great Britain of 1933 822,090 Germany of 1951 725,826 Great Britain of 1955 566,308 Belgium of 1958 OTHER REFERENCES Engineering News-Record, June 10, 1948 (pages 80, 81).
Engineering News-Record, September 1, 1949 (pages 183-187).
Civil Engineering, January 1953 (pages 42, 43).