US2261912A - Concrete pipe - Google Patents

Description

C. F. BUENTE CONCRETE PIPE New. 4, i941.

Filed March 12, 1940 C/Ia r/ e ifgzen fe M, W0 1M Patented Nov. 4, 1941 CONCRETE PIPE Charles F. Buente, Scott Township, Allegheny County, Pa.

Application March 12, 1940, Serial No. 323,567

4 Claims.

This invention relates to concrete pipe and, in particular, to that class of the product known as drainage pipe, i. e., pipe of relatively large diameter'adapted to be buried at substantial depths.

Concrete drainage pipe has been made heretofore in various shapes and sizes but the standard product is cylindrical, is provided with reinforcement of various kinds, and has a wall thickness varying with the inside diameter of the p p The present standards and formulas for the design of reinforced concrete pipe to sustain given loads, are based on an investigation made thirty years ago. These formulas have been generally adopted by pipe users and makers. The wall thicknesses now embodied in standard specifications for various sizes and classes of pipe were derived from the results of this investigation. Prior to that time, the accepted test for the strength of concrete pipe was to determine Whether the load for which each size and grade of pipe was designed, caused a crack extending thefull length of. the pipe. Because of the difiiculty in meeting this test, it was modified in the standard specifications to permit a crack having a width up to .01". These cracks in reinforced concrete pipe as now made, are accepted as unobjectionable because they cannot be avoided but do, nevertheless, seriously impair the utility of the pipe as a structural 'unit. The cracks are formed, of

course, because the steel reinforcement does not carry any substantial proportion of th stress in the pipe until the concrete has been stressed beyond its ultimate strength.

.Concrete pipe as now made is characterized by further shortcomings. Pipe of various sizes designed by present standards, when tested, exhibits a load-bearing capacity which is seemingly without relation to the specifications according to which it is made. A tabulation of numerous tests on various sizes and types of pipe indicates that the maximum load at which the standard cracking test was met, was virtually the same for all pipe in the range of sizes from 36" to 66", and that there is only a small variation in the ultimate load capacity of the pipe in the several classes. The relative strength of the pipe, furthermore, appears to be greater in the smaller sizes.

A further shortcoming of reinforced pipe as now. made is that an increase in the amount of reinforcement does not produce a proportional increase in the load-bearing capacity. As a matter of fact, the load-bearing capacity is increased only to the extent of-from 14% to 47% of the increase in-the amount selected sizes of pipe.

I have invented a novel form of concrete pipe which overcomes the aforementioned objections to pipe as now made. A preferred embodiment of the invention comprises a plain concrete pipe, the exterior of the section of which is generally oval, with a wall thickness varying from a maximum at the top and the bottom of the pipe to a minimum at the sides. signed on the basis of the strength of concrete alone in tension and compression and ordinarily no reinforcement is required. Some reinforcement may be employed if desired, in order to provide an increased factor of safety. The invention provides a pipe which, in all sizes, has the relatively high load-bearing capacity of the smaller sizes of pipe as made heretofore. Pipe according to the invention; furthermore, may be designed forincreased load-bearing capacity in the larger sizes. The pipe of my invention, finally, w-illnot-exhibit cracks when loaded, within the limits'for which it is designed, and will have an ultimate strength providing an adequate margin of safety.

- The following detailed description and explana tibn of the invention should be read with reference to the accompanying drawing illustrating several forms of the preferred embodiment outlined above, aswell as diagrams useful in explainof reinforcing, for certain ing my analysis ofthe theoryunderlying th performance of my invention. In the drawing,

Fig,- 1 is 'a transverse section through one form of pipe according to the invention;

Fig. 2 is a similar view showing a slightly different form of pipe;

.Fig. 3 is a similarviewshowing a still further different form of pipe; 7 Fig. 4 is a diagram illustrating the stress created in an ordinary cylindrical pipe when loaded; and g Fig. 5 isia. vector diagram showing the forces existing at a given point in the pipe.

Referring now in detail to the drawing, the pipelll shown in Fig. 1 has a section the exterior of Which-is definedb-ythe oval u having a major axis l2 and a minor axis' I3. The interior of the pipe section is defined by a circle l4 having its center at the intersectionof the axes l2 and I3 of the oval II. The pipe l0 thus has a Wall thick 7 ness-which varies fromamaximum at the top and bottom to a minimum at the sides. The

pipe shown in Fig. 1 is particularly adapted for I small sewer pipe and may be made in various sizes, without reinforcement. For an inside (11- Y The pipe is preferably 'deknown methods and apparatus, the only change in the latter being in the shape of the form.

Fig. 2 illustrates a slightly different form of pipe [5. The section of the pipe I5 is defined by concentric ovals l6 and I! having their major and minoraxes I8 and 19 in alinement and intersecting at a common point. The major and minor axes of the oval l6 differ by substantially the same amount from the diameter of a circle of substantially the same area. The major and minor axes of the oval I1 aresuch, relativeto:

siderably cheaper than ordinary reinforced concrete pipe. The manufacturing operations necessary for producing the pipe of my invention, furthermore, are not substantially different, nor any more expensive than those by which conventional pipe is made.

The section of the pipe of my invention has a width about 15% less than the diameter of cylindrical pipe of about the same sectional area. As a result, the area per unit length of pipe, which is projected on a horizontal plane is less than that of a cylindrical pipe. My pipe may, therefore, be designed for a lower earth load than that for which cylindrical pipe of the same flow capacity must be calculated. Not only is the .total earth load reduced, but the moment arm those of the oval IS, that the pipe l5 has a wall thickness which is a maximum1atthe top and bottom of the pipe asindicated at 20, and a minimum at the sides of the pipe, as indicated at;2l. The .wall thickness, furthermore, varies substantialy continuously between, the points and 2|; i r r t The pipe IS .in a size corresponding to a cylindrical pipe. having an inside diameterof 60", hasa minimurn'inside diameter of 52 and a maximuminside diameter of 68 The maxi mum wall thickness, furthermore, ,is 8" and the minimum Wall thickness, 5". iWith these dimensions,.the'pipe is capable of carrying a' load of 2000 pounds per square foot of projected area, when laid with the axis I8 vertical, with a margin of safety or 1 to 13/2, when loaded within the limit mentioned, furthermore, the pipe does not exhibit cracks, becausethe stress (both tension and compression) applied vto the concrete by such loading. is less thanthe ultimate strength thereof. The ends ofthe pipe l5 may, as previously indicated, be formed to provide bell and spigot 'oir'its... o V

Fig. 3 illustrates extra-strength pipe embodying my invention. The pipe 22 will withstand a load of 4000 pounds per square foot of projected area when laid with its major axis. vertical. The major and minor axes of the inner oval 23 thereof are the sameas those ofthe pipe I 5. The maximum 'Wall thickness of the pipe 22 is 9% and the minimum thickness, 6". Thev loadbearir'ig' capacity stated for pipe having the dimnsions given is based entirely on the ultimate "contrafiexure CF atwhich the tension and comstrength'of the concrete. At loads within the stated value, therefore, no cracks will be formed in the concrete. To take care of occasionalexcessive loads and provide: a factor of safety of -from .1 to 1 I employ light steel reinforcement 24. For the pipe shown, the section of each of the two. cages should amount to about .25 square inch per lineal foot of pipe; Cracks will result, of course, when the pipe is loaded beyond its rated capacity. 1

Concrete pipe embodying my invention is characterized by numerous advantages. In'the first place, the high degree of curvature of the pipe adjacent the top and bottom gives it the relatively high load-bearing capacity characterizing the smaller sizes of cylindrical pipe as made heretofore. The load-bearing capacity of the pip of my invention, furthermore, may be increased in the larger sizes by making the wall thicknessgreater. The invention also provides aconcrete pipe which is; not subject to cracking with all its attendant disadvantages, so long as theload is not greater than that for which the pipe is designed. The possibility of omitting ei i g ma e ei e of my in enti n om or span is similarly reduced. The actual bending moment is thus reduced by the square of the reduction in width of section, or about 22%.

By making the pipe thicker at the top and bottom, where cracks normally appear in ordinary concrete pipe, I provide greater strength toresist the'increase in effective vertical load which occurs when outward deflection of the sides is.

arrested by lateral earth pressure. v

The advantages of theinvention pointed out above may, in my opinion, be explained by the following analysis of the stresses involved, al-

though I do not wish to be bound by any theories of performance as stated herein.

Referring to Fig. 4, showing diagrammatically and for the purpose of explanation only, the

upper half of a plain, unreinforced, cylindrical concrete pipe 25, the stresses set up therein by the application of the load L are as indicated by I the arrows. At any vertical section there is a vertical shearing force S1. Tension T andcompression C exist in the fibers of the shell spaced from the neutral aXis N. The compression and a .tension are interchanged in passing the points of pression are zero. The longitudinal shearing force S2 representing the resistance of successive layers of the concrete to the force tending toslide one over the other, is tangential to theneutral axis at the plane of any section through" the pipe wall.

Following the analysis given by H001, Reinforced Concrete Construction vol. I, "p. 44, the vector diagram shown in Fig. 5 may be drawn to represent the forces at a point P in the neutral axis, the vertical and tangential shear at any The vertical 1 point being equal (op. cit., p, 43). shear S1, combined vectorially with the'tangential shear S2, produces resultant tension Trand' compression C1 perpendicular to the neutral axis N" and tangential thereto, respectively. The compression C however, is not equal to the tension- T1, as in Fig. 14 on page 44 of Hook. vOn the contrary, the compression C1 is much greater. than the tension Trand this compression acts; directly to resist the effect of the load. It is thus apparent that thereexists in a concrete pipean This force, together- Y with the force developed by the resistance to; bending, constitutes the total strength of the active shear resistance.

The pipe of my invention, by having the maximum curvature at the top and bottom, obtains '.'the greatest benefit from this shear resistance, By reason of the oval shape, the pipe of my indrical pipe of the same sectional area. v

By numerous tests, I; have'determined that the bending strength or modulus of rupture of the concrete in conventional reinforced concrete pipe is about 450 pounds per square inch. By the use of known formulas, it is possible to deter- As stated above, pipe designed in this manner will support its standard load without the formation of cracks. Since the design is based on the strength of the concrete represented by its mine from this figure the load carried by the 5 modulus of rupture, the tensile stress in the conbending resistance ofiered by any given pipe. Crete has no further leeway and it is desirable, After proper allowance for the small resistance to l f i to provide some light l'ffinforcement the formation of a crack in the concrete oifered as {ndlcated at 24 m Winch, W the Shear by the reinforcement, it is found that a resistance which is still effective, W111 produce the plain concrete pipe with a Wall thickness of 1 additional strength needed to provide a suitable safety factor. has an ultlmate mncenimted load strength. Although I have illustrated and described 4100 pounds. The portion of the load carried b the bendin resistanc det r d b herein but a preferred embodiment of the 1ny d 2 3 e as We vention, it will be understood that changes in m lone owever (my pounds rIjhe 15 the details thereof may be made without dedlfierence or 1450 pounds 3113139? to be c'arned parting from the spirit of the invention or the by the excess Of the Compression C1, (Fig cope of the appended claims over the tension T1. The following table gives 1 i Simllal figures for Various other P p Sizes; 1. Concrete drain-age pipe having a cross sec- Shell 3 Shear Size of pipe thick- Strength 33% Total resist- Total HESS aznce 811GB Inches Pounds Pounds Percent Pounds Percent Infinite (straight beam) 6% 4100 4100 100 0 0 6 0% 4100 3280 so 820 6 4100 3070 75 1030 4100 2860 70 1240 4100 2050 05 1450 4100 2450 00 1050 4100 2300 55 1x00 4100 2000 2040 50 4100 0 0 4100 100 To design pipe according to my invention, for 35 tion which is generally oval on the exterior and various loads, I start with the inside diameter interior and a wall thickness varying substanof a cylindrical pipe necsesary to give the detially continuously from a maximum at the top sired free sectional area. The major and minor and bottom to a minimum at the sides. axes of my oval pipe are then obtained by in- 2. Concrete drainage pipe having a generally creasing and decreasing this diameter by about 40 oval section and a wall thickness varying from the same amount, e. g., in the embodiment of a maximum at the top and bottom to a minimum Fig. 2, 8". The sectional area of the oval pipe at the sides, the wall of said pipe being sodithus obtained differs but slightly from that of mensioned that the concrete will withstand. any cylindrical pipe of the starting diameter. I then stress created therein by loads below the maxidetermine from the above table, the load ca- 45 mum for which the pipe is designed. pacity of the pipe due to shear resistance, pick- 3. Concrete drainage pipe having a generally ing the value for shear resistance to correspond oval section, the horizontal and vertical interior with the radius of curvature of the pipe at the dimensions of the section differing substantially top and bottom. The balance of the load for equally from the diameter of a circle having which the pipe is being designed must be car- 50 approximately the same area, the vertical dimenried by the bending resistance. Using standard sion being greater and the horizontal dimension structural formulas, and basing the bending reless than said diameter, the wall thickness of sistance on the modulus of rupture of the conthe pipe being greater at the top and bottom crete, i. e., about 450 pounds per square inch, the than at the sides. wall thicknesses along the major and minor axes 4. Concrete drainage pipe as defined by claim may then be determined. These will determine the dimensions of the exterior of the pipe.

2 characterized by a circular bore.

CHARLES F. BUENIE.

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