US2377055A - Pile cap - Google Patents

Description

J. H. THORN LEY PILE CAP May 29, 1945.

Filed May 31, 1944 2 Sheets-Sheet 1 INVENTOR.

May 29,1945. J. H. THORNLEY PILE CAP Filed May 5l, 1944 2 Sheets-Sheet 2 INI/ENTOR. Jaiep/z E 7720/71@ BY Patented May 29, 1945 rice l PILE CAP Joseph H. Thornley, Evanston, Ill.

Application May 31, 1944, Serial No. 538,120

(ci. ei-'m claims.

My invention relates generally to building foundations or building structures, and it has particular relation to means for distributing the load imposed by a column among a plurality of supporting columns or piles, or to other load su'pporting means.

The invention is not limited to use as a pile. cap. It is applicable to any situation where a heavy concentrated load, such as is carried in a column, is imposed upon a supporting body of concrete or the like, which is required to receive and distribute the concentrated load. n The principal object of my invention is to provide a structure for distributing the load from a column through a body of concrete or the like to .spaced apart load supporting means, which may two on more eccentric supports, the art heretofore has employed a cap or block on the theory that it is to serve as a beax'n or plate supported at the ends and loaded at the middle, and that ,nexure is to be resisted. I conceived that if material strong in compression were to be employed l economically, it should embody the theory of the arch instead of that of a beam. Experimental investigation of the failure of caps or blocks of various structures andy shapes confirmed my .theory that the most advantageous use of material such as concrete could'be made by stressing 35 the concrete as an arch and restraining the tendeney of the limbs thereofl to separate or spread by a reenforcing metal tie. When the load is to be carried on two or more spaced areas of support, or is distributed over an areaof support 40 greater than the loading area, a body of circular section, i. e., having a surface of revolution, and preferably a body which is an arch in any vertical l axial section, is a simple and highly effective form, since in that form annular reenforcing 45 rings having maximum, strength toward the supporting side can provide the tension strength with a minimum of material.

The body-of concrete preferably shaped like a sugar loaf or hemisphere, and without reenforcement or restraint when subjected to compressivel stress, tends to crush by shearing an inverted cone ,of material down through the center of the body, splitting the base in all directions. By proprogressively downward, I secure a base or cap of the desired load carrying ability with substantially a minimum of material.

This discovery applies not only to the transfer occurring between a concentrated load as from a column and eccentrically disposed supporting piles, but to any situation where the load is to be spread from the center to two or more points offset from the axis of the projected area of the column or concentrated load. In fact, it applies equally well to the spread of the concentrated column load to a supporting surface which comprises all of the bottom surface of the cap, as might be the case where the load of the column is to be spread at reduced unit pressure to all or 'a large part'of the bottom area of the cap. The invention thereby `provides a novel utility in spreading laterally, with or without decrease .in unit pressure, a concentrated vimposed load, and is particularly useful as a pile cap, because in that situation the supporting areas provided by the pile ends have no other tie between them to keep them from spreading apart.

While the preferred external form is the hemispherical or sugar loaf form, this is not the essential feature, but only a highly advantageous feature. The essential feature is concerned with the method of containing the bursting stresses developed in a block when the area or areas of support is or are not coincident in area and in line with the area of load imposition. Which is load and which is support is of no moment for into compression) tends to expand the radius of the area in the hoop, and that force is therefore resisted by the tension strength of the hoop. The hoop therefore is effective for all conditions of load support. of the central concentrated load. The hoop may be a distributive hoop, i. e., consist of a plurality of rings graduated in size and/or number to give the desired disposition of strength. Also, instead of an integral metallic ring. it may consist of spiral or helical coils of rodlike stock suitably disposed and distributed as will be under.- stood by those skilled in the art.

It is to be observed that the reenforcin'g ring or rings surround or embrace within their connes the free area or cross sectional area, whichever is the construction through which the load is viding the body with hoop strength increasing applied, and likewise the area or areas of support.

In brief, the compression stress falls inside the ring, and is therein converted into a generally radial force which is contained by the ring.

The ring above' mentioned may be a concentrated or a distributive reenforcement. When it is distributive as herein illustrated, graduation of the amount of reenforcement (i. e., cross sectional value) at various locations is facilitated.

For a more complete understanding of the nature and scope of my invention reference can be had to the following detailed description taken in connection with the accompanying drawings in which:

Figure 1 is a view, partly in side elevation, and

. partly in section, illustrating one embodiment of my invention;

Figure 2 is a top plan view of the construction shown in Figure 1;

Figure 3 is a view, partly in side elevation and partly in section, illustrating another embodiment of my invention; and

Figure 4 is a top plan view of the construction shown in Figure 3.

Referring now particularly to Figures 1 and 2 of the drawings, it will Dkbe noted that the refer-4 ence characters Il), Il and l2 designate columns or piles in aV building construction or foundationwhich are arranged to support a column I3 that is adapted to carry a portion of the building load. The arrows Il, l5 and I6 indicate, respectively, the direction of the reaction of the co1- umns or piles I0, Il and I2 to the load applied throlllgh the column I3 as indicated by the arrow 1.

With a view to distributing the load indicated by the arrow i1 among the columns or piles Ill, Il and i2, a cap, shown generally at 20, is provided. It will be 'readily understood that the cap 2l is subject not onlytc' compressive stress resulting from application of the load indicated by the arrow l1, but also it is subjected to tension stress at a maximum near the lower portion or base because ofthe positions of the axes of the columns or piles I0, Il and l2 spaced from the axis of'the column i3, as is more clearly shown in Figure 2 of the drawings. It is desirable that a'minimum weight and amount of maaevaoss application of the load, as indicated by the arrow il. lf desired, more or less than three concen-I tric rings can be used, and they are preferably of steel or other high tensile strength material.

Since the tension stress in the cylinder El decreases upwardly from the base thereof, the number of concentric rings is reduced, as shown, until at the top only a single ring is employed in each layer. It will be understood that a single ringlike tension member can be employed which decreases in cross section from a maximum at the base to a minimum at the top, or the graduation of reenforcement to stress distribution securedby various modifications of the hoop or ring structure. However, it is preferred to use a plurality of ringlike tension members of different diameter as shown embedded in the concrete.

It will be understood that the proportions of the cylinder 2l and' of the rings 24, 25 and 26 can be varied, and that the compositions thereof can be chosen in accordance with the character and magnitude of the imposed load, the dimensions of the column I3 and piles l0, H and i2'. It is important to so proportion the various elements entering into the design of the cap 2li thata proper balanced relation is maintained between the load carrying capabilities thereof and the distribution of the compression and tension characteristics thereof.

In Figures 3 and 4 of the drawings, the prepared embodiment of my invention is illustrated.y

-The construction there shown takes advantage of the load carrying ability of an arched conflgura= tion, as will be presently set forth.

Similarly to the arrangement previously described, columns or piles 30, 3l and nare disposed to support a column 33, the forces exerted thereby being indicated by the arrows N, Il and 36, respectively. The downward load or force exerted through the column is indicated by the arrow 31.

terlal be employed in making up the cap 20 for obvious reasons. However, it is necessary to take into consideration thecompressive and tension stresses and their distribution in the cap 2li and to make the same sumclently strong to resist them.

Preferably the cap20 is largely formed of a material which is relatively strong in compression and relatively weak in tension. For example, concrete is well suited for this purpose.

In the particular embodiment shown in Figures l and 2 of the drawings, the concrete is preformed as a solid of revolution, for example, a cylinder 2l with its longitudinal axis vertically disposed .substantially coaxially with the longitudinal axis of the'column i3. It will ybe noted that the column i3 rests upon the upper surface 22 of the cylinder 2| and that the columns or piles I0, Il and l2 bear against the under recessed surface 23 of the cylinder 2l.

As indicated, the tension stress is at a maximum at the base cf the cylinder 2l. The mate.- rial, such as concrete, of which the cylinder 2l largely made, is relatively weak in tension. .With a view to resisting this tension stress a series of concentric rings 24,. 25-and 26 of circular or other cross-section, are embeddeddn the cylinder 2l substantially coaxially with the direction of A cap, shown generally at 4l, is provided for distributing the load represented by the arrow I1 among the columns or piles 30, 3l and l2. The cap lll is generally dome shaped and, as shown, has an arched cross section in any vertical axial section. This permits substantially maximum use to be made of the material, such as concrete, in distributing uniformly the compression throughout the entire upper and outer portion of the cap 40. At the same time there is no unstressed portion, as is the case in the upper and\ outer portion of the cap 2li shown in Figure l.

ItV will be noted that the column 33 bears against the upper surface 42 of the cap l0. A recessed under surface 43 is provided for receiving the upper ends of the columns or piles $0, 3l and 32. C

With a view to resisting the tension stress developed in the base ofthe cap l0, concentric tension rings Il, I5, 46 and 41, preferably but not necessarily of rectangular cross section, and preferably of high tensile strength steel or the like circumscribe the indicated portions of the base. Since the tension stress decreases upwardly from the base, the number of the concentric rings in -each layer is reduced in accordance with the stress distribution. The load transferring member of my inventio optionally may be integral with one or more of the load imposing or load simporting elements or it n ay be a detached piece.

Now the external form is not the criterion of my invention. The criterion is the controlled disposition of reenforcing hoops, or' rings or their not appear-externally to conform to the struc- 'tures herein illustrated as typical embodiments.

. Also, it is to be noted that the structure illustrated inv Figures 3 and-ias formed externally' in the shape of a dome is an idealized form'. In practice, such afiorm-is dilcult to's'hape, and

without anydepartur'e from the broad aspects of v'my invention,u the cap maybe cylindrical, as in 2,877,055 'y equivalentin the'body which receives the concensubstantially coinciding with the vertical axis of "trated load and distributes the same. The body l Awhich If have shown as. a separate cap may be. lformed as a part oiramat or slab, and hence may.

said column'.4

2. 4A load .distributing p iie cap comprising a solid block of concretehaving' avertioal axis of i symmetryin line of which load vis-adapted to be imposed, and a substantially planar bottom face upon Asymmetrically disposed restricted areas of which supportisadapted tobe applied, said block Figure 1,'or rectangular, or pyramidal in ex 'o ternal form, and yet include`the 4structure and gain-the advantages of my invention Similarly',

the part or .column need not be circular in sec" tion, although that is the preferred form.

. In4v both embodiments .illustrated the distributed reenforcementrings overliethe areas of support l-ofr the'piles in a transverse direction, .whereby increased vstrength in shearis provided againstv failure by shear of the capabout the -areasl'of y support of said piles. This is not a feature of primary is optional..`

The preferred disposition oi the ringsfor hoops is as illustrated in-Iiisurosv land 2, with the rings coaxialiy--fon sucz'zessiv'e.A levels andfwith the number of concentric ring'sfon one 1 or more oi the lower-levels therebyto accomplish the desired increase of section v of .the reeniorcement. Since `other. modications of' the constru't'ztions- "shown and described-hereinbeforevoan'be made, without de parti'ng-'fromthe spirit-and scope ot .35 vthe body, said rings embracing the projected area the present inventiori-I itis intended that.allmat ter shown in theaccompanyingdrawin'gs and described hereinbefore shall be intellreted as ii1us. trative and not in aii'miting sense.

I claim as my invention:

1'. Iiloombinationlav central loaded column, 'I

. plurality of substantially symmetrically spaced` apart supporting piles for receiving the load of y .the column and a pile cap comprising a body o1 concrete' interposed between said column and said'piles and havingA` annularmetaliic 'reenforoef 4 mehr.-- symmetrically disposed therein increasing in. A: te cross section -fromgtop toward the i bottom;l the otfsaid-annular reenforcementiwith their axes Adisposed --substantially in' line ""verticalaxes on the bottom. side,

comprising vva'series of annular metallic reenforcernent members with their axes disposed in substantially the axisof the block eiec'tive as hoops embeddedin the block, said reenforcing .members being of aggregate cross section increaso ing from top towardthe bottom of the body of the -.cap-.

3.-.A load. solid .body of concrete of a form providing substantially a hemisphere in any vertical section,

said cap havingavertic'alaxis for a centrally' applied load, and having a base adapted to rest upon ayplurality of substantially symmetrically f disposed' supportin8 piles, .and hooplike reeni'orcing. rings embedded in said bo'dy of concrete with the axis of the cap and being of increasing 'aggregate cross section from top toward the bottom of the body..

`is A 10aa distributing pile mais comprising a solid body-of concretehaving a central area of loading on a vertical' axis on thetopside thereof and having separate areas of support on parallel enforcing graduated aggregate cross section increasing from top toward bottom 'of ofthe applied load and the lowermost rings exe tending across theprojectedareas of support.

5. A loaddistributing pile cap comprising a .solid body of concrete having a top central area of loading on a vertical axis and having' separate restricted bottom areas of support on parallel vertical axes and embodying reenforcing ringsl graduated in 'aggregate cross section to increasex5 from top toward bottom of said body, said rings being disposed on successive-levels with a greater number of rings on the' lower level thanon the bossen rr.y mommy.

distributing lpile cap comprising a-

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