US3283458A - Shear reinforcement in reinforced concrete floor systems - Google Patents

Description

Nov. 8, 1966 B. GERSOVITZ 3,28

SHEAR REINFORCEMENT IN REINFORCED CONCRETE FLOOR SYSTEMS Filed May 10, 1963 5 Sheets-Sheet 1 Lgh a (c+ 03} M56 00i 6/- Y Benjamin Gersovhz mfiW%# ATTORNEYS Nov. 8, 1966 sovrrz 3,283,458

SHEAR REINFORCEMENT IN REINFORCED CONCRETE FLOOR SYSTEMS Filed May 10, 1963 5 Sheets-Sheet 2 INVENTOR Benjamin Gersovitz ATTORNEYS Nov. 8, 1966 B. GERSOVITZ 3,283,458

SHEAR REINFORCEMENT IN REINFORCED CONCRETE FLOOR SYSTEMS Filed May 10, 1963 5 Sheets-Sheet 5 INVENTOR Benjamin Gersovirz EMMMEM ATTORNEYS B. GERSOVlTZ Nov. 8, 1966 SHEAR REINFORCEMENT IN REINFORCED CONCRETE FLOOR SYSTEMS Filed May 10, 1963 5 Sheets-Sheet 4 INVENTOR Benjamin Gersovirz wfi wgw ATTORNEYS SHEAR REINFORCEMENT IN REINFORCED CONCRETE FLOOR SYSTEMS Filed May 10, 1963 B. GERSOVITZ Nov. 8, 1966 5 Sheets-Sheet 5 INVENTOR Benjamin Gersovirz wgw ATTORNEYS columns merge directly with the ceiling.

United States Patent 1 Claim. (61. 52-260) This is a continuation-in-part of applicants U.S. application S.N. 790,960, filed February 3, 1959, and now abandoned.

This invention relates to a reticulated structure for providing shear reinforcement in reinforced concrete floors and footings and in particular is concerned with a prefabricated shearhead for providing shear reinforcement above columns supporting fiat plate floors.

Various types of reinforced concrete floor systems are in current use by the build-ing industry and are generally well known in the art. A suitable reference for the various types of floors, their names and uses is the brochure entitled Reinforced Concrete Floor Systems published by the Portland Cement Association. In this brochure, reference is made to flat slab floor systems and it is in connection with a particular variety of flat slab floor system known as the flat plate floor that the present invention finds particular, though not unique, employment.

Modern design methods for reinforced concrete building, using girderless floors base, the safety of such structures on their ultimate strength; and a theory based on a consideration of the ultimate flexural capacity of the slabs used in the floors and footings of such buildings, known as the yield line theory, has been evolved. It is a basic assumption of these design methods, and a premise of this theory, that the slabs used in a construction method will not fail in shear before the full flexural capacity of slab is reached. Therefore it becomes mandatroy for the safety of such buildings that means are provided to ensure that shear failure cannot occur. Since the maximum shear stress is generally reached in the neighborhood of the columns supporting the floors it follows that such shear reinforcement must primarily be provided at these columns.

One known method of providing such shear reinforcement is by the use of drop panels and column capitals, and a floor system utilizing these two types of reinforcement is known as the Flat-Sla system. Drop panels are thickened portions of the slabs around the columns and act to provide increased cross-sectional area and depth to resist negative moments and shears. Column capitals are flared shoulders at the tops of columns, often being used in conjunction with drop panels, and are effective to reduce the slab span between columns. The use of the drop panels and column capitals though able to provide the required measure of shear reinforcement is undesirable fora multiplicity of reasons. They are unsightly objects and restrict the freedom of the architect in planning the layout of the interior of the building; they project below the bottom of the slab and hence interfere with any mechanical work depending from the ceiling. It would therefore be extremely desirable to eliminate the use of drop panels and column capitals without increasing the danger of shear failure.

The type of flat slab floor'system which dispenses with the use of drop panels and column capitals is, by definition, known as flat plate floor system and in it the As well as overcoming the drawbacks of the above system, the fiat plat floor system is aesthetically purer and more economical than a flat slab floor system as it also permits a "ice reduction in the height of the stories and the lengths of vertical construction including columns, walls, etc.

A search has, therefore, been conducted for some time by the reinforced concrete industry for some means of solving the problem of providing shear reinforcement above columns in flat plate floor systems. One solution is offered in the R/C Bulletin entitled Shearhead Reinfol-cement for Flat Pilate Floors, published by the Portland Cement Association which consists of disposing concentrically around the column a number of V-shaped stirrups which provide an area of reinforcement surrounding the column. Though such .a type of shearhead should apparently afford suflicient protection against shear failure, the stirrups do not seem to have sufiicient anchorage as samples tested still failed in shear, with crushing taking place at the upper and lower edges of the stirrups.

Another attempt at solving the problem of providing shear reinforcement in flat plate floor systems is described in the article entitled Shearing Strength of Reinforced Concrete Slabs, by Richard Elstner and Edwind Hognestad which appears in the July, 1956 journal of the American Concrete Institute. Here in series IX of the tests conducted by the above researchers there is disclosed an embryonic form of the present invention in which steel bars are laid out in a cruciform pattern above the column. It was the apparent endeavour of these researchers to increase the ultimate shear strength of the floor in the neighbourhood of the column to the point where the slab would fail fiexurally, i.e. in compression rather than in shear. They were unsuccessful in their attempt for though they managed to increase the ultimate load capacity of the slab system by 30%, failure still oocured in shear in all samples tested, and the inference to be drawn from this article is that the researchers did not consider it practical to use steel bars in a reticulated pattern to obtain suflicient shear reinforcement so that the slabs would fail in compression rather than in shear.

The shearhead disclosed herein, although retaining some of the more elemental features of the structure described in the above article, increases the ultimate load capacity by 75% and failure occurs in compression not in shear, that is the flexural capacity of the slab system is fully developed and the problem of providing sufficient shear reinforcement for flat plate floor systems is thereby solved which the above researchers were unable to do.

According to the present invention there is provided a reinforced concrete structure comprising a fiat plate floor having an upper and lower surface, at least one supporting column, and shear reinforcement embedded in said concrete adjacent said column, said reinforcement comprising a shear head having a first set of reinforcing members consisting of a first series of bars arranged substantially parallel in equally spaced side-byside relationship and a second series of similarly arranged bars at right angles thereto, the bars in each series having an upper horizontal central portion substantially parallel to and adjacent the upper surface of said floor and opposed horizontal anchoring portions interconnected to said central portion by a pair of downwardly, outwardly inclined portion, said inclined portions of each series lying in a plane angularly disposed to the axis of the respecparent from the following description, when taken in conjunction with the accompanying drawings in which:

FIGURE 1 shows diagrammatically the primary shear stress pattern in an unreinforced flat plate floor.

FIGURE 2 shows the effect on this pattern of intro- I ducing an inclined reinforcement bar which intersects this primary pattern. FIGURE 2a shows a detail of a suitable reinforcement bar.

FIGURE 3 illustrates the etfect of introducing further reinforcement bars to intersect the secondary shear stress pattern.

FIGURE 4 is the structure of FIGURE 3 presented in a graphical form.

FIGURE 5 depicts a partial plan view of the underside of a flat plate floor supported by square columns.

FIGURES 6, 7, and 8 are used to illustrate the extent to which shear reinforcement must be provided around the top of the column.

FIGURE 9 shows a plan view of the prefabricated shearhead of the present invention.

FIGURE 10 shows an end elevation of this shearhead and FIGURE 11 is a perspective view of this shearhead as it would be placed above a column.

FIGURE 12 shows an alternative embodiment of this shearhead when used above a column which is situated along the outer wall.

Before embarking upon a description of the preferred shear reinforcement structure of the present invention it is necessary to describe in some detail the principle underlying its method of operation.

As is well known the shear stress in a flat slab floor system reaches a maximum value at the head of each column and decreases to a minimum at a point most remote from such columns. The planes of maximum stress at the head of a column commence at the junctions of the column with the underside or compression face of the floor and then proceed outwardly from the column through the floor, at an angle which is approximately 45 degrees, to the upper or tension face of the floor.

This is shown in FIGURE 1 where the shear pattern 8.

commences at the junction of the column C with the flat plate floor F and proceeds outwardly at an angle of 45 degrees. The linear distance between opposed points on the upper or compression surface Where the shear line S emerges isdenoted by I. Beyond the two boundary points where the shear line emerges the shear stress decreases as the distance from the column increases.

Without reinforcement it is quite easily possible for the shear stress induced at the head of the column to exceed the shear capacity of the concrete material forming the flat plate floor and therefore the floor would fail in shear at the line S of primary maximum shear stress. If, however, reinforcement consisting of a first set of reinforcement members is provided and arranged in a grid like pattern symmetrically with respect to the column and if these steel bars each has inclined portions which intersect this maximum shear line S, then this primary maximum shear line will be'moved outwardly from the column a certain distance to form a secondary shear stress pattern S which starts at the lower end of the inclined portion of the bar and then proceeds up-, ward, at an angle of 45 degrees, to the tension face of the concrete floor. As previously noted, the shear stress reduces as the distance from the column increases, accordingly, the new maximum shear stress in the floor, which is now the secondary shear stress along line 5;, may or may not fall below that value which can be safely home by the concrete. Such a single layer of reinforcement is shown in FIGURE 2 where the flat plate floor F is supported on column C in themanner described for FIGURE 1. In the event the secondary shear stress along S cannot be borne by the concrete, 3, second setof reinforcing members may be added. The second set. of reinforcing members is likewise arranged symmetricaL Due to the action of bar R, in offsetting the shear line,

the distance between opposed points on either side of the column where the shear line breaks through the upper surface of the floor, has now increased from the value I, shown in FIGURE 1, to a new greater value 1 and, of course, the shear stresses in the floor at this greater distance are reduced from those which occurred without reinforcement.

A suitable reinforcement bar R is shown in detail in FIGURE 2a. This bar has two inclined portions 2 disposed on either side of a central portion 1, and it is obvious that by varying the length of this central portion the distance apart of inclined portions 2 can be varied so that they will intersect the lines of maximum shear stress. Extending outwardly beyond the inclined portions 2 are end portions 3, each of which is shown terminating in an anchoring hook 4 which forms a useful though'not essential adjunct to the bar. The inclined portion subtends an obtuse angle -0 with the central portion and varying this angle obviously controls the horizontal extent of the inclined portion. In use in a concrete floor, the central portion and the two end portions, anchor the inclined portion and enable it to resist the shear stress. Though these anchoring portions are shown here in their preferred flat parallel forms, it should be remembered that their function is to anchor the opposite ends of the.

inclined portion.

If, as is generally the case, the reinforcement provided by the single bar of FIGURE 2 is not sufficient to reduce the shear stress to a value which can be borne by the concrete material which constitutes the floor, then, as previously mentioned, it becomes necessary to provide other steel reinforcing bars across the secondary pattemof maximum shear stress shown inFIGURE 2. This further reinforcement is shown in FIGURE 3, where once again, a flat plate floor F is supported upon a column C,

and a first set of reinforcing members is provided. A

second set of reinforcing is introduced wherein the bars 7 have. a longer central portion than in the first set so that mary and secondary planes of maximum shear stress are suflicient to provide adequate shear reinforcement but in certain cases it 'will become necessary to provide a third or even fourth set of shear reinforcement members wherein the inclined portion of the bars are positioned further from the column so as to. intersect further shear planes.

It will thus be appreciated that the basic object of this i invention is to provide reinforcement for a column supported fiat plate floor wherein such reinforcement includes rods having inclined portions symmetrically arranged about the column, the inclined portions in each series be-.

ing in a plan inclined with respect to the column and forming the frustrum surface of a regular pyramid, and in the usual type of construction where the column is square sided this frustrum surface will be that of a regular square pyramid. This reinforcement must of course be substantially uniform and continuous through this surface, and according to this invention the necessary reinforcement is supplied by the inclined portions of an appropriate number of reinforcement bars which extend upwardly in each frustrum surface. The reinforcement planar surfaces of each set are positioned symmetrically around the column so that the surface of one set will intersect the plane of maximum primary shear stress, and that the second set will intersect the plane of maximum secondary shear stress. Anchorange for each inclined portion is provided by forming two anchoring portions on the reinforcement bar extending inwardly and outwardly respectively with respect to the column into the two anchoring regions, and which, when the concrete floor material is poured, firmly anchor the inclined portions. In the preferred embodiment of the present invention for square sided columns, the anchoring portions are made flat and parallel, and the inwardly extending anchoring portions of opposed inclined portions in opposed frustrum surfaces are interconnected to form a common flat central portion; this latter type of reinforcement bar structure being in essence that shown in FIGURE 2a.

Shear reinforcement is provided in this manner, outwardly from the column in all directions where the shear stress exists till a point is reached where the shear stress is reduced to a value which can be borne by the concrete. Alternatively, the problem can be visualized as a need for increasing the cross sectional area concentric with the column over which the shear force is applied, by extending this concentric perimeter outwardly from the column until a point is reached where the maximum shear stress, which is equal to the total load on the column divided by the effective cross sectional shear area, is reduced to a value which can be comfortably borne by the concrete material used in the floor or footing.

It becomes necessary, therefore, in considering the type of reinforcement structure and the area over which such reinforcement must be applied to determine at what point, outwardly of the column, the shear stress has been reduced to an acceptable value.

The first consideration is that it must be known what value of shear stress can suitably be borne by the concrete material which constitutes the floor. This has been set by the American Concrete Institute at Section 807 of the AC1 code at three percent of the ultimate strength of the concrete 0.03 fc, where fc is the twenty-eight day ultimate strength of the concrete. In a typical case of concrete having a 28 day strength, at 3000 lbs., per square inch, the value of the shear stress which could comfortably be borne by the concrete is therefore given as 0.03 x3000 that is, 90 lbs. per square inch.

The next step is to determine, from a calculation based on the other known factors, at what point the shear stress, outwardly of the column, is reduced to this value of 0.03 is. The value of the shear stress is given by:

where P is the total shear load borne by the column b is taken equal to the perimeter of an area concentric with the column at that distance from the column at which it is desired to determine the value of shear stress. The factor d represents the distance from the compression surface of the floor to the centroid of the reinforcing steel, and j is a standard factor and is equal to the ratio of the distance between the centroid compression and the centroid of tension to the depth d, and is generally taken as being approximately .875.

The manner in which these various factors are calculated so that b, the unknown dimension, can be found from the above equation is as follows:

In FIGURE 5 is shown a typical section of a floor layout in which the columns are uniformly spaced at a distance n apart. It will be seen that the area supported by each column is equal to I1 From this area the column receives a live load due to the weight of materials. etc. placed upon the floor and a reasonable value can be assumed for this quantity, and in addition it receives a dead load due to the weight of the floor construction, which will vary with the thickness of the floor and the types of material used in its construction. Both the live and dead loads are conveniently expressed as so many pounds per square foot and the total load P on any column is given by the sum of the live and dead load units multiplied by the factor n where n is in feet.

As mentioned above, at is the distance from the compression surface to the centroid of the reinforcing steel floor and this is shown in FIGURE 6 where this steel T is located at a depth d below the upper surface of the floor and overall thickness t. Since, as is common practice, the steel is located at a distance of approximately 1.5 inches, above the lower surface of the floor, then d is given by t1.5 inches and for the case of a six inch floor would therefore be 6-1.5 i.e. 4.5 inches.

The significance of b is shown in FIGURE 7 which shows a plan view of a section of floor supported upon a square column having a side dimension c. The peripheral area b has been defined as the perimeter of an area concentric with the column and this area will thus take the shape of a square concentric with the column whose sides as shown are of length p, and b therefore given by 4p, where p is the distance between similar points on either side of the column -at which the shear stress is to be computed. Thus to calculate the dimension of the square over which shear reinforcement must be provided in order to reduce the shear stress to an acceptable level it is necessary to first determine b and from this the value of p the length of the side of the square.

If, in the above equation for the value of the shear stress the general value V of the shear stress is replaced by the specific value of the shear stress which can be borne by the floor which as stated, is generally taken as 0.03 fc then we may write This can be solved for b since all the other factors are known for any specific fi-oor configuration and so the perimeter of the area of reinforcement is determined.

As an example of the manner in which the above formula can be applied, consider the case of a flat plate floor 3000 lbs/sq. inch 28 day strength concrete composed of 6 inch slabs which is borne upon 14 inch square columns set apart at 20 foot intervals. If the live load on the floor is taken at lbs. sq. ft. it is known that the dead load for such a six inch slab is 75 lbs. per square foot, so that the total unit floor loading is lbs. per square foot. Thus the total load on any one column is given by 175 20 20 that is 70,000 lbs. The thickness t is equal to 6 inches so that d, the effective depth, may be taken as 61.5=4.5 inches. For a 3000 lbs/sq. in., 28 day strength concrete .03 is is equal to 90 lbs. per square inch and as we have seen above, j is taken as being equal to .875. Therefore, We may write a i.e. 49.4 inches It is interesting to compare this reduced value of shear stress along the perimeter of a square 2 concentric with the column with the maximum value of shear stress which would prevail in the absence of reinforcement. This maximum value of shear stress vC is given by the following equation:

where b is the specific perimeter of the median of the primary shear stress pattern and is equal to 4 (c-t-d) where c is the side length of the square column. In the example cited above, the column had a side of 14 inches so that:

Thus it will be seen, that the maximum shear stress in the floor has to be reduced by approximately 150 lbs. per square inch and to do this it is necessary to provide shear reinforcement extending outwardly to the edges of a square area of side approximately 50 ins. concentrically about the column to ensure that this reduction is accomplished.

A graphical representation of the significance of the above calculations is shown in FIGURE 8 in which is depicted a square slab of the floor of side p supported upon a square column of side c which has been raised from the floor. The reinforcing steel T located at a distance d above the lower surface of the slab floor, the thickness of the slabs being I.

As well as computing the extent over which shear reinforcement is necessary it is also a requirement to determine how many inclined reinforcement planes are needed to intersect the successive planes of maximum shear stress, and a graphical representation of how this is done is shown in FIGURE 4, here the reinforcement extends over a distance d, which as shown above is slightly less than t, the thickness of the floor. The shear stress perimeter is considered as being at the mid point of the shear line i.e. where this line intersects the inclined portion of the reinforcement bar, and so the distance between opposed intersection points which was designated earlier as p in the case of the square sided column, will be measured along the median of the floor or along a line running through the floor at a depth d/2 above or below the reinforcing steel.

From FIGURE 4 it will be seen that, for any given floor configuration, the primary plane of maximum shear stress must intersect the reinforcing bar at a fixed distance from the column of d/2, cot 45 i.e. d/2, the total span along the median between opposed bars intersecting the primary plane of maximum shear thus being approximately c-l-d, and accordingly independent of the angle of inclination of the inclined portion of the that.

A further distance beyond this primary span is however very much dependent upon 0 and for one side this in- =240 lbs/sq. in.

I number, and must be rationalixed to a whole number.

creased distance is given by d/2 cot +d/2, the total distance between opposed bars should thus be (c+d) +2.(g+g cot 9) (c+d)+1.5(g+g m a) the (c+d) quantity has not been reduced since this factor is itself already a reduced approximation from its geometric value of (c+t), and is reduced thereby in approximately the same order of magnitude.

For N reinforcement planes this distance becomes and this must be equaled to the distance between opposed reinforcement planes deduced as above from the computed value of the perimeter i.e. for a square sided column It should be noted that reducing the value of 0 increases the distance between reinforcement planes, and so its value is preferably low.

A reticulated structure formed as a preferred embodiment according to the disclosure of the present invention is shown in FIGURES 9, 10 and ll and provides the necessary area reinforcement to enable the shear stress present in a flat slab floor system to be borne without the use of drop panels or column capitals.- It will be seen that the structure is symmetrical in a horizontal plane about 2 axes at right angles to one another and comprises a first of mutually parallel bars across which is laid a second of mutually parallel bars substantially at right angles to the first. Each of these bars is formed in the manner shown in FIGURE 2a and has a flat central portion 1 on either side of which is disposed a downwardly and outwardly inclined portion at an angle of inclination of 1806 to the horizontal, outwardly beyond which the bar has two further spaced horizontal portions 3..

The bar generally terminates as shown at opposite ends in an anchoring hook 4 which insures thatit is firmly imbedded in the concrete. Such hooks are well known and do not form an essential feature of the present invention. By varying the length of the centre section 1, it is possible to move the opposed inclined portions 2 varying distances apart and thus by combining in each of the sets of bars a number of bars whose centrallength is varied, it is possible to provide reinforcing in a series of outwardly disposed inclined planes which lie across the successive planes of maximum shear stress and thus move the shear line outwardly from the column. The angle 0 may usefully lie anywhere in the range from 15 to 60 degrees but is preferably in the region of twenty degrees since this has the dual benefit of increasing the extent to which the shear plane is offset by each inclined plane and also re-- duces the crushing stress which appears at the junctions of the inclined portions with two flat portions.

The manner in which the bars of various lengths are interspersed, in each of two series of bars at right angles to one another and the total number of bars in each. series, is of course, dependent on the particular circum-.

stances of each case. However, since it is required to define progressively wider inclined areas of reinforcement, it follows that the outermost bars of each set will be progressively spaced further apart with the outermost.

'bars of the series having the longest centre section and that the bars intermediate to these outer bars will be: varied as to number and length of centre sections in con-. formity with the need for providing sufficient reinforcement in each inclined plane and for distributing the reinforcement substantially uniformly in each inclined plane.

The shear head shown in FIGURES 9 and 10 consists of a first and second set of members each set being in:

a grid like pattern. The first set consists of a first series of bars 11a, b, c, d, and e and a second series 13a, b, c,

d and e disposed at right angles thereto. Bars 11c and respectively form the respective axis ofa quadrature and bars 11a and b; 11b and e; 13a and b and 130 and d are respectively disposed symmetrically on either side thereof. The inclined portions of bars 11 and 13 define a fiirst inclined plane of reinforcement disposed transverse.

to the axis thereof.

The second set consists of similarly arranged and 12 having like alphabetical reference indicating a corresponding arrangement and forming respectively a first and second series. symmetrically placed on either side of the axis of the first series in the first set arev longer bars 10a, 10b, 10c and 10d forming a first series in the second set and whose inclined portions define a second inclined plane of reinforcement symmetrically spaced outwardly from the first inclined plane provided by the shorter set of bars 11 above. Bars 10a and 10d bars 10 form the two outer bars of this first series in the second set and bars 10b and 100 are disposed symmetrically inwardly of these two outer bars and are interposed between the shorter bars with bar 10b being positioned between shorter bars 11b and 11c of the first set and bar 100 being located between bars 11c and 11d.

A second series of bars in the second set consisting of bars 12a b, c and d are disposed symmetrically at right angles to the first series of the second set and are similarly arranged with respect to bars 13a, 13c, 13d, 132.

The diameter of the bars is determined from a consideration of a number of bars in the particular inclined reinforcement surface and the amount of reinforcement needed in this surface to resist, with an adequate margin of safety, the shear force on the surface, which may be suitably and conveniently expressed in square inches, in cross-section, of steel per surface. Thus in a specific shearhead designed according to this invention, it was found necessary to have 0.95 square inch of steel reinforcement in the first inclined surface, and 0.65 square inch in the second inclined surface, and, using the configuration described above employing five bars in the first set and four bars in the second set, these two amounts were provided by five A2 inch diameter bars supplying approximately a total of one square inch, and four /2 inch diameter bars supplying approximately 0.80 square inch respectively for the two surfaces.

This particular combination of number and size bars was chosen for their convenient approximation of the required reinforcement and because their resultant spacing seemed preferable, but equally, the first surface would have been reinforced with three inch diameter bars and the second surface with six /8 inch diameter bars. The number and size of the bars is therefore moderately flexible, the primary requirement being that, in the aggregate, they provide adequate shear reinforcement.

The first series of bars in each set are secured together at opposite ends by a tie :bar; the two tie bars in the case of the first series which comprises shorter bars 11 and long bars 10 being tie bars 14 and 15 and the corresponding tie bars for the second series being tie bars 16 and 17. These tie bars each have at either end a small vertical portion which depends downwardly below the general lower level of the shear-head and ensures that the inclined reinforcing surfaces of the shearhead are correctly positioned vertically with respect to the shear lines.

The shear reinforcement structure described above can be formed by positioning the bars in their correct respective stations actually on the job. However, this task is both time consuming and requires an undesirably high level of skill on the part of the workmen forming the structure and it is one of the virtues of the sheanhead structure formed according to the present invention that it can be prefabricated elsewhere in a suitable workshop employing jigs and braces of the appropriate size and shape, the bars either being tied or spot welded together when correctly positioned. The shearheads can be prefabricated and stored until required and then positioned on the job with a minimum of time and effort by relatively unskilled workmen.

FIGURE 11 shows a skeleton view of this shearhead structure M as it would appear when placed above a column C in a fiat plate fioor F. While the parts are the same as in FIGURES 9 and 10, the numerals are omitted for greater clarity and simplicity.

It is necessary to modify this structure when the column is located atone of the outer walls of the building and a typical manner in which this structure is adapted is shown in FIGURE 12, though this must be done without disrupting the requirement that the shear reinforcement be provided over a unitary inclined surface of reinforcement symmetric with respect to the column in all directions over which shear reinforcement is necessary.

Since the column is at the outer wall, no shear load is presented to it from beyond the wall, the full shear load is presented from a direction normal to the wall and a lesser shear load approximately half the full load is presented from direction on either side of the column parallel to the wall. Thus it logically follows that the number of bars in that series parallel to the wall may :be halved without any appreciable loss in shear reinforcement. Accordingly, those bars in the series parallel to the wall which would normally lie outwardly beyond this wall are removed and the ends of the bars [forming the series normal to the wall are bent downward through degrees until they lie in the thickened end portion W of the flat plate floor F above column C.

I claim:

A reinforced concrete structure comprising a flat plate floor having an upper and lower surface, at least one supporting column, and shear reinforcement embodied in said concrete adjacent said column, said reinforcement comprising a shear head having a first set of reinforcing members consisting of a first series of bars arranged substantially parallel in equally spaced side-by side relationship and a second series of similarly arranged bars at right angles thereto, the bars in each series having an upper horizontal central portion substantially parallel to and adjacent the upper surface of said floor and opposed horizontal anchoring portions interconnected to said central portion by a pair of downwardly, outwardly inclined portions said inclined portions of each series lying in a plane angula-rly disposed to the axis of the respective series to present a first plane of reinforcement and a second set of similarly arranged reinforcing members consisting of a first and second series of bars interspersed symmetrically with the respective series of said first set and having similarly inclined portions to present a second plane of reinforcement, said planes of reinforcement being located symmetrical with respect to said column, said second set of bars having an upper horizontal central portion of greater length than that of the first set whereby said second plane of reinforcement is located more remote from said column than said first set thereby to provide reinforcement respectively to intersect secondary and primary shear stresses in said floor adjacent said supporting column.

References Cited by the Examiner UNITED STATES PATENTS 729,299 5/ 1903 Ellinger et a1. 52252 896,963 8/ 1908 White 52721 938,393 10/1909 Martin 52251 1,033,797 7/1912 Hartman 52252 1,052,708 2/ 1913 Anderson 52260 1,072,532 9/1913 'Iurner 52251 1,078,510 11/1913 Luten 52252 1,119,406 12/1914 Danielson 52260 1,143,527 6/1915 Francis 52250 1,163,853 12/1915 Randall et a1. 52264 1,182,421 5/1916 Ramsey 52251 1,205,347 11/ 1916 Hincz 52252 1,244,641 10/1917 Pratt 5225 1 FOREIGN PATENTS 17,179 12/ 1929 Australia. 326,638 11/ 1902 France. 498,886 9/ 1954 Italy.

FRANK L. ABBOTT, Primary Examiner.

HENRY C. SUTHERLAND, Examiner.

I. L. RIDGILL, Assistant Examiner.

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