US2916910A - Steel reinforcement for reinforced concrete structures - Google Patents

Description

Dec. 15, 1959 w. BOYER ETAL 2,916,910

STEEL REINFORCEMENT FOR REINF ORCED CONCRETE STRUCTURES Filed March 14, 1957 'Y ,4 FIGA. F|G,3

INVENTORS WILHELM BOYER KuNo EISENBURGER WALTER HUF GL JOSEF RITT ATTORNEY,

United States Patent O ce STEEL REINFORCEMENT FOR REINFORCED CONCRETE STRUCTURES Wilhelm Boyer, Graz, and Kuno Eisenburger, Wels, Austria, Walter Hufnagl, Munich, Germany, and Josef Ritter, Graz, Austria, assignors to EVG Entwicklungsud Verwertungsgesellschaft m.b.H., Graz, Styria,

ustria Application March 14, 1957, Serial No. 645,977

9 Claims. (Cl. 72-l10) The invention relates to steel reinforcements for reinforced concrete structures, and has the main object of providing steel reinforcements which combine high permissible steel4 stresses with high adhesive strength between the steel reinforcements and the concrete.4

This is a continuation-impart of our application Serial No. 344,082, tiled March 23, 1953, and now. abandoned.

The development of reinforcing rods for reinforced concrete is based on the desireto apply higher stresses to the steel in the reinforcement and thus to reduce the total amount-of steel in the concrete. Calculations of the stresses on the rods are valid only if the forces which correspond to the increased stresses in the steel canbe transmitted from the steel to .the concrete or vice versa by an improved adherence or another anchorage of the reinforcing rods in the concrete.` L

The earliest and simplest form of a reinforcing rod for reinforced concrete is Va round rod having "a relatively smooth surface. When sucha round Vor sectional rod is used as a reinforcement in concrete, a tensile load will cause a transverse contraction of the rod, which results in loosening the reinforcement inthe concrete; the remaining `bond between the concrete and "steelis'sucient and pairs of rods which have been twisted together have been suggested. Whereas in both cases abetter anchorage is achieved thanwith round rods-and-enablesthe permissible steel stress to be increased to` about 28,000`

p.s.i., the anchorage forces are transmited only by the roughness of the surface and by the resistance to rotation at the helically twisted contact surface of the reinforcements. Thus oblique components of force are set up in the concrete, which can Aproduce wedge eiects resultingY in an unfavorable stress pattern in the concrete.

According to another proposal, a reinforcing rod having grooves in its sides is formed with transverse ribs in the grooves which form anchoring surfaces extending at right angles to the, rod axis. Whereas this provides for a good anchorage in the concrete, which. permits also of steel stresses up to 28,000 p.s.i., a further increase of the permissible steel` stresses is noty possible without. ditliculty because, in such reinforcing rods, an unfavorable ratio between the steel cross-section to the anchoring rib surface area is obtained. For various reasons that ratio can hardly be improved. This is so because an increase in the rib surface area would involve the danger of a local reduction in quality at the position of the ribs if the reinforcing elements are made of high-grade steel, and because more highly protruding ribs would not only be subjected to high bending stresses but would also render the manipulation and storage of such reinforcing 2,916,910 Patented Dec. l5, 1959 correspondingly large amounts of alloying constituents,A

involving high cost. Y

It is an object of the invention to lprovide a novel reinforcing element which ensures a perfectly safe anchor-A age in the concrete and a reliable transmission of forces between the concrete andV steel even if the adherence with the concrete is loosened by a transverse contraction under tensile stress, and which involves no difficultiesl in handling and can be manufactured with ease, particularly in a high-grade condition, i.e., with a high yield point, by cold drawing at low cost.

It is another object of the invention to provide welded steel reinforcements which combine a high yield point with good welding properties.

It is a further object of the invention to provide strip shaped reinforcement steels which allow comparatively easy shaping by bending out of the plane of the strip while being comparatively stiff in the plane therof.

I-t is yet another object of the invention to provide reinforcement grid systems which combine the characteristics of the individual reinforcements referred to hereinabove.

With these and other objects in View we provide a reinforcement steel strip vfor reinforced concrete structures comprising in combination: longitudinal rods consisting of steel of Va comparatively high yield point arranged parallel to one another, and transverse connectors inter-` welded with the said longitudinal rods at intervals. For

example the longitudinal rods may consist of a steel having a yield point of more than 70,00 p.s.i. and of a higher content in carbon and/or manganese than the transverse connectors. The distance between parallel longitudinal rods is of the same order of magnitude as their own thickness, about equal for rods of 0.8 inch thickness or more, but not less than 0.8 inch forfthinner rods. A spacing of 'about 0.8 inch'and 1.2 inches is suitable in most instances. The interval between the transverse4 connectors is usually of a higher order of magnitude than the distance between the longitudinal rods, and does not exceed 8 inches. p

The transverse connectors may be pieces of rods or of plates, the latter may be put edgewise across the longitudinal direction of the longitudinal rods with their flat faces extending at right angles to the longitudinal rods.

WhenV such a reinforcing element is embedded in concrete the latter will penetrate into the spaces between the longitudinal rods and will form between the adjacent transverse webs thick bridges on which the transverse webs are supported. The flat faces of the transverse webs or connectors prevent a transmission 'of detrimental oblique forces to the concrete. Then the reinforcing element is subjected to tensile stress only the adherencebetween the contacting concrete and steel surfaces can be eliminated whereas the anchorage formed by the transverse webs and concrete bridges will remain intact. That anchorage is so strong that any holding head previously required at the ends of the reinforcing element and pro-V vided mostly by the bending of end hooks can be omitted. As will be explained hereinafter the ratio of the spacing between the transverse connectors `or webs to the spacing between the longitudinal rods must be less than forming in the concrete does not exceed the permissible" maximum value of ym() inch. The optimum value for the spacing between the webs depends on the concrete quality, on the permissible steel stress and on the diam- 3 eter of the longitudinal` rods. If a value smaller than the optimum is chosen a larger amount of -steel is required but the reinforced concrete is not adversely affected thereby. The thickness of the transverse webs should be in the range from 0.4-0.75 inch.

In order to achieve a perfect welded joint between the longitudinal rods and transverse webs or connectors the latter should be made of a softer steel than the former. The longitudinal rods may be worked by cold drawing or their high yield point may be due to an appropriately high-content of alloying constituents such as C and/or Mn. The transverse webs should not contain more than a.V relatively small content of said elements.

It has been found that reinforcement strips and-grids according to the invention allow the use of a steel of a yield-point of about 4000 kilograms per square centimeter and of correspondingly high permissible tensile stresses, and at the same time give such an improved adhesive strength between the steel reinforcements and the concrete that the inavoidable cracks in the concrete do not exceed 0.15 to 0.20 millimeter in width whereby the danger of d'ampness or vapors penetrating right to the steel reinforcements and causing corrosion isgreatly reduced and the durability of the reinforced steel construction is improved. Owing to better adhesion of the concrete to the steel reinforcements the distances between adjacent cracks are also reduced. The width of the cracks measured on concrete structures using the reinforcement steel according to the invention has proved to be in good agreement with calculated values. For example, With a steel of a permissible tensile stress of 3000 kilograms per square centimeter and a modulus of elasticity of 2,100,000 kilograms per square centimeter and a distance 100 millimeters between transverse connectors the calculated width of cracks is =0.14 millimeter for the longitudinal members of the reinforcement stripsl according to the invention.

For a better understanding of the invention, reference may be had to the accompanying drawings inl which:

Figure 1 of the drawing is a perspective view of a reinforcing element embodying the present invention;

Figures 2 and 3 respectively are a top plan view and a cross-sectional view of a reinforcing element;

Figure 4 is a schematic illustration in perspective of a test set up for determining the reinforcing strength and the required dimensions of the reinforcing elements according to the invention;

Figure 5 is a cross-section view of the reinforcing element illustrating dimensions on which tests are based;

Figure 6 is a cross-section view of a concrete beam reinforced with a reinforcing element of the kind shown in. Figures l and 5;

Figure 7 is a side elevational view of the concrete beam supported at its ends and under load; and

Figure S and Figures 8a, 8b, 8c, and 8d illustrate schematically the stresses on the concrete beam when a crack occurs in the beam.

The reinforcement steel according to Figures l to 3 has a strip-like shape and consists of two longitudinal rods 11a and 11b which are parallel and are connected with one another by pieces of rod-12 or other web forming material lying between them and interwelded with the longitudinal rods. The transverse webs 12 are spaced from one another lengthwise of the rods 11a, 11b a distance s. The longitudinal rods 11a, 11b in particular consist of a steel having a high yield point. The crosssection of the longitudinal rods 11a, 11b may be round, or profiled, as the case may be; the cross-section of the oblong cross-section profiles being positioned on edge relative to the longitudinal dimension of the reinforcement strip, if desired. Thus each transverse web may consist of a separate form piece produced e.g., by pressing, stamping or the like.

The interwelded transverse webs 12 form an absolutely rigid connection between the two longitudinal rods 11a, 11b which completely prevents any sliding of the reinforcement relative to the concrete in the zone of the webs; the anchor resistance can be overcome only by complete destruction of the tension Zone of the concrete. By controlling the spacing between the transverse web 12 the permissible width of cracks can be determined, assuming an appropriate quality of the steel. The spacing between the transverse webs should be less than 200 millimeters (8 inches).

As regards the spacing a between'the two longitudinal rods 11a and 11b, it should be equal to the diameter or thickness of the rods; with a rod diameter below 20 millimeters the clearance spacing should not be less than 20 millimeters (0.8 inch). When two longitudinal rods 11a, 11b interconnected by one or more interwelded transverse webs 12 are embedded in a concrete cube 13, as is shown in Figure 4, and the protruding longitudinal rods are subjected to tension in the sense of the arrow P, the first slight movement of the reinforcing element will occur with any concrete quality and independently of the diampressure of the concrete on the transverse web 12 exceeds the value where fc is the ultimate compressive strength of concrete. Said movement can be observed with a dial gauge 14.

Whenlthe average pressure of the concrete on the transverse web` exceeds the value the concrete. will be destroyed and the longitudinal rods as well as theA transverse web will be pulled out ofthe concrete.

Y To 'confirm the dimensions specified for the reinforcing elements according to the invention the cross-section of a reinforcing element according to the invention as shown in Figure 5 will be considered rst. The diameter of the longitudinal rods 11a, 11b is designated ip, the spacing between said rods is designated a. The dimension a is g at the-SametimeV a sufficiently accurate specification of the length of the transverse webs 12, which are interwelded to the longitudinal rods 11a, 11b. On the one hand the width of the transverse webs should be as large a's possible to ensure that the transverse web provides the largest possible anchoring surface area for reinforcing'ele- The entire steel cross-section A, of the two longitudinal rods is 2f As-b 2 (4) Therefore:

v F, 2 fyi-fri 5 In the` ensuing explanation, and as used elsewhere throughout this application, have the following meaning:

fc-compressive stress of concrete fc-ultimate compressive strength of concrete fet-tensile stress of concrete icy-ultimate tensile strength of concrete Figures 6 and 7, respectively, are a cross-sectional view and a side elevation showing a rectangular concrete beam having the width b and the height d and reinforced in the tension zone witha reinforcing element according to the invention. As is apparent from Figure 7 that beam is supported at its ends and loaded. An increase inthe load on the beam will cause a crack to'form in the tension zone, e.g., at the point x, becausethe tensile stress fet has exceeded the ultimate tensile strength fet of concrete. At that point x the reinforcement must take up the entire tensile stress because the locally cracked concrete canV no longer take up any stress. At the point y, spaced by the distance e from the point x no crack has beenformed at that time and high tensile stresses are effective in the concrete so that the reinforcement is subjected only to little stress. In that condition, therefore, the stress distributions fc and ict, respectively, shown below Figure 7 (in Figure 8c for the cracking position x, in Figure 8d for the undamaged position y) are obtained. The stresses actually taken up by the concrete are shown as hatched areas, while the tensile stress taken up by the reinforcement is shown as a clear area. If the tensile stresses fet in the concrete exceed the ultimate tensile strength fet the values fc, fc', fat ardifct' also at the point y a second crack will be formed and the stress distribution shown in Figure 8c in'Figure 7 below point x will be obtained also at point y as shown in Figure 8d. t

If several transverse webs 12 of the reinforcing element according to the invention are disposed between the rst cracking point x and the point y considered, a stepped stress distribution fs and fet according to Figures 8a and 8b will be obtained in the longitudinal rodsV and vin the concrete before a crack occurs at y. The stepwise stress reduction in the longitudinal rods corresponds to the stepwise increase of the tensile 'stresses in the concrete. The stress steps are disposed at the positions of the transverse webs, which absorb the stress step with their effective web surface areas Fs. Regarding the test results explained with reference to Figure 8 a transverse web having an effective area Fs can transmit the force F5212 without moving in the concrete.

If the spacing of the transverse webs is designated s, the number of transverse webs in the distance e equals The total force which can be transmitted bythe transverse Webs in the distance e without causing a movement of the reinforcement is, therefore,

On the other hand it is apparent from the diagram shown in Figures 8, 8c and 8d that the force required to set up the stress ict in the outermost fiber in tension 1n the tension zone of the beam is determined by f swf..

5ov`-`together with the standarized permissible concrete stress 75 can be computed'for thesetwo extreme cases.

or, with (5) (If the beam is reinforced with several reinforcing eleirnents, As is to be replaced by 2Aa and Fs by EFS.)

If it is further considered thatthe ratio, fc5/fc amounts for several concrete qualities, on an average, to

then the equalization of the Expressions 7 and 8 and the substitution of (9), (10) and solution-for s leads tothe equation s: 96epa #M1-.kl In order to obtain an interpretable relation between the parameters s, a and 4:' of the reinforcing element the values e, k and p must be determined.l

` The determination -of the distance e between cracks is t `based on the rule that cracks in the tension zone of a reinforced concrete structure must not exceed a width of 1/100 inch but are permissible up to that harmless width. Because the concrete remains stressless in the tension zone between two cracks the width of thecrack depends on the elongation of the reinforcing element on the distance e between the two cracks. If the steel stress is designated with fs and the modulus of elasticity is designated with Es an elongation of the reinforcing element by 1,myinch t will be obtained at For the determination-of the value k the extreme cases of `concrete quality in reinforced concrete structures may be considered, i.e., the lowest quality fmm` '1800 p.s.i.

f lnm =800 p.s.i., and the highest concrete quality at present achievable, fcmax =6500 p.s.i. and fc max =3000 p.s.i. The value k is calculated according to the known equals the relation between the moduli of elasticity of steel and concrete; for the poorest concrete considered holds n=15, for the best concrete n may be considered as equalling 6.

If the stated Values of fc and n arel substituted in (14) and the steel stress is assumed according to the invention at its lower limit f,=35,000 p.s.i. but at its upper limit in conjunction with the highest concrete quality as s:50,000 p.s.i., a mean k value of l v k-og3 effi-Ima?V (15) the outermost `ber in tensionA accord- .Areinforced concrete'bearnl is .correctly reinforced if the tensile force T in the reinforcement is as: great'as vthe highest-permissible compressivefforce C in the concrete.

The tensile force inthe reinforcementequals According to Figure 8 the compressive force in the concrete equals l 0.16.1010 s 0.6.10*o

effa agi-?? '(20) and, for arm-1&- inch:

giml'iom 21) Frtliermore, if the limits of are calculated for the poorest and best concrete and steel qualities assumed, (19) gives with a small margin of tolerance The lower limit for .al d

applies to relatively poor 'qualitiesv of the materials,fthe upper limt for the best qualities.

The spacing a of the longitudinal rods should be small to--enable the reinforcing element according to the invention to be handled like a rod-shaped reinforcing element according to Figures 1 to 4. According to United States Standards it must not be less than 1 inch to ensure a safe penetration even of coarse concrete into the space between the longitudinal rods.

For the dimension a=1 inch, Equation 22 shows that Thus the spacing of the transverse webs for 1/2Y inch should ybebetween 2.6:inches and 4.8 inches, depending on'. the concreteiquality. `AIn this connection, care is to be taken .above all toremain `.below the upper limit of 8 inches for s because tlie width of cracks in the concrete would be increased if.that limitis exceeded. A value below the lower limit is not critical because it will lead only to a somewhat increasedconsumption Aof web materialV but has" .no detrimental consequences f forl the-'reinforced concrete structure.

The thickness i of the transverse Webs should be such that the section modulus of said webs at the Welded joint should be capable of aborbing the fixed-end moment of the transverse Webs, which are considered beams gripped between the longitudinal rods, at the load obtained under the calclulated stress condition, taking into account the yield point of about 30,000 p.s.i. of the steel used for the web. Since the static consideration of this problem involves great difficulty the tolerances for the thickness of the web have been determined experimentally. It was found that the thickness i of the web should meet the relation Compared tovknown reinforcements for concrete the reinforcing element according to the invention enables a reduction of'steel in the range of LlO-65%, with much reduced transport and handling costs.

The derivation of Formula 19 has been based on the assumption that the tensile force in the reinforcement of the reinforced concrete beam s to equal the maximum permissible tensile strength in the concrete. That requirement is not always fulfilled. There are many cases where the concrete is not loaded to its full carrying capacity. Owing to the vunderload on the concrete a smaller steel cross-section is required in such cases. Also'in that case, howeverfthe reinforcement should be designed so that the permissible width of crack will not be exceeded.

In order to observe the permissible width of crack on which Formula 16 is based even under the changed load and reinforcement conditions the total of the effective web areas, represented as the product of the individual web areas and theV number of Webs on a predetermined distance must be the same as under the optimum load conditions. This can be achieved by providing a graded series of reinforcing elements according to the invention, having different longitudinal rod diameters, and designing the distance of transverse webs in that specimen of the series which has the greatest longitudinal rod diameter according' to Formula 16; that specimen is usedY for fully utilized concrete; the reinforcing elements of the series having longitudinal rods smaller in diameter have the same spacing of the transverse-webs as the stronger reinforcing element designed according to Formula 16 and are used in less highly loaded concrete.

Equations 7 and 8 state that a certain web area F, is required to obtain a width of crack not exceeding to M00 inch. A reduction of the steel cross-section involves also a reduction in the web area whereas Equa- .tion 5 shows the ratio Fs As to increase with a decrease in the longitudinal rod diameter. This enables the total web area required to be ensured by maintaining the web spacing s constant when the longitudinal rod diameter is decreased.

If the concrete cross-section is fully utilized a longitudinal rod diameter of =1/2 inch, c g., involves a ret quired transverse web area Fs to produce a crack not exceeding JA00 inch in width. If the concrete crosssection is utilized only by half so that only half the steel cross-section is required the use of two reinforcing elements having a longitudinal rod diameter of 1A inch will reduce the steel cross-section by half whereas the total transverse Web area'required according to Formula 5 for a web spacing calculated according. to Equat1on 19 for 95:1/2 is retained.

The 'new'reinforceme'nt steel maybe bent into the form of lines desiredirnore easily-than the profile steels of round or square cross-section because it has a flat shape, and it `hasinoreover the advantage ascompared with the known'cold-worked reinforcement steels which have been preferred because of their higher yield point values, that init a high yield point for the purpose of attaining higher permissible steel stresses appears combined with the advantage of betteradhesive strength. This adhesive strength makes it for example possible to dispense with the turning up of end hooks for the purpose of anchorage. Itis likewise possible to dispense with an anchorage in Vthe compression zonewhich is indispensible with all known profile steels, and tests have proved that the profile steel according to the invention need not extend beyond the end point according to calculation. A further particularly valuable advantage arises from the use of reinforcement steel according to the invention as a prestressed reinforcement in concrete constructions. This advantage consists in that owing to the repeated direct anchorage along the entire length of the reinforcement steel stresses are reliably transmitted between the concrete and the steel in contradistinction to the smooth drawn prestressed wires where such possibility of transmission does not exist, or as compared with the known reinforcements consisting of intertwisted wires and spacers arranged between the same at places, where such a possibility of transmission exists to a small extent only. Accordingly the holder heads required with pre-stressed smooth wires can also be dispensed with.

Besidse, at least the longitudinal rods may be subjected at one stage of the production process of the reinforcement steel according to the invention to an improving treatment increasing its strength properties,

. and having a width substantially equal to the diameter of the longitudinal rods, the ratio of the spacing between the transverse webs to the spacing between the longitudinal rods fulfilling the relation e.g., to a thermal treatment such as artificial ageing or patenting The same effect can alternatively be attained by the addition of appropriate strength-increasing alloy elements.

The new reinforcement steel is suitable for reinforced concrete structures of any kind.

We claim:

1. An integral reinforcing construction for reinforced concrete, which comprises two longitudinal rods of steel, each of a diameter ranging between 0.13 inch as a minimum and 1. 2 inches as a maximum, and havinga yield point exceeding 70,000 p.s.i. which extend parallel to each other, 0.8 to 1.2 inches apart, and transverse webs of softer steel interwelded at intervals to said longitudinal rods and extending at right angles to the longitudinal rods and having substantially at side faces extending at right angles to the longitudinal rods and a width substantially equal to the diameter of the longitudinal rods, the ratio of the spacing between the transverse webs to the spacing between the longitudinal rods being less than inches where qa is the diameter of the longitudinal rods in inches.

2. A reinforcing construction as set forth in claim 1, in which the thickness of the webs is between 0.4 to 0.75 o.

3. A reinforcing construction as set forth in claim 1, in which the longitudinal rods consist of cold-drawn steel.

4. A reinforcing construction as set forth in claim 1, in which the steel of the longitudinal rods has a relatively high content of alloying constituents selected of the class consisting of carbon and manganese and the steel of the transverse webs has a relatively low content of said alloying constituents.

5. An integral reinforcing construction for reinforced concrete, which comprises two longitudinal rods of steel, each of a diameter ranging between 0.13 inch as a minimum and 1.2 inches as a maximum, and having a yield point exceeding 70,000 p.s.i., which extend parallel to each other with a spacing of about 0.8 to 1.2 inches and wherein fs is the permissible steel stress in p.s.i., the diameter of the longitudinal rods, sis the spacing between the transverse webs and a is the spacing between said rods. l

6. An integral reinforcing construction for reinforced concrete, which comprises two longitudinal rods of steel, each of a diameter ranging between 0.13 inch as a minimum and 1.2 inches as a maximum, and having a yield point exceeding 70,000 p.s.i. which extend parallel to each other about 0.8 to 1.2.inches apart and transverse webs of softer steel interwelded at intervals to said longitudinal rods and extending at right angles to the longitudinal rods and having at side faces extending at' right angles to the longitudinal rods and a width substantially equal to the diameter of the longitudinal rods, the ratio of the spacing s of the transverse webs to the clearance a between the longitudinal rods fulfilling the relation 0.32.10lo 1.2.1010 aff ff each other with a spacing between them about equal to the diameter of said rods, and transverse webs of softer steel interwelded at intervals to said longitudinal rods and extending at right angles to the longitudinal rods and having substantially fiat side faces extending at right angles to the longitudinal rods and a width substantially equal to the diameter ofthe longitudinal rods, the spacing of the transverse webs being less than about 8 inches.

8. A graded series of reinforcing elements for reinforced concrete, in which each element comprises two longitudinal rods of steel, each of a diameter ranging between 0.13 inch as a minimum and 1.2 inches as a maximum, and having a yield point exceeding 70,000 p.s.i., which extend parallel to each other with a spacing between them of about 0.8 to 1.2 inches and transverse webs of softer steel interwelded at intervals to said longitudinal rods and extending at right angles to the longitudinal rods and having fiat side faces extending at right angles to the longitudinal rods and a width substantially equal to the diameter of the longitudinal rods, the element having the longitudinal rods largest in diameter fulfilling the relation wherein s is the spacing'between said webs, a is the spacing between said rods, and i is the diameter of said rod in inches whereas in the elements having longitudinal rods smaller in diameter, the transverse webs have the same spacing as in the element having the longitudinal rods largest in diameter.

9. A graded series of reinforcing elements according to claim 8, characterized in the element having the longit 2181` l 1 tudinal` rdslalrgest in diameter the diametel of the longitudinal rods is 1/ziinchand the value Y Stephenson Feb. 16, 1892 Cummings May 12,1914 Goeltz May 23', 1933 Causey Apr. 13, 1943 FOREIGN PATENTS France 1 June 13, 1925 Australia Apr. 27, 1938

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