BY : SAURABH SHAH CODE: 1710 FACULTY : PROF. R.J.SHAH Definition: y Shear failure is actually a diagonal tension failure that is brittle in nature and should be avoided. y To better understand diagonal tension consider the basic mechanics of a beam with no shear web reinforcing: Recall from Mohr's circle - an equivalent state of stress different than that shown above is obtained by rotating the differential element 45°. y This rotated element yields principal tension and compressive stresses which are occurring simultaneously with the previous maximum fv state of VQ / Ib. y y In general this ft will exceed the inherent tensile strength of masonry, before fv exceeds masonry shear strength. y When this happens, diagonal cracks, originating at the N.A begin to occur and grow with increases in beam loading. y Introduction y Shear Transfer Action and Mechanisms y Failure Modes in Shear (Without Web Reinforcement) y Factors affecting Shear y Failure of corbels in shear y Case study y Few pictures of shear failure y Bibliography . The early use of reinforced concrete was characterized by a large number of patented ³systems´. . the design methods of which were generally not brought to public attention. Study of the historical development of shear design was required so as to find out the problems in basic shear design from the early days. The equations were brought to the codes after many experiments and research work done by numerous people. . Early pioneers of reinforced concrete before the year 1900 developed two schools of thought pertaining to the mechanism of shear failures in reinforced concrete members. One school of thought considered horizontal shear as the basic cause of shear failure . considered diagonal tension the basic cause of shear failures. accepted by nearly all engineers today. . The second school of thought. bounded on one side by a diagonal crack. is resisted by the combination of 1. A shear force across the compression zone (Vc) 2. A dowel force transmitted across the crack by the longitudinal reinforcement (Vd) . It may be seen that the total external transverse force V. can be identified.Equilibrium in the Shear Span of a Beam: The internal and external forces that maintain equilibrium for this free body. Thus.3. The vertical components of inclined shearing stresses transmitted across the inclined crack by means of interlocking of the aggregate particles. V = Vc + Vd + Va . e. Dowel action (Vd).i.e.(20% to 40% of the total resistance) ± it is the shear resistance of the uncracked section above the crack. i. Aggregate interlock (Va). .e the resistance of the longitudinal reinforcing to a frictional force of shear i. i.e.(33% to 50%) 3.(15% to 25%) 2.As the shear force increases each of the above resistances reach their capacity in the following order in the absence of web reinforcement. the tangential friction developed due to the interlocking of the aggregate on the concrete surfaces on opposite sides of the crack i. Shear capacity of concrete in compression zone (Vc).e. 1. As load is applied to the beam. the noticeable change is the formation of practically vertical tension cracks in the region of maximum moment. . Here the cracks develop almost perpendicular to the axis of the beam. additional cracks form closer to the supports and some of the cracks become slightly inclined toward the load. With increasing load. These are called flexural cracks. . the inclined cracks develop as an extension of the flexural and are called web shear cracks. These are the regions of high shear and low moments and due to the diagonal tension. The cracks developed in the intermediate region and starts at the top of an existing flexural cracks and propagates into the beam are called flexure shear cracks. The stirrup passing across the cracks carries shear directly. by resisting the growth of the inclined crack. it enhances and preserves shear transfer by aggregate interlock. Vs. . It increases the magnitude of the interface shear. Va.The contribution of shear reinforcement to the shear strength of a reinforced concrete beam can be described as follows: It resists part of the shear. By limiting the opening of diagonal cracks within the elastic range. A stirrup can effectively support a longitudinal bar that is being crossed by a flexural shear crack close to it. thereby improving the dowel action. It restrains the bars from prying off the covering concrete. It suppresses flexural tensile stresses by the means of the diagonal compression force resulting from truss action. It increases the dowel force. The holding together of the concrete on the two sides of the cracks help keep the cracks from moving into the compression zone of the beam. . Vd in the longitudinal bars. The confining action of the stirrups in the compression concrete may increase its strength. of the beam mechanism. allowing additional shear forces to be resisted by truss mechanism. These stirrups wrapped around the core of concrete act like hoops and thus increases the beam s strength and ductility. It can be said that suitably detailed web reinforcement will preserve the integrity. . therefore the strength. The confining action of stirrups on the concrete increases the rotation capacity of plastic hinges that develop in a member at ultimate load and increases the length over which yielding takes place. The various failure modes in shear without web reinforcement area) Diagonal Tension failure y The shear failure always in the shear span when the a/d ratio is above 2. y Such a crack comes not proceed immediately to failure. The diagonal crack starts from the last flexural crack and turns gradually into a crack more and more inclined under the shear loading. . although in some of the longer shear spans this either seems almost to be the case or an entirely new and flatter diagonal crack suddenly causes failure. This type of failure has been designated as a shear compression failure because the shaded area in Fig. called a web shear crack. across the neutral axis before a flexural crack appears.0 and 2.b) Shear compression failure y A large shear in short shear spans may initiate approximately a 45 degree crack. Such a crack crowds the shear resistance into a smaller depth and thereby increasing the stresses. tends to be selfpropagating until stopped by the load or reaction. . This failure occurs at a range of a/d between 1. A compression failure finally occurs adjacent to the load. also carries most of the shear and the failure is caused by the combination.5. The ultimate load is sometimes more than twice at diagonal cracking. c) Splitting or true shear failure When the shear span is less than the effective depth d, the shear crack is carried as an inclined between load and reaction that almost eliminates ordinary diagonal tension concepts. Shear strength is much higher in such cases. The final failure, as shown in Fig, becomes a splitting failure or it may fail in compression at the reaction. The analysis of such an end section is closely related to the analysis of a deep beam. This failure occurs when a/d is less than unity. Influence of Shear span to depth ratio on Shear Shear resistance of beams decrease with the increase of shear span to depth ratio It is well established in both British and American design practice ( Evans and Kong, 1967) (ACI-ASCE Committee 426, 1973) that the failure mode of rectangular reinforced concrete beams without shear reinforcement is strongly dependent on the shear span/depth ratio. (a) for a/d > 6, failure usually occurs in bending; (b) for 6 > a/d >2.5. the development of a flexural crack into an inclined flexure-shear crack results in diagonal tension failure, (c) for 2.5 > a/d > 1, a diagonal crack forms independently but the beam remains stable until shear-compression failure occurs; (d) for a/d < 1. the behaviour approaches that of deep beams In addition to the shear-span to depth ratio. the contribution of the concrete to the shear strength, Vc, is dependent on a number of other factors including the concrete strength (fi) the main tension reinforcement ratio (p) and the beam size (b.d). These factors are represented in both the ACI and BSI design formulae for Vc. The shear strength of reinforced concrete beams may be substantially increased by the provision of suitable shear reinforcement, usually in the form of stirrups or links, which serve to intercept the diagonal shear crack. Thus, the external shear force, V, is resisted partly by the concrete, Vc, and partly by the shear reinforcement, V. such that V=Vc +Vs does not increase in the same proportion as the beam size. that is .d. It is believed that this is because the aggregate interlock contribution to shear strength Vc. . Design shear stress values in BS8110 allow for the influence of the effective depth .Influence of Beam size on Shear y It has been shown by Kani (1967) and Taylor(1972) that larger beams are proportionally weaker in shear than smaller beams. The proportion of the strength that the bonded shear plates contributes is also likely to change. the ultimate shear stress reduces with beam depth . depending on the number of stirrups intercepted by the primary shearcrack (Tompasand Frosch 2002). In particular . It is known that the opening of the critical shear crack is not same along its length. where the stirrups could not reach the yield stress . where the stirrups yield . the contribution of steel shear reinforcement can be estimated on the basis of the cracking pattern . and a low value at the end of the crack near the compressed zone. the opening of the shear crack has the maximum value at the initiation of the critical crack.Web reinforcement contribution to Shear Strength y Stirrups provide a contribution to shear strength if crossed by a diagonal crack: therefore . Compression from support at bottom of beam tends to close crack at support . b and c and are mentioned above.2.6. where d is the effective depth of the beam.a. the shear force computed at a distance d from the face of the support is to be used for the design of sections located at a distance less than d from the face of the support.6. y (ii) When the reaction in the direction of the applied shear introduces compression into the end region of the member .2 and 22.Clauses 22. y . y (i) When the reaction in the direction of the applied shear introduces tension into the end region of the member. the shear force is to be computed at the face of the support of the member at that section.1 stipulate the critical section for shear and are as follows: y For beams generally subjected to uniformly distributed loads or where the principal load is located further than 2d from the face of the support. the critical sections depend on the conditions of supports as shown in Figs. . . cls. respectively) are given below: . the shear strength of reinforced concrete with the reinforcement is restricted to some maximum value cmax depending on the grade of concrete.1 and 40. These minimum and maximum shear strengths of reinforced concrete (IS 456. This shear strength ( c) depends on the grade of concrete and the percentage of tension steel in beams.y Recent laboratory experiments confirmed that reinforced concrete in beams has shear strength even without any shear reinforcement.2.3. On the other hand. 40.2. 01 .75 1.81 0.29 0.84 0.96 0.78 0.74 0.82 0.75 2.56 0.36 0.50 0.80 0.74 0.59 0.29 0.82 0.86 0.68 0.00 0.92 0.91 0.75 0.94 0.72 0.88 0.67 0.15 0.79 0.92 0.59 0.90 0.36 0.00 2.50 2.50 0.70 0.76 0.82 0.50 0.84 0.29 0.82 0.Table 6.28 0.00 1.71 0.64 0.98 1.37 0.90 0.50 1.88 0.25 2.75 3.1 Design shear strength of concrete.79 0. c in N/mm2 (100 As /b d) Grade of concrete M 20 M 25 M 30 M 35 M40 0.51 0.78 0.25 1.93 0.66 0.38 0.30 0.60 0.85 0.88 0.67 0.62 0.49 0.57 0.82 0.48 0.25 0.37 0.73 0.99 0.96 0.95 0. 0 .1 3.8 3. N/mm2 2.5 3. cmax in N/mm2 Grade of concrete M 20 M 25 M 30 M 35 M 40 and above cmax.7 4.y Maximum shear stress. grade. Concrete Strength Tensile reinforcement ratio Shear arm ratio Tensile reinforcement type. arrangement Shape of cross section Section dimensions Web reinforcement types Spacing and arrangement of web reinforcement Loading configuration Beam types . as follows : y The shear span/depth ratio is less than 1. it makes the corbel behave in two-dimensional manner. y Shear deformation is significant is the corbel. .BEHAVIOR OF CORBEL The followings are the major items show the behaviour of the reinforced concrete corbel.0. failure of concrete by compression or shearing and bearing failure. y The cracks are usually vertical or inclined pure shear cracks. y Bearing failure due to large concentrated load. y The mode of failure of corbel are : yielding of the tension tie. failure of the end anchorage of the tension tie. y There is large horizontal force transmitted from the supported beam result from long-term shrinkage and creep deformation. . Flexural tension in corbels.It is very common that corbels are failed in shear. Failure mechanism in corbels as under. 1. . Sliding shear. 3. Diagonal splitting .2. 4. 5.anchorage splitting. crushing due to bearing . Diagonal shear reinforcement in corbel. . .Shear reinforcement should be provided in the form of horizontal links distributed in the upper two-third of the effective depth of root of the corbel. this reinforcement should be not less than one-half of the area of the main tension reinforcement and should be adequately anchored. . Other warehouses. 1955 y Two warehouse roofs at Air Force Bases in Ohio and Georgia cracked and collapsed under combined load. These failures led to more stringent shear reinforcing steel requirements in subsequent editions of the ACI Building Code. and the members had no shear capacity once they cracked (McKaig 1962. with no stirrups. the concrete alone. 122 m (400 ft) lengths of reinforced concrete roof girders functioned as single units because of defective expansion joints. 1997). In the warehouse structures. was expected to carry the shear forces. and thermal effects in 1955 and 1956. built to the same plans. Feld and Carper. shrinkage. . survived because separation between adjacent two-hundred-foot bays was maintained by functioning joints.Wilkins Air Force Depot. with a modification to reinforcement made in March 1954.y At the Wilkins Air Force Depot in Shelby . 25).000 ft2) of the roof collapsed suddenly on August 17. Ohio . The haunched rigid frames each had six 20 m (67 ft) spans.000 ft) long. It was a six-span rigid frame building. The original design was developed in April 1952. and were spaced approximately 10 m (33 ft) on center. p. . about 370 m2 (4. there were no loads other than the self-weight of the roof (Feld 1964. y The Air Materiel Command (AMC) built warehouses to a common design at many Air Force bases and depots. 1955. The Ohio warehouse had been built to the original 1952 design. At the time of collapse. 122 m (400 ft) wide and 610 m (2. but they may not have been effective (Feld 1964.The concrete for each frame was placed continuously in a single working day. so the girder had been supported by temporary shoring. . pp. A typical AMC warehouse frame is shown below. y Severe cracking had been observed two weeks before the collapse. The cracks occurred about 0. Vertical steel plate construction joints were set at the center of each span before concrete placement. p.45 m (1 ½ ft) past the end of the cutoff of the top negative reinforcement over the columns (Feld 1964. 25). 26 27). . This warehouse had been built to the revised design. at a volume of about 0. Georgia .000 ft2) of the roof. for the length of the frames. The revision added top bars and nominal stirrups.06 %.y A second warehouse roof collapse took place at Robins Air Force Base near Macon . This collapse included two adjacent girders and about 560 m2 (6. Feld (1964. cracks in the concrete girders that reached 13 mm (½ in) in width had been observed. Before the collapse occurred. 25) suggests It seems that the extent of shrinkage and resulting axial tensions may be somewhat related to the speed of concreting or to the extent of each separate placement. . 1956. early on the morning of September 5. p. In other words. the failures had still occurred. However. Circumstantial evidence suggested that high friction forces were developed in the expansion joint consisting of one steel plate sliding on another. p 27) believed that failure took place by a combination of diagonal tension (shear) due to dead load and axial tension due to shrinkage and temperature change. Feld (1964. and workmanship were up to the codes and standards of the day. some plates showed no indication of relative displacement since their installation.y In both cases. the design. materials. . the expansion joints locked and did not function to relieve stress. . the collapse occurred slowly enough for most of the other workers to run to safety. planning. Four workers died after a failure on the roof instigateda progressive collapse all the way to the basement. Boston. . Low concrete strength due to inadequate protection against cold weather contributed to low punching shear strength of the flat slab. Massachusetts. where the men were found. Fortunately. quality control. and supervision were for all practical purposes absent from the project. Inspection. two thirds of a 16-story apartment building collapsed while under construction at 2000 Commonwealth Avenue.y On January 25. 1971. The surviving workers' descriptions of the failure provide a textbook definition of punching shear. y The high-rise apartment building was made of cast-in-place reinforced concrete flat slab construction with a central elevator shaft. Excavation had been partially started a few years earlier. Construction began on the site late in the fall of 1969. Brickwork was completed up to the sixteenth floor and the building was mostly enclosed from the second to fifteenth floors. Only one representative from the General Contractor was on site during construction. 1971). heating and ventilating systems were being installed throughout various parts of the building. A swimming pool. ancillary spaces and one apartment were located on the first floor and one hundred thirty two apartments were on the second through sixteenth floors. The structure also had two levels of underground parking. At the time of collapse. y . Most of the work was subcontracted to area specialists. Plumbing. It is estimated that one hundred men were working in or around the building at the time of failure (Granger et al. was designed to be sixteen stories with a mechanical room above a five-foot crawl space on the roof. construction was nearing completion. . . at about three o clock. Shortly after the coffee break. Only two concrete finishers remained on the pouring level near line 4-1/2. the two men felt a drop in the mechanical room floor of about one inch at first and then another two or three inches a few seconds later. Later in the afternoon. Placement started at the west edge and proceeded east. wall beams. Column E5 is located directly below where the concrete was being placed for the mechanical room floor slab on the east side of the building as shown in the following figure (Granger et al. wall. most of the workers went down to the south side roof for a coffee break. and brackets. The slab had dropped five or six inches around the column and there was a crack in the bottom of the slab extending from column E5 toward column D8. That is when the punching shear was noticed around column E5.Punching Shear Failure in the Main Roof at Column E5 y At about ten in the morning. concrete was being placed in the mechanical room floor slab. 1971). This type of failure is caused by unbalanced moments transferred between the column and flat-plate (Megally and Ghali 2000) and was a result of non-conformities to the design documents. Engineering News Record reported that there were three possible causes of structural failure under investigation: formwork for the penthouse floor slab collapsed onto the roof.y A week after the collapse. the mayor's commission concluded that there were many design and construction flaws that attributed to the collapse. The committee determined that punching shear failure at column E5 triggered the initial collapse. or concrete placed during previous cold days had failed (ENR. . However. a heavy piece of equipment fell from a crane and started the progressive collapse. after an extensive investigation. February 4. 1971). . . . It is a cantilever pier cap. the top reinforcement of the cantilever beam had an L bend of 500 mm only. The shop owners put up resistance against casting of the other leg of the portal and it was subsequently decided by DMRC that this would be changed to a cantilever pier. y .The pier cap of affected pier (P-67) has sheared from the connection point of the pier and pier cap. It was informed by the contractor and DMRC representatives that the support system for via-duct was initially designed as portal pier till the casting of the pier was over. As learned from the sources. y It was noticed that the prop support of the cantilever has failed from its connection to the pier. y The top reinforcement of the cantilever beam does not have any development length into pier concrete. similar to P-68 which is still standing at site. . y The boom of the crane. has failed in bending and shows clear sign of overloading. Also. y . y The launching girder has fallen below with the failure of pier cap. 2009. supported by the ground at one end and pier cap (P-68) on the other.There was very nominal (or no trace) of shear reinforcement at the juncture. the span between P-67 and P-68 has fallen inclined. used for lifting the launching girder on 13 July. The cantilevered deck shows signs of failure as crack. between pier 66 67 Probable Cause insufficient lap of deck top tension rebars with pier s projected rebars. The work is stopped for almost 2 months for deciding rectification measure .e.Top deck develops crack while erecting segments of previous stretch i. . . . More emphasis should be given on detailing of reinforcement to cater for connections and behaviour of the structural components. e. structure should be as far as practicable abandoned and new structure should be built up c. Once failure observed. Structural designs should be proof checked by experienced structural engineer. deep beams. Any make-shift arrangement to save a failed structure should be avoided. Reinforcement detailing in corbels.y What it taught us? a. cantilever structures should be checked as per the provisions of more than one type of Standards (both IS & BS should be followed). b. d. Adequately experienced Engineer / Forman should be deployed for erection works. f. . . . this leads to shear crack at the junction of bearing.Shear crack Bearing The bearing of bridge attains some fixity and do not transfer moment by rotation. . . . . Failure of column shear crack through Direction of shear forces . Shear failure of column . ) on SHEAR STRENGTH OF R. of Civil Engg. Palakkad. Carper Paper by dileep kumar (P.C BEAMS WITHOUT WEB REINFORCEMENT www.nau. Govt.iitm.C.forensic case studies for civil engineers By Norbert J. Dept. College.Kenneth L. Engg. Delatte Construction failure By Jacob Feld.nptel.google.edu Pictures from (www.in Shear study by : (Dr.y y y y y y y Beyond failure. Devdas Menon) http://matdl.ucc.G M.Tech Lecturer.org/failurecases (Case studies) http://jan. Amlan K Sengupta and Prof.com\images ) .