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2008 ASCEStructures 2008: Crossing BordersRecent Changes to Concrete Shear Strength Provisions of AASHTO-LRFD Bridge Design Specifications Authors: Neil Hawkins, University of Illinois at Urbana-Champaign, 2634 86th Ave NE, Clyde Hill, WA, 98004, nmhawkinDaniel Kuchma, University of Illinois at Urbana-Champaign, NCEL, 205 N. Mathews, Urbana, IL, 61801, kuchmaBACKGROUND High strength concrete (HSC) offers considerable economic advantages for the design, construction, and maintenance of bridge structures. The use of HSC, rather than normal strength concrete, enables a section of a given size to support larger loads or span longer distances. The improved durability associated with HSC increases the lifespan of structures and their ability to meet larger future loading demands. While concrete with compressive strengths up to 24 ksi are now commercially available from suppliers until recently the AASHTO LRFD Bridge Design Specifications (LRFD Specifications) limited the cylinder compressive strength that could be used in design expressions to 10 ksi. The principal reason for this limitation was the lack of experimental data from tests on specimens cast with concrete strengths higher than 10 ksi. The authors were the principal investigators for two US National Cooperative Highway Research Program (NCHRP) projects concerned with the extension of the shear provisions of the LRFD Specifications to concrete strengths greater than 10 ksi. The first study was Project 12-56 “Application of LRFD Bridge Design Specifications to High-Strength Structural Concrete: Shear Provisions” and the second study was Project 12-61 “Simplified Shear Design of Structural Concrete Members.” As a result of these studies the 4th Edition of the LRFD Specifications now permits the use of the shear provisions therein for concrete strengths up to18 ksi and also contains provisions for shear design using both a Sectional Design Model and Simplified Provisions. The Sectional Design Model (SDM) was developed in Canada by Professors Michael Collins and Denis Mitchell and was introduced to the U.S. bridge community, as the specified procedure for determining required amounts of shear reinforcement, with the 1st Edition of the LRFD Specifications in 1994. The model was derived from the Modified Compression Field Theory (MCFT), which is a comprehensive behavioral model for predicting the shear response of diagonally cracked concrete. Compared with the traditional shear design model of the AASHTO Standard Specifications for Highway Bridges, 17th Edition, (Standard Specifications), AASHTO, 2002 and the ACI 318-05 Building Code Requirements for Structural Concrete, (ACI 318), the SDM provided newer strain-based relationships for evaluating the contributions of concrete and vertical transverse reinforcement to the shear capacity, as well as newer limits for minimum shear reinforcement and maximum shear design strength. Although the SDM provides a unified treatment for the design of reinforced and prestressed concrete structures and offers the potential for some significant performance advantages, the procedure is unfamiliar to Downloaded 19 Mar 2009 to 50. Redistribution subject to ASCE license or copyright; see 2008 ASCEStructures 2008: Crossing BordersUS bridge designers, more complicated than the shear design procedures of the Standard Specifications, and often requires an iterative solution. The Simplified Provisions (SP) are a modified version of the Standard Specifications and ACI 318 provisions for shear design of prestressed concrete members. They supplement the LRFD methods by providing a direct solution for the transverse shear reinforcement requirements for concrete structures, both prestressed and non-prestressed, of common proportions. The shear strength provisions for prestressed concrete of the Standard Specifications and ACI 318 were developed in the U.S. by Professors Chester Siess and Mete Sozen and the concepts of those provisions were introduced to the U.S. bridge community with the Bureau of Public Roads first 1959 publication on design of prestressed concrete bridges and to the U.S. building community with ACI 318-63. The SP provide a mechanistic model that can be applied to the shear design of bridge members containing at least the minimum required amount of shear reinforcement. The SP allow designers to develop an intuitive feel for shear reinforcement requirements, a feel that is difficult to develop with the SDM, and readily permit the verification of design solutions developed using automated design software. The work for Project 12-56 was initiated before that for Project 12-61. However, because there were considerable commonalities in some of the issues addressed in the projects, the work for one project provided crucial information for the other project and vice versa. The first step for both projects was the assembly of a large experimental database of existing results from prior shear tests reported in the literature. That database was then used to plan the experimental studies that were the crucial part of Project 12-56 and to examine alternatives for the development of simplified shear design provisions that was the objective of Project 12-61. In turn, the proposed simplified shear design provisions developed in Project 12-61 were crucial in selecting the properties of the beams tested in Project 12-56 while the results of those tests were crucial to the validation of the simplified provisions developed in Project 12-61. As a consequence it was possible to complete the work on Project 12-61 and published it as NCHRP Report 549 Hawkins et al., 2005 before all implications of the experimental results reported in Project 12-56 were understood and published in NCHRP Report 579 Hawkins and Kuchma, 2007. With the completion of those two reports, their recommendations were submitted to the AASHTO Bridge Committee T-10 as a series of proposals for change to the LRFD Specifications. The final versions of those changes, as approved by T-10, are now included in the 4th Edition of the LRFD Specifications AASHTO, 2008. PROJECT 12-56 The first tasks in this project were to review relevant HSC experience and identify potential barriers to the use of the Sectional Design Model (SDM) for evaluation of the shear strength of HSC beams. Five potential barriers were identified: the contribution of concrete; the contribution of shear reinforcement; minimum shear reinforcement requirements; maximum shear strength limits; and the validity of assumptions made in the SDM. In the SDM the concrete contribution depends on aggregate interlock along the inclined cracks that develop in the web of a beam and the spacing of those cracks. Such cracks are Downloaded 19 Mar 2009 to 50. Redistribution subject to ASCE license or copyright; see 2008 ASCEvcc Structures 2008: Crossing Borderslikely to be smoother and more widely spaced with HSC than with regular strength concrete and that could lead to a reduced concrete contribution with increasing concrete strength. In most codes of practice, a parallel chord truss model is used to evaluate the contribution of shear reinforcement to shear strength and for both the Standard Specifications and ACI 318 truss chords at 45 degrees are used in that model. In the SDM the truss chords can be assigned angles as low as 18.1 degrees. The shear reinforcement contribution is then three times that for the Standard Specifications and ACI 318. For HSC, the accuracy of the truss chord angle assumption becomes more critical as the much higher shear design forces permitted by the SDM, as compared to the Standard Specifications and ACI 318, result in requirements for increased shear reinforcement contributions. The LRFD Specifications increase the minimum amount of shear reinforcement by about one third over that required by the Standard Specifications and ACI 318. There was a real concern as to whether the same minimum amount was appropriate for HSC since the energy released at diagonal cracking was considerably larger and the cracks smoother than for regular strength concrete. In the Standard Specifications and in ACI 318 the maximum nominal shear force is limited to 8fcbwd where fc is the compressive strength in psi, bw is the web width and d is the effective depth. By contrast the maximum nominal shear design force in the LRFD Specifications is limited to 0.25fcbwdv where d is the effective shear depth and equal to about 0.9d. The ratio of those two limits increases rapidly with increasing concrete strength from 1.4 for fc equal to 4 ksi to 3.1 for f equal to 20 ksi. Investigation of this limit was essential for HSC use. While the SDM is derived from a comprehensive behavioral model (MCFT) there is considerably less experience with the use of that model than the model of the Standard Specifications. Therefore, the extension of that model to HSC could reveal limitations to the assumptions used in that behavioral model. Based on the review of the existing literature and the foregoing five barriers a program of tests were conducted on large uniformly loaded precast prestressed concrete bulb-tee girders. The precast girders were 63 in. deep and 52 ft long and had a 10 in. deep by 42 in. wide deck slab attached to them in the laboratory. The principal variables were the design shear stress level, (v/fc ranging from 0.05 to 0.25), the concrete strength, (f from 10 to 18ksi), end anchorage conditions for the strand, (straight and bonded, some debonded, some draped, additional longitudinal steel in the end at various locations, and steel spirals around the transfer length of the strands). The girders were designed to evaluate all possible shear failure modes recognized by the SDM and all satisfied the LRFD Specifications 2nd Edition. The girders were extensively instrumented with strain gages, displacement transducers, and distributed deformation measuring systems. The instrumentation allowed determination of loss of prestress with time, and the distributed deformation systems allowed accurate determination of the strain and deformation distributions over the depth of the girder for a significant length adjacent to the support. A total of 10 girders were tested with the shear design conditions for the two ends of each girder differing. After failure occurred at one end, that end was repaired and strengthened and testing continued until the second end failed. Thus a total of 20 shear test results were obtained. The girder tests were accompanied by a comprehensive program of materials testing including compression tests, split cylinder tests, fracture tests, and shear friction tests. Comprehensive details of the test measurements and test results are provided in Hawkins and Kuchma, 2007. Downloaded 19 Mar 2009 to 50. Redistribution subject to ASCE license or copyright; see 2008 ASCEStructures 2008: Crossing BordersPrimary observations from the tests were as follows: Inclined cracks could be divided into web-shear cracks and flexure shear cracks as prescribed in the Standard Specifications. Web-shear cracks occurred suddenly along straight lines, and with a significantly loud “pop”. Flexure-shear cracks were slower to develop, occurred towards midspan, and had a quadratic curve shape. Flexural cracking loads were reasonably well predicted by Mcr when the tensile cracking stress was taken as 7.5fc in psi units. The first web-shear crack usually occurred within a longitudinal distance equal to the overall height of the beam from the center of the support. This shear cracking was within a region of discontinuity, a D region, as defined in strut and tie modeling procedures. The average angle of this first crack was 36.2 degrees and the closer the crack was to the support, the steeper was its angle. The web-shear cracks in the first shear design region, (Article of the LRFD Specifications) a B region, as defined by strut and tie modeling procedures were reasonably constant and flatter than the cracks in the D region. Crack angles ranged from 23 to 32 degrees and averaged 27.8 degrees. The angle of diagonal cracking could be accurately predicted using Mohrs circle of stress for the conditions existing at the centroidal axis of the beam, and typically the angle was a little steeper than the angle of diagonal compression calculated using the SDM. The spacing of the cracks in the web was on average about half of the values predicted using the CEB-FIP expression for crack spacing CEB, 1978. The Standard Specifications provided a reasonably accurate estimate of the web-shear cracking load, Vcw, even when the full prestress was used in calculations. At beam ends the full prestress was unlikely to be acting at the centroidal because the transfer length for the strands was about 30 inches. The Standard Specifications marginally overestimated the flexure-shear cracking loads, Vci. The first web-shear cracking occurred at between 33 and 87 percent of the LRFD shear design stress. Values can be a much lower percentage of the shear design stress than permitted by the Standard Specifications. Upon initiation, the measured width of the first web-shear crack ranged from 0.3 to 0.5 mm. Initial cracks widths were wider for members with less shear reinforcement. Once flexure-shear cracks occurred they opened faster than web-shear cracks. With one limitation, the SDM provided relatively accurate estimates of shear capacity regardless of the concrete strength and the draping or debonding of strands in the member. The one strength limitation was that the SDM became slightly unconservative where the design shear stress exceeded 0.2fc. That unconservatism was due to the funneling into the support of the diagonal compressive stresses above the support. The funneling lead to local diagonal crushing and very high shear stresses at the interface between the bottom flange and the web. In that situation failure occurred before yielding developed in a band of web reinforcement forming a critical shear plane within the web. The local crushing often initiated a sudden and explosive failure of the web concrete in the end of the beam. Design of the end region of the beam, including consideration of the consequences of using draped strands, debonded strands, and added longitudinal deformed bar reinforcement, had a significant effect on the overall shear strength of the beams. The use of draped strands, Downloaded 19 Mar 2009 to 50. Redistribution subject to ASCE license or copyright; see 2008 ASCEStructures 2008: Crossing Bordersparticularly strands that are draped over the depth of the web in the end region, significantly improved the behavior and shear capacity of the end region. Selected crack-based free body diagrams and measured stirrup strains were used to assess what portion of the shear load was carried by the transverse reinforcement and then, by subtraction, what remaining component was carried by the concrete. The results indicated that the amount of stirrup reinforcement had a significant effect on the concrete contribution and that therefore suggests that the level of interface shear resistance on cracks is influenced by the amount of shear reinforcement. This result is consistent with the concepts of the MCFT but in the SDM that effect is neglected in order to simplify the design procedure. The free body results also suggested that the concrete contribution to shear strength by the bottom bulb could be significant. PROJECT 12-61 This project to develop new simplified shear provisions began with a review and evaluation of some of the most prevalent methods for calculating shear strength. The methods included; ACI 318-02; Standard Specifications; Canadian Standard CSA A23.3-04; LRFD Specification; Eurocode EC2; German Code DIN, 2001; Japanese Code JCSE, 1986. The evaluation of the accuracy of predictions was made using the results of a large experimental database. In addition a survey was conducted of practitioners on the use of the shear provisions of the LRFD Specifications and the Standard Specifications. The review, evaluations, and survey resulted in the following observations used subsequently for developing the simplified provisions: Since the 1960s U.S. bridge and building design codes have used the diagonal cracking strength, Vc, as a measure of the concrete contribution to shear resistance at ultimate and the 45-degree parallel chord truss model for calculating the contribution of shear reinforcement to shear capacity. For the experimental database these empirical design approaches provided reasonably accurate and conservative estimates of the shear capacities of beams with shear reinforcement. However, the same methods were unconservative at predicting the shear capacity of non-prestressed (reinforced) concrete beams without shear reinforcement. Basing the concrete contribution at ultimate on a conservative value of the diagonal cracking strength enables the designer to check whether or not a member will be cracked under service loads and simplifies the condition assessment of structures in the field. Further, characterizing the diagonal cracking as web-shear or flexure shear is useful for describing shear behavior and for condition assessments. The SDM of the LRFD Specifications and the CSA procedure provide similar estimates of shear capacity and of all the methods produced the most accurate estimates of the measured database capacities. Overall these methods had only about a 10% probability of being unconservative. The LRFD Specifications require a larger minimum amount of shear reinforcement than the other codes. This higher requirement was found to be desirable for assuring reliable shear behavior based on the experimental results of the database. Downloaded 19 Mar 2009 to 50. Redistribution subject to ASCE license or copyright; see 2008 ASCEStructures 2008: Crossing Borders The CSA method, LRFD Specifications, Eurocode 2, JSCE, and DIN methods, all enable the designer to use an angle of diagonal compression in a parallel chord truss model that is flatter than 45 degrees for evaluations of the contribution of shear reinforcement to shear capacity. The LRFD Specifications, DIN and Eurocode 2 allow the design of members supporting much larger shear stresses than those permitted by other codes. The shear stress limit is intended to guard against a diagonal compression failure. In the LRFD Specifications, the shear design stress limit is 0.25fc plus the vertical component of the prestressing. In ACI 318 and the Standard Specifications, the same limit is approximately 12fc. The LRFD limit is adequate to prevent web crushing where there is a uniform field of diagonal compression. However, as demonstrated in Project 12-56, this limit can be unconservative near beam ends where the diagonal compression may have to funnel into a support. The changes incorporated in 2004 into the CSA Code greatly simplified the MCFT procedures. Although the CSA Code uses an approach that is functionally identical to the SDM, the tables in the SDM for evaluating the coefficient , defining the concrete contribution to shear strength, and the value of , defining the angle of diagonal compression, are replaced by simple algebraic expressions. Further, the procedures for calculating and are made non-iterative by removing the dependency of the calculated longitudinal strain at mid-depth on . The survey of practice found that few U.S. organizations had experience in using the SDM. Further all agreed that the SDM had to be automated with software if it was to be used in production design. That automation lead to a loss of comfort with respect to the checking of designs because the SDM cannot be readily executed by hand. While the Standard Specification procedure for prestressed concrete design also had to be automated for effective production work, designs could be easily checked by hand. The primary objection of designers to the SDM was a loss of their physical “feel” for shear design due to the complexity of the provisions and the need for automation. The primary simplification that designers wanted was elimination of the iterative procedure for determining . Researchers have not tested the broad range of structures that are built with design provisions and thus available test data alone cannot provide a reliable assessment of the suitability of provisions. Researchers continue to members that are convenient to test and not members representative of what is being built. Most members that have been tested are small (less than 15 in. deep), have rectangular cross-sections, are simply-supported, are stocky, do not contain shear reinforcement, are loaded by point loads at small shear spans, and are supported on bearings positioned beneath the members. By contrast, a large fraction of the bridge members in the field are large, continuous or made continuous, have top flanges, are essentially uniformly loaded, and are often built integrally at their ends into diaphragms and piers. In addition, members in the field must be designed for shear over their full length. They have shear critical sections at regions away from the support and at points of inflexion, as well as at regions near supports. Because most code provisions are ultimately validated by test data, and because the existing test data does not represent what is built using design codes, there is great uncertainty about the safety, economy and validity of existing shear provisions. For example, existing codes predict wide variations in the shear requirements for the region of contraflexure of a continuous beam. Also, the codes give wide variations in shear reinforcement with the amount required by one code being to two to three times that required by another code for the same section and the same factored sectional forces. Downloaded 19 Mar 2009 to 50. Redistribution subject to ASCE license or copyright; see 2008 ASCEStructures 2008: Crossing BordersBased on the foregoing findings two changes were proposed to the LRFD Specifications. The first change has now been incorporated into the 4th Edition of the LRFD Specifications and consists of simplified provisions for shear design of beams, prestressed, partially prestressed and non-prestressed, that are modified versions of the Standard Specifications provisions for prestressed members. These simplified provisions differ from the Standard Specification provisions in four ways. (1) The expression for calculating the web-shear cracking strength is made more conservative than in the Standard Specification, and is applicable calculating the concrete contribution at ultimate shear strength for all concrete member types. (2) A variable angle truss model is used to evaluate the contribution of the shear reinforcement in web-shear cracking regions. In flexure-shear cracking regions, and in all regions where Mu Mcr the truss angle is taken as 45 degrees. (3) The maximum permissible shear design stress is made the same as that for the SDM. (4) The minimum shear reinforcement requirements are made the same as those of the SDM. Comparisons with the shear database showed that the simplified provisions have a six percent probability of being unconservative. The AASHTO T-10 committee is currently considering the second proposed change. That change modifies the SDM by inserting the CSA relationships for and and the CSA method for calculating the longitudinal strain at mid-depth. Those changes greatly improve the simplicity of design using the SDM. Both the simplified provisions and the SDM are retained because the former is applicable only to beams subject to bending and shear and containing at least the minimum required amount of shear reinforcement. However, the SDM is applicable for the design of sections for shear for any combination of axial load, moment and level of prestressing, for members both with and without shear reinforcement. CONCLUDING REMARKS Based on the results of the foregoing studies the AASHTO LRFD Specifications for the 4thEdition were revised as follows: The limit of 10 ksi on the concrete compressive strength that could be used for shear design in the LRFD Specifications through the 3rd Edition was raised to 18 ksi. The limit on the maximum shear stress for shear design was reduced from 0.25fc plus the effect of the vertical contribution of the prestress force to 0.18fc plus