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11 Although the LRFD specifications were derived from the to the centroid of the compression block (i.e., the flexural MCFT, because of the significant simplification and assump- level arm). In developing the Standard Specifications, d was tions used in developing this method, the shear capacity used rather than the flexural lever arm for the sake of sim- determined using the LRFD Sectional Design Model should plicity and also because the provisions still proved to be not be considered equivalent to the shear capacity calculated conservative with the use of d. In the LRFD specifications, by the MCFT. dv is used as the flexural lever arm and is typically taken as 0.9d. LRFD Raises Minimum Shear Reinforcement Requirement 1.1.3 Comparison of AASHTO LRFD and The LRFD shear design provisions require a substantially AASHTO Standard Specifications larger amount of minimum shear reinforcement (typically The LRFD Sectional Design Model provides a complete 50 percent more), than do the Standard Specifications, as shear design approach for structural concrete in which the shown in Figure 7. This difference is particularly important actions of axial loading, moment, and prestressing are con- for prestressed concrete members for it is common that large sidered explicitly. This approach is a significant departure portions of the length of prestressed concrete members from the shear design procedures of the AASHTO Standard require minimum shear reinforcement only. Specifications and ACI318-02. LRFD Introduces Longitudinal Reinforcement Require- The key differences between the AASHTO LRFD and ment Check Standard Specifications are as follows: In the Standard Specifications, anchorage rules for longi- tudinal reinforcement have been used to account for the LRFD Eliminates Approach of Evaluating Vc Based on demands that shear imposes on the longitudinal reinforcement the Diagonal Cracking Load requirements. In the LRFD Specifications, the demand that In the AASHTO Standard Specifications, the concrete shear imposes on longitudinal reinforcement requirements is contribution to shear resistance, Vc, is taken as the load at taken into account directly. The difference between these which diagonal cracking is expected to occur. In this approaches is particularly significant at the ends of simply approach, Vc is taken as the lesser of the force required to supported prestressed members where the horizontal compo- cause web-shear cracking, Vcw, or flexure-shear cracking, Vci. nent of the diagonal compression force can be large and yet, In the LRFD approach, Vc is taken as a measure of the con- by the LRFD Specifications, only the developed portion of crete contribution at ultimate. A significant effect of this dif- the strands may be considered to provide the required resis- ference is that with LRFD the state of shear cracking in a tance (see Figure 8). member cannot be used to estimate the force that it has sup- LRFD Enables Design for Much Higher Shears ported nor can the designer evaluate whether or not the mem- One of the greatest differences with the LRFD Specifications ber is likely to be cracked in shear under service loads. is that it enables members to be designed for shear stresses that LRFD Introduces Use of a Variable Angle Truss Model can exceed 2.5 times those permitted by the Standard Specifi- In the Standard Specifications, the contribution of the cations. In the Standard Specifications, the contribution of the shear reinforcement to capacity is determined using shear reinforcement is limited to 8 fcbw d so as to guard against a 45-degree parallel chord truss model. In this way, the number of stirrups considered to lift the diagonal compres- sion across a single shear crack is taken as d/s where d is the depth of the member and s is the spacing of the shear reinforcement. In the LRFD Sectional Design Model, the angle of diagonal compression can be taken as ranging from 18.1 to 43.9 degrees and where the number of stirrups considered to lift the diago- nal compression force is taken as dvcot/s. Because cot(18.1 degrees) is 3.06, a given number of stirrups can be calculated by the LRFD Specifications to provide about three times as much shear capacity as would be calculated by the Standard Specifications. Evaluation of Shear Depth In the LRFD Specifications, the shear depth is taken as dv, rather than d, to overcome a previous simplification in the Standard Specifications. In accordance with the parallel chord truss model, the shear depth is equal to the distance Figure 7. Minimum required amount of shear from the centroid of the longitudinal tension reinforcement reinforcement.