shear design AASHTO

# shear design AASHTO - SHEAR DESIGN IN AASHTO LRFD CODE 1...

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1. INTRODUCTION Shear design in the LRFD AASHTO Code can be performed using sectional models or full member/region models such as the strut- and-tie method. In this section, an overview is given of both the sectional and strut-and-tie methods in the AASHTO LRFD provisions. In addition, background information on the Modified Compression Field Theory, which is used as the basis for the development of the AASHTO LRFD general sectional method and strength factors in strut-and-tie models, is given. 1.1 Sectional Method Sectional models are used for shear design of concrete sections when it is reasonable to assume that plane sections remain plane after loading. In the AASHTO LRFD Code, the use of sectional models for shear design is described in Section 5.8.3. 1.2 Strut-and-Tie Method The strut-and-tie method can be used for the design of full structural systems, members or regions. It is typically used in situations where the assumption of plane sections remaining plane after loading is not valid (nonlinear distribution of strains). However, it could also be used for design of sections where classical beam theory applies. Provisions for the use of the Strut-and-Tie Method are given in Section 5.6.3 of the AASHTO LRFD Code. In the following, a discussion on both the sectional and strut-and-tie methods for shear design of concrete members following the AASHTO LRFD provisions is given. SHEAR DESIGN IN AASHTO LRFD CODE

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2. SECTIONAL MODELS (AASHTO LRFD SECTION 5.8.3) Sectional modeling is applicable to typical bridge girders and slabs where the assumption of plane sections remaining plane after loading is reasonable. Typically, the assumption of linear strain distribution is valid for regions of members away from a source of disturbance, i.e. change of geometry, application of a concentrated load. Using St. Venant’s principle, the region where classical beam theory is not applicable can be assumed to extend over approximately one member depth from the source of disturbance. Therefore, regions farther than one member depth from the source of disturbance can be assumed to experience a linear strain distribution. In general, the assumption of plane sections remaining plane after loading is acceptable for typical slender elements, such as concrete girders and slabs. On the other hand, this assumption becomes less accurate as the ratio between member span and depth decreases. More detailed information on the appropriateness of assuming a linear strain distribution for member/region design is given in Section 3 of this document. 2.1 Nominal Shear Resistance (AASHTO LRFD Section 5.8.3.3) Nominal shear resistance, V n (kips), is expressed as the summation of the contributions assigned to the concrete, steel and prestressing steel as follows, p s c n V V V V + + [1] p v v c n V d b f' .25 0 V + [2] where V c : (kips) commonly referred to as “concrete” contribution to shear strength. This term has traditionally been associated with contributions from aggregate interlock, dowel action and shear
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