Simplified Shear Design of Structural Concrete Members (2005) / Chapter Skim
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From page 5... ...
Section 1.3 defines project objectives, the approach used for meeting these objectives, and project tasks. 1.1 THE AASHTO LRFD SHEAR DESIGN SPECIFICATIONS 1.1.1 Summary of the LRFD Sectional Design Model (S5.8.3)
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The LRFD Sectional Design Model introduced a new requirement into shear design provisions -- the direct consideration of shear in determining the required capacity of the longitudinal reinforcement at any point along the length of the member (see Equation 9)
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/2 from the support. Figure 4 is a flowchart of the entire procedure for use of the LRFD Sectional Design Model.
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Design regions and shear demand using the sectional design model. dvcotθ 0.5dvcotθ0.5dvcotθ 0.5dvcotθ2 0.5dvcotθ2 dvcotθ2 Sh e a r Location Design Span Design Span Vu Vr Design Section Design Section dvcotθ dvcotθ2 Sh e a r Location Design Span Design Span Vu Vr Design Section Design Section
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Figure 5. Design example implementing the LRFD sectional design model.
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The most significant of these was that the distribution of shear stress over the depth of the section was taken as the value at mid-depth as calculated by the MCFT using the designer-calculated longitudinal strain, x, at mid-depth. Additional assumptions that were made in the development of the LRFD Sectional Design Model were that the shape of the compressive stress-strain response of the concrete was parabolic with a strain at peak stress of −0.002, and, for members with Av ≥ Av,min, that the spacing of the cracks was 12 inches and the size of the maximum aggregate was 0.75 inches.
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From page 11... ...
In this way, the number of stirrups considered to lift the diagonal compression 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 diagonal compression force is taken as dvcotθ/s.
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From page 12... ...
12 • Further Iteration Required for Capacity Evaluation In the LRFD Sectional Design Model, x and thus β and θ are functions of Vu. Thus, the shear design force must be known in order to evaluate Vc, Vs, and the nominal shear strength.
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Shear strength of RC beams with shear reinforcement.
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493 (29) , where the data that forms the basis for the prestressed concrete shear design concepts of the Standard Specifications and ACI 318-05 (30)
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design practice, and in the LRFD Sectional Design Model, the contribution of the concrete to Figure 13. Mechanism of shear resistance.
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practice for beams without shear reinforcement is also dependent on the parameters of beam depth, concrete strength and maximum aggregate size, not considered in that expression. Influence of Axial Loads The influence of axial compression and tension on shear capacity is examined in Figures 16 (44)
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Comparing the required amount of shear reinforcement by different design approaches for each design case can reveal where prospective provisions may be unconservative or overly conservative. It is also useful to compare these required strengths of shear reinforcement (pv fy)
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A refined work plan was established for developing and refining the selected proposed simplified provisions. This plan included the use of a design database to assess the effect of different potential approaches on the efficiency and conservatism of codes.
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The tuning of these parameters was performed by considering the fit of the proposed simplified provisions with the test results in the refined experimental database and by comparing the required amounts of shear reinforcement for members in the design database with the requirements by other design methods, including the current LRFD Sectional Design Model, the AASHTO Standard Specifications, and Response 2000. The research team prepared eight design examples that covered both prestressed and non-prestressed members, simple span and continuous members, different types of structural components and both stocky and slender members.
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