Simplified Shear Design of Structural Concrete Members (2005) / Chapter Skim
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From page 20... ...
This led to the development of the proposed changes to the LRFD Sectional Design Model and the Proposed Simplified Provisions presented in Chapter 3. Chapter 2 summarizes the findings.
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A further complication is that the experimentally measured concrete contribution to shear resistance used to calibrate this type of model also requires an assumption for the angle of diagonal compression to be used in calculating the concrete contribution to shear resistance. Thus, the concrete contribution to shear strength Vc cannot be clearly established by this approach.
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By contrast, the completion of a shear design by the LRFD Sectional Design Model is a comparatively simple hand-based procedure. Developing this hand-based procedure from the MCFT required several assumptions.
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3. In the derivation of the LRFD Sectional Design Model for members with shear reinforcement, the average crack spacing was assumed to be 12 inches.
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24 TABLE 3 Comparison of different design approaches (units: psi, in, lbs)
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This shear stress limit is intended principally to guard against diagonal compression failures. In the AASHTO LRFD, the shear design force limit is 0.25f ′c plus the vertical component of the prestress, while in ACI 31802 the limit is approximately plus the vertical component of the prestress when the web-shear crack12 ′fc TABLE 3 (Continued)
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2.3 EVALUATION OF SHEAR DESIGN METHODS USING TEST DATABASE To evaluate the accuracy of different national codes of practice, a large experimental database was used to evaluate the shear strength ratio (Vtest /Vcode) for six different Codes for 1,359 selected beam tests results.
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This database is also useful for evaluating the maximum shear stress design limit. The LRFD Sectional Design Model enables the design of members for up to 2.5 times the maximum shear design stress permitted in the AASHTO Standard Specifications.
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Conse28 quently, many organizations are actively beginning the conversion process, and thus are on, or just beginning, the learning curve with the LRFD Sectional Design Model (Section 5.8.3.3 of the LRFD specification)
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Several respondents indicated that the revisions to the β and θ values incorporated into the second edition of LRFD helped bring the steel contents into parity with the Standard Specifications method. Regarding the use of the simpler 1979 AASHTO shear design procedure allowed by the Standard Specifications in a footnote to Section 9.20, about one-third indicated that they still used this method.
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The following states responded to the questionnaire: Alaska, Arkansas, California, Delaware, FHWA CFLHD, Florida, Georgia, Illinois, Kansas, Kentucky, Mississippi, Missouri, Montana, Nevada, New Hampshire, New Jersey, Oregon, Pennsylvania, Tennessee, Texas, and Washington. 2.5 CRITERIA FOR PROPOSED SIMPLIFIED PROVISIONS Based on the experiences of practicing engineers, the review of shear design methods in codes of practice, the analysis of experimental test data, and a comparison of the required amounts of shear reinforcement for sections in a design database (presented in Section 2.9)
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