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OCR for page 18
18 1.3 PROJECT OBJECTIVES AND TASKS Detailed knowledge of a broad base of mechanistic models for shear including U.S., Canadian, and Euro- 1.3.1 Project Motivation and Objectives pean approaches; Detailed understanding of the Modified Compression The LRFD shear design provisions provide some defi- Field Theory and how the LRFD provisions were nite advantages over the methods in the AASHTO derived from this theoretical model for behavior; Standard Specifications. The Sectional Design Model Custodians of the largest and more detailed shear test provides a comprehensive approach for shear design of database yet assembled; sections subjected to the actions of axial load, prestressing, Not committed to a single line of thinking on the final and moment while the strut-and-tie method provides a structure of the simplified provisions or on the mecha- completely general design method for regions in which the nistic model (or models) on which these provisions flow of forces is more complex, such as near geometric should be based; and discontinuities or near concentrated forces and reactions. Familiarity with the use of non-linear numerical tools for However, the Sectional Design Model requires an iterative predicting the capacity of members in a design testbed. design procedure that involves selecting and values from tables. Some designers consider this procedure There were 8 tasks listed in the request for proposals that complex to use and difficult to understand, with the effect were required for meeting project objectives. The researcher's that some design engineers lose a feel for what they are approach on each of these tasks follows the description of evaluating. With the strut-and-tie method, concerns have each of these individual tasks. been expressed that solutions require an iterative approach The researchers conducted a survey of practicing engi- and are non-unique. neers concerning their experiences in the use of AASHTO The overall objective of this research was to provide sim- Standard and LRFD specifications, collected codes of prac- plified procedures for the shear design of the most common tice, and then conducted a preliminary review and assess- concrete structures, including reinforced concrete T-beams; ment of different shear design approaches using an extensive prestressed concrete I girders continuous for live load; pre- experimental database of shear test results. stressed concrete box beams; and cast-in-place post-tensioned A refined work plan was established for developing and box girders, hammerhead piers, and concrete bents. These refining the selected proposed simplified provisions. This simplified provisions were expected to be in a form similar to plan included the use of a design database to assess the effect the standard specifications and to be applicable for both pre- of different potential approaches on the efficiency and con- stressed and precast members up to concrete strengths of servatism of codes. The researchers produced a tentative list 18 ksi and cast-in-place concrete strengths up to 10 ksi. of design examples for consideration by the project review Although there are recognized challenges to the application panel from which the final design examples were selected. of the strut-and-tie method, there was no project objective to An Interim Report was submitted and then, following a refine the strut-and-tie design provisions. request by the Project Panel, a more comprehensive interim report was submitted containing an initial proposal by the 1.3.2 Research Approach and Project Tasks contractor for the simplified provisions. These proposed sim- plified provisions were essentially the same as those devel- There are many approaches for shear design, underlying oped by Michael Collins, a developer of the MCFT, for the theories for explaining how shear is carried in structural 2004 Canadian Standards Association "Design of Concrete concrete, and the primary factors that influence the mecha- Structures." These simplified provisions consisted of equa- nisms of resistance. The approach on this project was to tions for and and an elimination of the dependency of x investigate and then select the most suitable simplified shear on , thereby eliminating the iterative nature of the LRFD design provisions based on a detailed review of existing design procedure. shear design approaches and an evaluation of these The researchers conducted the plan approved by the proj- approaches by comparison with both experimental test data ect panel. This plan consisted of: and with the predictions from numerical methods. The members of the research team were selected so that differ- Reviewing shear design provisions additional to those ent points of view and experiences were represented. Sev- examined in the Interim Report; eral of the important, if not essential, attributes of the Developing a refined experimental database of shear research team were as follows: test results of large members with shear reinforcement; Developing an expanded member design database; Leadership experience in the developing code provi- Developing alternative provisions to the CSA method sions for shear, including the AASHTO LRFD specifi- proposed in the Interim Report; and cations, the AASHTO Standard Specifications and the Developing detailed criteria for selection and verifica- ACI 318-05 provisions; tion of the simplified specifications.

OCR for page 18
19 Based on the results of their analytical and design investi- of only a few parameters and limits. The tuning of these gations, the researchers (1) developed a new simplified shear parameters was performed by considering the fit of the pro- design procedure for members with minimum shear rein- posed simplified provisions with the test results in the forcement, (2) verified the need for the existing limit on the refined experimental database and by comparing the required minimum amount of shear reinforcement, (3) veri- required amounts of shear reinforcement for members fied the need for a new lower limit on the maximum shear in the design database with the requirements by other stress that can be used in design if members are not supported design methods, including the current LRFD Sectional over their full depth at the ends, and (4) developed modifi- Design Model, the AASHTO Standard Specifications, and cations to simplify the existing General Procedure for Response 2000. sectional shear design of Article 5.8.3.4.2 of the LRFD The research team prepared eight design examples that Specifications. covered both prestressed and non-prestressed members, Based on the final form of the proposed simplified spec- simple span and continuous members, different types of ifications, the goal of the regression testing was the setting structural components and both stocky and slender members.