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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.
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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.
