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OCR for page 47

47
CHAPTER 4
CONCLUSIONS AND SUGGESTED RESEARCH
4.1 CONCLUSIONS find the angle of diagonal cracking. It has further been
argued by researchers that the number of stirrups that
The conclusions presented here go beyond the scope of should be considered to form a given leg in the ideal-
Project NCHRP 12-61. These conclusions principally iden- ized truss should be dvcot()/s - 1 because diagonal
tify deficiencies that the research team considers will remain, cracks often form from the top of one stirrup to the bot-
even if the change proposals recommended in this document tom of another.
are adopted. The conclusions are presented in four cate-
gories: basis of design provisions; role of experimental Av f y d v cot()
research and field experience; role of design database and Vs =
s (Eq. 35)
numerical tools; and differences in shear design provisions.
4. For members with shear reinforcement, the equation
4.1.1 Basis of Design Provisions developed for Vc in provisions must account for the
rules used for evaluating the angle of diagonal com-
1. Although researchers may agree about the components pression. For example, in current U.S. practice where
that contribute to shear resistance, there is considerable the angle is assumed to be 45 degrees for prestressed
disagreement about the relative magnitude of these con- concrete structures, the calculated contribution of the
tributions, the factors that influence these contributions, shear reinforcement is less and Vc can afford to be
and their significance for different design conditions. larger than in other codes such as the LRFD Sectional
2. The diagonal cracking strength is not a measure of the Design Model where may be as low as 18 degrees.
concrete contribution at ultimate for members with 5. For members without shear reinforcement, there is a
shear reinforcement. Thus, provisions in which Vc is large debate about how to evaluate the contribution
related to the diagonal cracking strength for members of the concrete at the ultimate limit state. Some re-
with shear reinforcement are purely empirical and need searchers argue that it should be the load required to
to be validated by comprehensive test data. form or propagate a diagonal crack. Others suggest that
3. The parallel chord truss model provides a direct means it should be based on the shear-slip resistance of the
of calculating the contribution of shear reinforcement diagonal crack while others suggest that it is best eval-
to shear capacity. That contribution can be calculated uated by considering the shear force that can be trans-
as the yield strength of a stirrup times the number of mitted in the uncracked compression zone. Regardless
stirrups in one leg of the idealized truss (see Equation of which method is used, there is a significant depth
35 where the angle is the angle of diagonal compres- effect in shear for members without transverse rein-
sion in the idealized truss relative to its longitudinal forcement and little depth effect for members with
axis); however, there is no agreement on how to calcu- shear reinforcement; members without shear reinforce-
late the angle . In ACI 318-05 and the AASHTO Stan- ment and with a unit depth of three can fail at one half
dard Specifications, is assumed to be 45 degrees. In the stress of a geometrically similar member with a unit
the Eurocode, in which the shear design provisions are depth of one. However, there is significant debate over
partially based on plasticity theory, the angle can be the types and sizes of members for which the depth
selected by the designer to be as low as 18 degrees or effect in shear must be considered.
that when the diagonal compressive stress reaches a
limit equal to about 60 percent of the concrete com-
pressive strength. In the LRFD Sectional Design 4.1.2 Role of Experimental Research
Model, this angle is calculated using the MCFT and and Field Experience
using the longitudinal strain at mid-depth. In the pro-
posed simplified provisions, this angle is calculated in 1. What researchers have tested and continue to test in
the web-shear region using Mohr's circle of stress to laboratories is not representative of what is built using