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1. INTRODUCTION
Shear design in the LRFD AASHTO Code can be performed using
sectional models or full member/region models such as the strut
andtie method. In this section, an overview is given of both the
sectional and strutandtie methods in the AASHTO LRFD provisions.
In addition, background information on the Modified Compression
Field Theory, which is used as the basis for the development of the
AASHTO LRFD general sectional method and strength factors in
strutandtie models, is given.
1.1 Sectional Method
Sectional models are used for shear design of concrete sections when
it is reasonable to assume that plane sections remain plane after
loading. In the AASHTO LRFD Code, the use of sectional models for
shear design is described in
Section 5.8.3.
1.2 StrutandTie Method
The strutandtie method can be used for the design of full structural
systems, members or regions. It is typically used in situations where
the assumption of plane sections remaining plane after loading is not
valid (nonlinear distribution of strains). However, it could also be
used for design of sections where classical beam theory applies.
Provisions for the use of the StrutandTie Method are given in
Section 5.6.3
of the AASHTO LRFD Code.
In the following, a discussion on both the sectional and strutandtie
methods for shear design of concrete members following the AASHTO
LRFD provisions is given.
SHEAR DESIGN IN AASHTO LRFD CODE
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View Full Document2. SECTIONAL MODELS (AASHTO LRFD SECTION 5.8.3)
Sectional modeling is applicable to typical bridge girders and slabs
where the assumption of plane sections remaining plane after loading
is reasonable. Typically, the assumption of linear strain distribution
is valid for regions of members away from a source of disturbance,
i.e. change of geometry, application of a concentrated load. Using St.
Venant’s principle, the region where classical beam theory is not
applicable can be assumed to extend over approximately one member
depth from the source of disturbance. Therefore, regions farther than
one member depth from the source of disturbance can be assumed to
experience a linear strain distribution. In general, the assumption of
plane sections remaining plane after loading is acceptable for typical
slender elements, such as concrete girders and slabs. On the other
hand, this assumption becomes less accurate as the ratio between
member span and depth decreases. More detailed information on the
appropriateness of assuming a linear strain distribution for
member/region design is given in Section 3 of this document.
2.1 Nominal Shear Resistance (AASHTO LRFD Section 5.8.3.3)
Nominal shear resistance, V
n
(kips), is expressed as the summation of
the contributions assigned to the concrete, steel and prestressing
steel as follows,
p
s
c
n
V
V
V
V
+
+
≤
[1]
p
v
v
c
n
V
d
b
f'
.25
0
V
+
≤
[2]
where
V
c
:
(kips) commonly referred to as “concrete” contribution to shear
strength. This term has traditionally been associated with
contributions from aggregate interlock, dowel action and shear
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 Winter '09
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