Civil Engineering Reference
In-Depth Information
In contrast to the long and lightly loaded girder in Example 1, the girder in Example 2 is
short and is subjected to heavy load. The UH method indicates that this girder in Example
2 fails in web-shear at a critical section at distance
d
from the support. The stirrup spacing
increases slowly from the support toward the quarter-point of the span. Using the ACI and
AASHTO methods, the girder also fails in web-shear near the support, but the stirrup spacing
increases much faster from the support toward the quarter-point of the span.
Example 3 uses two large AASHTO Type VI prestressed girders, each with a large height
of 1.83 m (6 ft) and a large span of 33.5 m (110 ft), made continuous for live loads (Modjeski
and Masters Inc., 2003). The shear forces and bending moments in this third example are
calculated using influence lines for HS-20 truck load and uniform lane load defined by the
AASHTO-LRFD Specifications. Such girders are found to be critical in flexural-shear near
the quarter point of a span. In contrast, the ACI and AASHTO methods would indicate that
these girders would fail in web-shear near the support.
In short, the shear designs of girders in Examples 1 and 3 using the ACI and AASHTO
methods would wrongly identify these girders as critical in web-shear failure near the supports,
rather than flexural-shear failure near the quarter-points of their spans. As a result, these two
shear design methods would over-reinforce the regions near the support and would under-
reinforce the regions near the quarter point of the beam.
