A different design

Changes to LRFD specifications call for new automated tools

Toorak Zokaie, P.E., Ph.D / May 17, 2002

The American Association of State Highway &
Transportation Officials (AASHTO) LRFD specifications have been accepted by
several states as the design specification of choice since their adoption in
1994. However, the experience with the LRFD specifications for the design of
cast-in-place (C.I.P.) post-tensioned concrete box girder bridges is limited.
Several major changes have been made to the LRFD design specifications that
have a direct effect on the design and safety of these bridges and have imposed
a new set of design tasks to be performed. New automated design tools are
needed to assist bridge engineers and facilitate the transition to the LRFD
specifications.

Some of the design specification modifications that will
have an effect on the analysis and design of C.I.P. post-tensioned concrete box
girder bridges are as follows:

               Design
live loads:
The design live load in the LRFD specifications is considerably
different from the standard specifications. An additional loading criterion
consists of two trucks running back-to-back, referred to as “dual
truck” loading. The effect of the “dual truck” loading on
negative moment and interior supports usually translates into higher negative
moments and additional reinforcement.

In the West Coast, it is customary to design box girder
bridges for stresses for the design live load and check the moment capacity for
the standard overload truck. This issue is not directly addressed in the LRFD
specifications and may alter the level of safety of the bridge;

               Live
load distribution:
The live load distribution according to the standard AASHTO specifications is dependent only on the girder spacing. The LRFD specifications
realize the effect of bridge length, superstructure depth and number of cells
on lateral live load distribution. In addition, the LRFD specifications allow
for reduction of live load moment due to skewed supports and require an
increase in shear forces at obtuse corners;

              
Full-width vs. single girder design: AASHTO specifications are based on an individual girder design, whereas design practice in some states, such as California, is to design the entire bridge cross section as one unit. A recent update to the LRFD specifications has resulted in provisions to allow whole width design, treating the entire section to act as one unit;

               Partial prestressing: Box girder bridges are typically designed with post-tensioning force to satisfy the stress requirements and additional mild reinforcement to satisfy capacity requirements. The standard AASHTO specifications do not directly deal with a combination of both post-tensioning and mild steel. The LRFD specifications treat this issue with the concept of partial prestressing
and provide guidance for calculation of loss of prestress and capacities using
a combination of post-tensioning and reinforcement;

               Loss
of prestress:
The prestress loss equations in LRFD are slightly different from
the standard specifications, and the lump sum losses include the effect of
partial prestressing. In general, the LRFD specifications tend to result in
higher prestress losses, which is more conservative in most cases;

               Shear
capacity calculations:
The LRFD specifications include modifications to the
shear capacity calculation procedures. Whereas the standard AASHTO
specifications are based on empirical formulas, the LRFD specifications have a
sound theoretical basis. In addition, the need for longitudinal reinforcement
to develop the shear reinforcement is present in LRFD specifications. These new
procedures result in additional shear reinforcement; and

               Effect
of skewed supports:
Skewed supports affect the response of the bridge
superstructures in several ways. The most important effects on C.I.P.
post-tensioned concrete box girder bridges are reduction of moment, increased
shear in obtuse corners and additional torsion. The LRFD specifications address
the first two through live load distribution factors and address the latter
through procedures for combined shear and torsion design.

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