Redundancy analysis - Computational Analysis and Design of Bridge Structures - page 463

Thus, a three-dimensional (3D) nonlinear model of the truss bridge is
recommended for the structural analysis. In this chapter, the safety analysis
is conducted as follows:
1. Member failure check, ultimate capacity check, and functionality
check. It is proposed that the check of member failure be performed
using a 3D elastic analysis of the structural system [ANSYS (2012)
or SAP2000 (2007)]. Member capacity is calculated using AASHTO
member strength formulas (AASHTO 2012, 2013). Penn DOT load
combination extreme event III will be applied on a linear elastic struc-
tural model.
2. Damaged condition check. It is proposed that the check be performed
using ANSYS (2012) or SAP2000 (2007) to analyze the damaged
structure on a structural model. ANSYS (2012) or SAP2000 (2007)
also may be applied using several degrading models to simulate the
incremental analyses. Penn DOT load combination extreme event IV
will be applied on a nonlinear elastic structural model.
The intention is to prove that although this bridge geometry does not sat-
isfy redundancy criteria, the conservatives of the member design ensure that
enough system safety is still available. Note that extreme events III and IV
described here can be replaced by any extreme cases described in other codes.
15.2.1 analysis cases
When possible, alternate load paths should be included in the design.
Though this is not always an option, special consideration is warranted
during the design of nonredundant structures. Due to the criticality of
the primary load-carrying members, attention should be paid to fatigue,
and effort should be made to eliminate detrimental details when possible.
Sophisticated analyses have been performed in the past with some effec-
tiveness to determine if two-girder systems are truly nonredundant or not,
to account for the membrane action of the deck and to determine load-
shedding properties of secondary members. These analyses are rather gru-
eling and are not suggested as part of a typical design to avoid the penalties
associated with the use of nonredundant members and FCMs.
Before 1998, there was some discussion but little guidance on the assess-
ment of redundancy. The AASHTO LRFD Specifications (AASHTO 2013)
specifications for the design of highway bridges recognize the importance
of redundancy and require its consideration when designing steel bridge
members. The specifications state that a structure is nonredundant when
the failure of a single element could cause collapse.
The AASHTO LRFD specifications (AASHTO 2013) and Penn DOT
Design Manual Part IV (Penn DOT 2000) proposed a format explaining