Redundancy analysis - Computational Analysis and Design of Bridge Structures - page 464

how redundancy can be included in the design process by using load factor
modifiers η R , where this redundancy factor ≥1.05 for nonredundant mem-
bers, = 1.00 for conventional levels of redundancy, and ≥0.95 for excep-
tional levels of redundancy.
In 1998 NCHRP Report 403 was published, entitled “Redundancy in
Highway Bridge Superstructures” (NCHRP 1998). A clear guideline for a
redundancy check was given. The limit states that should be checked to
ensure adequate bridge redundancy and system safety are defined as
1. Member failure. A traditional check of individual member safety
using elastic analysis and nominal member capacity.
2. Ultimate limit state. The ultimate capacity of the intact bridge sys-
tem. It corresponds to the formation of a collapse mechanism for
bridges.
3. Functional limit state. A maximum acceptable live load displacement
in a main longitudinal member equal to the span length/100.
4. Damaged condition limit state. The ultimate capacity of the bridge
system after damage to one main load-carrying element.
Penn DOT Design Manual Part IV (Penn DOT 2000) has an even more
explicit statement on the checking of redundancy for truss bridges, which
are as follows:
1. Provision of a third line of trusses where possible
2. Use of stitched built-up components, which are designed to support
the entire component load with any one element assumed to be bro-
ken and for which joints and splices have been designed to transmit
component loads with any one element of the component assumed to
be broken (based on load combination extreme event III)
3. Demonstration through 3D analysis that failure of any tension com-
ponent, or other components designated by the department, of a two-
truss system will not cause the collapse of the entire structure (based
on load combination extreme event IV)
A series of analysis cases were defined to match the proper analysis meth-
odology with the appropriate truss configuration.
15.2.2 finite element modeling
For steel bridge redundancy analysis, two levels of analysis should be made.
First level is the 2D or 3D linear analysis to identify FCMs, as shown in
Figure 15.1. Second level is the 3D nonlinear analysis to check the perfor-
mance under loading. In the 3D nonlinear analysis, steel plastic behavior is
described by bilinear kinematic hardening material model.