Redundancy analysis - Computational Analysis and Design of Bridge Structures - page 462

the term progressive collapse has constantly been used in the redundancy
analysis. As stated in ASCE 7-10 (2010), progressive collapse is defined as
the spread of an initial local failure from element to element, eventually
resulting in the collapse of an entire structure or disproportionately large
part of it. Progressive collapse due to earthquake loading will be discussed
more in Chapter 17—Dynamic/Earthquake Analysis.
To achieve targeted integrity during blast, the redundancy of the gravity
load-carrying structural system takes center stage in tackling the issue of pro-
gressive collapse. This is not explicitly addressed in any code. However, ASCE
7-10 (2010) implies a desired alternate load path in the event one or more
beams and/or columns of a building fail as a result of a blast. The structure
should be able to remain stable by redistributing the gravity loads to other
members and subsequently to the foundation through an alternate load path,
while keeping building damage somewhat proportional to the initial failure.
For performance-based designs, factors considered include life safety
issues, progressive collapse mechanisms, ductility of certain critical compo-
nents, and redundancy of the whole structure. Blast load damages structures
through propagating spherical pressure waves, which can be simulated by a
series of equivalent loads. Performance of bridge elements under equivalent
static loads can be considered as reasonably similar to that under the origi-
nal dynamic blast loads. For the evaluation of the existing bridges under
blast loading, the structural performance levels, the immediate occupancy
(IO) level, life safety (LS) level, and the collapse prevention (CP) level,
adopted by FEMA (1998) for the seismic evaluation of buildings, are used
here. More details about these three levels will be discussed in Chapter 17.
15.2 PRinciPle and Modeling of
BRidge Redundancy analysis
The emphasis of this chapter is to illustrate how to conduct a bridge redun-
dancy analysis. NCHRP Report 403 (1998) proposed a series of tables
for system factors to be used in the design and evaluation equations for
common-type bridges. The system factor tables developed in the NCHRP
study are applicable to standard prestressed concrete and steel bridges.
Bridges with configurations that are not covered by the tables have to be
checked by performing a detailed incremental structural analysis. A steel
truss bridge was mentioned specifically in this report to illustrate how the
direct analysis can be applied in practice. This approach is allowed by Penn
DOT Design Manual Part IV, Section 3—Loads and Load Factors (Penn
DOT 2000). Commentary for extreme event IV states that
For this extreme event, a 3D analysis is required. The objective of this
analysis is survival of the bridge (i.e., the bridge may have large perma-
nent deflections, but it has not collapsed).