A comparison of single-run pushover analysis techniques for seismic assessment of bridges

The term ‘pushover analysis’ describes a modern variation of the classical ‘collapse analysis’
method, as fittingly described by Kunnath [1]. It refers to an analysis procedure whereby an
incremental-iterative solution of the static equilibrium equations has been carried out to obtain
the response of a structure subjected to monotonically increasing lateral load patterns. Whilst the
application of pushover methods in the assessment of building frames has been extensively verified
in the recent past, nonlinear static analysis of bridge structures has been the subject of only limited
scrutiny [2]. Since bridges are markedly different structural typologies with respect to buildings,
observations and conclusions drawn from studies on the latter cannot really be extrapolated to the
case of the former, as shown by Fischinger et al. [3], who highlighted the doubtful validity of
systematic application of standard pushover procedures to bridge structures.
Recent years have also witnessed the development and introduction of an alternative type of
nonlinear static analysis [4–10], which involve running multiple pushover analyses separately, each
of which corresponding to a given modal distribution, and then estimating the structural response
by combining the action effects derived from each of the modal responses (i.e. each displacement–
force pair derived from such procedures does not actually correspond to an equilibrated structural
stress state). As highlighted by some of their respective authors, the main advantage of this
category of static analysis procedures is that they may be applied using standard readily available
commercial software packages, since they make use of conventional analysis types. The associated
drawback, however, is that the methods are inevitably more complex than running a single pushover
analysis, as noted by Maison [11], for which reason they do not constitute the scope of the current
work, where focus is instead placed on single-run pushover analysis procedures, the simplicity
of which renders them an even more appealing alternative, or complement, to nonlinear dynamic
analysis [12].
In this work an analytical parametric study is thus conducted applying different single-run
pushover procedures, either adaptive or conventional, on a number of regular and irregular continuous
deck bridges subjected to an ensemble of ground motions. The effectiveness of each
methodology in reproducing both global behaviour and local phenomena is assessed by comparing
static analysis results with the outcomes of nonlinear time-history runs. Adaptive pushovers are
run in both their force-based [13–17] and displacement-based [18, 19] versions. With respect to
the latter, it is noted that, contrary to what happens in a non-adaptive pushover, where the application
of a constant displacement profile would force a predetermined and possibly inappropriate
response mode that could conceal important structural characteristics and concentrated inelastic
mechanisms at a given location, within an adaptive framework a displacement-based pushover
is entirely feasible, since the loading vector is updated at each step of the analysis according to
the current dynamic characteristics of the structure. The interested reader is referred to some of
the aforementioned publications for details on the underlying formulations of adaptive pushover
algorithms.
It is observed that whilst for regular bridge configurations some conventional single-run pushover
methods may manage to provide levels of accuracy that are similar to those yielded by their more
evolved adaptive counterparts, when irregular bridges are considered the advantages of using the
latter become evident. In particular, the displacement-based adaptive pushover (DAP) algorithm is
shown to lead to improved predictions, which match more closely results from nonlinear dynamic
analysis.
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