Integral bridges - Computational Analysis and Design of Bridge Structures - page 496

analysis is usually combined with a nonlinear soil-structure interaction
analysis of the foundation elements. A simplified way to approach this is to
separate the foundation analysis from the rest of the structure and consider
the foundation elements independently. For the case of pile or drilled shaft
foundations, this lateral analysis would be accomplished via a laterally
loaded pile analysis, often facilitated by a standardized computer model
based on a p - y curve analysis of the lateral response of the soil until the
laterally loaded pile analysis and the structural analysis converge.
A more rigorous approach to a comprehensive analysis might involve the
modeling of the soil response directly in the structural analysis model. This
step eliminates the tedious iterations of exchanging information manually
between the geotechnical and the structural analysis models, but the result-
ing soil-structure interaction model can become fairly complex (FHWA
2012). Often a simple two-dimensional (2D) model is a sufficiently compre-
hensive approach to the soil-structure interaction analysis. For a skewed
or curved bridge, a full three-dimensional (3D) analysis may be warranted.
In many integral abutments with foundations on steel piles, longitudinal
movements of the bridge will cause sufficiently high internal loads so that
the plastic moment capacity of the pile is exceeded. In those cases, the com-
mon assumption is to allow a plastic hinge to form during the analysis, which
provides significant moment relief for any movements above those that cause
yielding of the piles. Some designers have pointed out that allowing a plastic
hinge at the pile-abutment interface while simultaneously sizing the pile to pre-
vent even a nominal overstress in terms of bending-axial interaction lower in
the pile represents an inconsistent design approach, but to date there have been
no known significant in-service problems for piles designed in this fashion.

16.2.1 force analysis
In this section, a sample design calculation to attain moment and shears of a
semi-integral abutment is provided for better understanding of the loading due
to earth pressure. Figure 16.6a shows the cross section of the superstructure,
and Figure 16.6b shows the elevation view of the semi-integral abutment. The
calculation steps for the backwall moments and shears are shown here:
Earth pressure resultant per unit width

1
2

( )

2

w K H

p

= (16.1a)

backwall

For unit weight of soil γ = 145 pcf (2325 kg/m 3 ), K p = 4, assuming the use of
Expanded Polystyrene (EPS) material behind backwall, and backwall height
H backwall = 1.93 m (6.33′), calculated w = 11.6 klf (169.2 kN/m). This can be
assumed a distributed line load applied along the abutment.