D. Pak/H. Bigelow/M. Feldmann · Design of composite bridges with integral abutments
ferent countries. In the USA as well as in some European
countries, bridges are called semi-integral when they have
either expansion joints or bearings, but not both. Bearings
or expansion joints are arranged solely at the abutments.
In Germany bridges that do not comply with the definition
of fully integral bridges and in which the piles are monolithically
connected to the superstructure on at least two
axes are referred to as semi-integral bridges (Fig. 6) .
Integral abutment bridges are generally designed
based on two different concepts:
1. Low flexural stiffness of piles / low degree of restraint
In the USA especially, abutments and columns are supported
by single rows of flexible steel piles. The bridge
structure can be considered to be a continuous frame.
As the columns are quite flexible, the continuous superstructure
may be assumed to have simple or hinged supports.
Consequently, except for the design of the continuity
connections at abutments and columns, frame
action can be ignored when analysing the superstructure
for superimposed dead and live loads . Furthermore,
as only low moments need to be conducted
through the abutment’s corner, the design of that detail
becomes rather simple. The main advantage here is seen
to be the absence of bearings and joints.
2. High flexural stiffness of piles / high degree of restraint
The more slender the superstructure is intended to be
constructed, the stiffer the substructure system has to be
. In order to increase the corner moment of the bridge
and to decrease the span moment, the horizontal member
(the continuous superstructure) is partly restrained
24 Steel Construction 10 (2017), No. 1
by the stiff vertical members. This frame concept is
widely employed in Europe, calling for stiff concrete
foundation piles. For bridges with short and medium
spans especially, the main advantage here is seen to be
the slender superstructure and the absence of the middle
support (Fig. 1).
A typical integral abutment bridge has one span and can
be founded on piles or footings (Fig. 3). In the case of long
spans especially, piled foundations are preferred for reason
of their more flexible horizontal bedding, as constraint
forces due to temperature loading and support settlement
can be absorbed by flexible structures more effectively .
For reasons of aesthetics, but also to optimize visibility
for traffic, the abutments are often designed with inclined
faces (Fig. 4). Inclining the abutments to the back
effectively creates a smaller mid-span moment as the superstructure
is dimensioned for span ls2, which results in superstructures
that are very slender visually .
In the case of multi-span structures, a deep foundation
should be provided for the columns and, most importantly,
for the abutments (Fig. 5). Alternatively, the abutments can
be separated from the superstructure by bearings, resulting
in a semi-integral structure (Fig. 6), although this kind of
bearing system is less efficient in some cases. For example,
in the case of semi-integral bridges, high braking forces
from railway traffic can only be absorbed by the piles. In
the case of fully integral bridges, braking forces are transferred
directly by the pile block supporting the abutment
into backfill and subsoil (Fig. 7) .
Fig. 1. Typical composite bridge with integral abutments
Fig. 2. Old framed bridges in Germany dating from the 1930s: a) Wilhelmsbrücke crossing the River Neckar in Stuttgart
Cannstatt, b) crossing over Hardenbergstraße at Zoologischer Garten railway station in Berlin