Applied Loads and Stability of Steel and Steel-Concrete Composite Bridges - Finite Element Analysis and Design of Steel and Steel-Concrete Composite Bridges - page 141

3.5.2 Dynamic Loads
3.5.2.1 General
The static stresses and deformations (and associated bridge deck acceleration)
induced in a bridge are increased and decreased under the effects of moving
traffic by the following three main parameters specified in EC1 [ 3.1 ] : The
first parameter is the rapid rate of loading due to the speed of traffic crossing
the structure and the inertial response (impact) of the structure. The second
parameter is the passage of successive loads with approximately uniform
spacing that can excite the structure and under certain circumstances create
resonance (where the frequency of excitation (or a multiple thereof) matches
a natural frequency of the structure (or a multiple thereof), there is a possi-
bility that the vibrations caused by successive axles running onto the struc-
ture will be excessive). Finally, the variations in wheel loads result from track
or vehicle imperfections, including wheel irregularities. For determining the
effects (stresses, deflections, bridge deck acceleration, etc.) of railway traffic
actions, the aforementioned parameters shall be taken into account. Accord-
ing to EC1, the main factors that influence dynamic behavior are the speed
of traffic across the bridge; the span L of the structural element and the influ-
ence line length for deflection of the element being considered; the mass of
the structure; the natural frequencies of the whole structure and relevant ele-
ments of the structure and the associated mode shapes (eigenforms) along the
line of the track; the number of axles, axle loads, and the spacing of axles; the
damping of the structure; vertical irregularities in the track; the unsprung/
sprung mass and suspension characteristics of the vehicle; the presence of
regularly spaced supports of the deck slab and/or track (cross girders,
sleepers, etc.); vehicle imperfections (wheel flats, out of round wheels, sus-
pension defects, etc.); and the dynamic characteristics of the track (ballast,
sleepers, track components, etc.).
3.5.2.2 Dynamic Loads on Railway Bridges
Looking at dynamic loads acting on railway bridges, as an example, EC1
[ 3.1 ] provides some requirements for determining whether a static or a
dynamic analysis is required based on V , which is the maximum line speed
at the site in km/h; L , which is the span length in meters; n 0 , which is the first
natural bending frequency of the bridge loaded by permanent actions in Hz;
n T , which is the first natural torsional frequency of the bridge loaded by per-
manent actions in Hz; v , which is the maximum nominal speed in m/s; and
( v/n 0 ) lim , which is given in Annex F of EC1. The requirements are valid for