Applied Loads and Stability of Steel and Steel-Concrete Composite Bridges - Finite Element Analysis and Design of Steel and Steel-Concrete Composite Bridges - page 143

steel-concrete composite bridges, in accordance with a loading period
appropriate to the natural frequency of the bridge.
According to EC1, the dynamic factor F takes account of the dynamic
magnification of stresses and vibration effects in the structure but does not
take account of resonance effects. Where a dynamic analysis is required,
there is a risk that resonance or excessive vibration of the bridge may occur
(with a possibility of excessive deck accelerations leading to ballast instability
and excessive deflections and stresses). For such cases, a dynamic analysis shall
be carried out to calculate impact and resonance effects. Quasi-static
methods that use static load effects multiplied by the dynamic factor F
are unable to predict resonance effects from high-speed trains. Dynamic
analysis techniques, which take into account the time-dependent nature
of the loading from the high-speed load model (HSLM) and real trains
(e.g., by solving equations of motion), are required for predicting dynamic
effects at resonance. Bridges carrying more than one track should be consid-
ered without any reduction of dynamic factor F . The dynamic factor F that
enhances the static load effects under Load Models 71, SW/0, and SW/2
shall be taken as either F 2 or F 3 . Generally, the dynamic factor F is taken
as either F 2 or F 3 according to the quality of track maintenance as follows:
(a) For carefully maintained track,

1 : 44

F 2 ¼ p

L F 2 +0

: 82 with : 1 : 00 F 2 1 : 67 ð 3 : 20 Þ

0 :

(b) For track with standard maintenance,

:

2 16

F 3 ¼ p

L F 2 +0

: 73 with : 1 : 0 F 3 2 : 0 ð 3 : 21 Þ

0 :

where L F is the “determinant” length (length associated with F )definedin
Table 3.10 in meters. The dynamic factors were established for simply sup-
ported girders. The length L F allows these factors to be used for other structural
memberswithdifferent support conditions. If nodynamic factor is specified, F 3
shall beused.The dynamic factor F shall not beusedwith the loadingdue to real
trains, the loading due to fatigue trains, HSLM, and the load model “unloaded
train.” The determinant lengths L F to be used are given in Table 3.10 .

3.5.3 Accidental Forces
3.5.3.1 General
Steel and steel-concrete composite bridges may be subjected to forces result-
ing from accidental situations. The situations comprise vehicle collision with