Applied Loads and Stability of Steel and Steel-Concrete Composite Bridges - Finite Element Analysis and Design of Steel and Steel-Concrete Composite Bridges - page 140

In the United States, AASHTO [1.23,1.24] specifies that the fatigue
load shall be one design truck or axles but with a constant spacing of
9000 mm between the 145,000 N axles, with the dynamic load allowance
applied to the fatigue load. The frequency of the fatigue load shall be taken
as the single-lane average daily truck traffic (ADTT SL ). This frequency
shall be applied to all components of the bridge, even to those located
under lanes that carry a lesser number of trucks. AASHTO specifies that
in the absence of better information, the single-lane average daily truck
traffic shall be taken as

ADTT SL ¼ p ADTT ð 3 : 18 Þ

where ADTT is the number of trucks per day in one direction averaged over
the design life and p is the fraction of truck traffic in a single lane, which is
equal to 1.00, 0.85, and 0.8 for the number of lanes available to trucks equal
to 1, 2, and 3 or more, respectively.

3.5.1.2 Fatigue Loads on Railway Bridges
Fatigue loads on railway steel bridges are also covered by EC1 [ 3.1 ] that
recommends that a fatigue damage assessment shall be carried out for all
structural elements of railway bridges subjected to fluctuations of stress.
For normal traffic based on the characteristic values of Load Model 71,
including the dynamic factor F , the fatigue assessment should be carried
out on the basis of the traffic mixes, “standard traffic,” “traffic with
250 kN axles,” or “light traffic mix” depending on whether the structure
carries mixed traffic, predominantly heavy freight traffic, or lightweight
passenger traffic in accordance with the requirements specified. Details
of the service trains and traffic mixes considered and the dynamic enhance-
menttobeappliedaregiveninAnnexDofEC1.Eachofthemixesisbased
on an annual traffic tonnage of 25 10 6 t passing over the bridge on each
track. For bridges carrying multiple tracks, the fatigue loading shall be
applied to a maximum of two tracks in the most unfavorable positions.
The fatigue damage should be assessed over the design working life.
ThedesignworkinglifemaybespecifiedintheNationalAnnexand
100 years is recommended. Vertical railway traffic actions including
dynamic effects and centrifugal forces should be taken into account in
the fatigue assessment. Generally, nosing and longitudinal traffic actions
may be neglected in the fatigue assessment.