ASSESSMENT OF RESIDUAL LIFE OF GIRDERS OF BRIDGE No

d) The fatigue life of standard spans has been assessed by calculating the
design life factor k1. This factor has been worked out as σRmax/(σ0 x k3)
and fatigue life calculations have been done by inversion, using the
equations given in Clause 9.2.3 of BS-5400 Pt.-10 by taking fatigue life as
minimum of the following:
Fatigue Life
or
120
…………………………………………………………..(2)
m
k1
Fatigue Life
120
…………………………………………………………(3)
2
k
m
1
Where m = 3.0 taken from Table-8 for detailed Class ‘D’.
e) Value of RU loading factor k3 has been taken from Table-4 of the code
considering the case of heavy traffic loading, corresponding to the base
length (L) of the influence line diagram for the girder.
f) Value of GMT factor, k4 is assumed as 1.0 for GMT of 18 to 27 million
tonnes. Actual GMT data suggests that this assumption is very much on
conservative side.
g) For single lane loading value of lane factor, k5 is taken as 1.0.
4.2 Method-2: Fatigue life assessment with damage calculations :
4.2.1 General : This method involves a calculation of Miner’s summation
and may be used for any detail for which S-N relationship is known for any
known load or stress spectrum. This method may be used as a more precise
alternative to the simplified procedure as described in para. 4.1of this report.
The essential information which is required to assess the fatigue life of a
structure is the pattern of stresses likely to be observed in it, during the
passage of normal traffic and the relationship between the applied stress
cycles and the number of times these can be withstood by the material of the
structure. This can be achieved by subjecting the representative samples
not only to varying constant amplitude cycles or stresses but also to
complete stress spectrum. Using test results and curve fitting technique, S-N
relationship can be obtained. Under normal service conditions, Railway
bridge structures are subjected to spectrum of varying stress – amplitude
and therefore, a process of damage accumulation continues. The fatigue
damage depends on the combined effect of the frequencies of different
stress ranges, likely to be observed by the structure under service loading.
Referring to Palm-gren Miner’s theory of linear damage accumulation , the total
damage to the structure is given by D= ∑ ni/Ni.
Where , ni = Observed number of stress cycles for different stress
ranges
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