The Design of Modern Steel Bridges - TEDI

72 The Design of Modern Steel Bridges
forces. The load combinations and the partial load factors for the ultimate limit
state are given in Table 3.8.
The permanent loads and their factors are not mentioned in the table;
these are:
1.05 for dead load of steelwork
1.15 for concrete
1.75 for superimposed dead load, but may be reduced to 1.20
1.50 for earth pressure
1.20 for creep and shrinkage of concrete.
The different design parts of the British Standard BS 5400 allow the effects of
differential temperature, differential settlement and creep and shrinkage effects
of concrete to be ignored in the ultimate limit state in those types of structural
design that are capable of redistributing the self-equilibrating internal stresses
set up by these effects. Snow load, loading due to stream current and floating
ice, earthquake and shipping collision forces are not included in the specified
load combinations, as few bridges in the British Isles are subjected to these
loadings; where such loading is likely special load combinations and partial load
factors are adopted.
In the AASHTO specification[5], nine load combinations are specified. Each
load is multiplied by one g-factor particular for the load combination under
consideration and one b-factor which varies for different loads in the same
combination. Thus the total load effect in combination N is given by
X
ðbFNFÞ
g N
Table 3.8 Load combinations and partial load factors for ultimate limit state in
British Standard BS 5400
Combination LL 1
LF CF CL W T F
1 1.5 – – – – – –
2 (a) – – – – 1.4 – –
(b) 1.25 – – – 1.1 – –
3 1.25 – – – – 1.3 2
–
4 (a) 1.5 3
– 1.5 – – – –
(b) 1.25 3
1.25 – – – – –
(c) 1.25 3
– – 1.25 – – –
5 – – – – – 1.3
Load combination in British Standard BS 5400: LL ¼ live load, LF ¼ longitudinal force,
CF ¼ centrifugal force, CL ¼ collision force, W ¼ wind loading, T ¼ temperature effects, F ¼ bearing
friction.
1
Factors for HA loading.
2
Load factor for differential temperature effects is 1.0.
3
These factors are used on reduced live loading.