Loads and Forces on Steel Railway Bridges - Design of Modern Steel Railway Bridges - page 143

TABLE 4.5
Load Combinations for Steel Railway Superstructure Design
Load Case Load Combinations Members F L

DL + LL + I + CF A1 All members 1.00

A2 DL + LLT + I + CF Truss web members 1.33

B1 DL + LL + I + W + LF + N + CWR All members, except floorbeam hangers

and high strength bolts

1.25

B1A DL + LL + I + W + LF + N + CWR Floorbeam hangers and high strength

bolts

1.00

B2 DL + LLT + I + W + LF + N + CWR Truss web members, except floorbeam

hangers

1.66

C ( LL + I) range All members f fat

D1 SL + N + CF Members resisting overall instability 1.50

D2 Q Members resisting overall instability 1.50

E1 DL + EQ All members 1.50

E2 DL + LL + I + CF + EQ Members in long bridges only 1.50

F W or LV Members loaded by wind only 1.00

G DF Cross frames, diaphragms, anchor rods 1.50

H1 DL Members stressed during lifting or

jacking

1.50

H2 DL Members stressed during erection 1.25
H3 DL + W Members stressed during erection 1.33
F L = Allowable stress load factor (multiplier for basic allowable stresses), DL = Dead loads (self weight,
superimposed dead loads, erection loads) (see Section 4.2) , LL = Live loads (see Section 4.3.1) , I = Impact
(dynamicamplification)(see Section4.3.2) , CF = Centrifugalforce(see Section4.3.4) , W = Windforces(on

live load and bridge) (see Section 4.4.1) , LF = Longitudinal forces from equipment (braking and locomotive

traction) (see Section 4.3.2.2) , N = Lateral forces from equipment (nosing) (see Section 4.3.2.3) , CWR =

Forces from CWR (lateral and longitudinal) (see Section 4.4.3) , EQ Forces from earthquake (combined

transverse and longitudinal) (see Section 4.4.4) , DF = Lateral forces from out-of-plane bending and from
load distribution effects (see Section 4.4.5.1), LV = “Notional” lateral vibration load (see Section 4.4.2),
LLT = Live load that creates a total stress increase of 33% over the design stress (computed from load
combination A1) in the most highly stressed chord member of the truss. This load ensures that web members
attain their safe capacity at about the same increased live load as other truss members due to the observation
that in steel railway trusses, the web members reach capacity prior to other members in the truss. This live
load, LLT, is based on the requirements discussed in Chapter 5, Section 5.3.2.3.4, SL = Live load on leeward
track of 1200 lb/ft without impact, I (see Section 4.4.5.2) , Q = Derailment load, f fat = Allowable stress based
on member loaded length and fatigue detail category.

=

REFERENCES
American Railway Engineering Association (AREA), 1949, Test results on relation of impact
to speed, AREA Proceedings , 50, 432-443.
American Railway Engineering Association (AREA), 1960, Summary of tests on steel girder
spans, AREA Proceedings , 61, 151-178.
American Railway Engineering Association (AREA), 1966, Reduction of impact forces on
ballasted deck bridges, AREA Proceedings , 67, 699.