Loads and Forces on Steel Railway Bridges - Design of Modern Steel Railway Bridges - page 117

a single moving lateral force of 25% of the heaviest axle of the Cooper's E80 load,
a notional load of 200 lb/ft applied to the loaded chord or flange and 150 lb/ft on the
unloaded chord or flange is recommended.

4.3.3 D ISTRIBUTION OF L IVE L OAD
Unlike highway loads, which may move laterally across the bridge deck, railway live
loads are generally fixed in lateral position. However, they are a longitudinal series of
large magnitude concentrated wheel loads, and longitudinal and lateral distribution
to the deck and supporting members must be considered.

4.3.3.1 Distribution of Live Load for Open Deck Steel Bridges

For open deck bridges, no longitudinal distribution is made and lateral distribution
to supporting members is based on span cross-section geometry and type of lateral
bracing system. Lateral bracing between longitudinal beams should be made with
cross frames, or for spans with shallow beams, rolled beams, and/or close beam
spacing, solid diaphragms. ∗ The cross frames and diaphragms should not have a
spacing exceeding 18 ft. In some cases,AREMA (2008) recommends that diaphragms
and cross bracing be fastened to the beam or girder flanges. When the lateral bracing
systemmeetsthesecriteriaandisproperlydesignedforthelateralforces(seeChapters
5 through 7), all beams or girders supporting the track are considered as equally
loaded.

4.3.3.2 Distribution of Live Load for Ballasted Deck Steel Bridges

For ballasted deck bridges, longitudinal and lateral distribution of live load to the
deck is based on tests performed by the Association of American Railroads (AAR)
(Sanders and Munse, 1969). Axle loads are distributed over a given width at a 2:1
( V / H ratio) distribution through ballast rock and the deck material, as shown in
Figure 4.17.
The longitudinal deck distribution width, ( 3 + d b ) , should not exceed either 5 ft
or the minimum axle spacing of the design load. The lateral deck distribution width,
(Length of the tie

d b ), should not exceed 14 ft or the distance between adjacent
track centerlines or the width of the deck.
The longitudinal distribution of live load to members supporting the deck in the
transversedirection (Figure4.18) isgivenintermsofaneffectivebeamspacing,which
is dependent on deck material, beam span, and spacing, and for concrete decks, the
stiffness of beams and deck, and the width of the deck.

+

1.15 AD
S

P = , (4.33)

∗ Channels and coped flange wide flange shapes are often used for diaphragms between longitudinal
beams. Plates are generally not used due to the absence of flanges and low bending strength.