Planning and Preliminary Design of Modern Railway Bridges - Design of Modern Steel Railway Bridges - page 62

materials at piers can be estimated as
d l = 2 yK 1P K 2P K 3P b pier
y

0.65

F 0.43 . (3.15)

Local scour depth for cohesionless materials at abutments can be estimated as
(Richardson and Davis, 1995)

4 y K 1 A
0.55

K 2A F 0.33 ,

d l = (3.16)

where K 1P is the pier nose geometry adjustment factor (from experimental values),
K 2P is the angle of the flow adjustment factor (from experimental values), K 3P is the
bed configuration (dune presence) adjustment factor (from experimental values), K 1A
is the abutment type (vertical, sloped, wingwalls) adjustment factor (from experimen-
tal values), K 2A is the abutment skew adjustment factor (from experimental values),
y is the depth at pier or abutment, and F is the Froude number calculated at pier
or abutment.

3.2.3.2.2.3 Total Scour The total scour depth, d t , is then estimated as

d t = d g + d l = d d + d c + d l , (3.17)

where d g is the general scour, d d is the degradation depth, d c is the contraction scour
(under live-bed or clear-water conditions), and d l is the local scour at substructure
(abutment or pier).
In some cases, substructure depth must also be designed anticipating extreme
natural scour and channel degradation events.Therefore, for ordinary railway bridges,
spread footings or the base of pile caps are often located such that the underside of
the footing or cap is 5-6 ft below the estimated total scour depth, d t . Also, it is often
beneficial to consider the use of fewer long piles than a greater number of short piles
when the risk of foundation scour is relatively great.

3.2.3.3 Highway, Railway, and Marine Clearances

Railway bridges crossing over transportation corridors must provide adequate hor-
izontal and vertical clearance to ensure the safe passage of traffic under the
bridge. Railway and highway clearances are prescribed by States and navigable
waterway clearance requirements are the responsibility of the United States Coast
Guard (USCG). Provision for changes in elevation of the under-crossing (i.e., a
highway or track rise) and widening should be considered during planning and
preliminary design.
The minimum railway bridge clearance envelope recommended by AREMA
(2008) is generally 23 ft from the top of the rail and 9 ft each side of the track center-
line.AREMA (2008, Chapters 15 and 28) outline detailed clearance requirements for
railway bridges. These dimensions must be revised to properly accommodate track
curvature.