Civil Engineering Reference
In-Depth Information
• Rolled or welded beams for spans up to about 50 ft in length (cover plates
may increase strength to reduce span and/or depth of construction) (often
used in floor systems of through plate girder and truss spans)
• Bolted or welded plate girders for spans between 50 and 150 ft
• Bolted or welded trusses for spans between 150 and 400 ft
∗
Steel freight railway bridge girder spans can be economically designed with a mini-
mum depth to span ratio of about 1/15. Typically, depth to span ratios in the range of
1/10 to 1/12 are appropriate for modern short- and medium-span steel girder freight
railway bridges. Beam, truss, and arch railway bridges can be constructed with open
or closed (i.e., ballasted) decks.
3.3.2.1
Bridge Decks for Steel Railway Bridges
3.3.2.1.1 Railway Track on Bridge Decks
Rails with elastic fasteners seated on steel tie plates fastened to wooden ties or
embedded in prestressed concrete ties are typical of modern North American rail-
road tracks. Steel ties have also been used and preclude the need for steel tie plates.
†
On ballasted deck bridges the wood, steel, or concrete ties are bedded in compacted
granular rock ballast for drainage and track stability. Concrete ties may require damp-
ing devices, such as rubber pads applied to the bottom of ties with adhesive (Akhtar
et al., 2006), on ballasted decks and are generally discouraged by railroads for use
in open deck applications. Wooden ties are used in both open and closed or ballasted
deck construction.
3.3.2.1.2 Open Bridge Decks
Open decks using wooden ties
‡
are still used in many instances on modern railway
bridges. Open deck bridges are often used in situations where new superstructures are
being erected on existing substructures where it is necessary to reduce dead weight
to preclude substructure overloading and foundation creep. On open bridge decks the
ties are directly supported on steel structural elements (i.e., stringers, beams, girders)
(
Figures 3.13a
and b). Dead load is relatively small, but dynamic amplification of live
loadmaybeincreasedbecausethetrackmodulusisdiscontinuous.Bridgetiesizescan
be large for supporting elements spaced far apart and careful consideration needs to be
given to the deck fastening systems. Most railroads have open bridge deck standards
based on the design criteria recommended by AREMA (2008, Chapter 7—Timber
Structures).
Open bridge decks are often the least costly deck system and are free draining.
However, they generally require more maintenance during the deck service life. Con-
tinuous welded rail (CWR) on long span bridges can create differential movements
causing damage to, and skewing of, open bridge decks (see Chapter 4).
∗
Based on the upper limit of span length that AREMA (2008, Chapter 15) recommendations consider.
†
Steel ties may also allow for a reduction in ballast depth on bridges.
‡
Steel, concrete, and composite ties may also be used but, due to their relatively large stiffness, may
require a detailed analysis of structural behavior. Wooden ties may be used in accordance with the
recommendations outlined in AREMA (2008, Chapter 7—Timber Structures).
