Civil Engineering Reference
In-Depth Information
heightsmightberequiredatsomecrossings. Handrailsandpostsconsistingoftubular,
pipe, or angle sections are often used for railway bridge guardrails where safety with-
out the need to consider aesthetics is acceptable. The designer should consult with the
railroad company and applicable regulations concerning specific safety appliances
that are required.
3.3.5 G
ENERAL
D
ESIGN
C
RITERIA
In North America, elastic structural analysis is used for freight railway steel bridge
design based on the allowable stress design (ASD) methods of AREMA (2008).
∗
The AREMA (2008) design criteria outline has recommended practices relating to
materials, type of construction, loads, strength, serviceability, and fatigue design of
steel railway bridges.
Dynamic amplification of live load (commonly referred to as impact) is very
large in freight railway structures (see Chapter 4). Serviceability criteria (vibrations,
deflections) and fatigue are important aspects of steel railway bridge design. Railway
equipment, such as long unit trains (some with up to 150 cars), can create a signifi-
cant number of stress cycles on busy rail lines, particularly on bridge members with
relatively small influence lines (see Chapter 5). Railroads may limit span deflections
based on operating conditions. Welded connections and other fatigue-prone details
should be avoided in the high-magnitude cyclical live load stress range regime of
freight railroad bridges (see Chapters 5, 6, 7, and 9).
AREMA (2008) recommends a performance-based approach to seismic design.
Steel freight railway bridges have performed well in seismic events due to the type of
construction usually employed.Typically, steel freight railway bridges have relatively
light superstructures, stiff substructures, large bridge seat dimensions, and substantial
bracingandanchorbolts(usedtoresisttheconsiderablelongitudinalandlateralforces
associated with train operations). In general, steel railway bridges have suffered little
damage or displacement during many recent earthquake events (Byers, 2006).
3.3.6 F
ABRICATION
C
ONSIDERATIONS
Thesteelfabricationprocesscommenceswithshopdrawingsproducedbythefabrica-
tor from engineering design drawings. The approved shop drawings are then used for
cutting, drilling, punching, bolting, bending, welding, surface finishing, and assembly
processes in the shop. Tolerances from dimensions on engineering drawings concern-
ing straightness, length, cross section, connection geometry, clearances, and surface
∗
Recommended practices for the design of railway bridges are developed and maintained by theAREMA.
Recommended practices for the design of fixed railway bridges are outlined in Part 1—Design and those
for the design of movable railway bridges are outlined in Part 6—Movable Bridges, in Chapter 15—Steel
Structures, of the AREMA MRE. Chapter 15—Steel Structures provides detailed recommendations for
the design of steel railway bridges for spans up to 400 ft in length, standard gage track (56.5
), and
North American freight and passenger equipment at speeds up to 79 and 90 mph, respectively. The
recommendations may be used for longer span bridges with supplemental requirements. Many railroad
companies establish steel railway bridge design criteria based on, and incorporating portions of these,
recommended practices.
