Civil Engineering Reference
In-Depth Information
contact must be adhered to during fabrication.
∗
Design and shop drawings should
indicate all FCM since these members require specific material (see Chapter 2)
and fabrication considerations. Fabricators must make joints and connections with
high-strength steel bolts in accordance with ASTM A325 or A490
†
—Standard Spec-
ifications for Structural Bolts or welds in accordance with ANSI/AASHTO/AWS
D1.5—Bridge Welding Code. Steel freight railway bridges are generally designed
with slip critical connections and pretensioning is required for bolt installation (see
Chapter 9). Bolts should be installed with a minimum tension
‡
by turn-of-nut,
tension-control bolt, direct-tension-indicator, or calibrated wrench tensioning.
Welding procedures, preparation, workmanship, qualification, and inspection
requirements for steel railway bridges (dynamically loaded structures) should con-
form to the ANSI/AASHTO/AWS D1.5—Bridge Welding Code. In particular, for
FCM, additional provisions concerning welding processes, procedures, and inspec-
tion merit careful attention during fabrication.
§
Welding procedures typically used
for steel railway bridges are SMAW, SAW, and flux cored arc welding (FCAW) (see
Chapter 9). Railroad companies often prescribe limitations concerning acceptable
welding procedures for superstructure fabrication.
Steel railway bridge fabrication, particularly for FCM, should be accompanied by
testing of materials, fastenings, and welding. Material mill certifications should be
reviewed to confirm material properties such as ductility, strength, fracture tough-
ness, corrosion resistance, and weldability. Bolted joints and connections should be
inspected by turn, tension, and torque tests to substantiate adequate joint strength.
Quality assurance inspection of welding procedures, equipment, welder qualifica-
tion, and nondestructive testing (NDT) is also required to validate the fabrication.
NDT of welds is performed by magnetic particle testing (MPT), ultrasonic testing
(UT), and/or radiographic testing (RT) by qualified personnel. Railroad companies
often have specific criteria regarding the testing of fillet, complete joint penetration
(CJP), or partial joint penetration (PJP) welds.
Steel bridges fabricated with modern atmospheric corrosion resistant (weathering)
steels are often not coated, with the exception of specific areas that may be galvanized,
metallized, and/or painted for localized corrosion protection.
∗∗
Where required, mod-
ern multiple coat painting systems are used for steel railway bridge protection and
many railroads have developed their own cleaning and painting guidelines or speci-
fications. Many modern steel railway bridges are protected with a three-coat system
consisting of a zinc rich primer, epoxy intermediate coat, and polyurethane topcoat.
Recommendations related to the fabrication of steel freight railway bridges are
included inAREMA (2008).
††
Engineers should consult with experienced fabricators
∗
These tolerances are outlined in AREMA (2008, Chapter 15, Part 3).
†
ASTM A490 bolts are sometimes discouraged or prohibited by bridge owners due to brittleness
concerns.
‡
For example, AREMA (2008) recommends a minimum tension force of 39,000 lb for 7/8 in diameter
A325 bolts.
§
FCPs are usually specified to ensure that FCM fabrication is performed in accordance with the
additional requirements indicated by AREMA (2008) and ANSI/AASHTO/AWS D1.5.
∗∗
For example, at bearing areas or top flanges of open deck spans.
††
Recommended practices for the fabrication of steel railway bridges are outlined in AREMA (2008,
Chapter 15, Part 3).
