Structural Analysis and Design of Steel Railway Bridges - Design of Modern Steel Railway Bridges - page 222

members in trusses should be proportioned for the force transmitted, but should not
be less than 0.50 in thick (see Chapter 9).
Wherecomponentsaresubjecttocorrosiveconditions,theyshouldbemadethicker
than otherwise required (as determined by judgment of the design engineer) or pro-
tected against corrosion by painting or metallic coating (usually zinc or aluminum).
Atmospheric corrosion resistant (weathering) steel (see Chapter 2) does not provide
protection against corrosion by standing water, and/or often wet or corrosive environ-
ments. Therefore, the design engineer should also carefully consider drainage holes
and deck drainage in the design of a bridge.
5.3.2.3.3 Camber
Camber is a serviceability-related criterion. AREMA (2008) recommends that plate
girder spans in excess of 90 ft long be cambered for dead load deflection. Trusses are
recommended for greater camber based on dead load deflection plus the deflection
from a uniform live load of 3000 lb per track foot at each panel point. ∗
5.3.2.3.4 Web Members in Trusses
AREMA (2008) recommends that truss web members and their connections be
designed for the live load that increases the total stress by 33% over the design
stress in the most highly stressed chord member of the truss. This live load ensures
that web members attain their safe capacity at about the same increased live load
as other truss members due to the observation that, in steel railway trusses, the web
members reach capacity prior to other members in the truss (Hardesty, 1935). This
recommendation is reflected by design load casesA2 and B2 inTable 4.5 of Chapter 4.
An example of the calculation is shown in Example 6.4 of Chapter 6. In that example,
the increase in gross area, A g , and effective net area, A e , are less than 1.5% based
on this recommended design load case. In parametric studies of some recent railway
bridge designs the effect was similar (Conway, 2003). In any case, when tensile stress
ranges are present, fatigue criteria often governs truss web member design (also see
Example 6.4).
REFERENCES
American Railway Engineering and Maintenance-of-Way Association (AREMA), 2008, Steel
structures, in Manual for Railway Engineering , Chapter 15, Lanham, MD.
Anderson T.L., 2005, Fracture Mechanics , 3rd Edition, Taylor & Francis, Boca Raton, FL.
Armenakas, A.E., 1988, Classical Structural Analysis , McGraw-Hill, NewYork.
Barsom, J.M. and Rolfe, S.T., 1987, Fatigue and Fracture Control in Steel Structures , 2nd
Edition, Prentice-Hall, Englewood Cliffs, NJ.
Basquin, O.H., 1910, Exponential law of endurance tests, Proceedings, American Society for
Testing Materials, Vol. 10, Part 2, ASTM, West Conshohocken, PA.
Bleich, F., 1952, Buckling Strength of Metal Structures , 4th Edition, McGraw-Hill, NewYork.
Christy, C.T., 2006, Engineering with the Spreadsheet , ASCE Press, Reston, VA.
∗ This can be accomplished during fabrication by vertically offsetting truss joints by changing the length
of the truss members.