Limit State Design and Lateral Buckling Analysis of the Steel Pony Truss Bridge

The design of through truss bridges was popular for its economical material use and ease of construction before World War II. However, this type of bridge loses its competitive advantage when compared to concrete box girder bridges, prestressed concrete girder bridges, and steel plate girder bridges because of the considerable amount complexity required in both the design and fabrication processes of the through truss bridge. A steel pony truss bridge becomes an attractive alternative only if the aesthetics criteria prevail. The majority of bridge engineers practicing their expertise today are rarely exposed to the design and analysis of through truss bridges. The out-of-plane buckling of the compression chord governing the strength of a pony truss bridge presents a design challenge. The current AASHTO specifications do not specifically address a direct design guideline. To overcome the design problem, an energy approach using the analogy of the buckling of a bar on an elastic foundation under distributed axial loads is proposed to analyze the upper chord of a pony truss bridge. The energy approach results in a satisfactory design on the new downtown Kirkwood, Missouri. There are thousands of pony truss bridges that were built half a century ago. Many of these bridges still serve the public as important traffic carriers on secondary highways and countryside roads around the world today. It is the intent of the author to present the unique design approach applying the theory of lateral stability to a pony truss bridge. Illustrated in this paper is the development of an optimized of design methodology of these structures' elements through the implementation of a spreadsheet to perform several iteration processes. It is also anticipated that the methodology developed herein can also be used to calculate the demand/capacity ratio to be used for rating through truss bridges from the bridge maintenance and repair standpoint.

• Abstract

• Background information

• The new bridge

• Design issues

• Engineering solutions

• The pony truss bridge design sequences

• A numerical design example

• Limits and constraints to applying the theory

• Conclusion

• References