Pony truss bridge top chord capacity

[VoyageofDiscovery]

Does anyone have any information to find the critical buckling capacity of this top chord using elastic supports as a function of vertical and floorbeam stiffness?

VOD

 

[bridgebuster]

Here's what the AASHTO "Guide Specifcations for Design of Pedestrian Bridges" says:

1.3.6 Half-Through Truss Spans

1.3.6.1 The vertical truss members and the floorbeams and their connections in half-through truss spans shall be proportioned to resist a lateral force applied at the top of the truss verticals that is not less than 0.01K times the average design compressive force in the two adjacent top chord members where K is the design effective length factor for the individual top chord members supported between the truss verticals. In no case shall the value for 0.01K be less than 0.003 when determining the minimum lateral force, regardless of the K-value used to determine the compressive capacity of the top chord. This lateral force shall be applied concurrently with these members’ primary forces.

End posts shall be designed as a simple cantilever to cany its applied axial load combined with a lateral load of 1.0% of the axial load, applied at the upper end.

1.3.6.2 The top chord shall be considered as a column with elastic lateral supports at the panel points. The critical buckling force of the column so determined shall be based on using not less than 2.0 times the maximum design group loading in any panel in the top chord.*

*For a discussion of half-through truss designs, refer to Galambos, Guide to stability Design Criteria for Metal Structures, 4th ed., l9û8, New York: John Wdey and Sons,
Inc., pp. 515-529.

Commentary
1.3.6 Half-Through Truss Spans

This article modifies the provisions of AASHTO Article 10.16.12.1 by replacing the 300 pounds per linear foot design requirement for truss verticals with provisions based on research by Holt and others. These provisions establish the minimum lateral strength of the verticals based on the degree of elastic lateral support necessary for the top chord to resist the maximum design compressive force. The use of 2.0 times the maximum top chord design load to determine the critical buckling force in the top chord is in recognition that under maximum uniform loads, maximum compressive stresses in the top chord may
occur simultaneously over many consecutive panels.