Buckling of laterally unrestrained truss member 1

[N G P]

I have been reviewing an aluminium pedestrian bridge structure of 20m span and a width of 1.7m. The structure is composed of two warren type trusses and a platform for the movement (floor beams) that is connected to the truss's bottom chords as shown in the figure. The top chords are not connected and not restrained in the lateral direction. I am confused about the effective length which needs to be considered for the design of the top chord member in terms of buckling. The struts of the trusses are spaced at 1.5m and the sections used are box sections (SHS 160x5.0). I would like to get your valuable advice.

Regards!

 

[Settingsun]

This is called a 'through-truss' or 'pony truss'. Searching for those terms should get you good information. The top chord is restrained by the stiffness of the deck and the vertical truss members acting as a frame.

My preference is to model the 3D structure and do an elastic buckling analysis. That will either (depending on the software) give you the effective length, or buckling load which you use to calculate the effective length. Then feed that into your design code's calculation for member capacity.

The US aluminium design manual used to give a hand method up to the 2005 version, but it appears to have been deleted in 2010 when the code format changed. Similar methods are in some bridge codes, eg BS5400 and the Australian code. It doesn't matter that those are steel codes as it's simply an elastic stiffness calculation. Just use the lower aluminium stiffness in the equations.

 

[Settingsun]

I remembered the hand methods wrong. They're for beams but the principle is still there. Here's the Australian version.