Pony Truss Bridge First Mode of Buckling

[SteynvW]

Hi All

When doing a elastic buckling analysis of a pony truss my second mode of buckling
is the expected sinusoidal shape as expected. I could then back calculate the effective length
and design the top chord appropriately.

However, my first mode of buckling causes my structure to rotate about the longitudinal axis.

Is this because the top chords buckling asymmetrically? And cause rotation and this buckling factor should then be used?

 

[Settingsun]

That looks realistic. I suggest you also put small loads at top chord midspan to simulate an imperfection causing them to buckle in opposite directions over the full length. Buckling the same way as in your first mode puts the deck member in reverse curvature. Chords buckling in opposite directions puts the deck in single curvature and is equally likely but an ideal computer model mightn't find this mode.

Edit: Both notional loads acting inwards to match gravity load deflections (I think is critical).

 

[saplanti]

I suggest you to understand the behaviour of the Pony Truss, first you increase the bottom two cord cross section areas to higher value, and top two cord lateral stiffnesses to higher values. Compare with the images above with the new one.

Similar can be said for the trusses on sides. The capacity of the side truss can be improved by increasing the cord member cross section areas in addition to the increase in the lateral stiffness of upper cord member. Increasing the stiffness of upper cord member will provide more resistance against lateral torsional buckling of the side truss. To increase this resistance some designers put additional bracing system between upper cord members.

Playing with area and stiffnesses you can reach the buckling mode that you are after.

 

[saplanti]

Am I seeing right? Last two bays do not have bracing neither on the floor (the last bay only) nor on the side trusses. Why is that?

 

[SteynvW]

SteveH

Thanks for the suggestion.

Saplanti

Unfortunately I cant put bracing between the top chord members (it is a pedestrian bridge), the last to bays do have bracing.
I see they were just turned off when I took the screen shot. Thanks for noticing. I will play around more with different sections.