Chord Buckling in an Oval Truss

[yxc]

I'm designing a 2.5 m high oval shaped truss (see attached). The top and bottom chords of the truss are radiused W-sections, with the strong axis oriented vertically. The bottom chord is braced along it's entire length, the top chord is not braced at all. In some places the top chord is in compression, and I'm concerned about buckling. How do you check for buckling of the top chord?

Thanks!

 

[hokie66]

Aside from the buckling check, why not turn the top chord with the strong axis horizonally? That way, when the chord wants to buckle, it will be bending and buckling about the same axis.

 

[grantstructure]

In the past, I have checked buckling of pony trusses by providing some fixity at the base of the vertical web members, and counting on them to cantilever up to brace the top chord. You then have a discrete brace system where the stiffness of the cantilevered web member is critical, including the rotational stiffness of the base fixity of those members.

http://www.scribd.com/doc/63965314/Fundamentals-of-Beam-Bracing-1

The link is to Yura's Bracing for Stability, which I think has the equation you'll need to determine the required brace stiffness and strength. You do also need to check torsional buckling of your top chord since the restraint to that mode will be a matter of how you choose to detail it.

The fact that you have an oval certainly helps but I have no idea how to make effective use of that contribution.

 

[yxc]

@ hokie66 I agree it probably makes sense to align the strong axis horizontally.

@ grantstructure
We wanted to decouple the truss from the members framing into the bottom chord. This is mainly for deflection compatibility. Providing a fixed connection would add a large torsional force to the bottom chord.

I would also be open to any suggestions for modelling this behaviour in SAP. I'm not sure if it's possible to account for this buckling type failure.

 

[BAretired]

I agree with hokie and grantstructure. The top chord should be oriented with the web horizontal and there should be lateral and rotational bracing at each vertical member. I do not believe this would add a "large torsional force to the bottom chord".

BA

 

[paddingtongreen]

I agree on rotating the top chord, also that there will be only small torsional moments because the truss has little resistance to rotation.

I question having single diagonals all facing the same way. The top chord ring will try to rotate, relative to the bottom chord, about the vertical axis of the structure. If not circular, this would twist the trusses.

Michael.
Timing has a lot to do with the outcome of a rain dance.