I want to ask about the length that I should take to calculate the slenderness of the top chord since it's in compression and only restrained at the bottom chord.

I've tried assuming that the vertical truss member as a column that supports the top chord. Since the capacity of the vertical member is adequate, can I assume that the top chord is laterally restrained at the same point?

Lets say the restraint between support is 5m (max) for bottom chord, can I take the length for the top chord as 5m as well?

As for the corner where it is supported by perpendicular truss member, can I take the condition as laterally and torsionally restrained at support and torsionally restrained at tip? I used the condition from BS5950 Table 14 - Effective length for cantilevers without intermediate restraint.

The answer I would give to all your questions is that I wouldn't.  

You need to have members placed orthoginally to the axis of the top chord that have a horizintal component to their placement, otherwise, there is no lateral support.  

Or, the top chord member has to be large enough to span the full distance from end to end without buckling.

Mike McCann
MMC Engineering

 

Or you can step on the stressed skin assumption in case there is some trapezoidal sheeting over the top chord.

whken,

I would say you definately could rely on the vertical web members to support the top chord of a truss as long as they have sufficient capacity and stiffness to resist the out of plane forces.

This method has been used on hundreds of truss and plate girder bridges throughout the UK.

Just make sure that an accidental impact force will not overlaod it and lead to disproportionate collapse

(Some of this is rather elementary, and is not intended to imply that you don't know it, it was just part of me getting to the conclusion.)
If the top chord is straight and in compression, it will bend laterally at some load if not restrained.  For a pin-connected truss, the member would need to be designed for an unbraced length equal to the effective compression length of the chord, for the axis which is free to move.  This may be longer or shorter than the physical member.

Where the continuous truss has intermediate support, the top chord force varies from tension to compression, but the structure is indeterminant and this whole scenario goes out the window as live loads move across the truss.

I recommend running appropriate models, making sure to model the joints as fixed, since this design looks like it will be functioning as a hybrid vierendeel truss, so typical truss rules are not effective.  This will tell you what length, if any, is in compression. [Intuitively, I think that with only self-weight, the bottom chord will be in compression at the supports (B', C', 3, 4?) and the top chord might have a little compression near midspan with live loads applied, if I'm looking at it correctly.]

The vertical members appear to restrain local twisting about the top chord member's axis, which would prevent LTB.  But you can only use the vertical member to resist a lateral-only buckling mode to the extent that the structure will resist lateral movement of the chord member.

The degree of lateral restraint provided by the vertical member would be a function of the ability of any torsion-resisting connection at the bottom chord to resist forces presented at the top chord.

Once you determine the force required to resist lateral buckling of the top chord, model the cross section of the assembly to see what happens when the force is applied at the top chord. Essentially, place a sidewards force, equal to the buckling force, onto BOTH top chords, and check inward, outward, and one of each simultaneously and individually on the chords at the same location.

You need to verify that the stiffness of the structure provides sufficient restraint, including 2nd order effects.