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Re: Theatre Roof

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> From: Daryl Richardson <h.d.richardson(--nospam--at)>

>         I'm designing a theatre which will have exposed steel trusses in
> the order of 20' centres radiating from the general area of the stage.
> The span at the centreline ridge is 82'; there are four additional

> 1.)  Can I design the truss verticals and the purlins as a rigid frame
> "T" to provide support for the bottom chord via this frame action?


> 2.)  Can I frame a "knee brace" from the mid height of the truss
> vertical to purlins on one or both sides?  I have done this before for

Yes. This is a minor variation on x-bracing between joists. Watch
bending stiffness in the vertical.

> Neither the AISC or the CISC codes address this problem directly (at

Stability bracing. Lateral-torsional. See Yura/Hellwig and the SSRC
Guide to Stability Design Criteria for Metal Structures. Note that their
recommendations are conservative. It depends on how much time you want
to spend to optimize the design.

>         I have designed triangular trusses with two separate top chords
> and one bottom chord before and I could do it again, however, the
> changes in slope complicate the construction and it may not look too
> good.

Sounds like an inverted bridge truss. Pony Truss? The knee-brace
approach will probably be more material/cost efficient but the T may be
more appealing to the architect. The bottom chord rotational restraint
will be a function of purlin bending stiffness.

Don't forget to resolve the truss lateral forces between stabilizers
(e.g. roof diaphragm or other bracing).

Paul Ransom, P. Eng.
Burlington, Ontario, Canada
<mailto:ad026(--nospam--at)> <>

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