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Theatre Roof

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Fellow engineers,

        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
trusses of lessor spans, all of which which slope gently downwards
toward the stage.  To complicate things the roof slope changes, becoming
less steep at each truss.

        For aesthetic reasons bottom chord bridging is undesirable.

        My question(s):

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
space frame structures like oil derricks but not for roof structures.

Neither the AISC or the CISC codes address this problem directly (at
lease I can't find the reference) so this will be a design from first
principals.  This does not bother me but there may be other issues I am
not considering.

        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

        Any thoughts any of you might have will be appreciated.


H. Daryl Richardson

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