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Re: Very Old Timber Trusses

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Mark,
It is impossible to say if the other engineers were being ultra
conservative. From your post, it sounds like you have access to the
original calculations, so you should be able to see where you differ
with them.  However, here are some things you might want to consider:

1. Impact factor. A very conservative loading condition is produced when
all the linesets, or battens, in a counterweight rigging system are
assumed loaded to full capacity, and then all these loads are increased
by an impact factor.  Impact is due to moving loads, of course, and in a
rigging system only a small number of battens are moving at one time.

2. All of the linesets in a counterweight rigging system will never be
loaded to full capacity simultaneously. Some will carry lighting, some
will carry scenery, some will carry curtains, and some will probably be
unloaded. The average load that the structure sees is some fraction of
the fully loaded condition. I doubt that your theater people actually
overloaded ALL the linesets simultaneously. More likely, they overloaded
a few of them, but others were probably underloaded at the time.  This
is sort of like the case of the layman who doesn't understand why a 100
psf floor live load isn't exceeded when he stands with his 200 pounds on
a one square foot area.

Rick Burch


Mark Nowmos wrote:
> 
> Nels,
> Thanks for the help.  I guess one thing I'm curious about is if the Wood
> Handbook you mentioned makes any note of old-growth wood species and the higher
> strength they possess, which I'm assuming is what I'm dealing with.  My
> grammatical error reminds of the Churchill quote:  "There are some
> things up
> with which I will not put."  Anyway, I'll give you a little more
> background in
> the hopes that it will better illustrate my concerns:
> This theater was renovated in 1970 and since then, 3 engineering firms issued
> reports / calculations regarding the maximum capacity of the gridirons (rigging
> support floors).  Each one seemed to concur with the previous report and one
> stated that the maximum load should be decreased for an additional
> factor of
> safety.  The original calculations made during the renovation had some fairly
> conservative assumptions (I don't blame him), but since then, these trusses
> have been reinforced en masse with steel channels and plates.
> Connections were
> beefed up with steel and additional web members were added.  From my
> preliminary analysis using the anticipated higher loads, I am not getting
> stresses that are dangerously large.  Granted, I have not visually
> checked all
> the connections, yet, but my question is, does anyone think these other firms
> were being ultra-conservative?  Or am I inviting catastrophe.  Some of the
> theater people have told me that they have exceeded the recommended maximum
> load on occasion, while chugging bottles of Maalox, but I just thought
> this was
> due to a bad cheesesteak from the food trucks.  To me, it just seems
> that all
> that is needed is some additional lateral bracing for the trusses and gridiron
> framing, and possibly some beefing up of some connections.  My general thought
> is, since these other firms didn't want to deal with this potential liability,
> am I missing something here?  I could go on, but I won't - you all have your
> own problems to deal with, although any comments on mine would be much
> appreciated, again.
> 
> Mark Nowmos
>

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