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Re: Very Old Timber Trusses

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Rick and Nels,
Thanks for the response.  I'll address each one of your posts individually.
Rick,
Due to the age of these trusses and because other firms have decided to not go
out on a limb, I analyzed the truss that supports both the high and low grids
and assumed that at worst case (certainly not realistic, and my letter to the
theater will state specific loading criteria) this truss would support the
maximum tributary load acceptable on both grids simultaneously, 17,000#, along
with a snow drift load, and a slate roof that doesn't exist anymore.  After
going to the site this morning and talking with the theater's chief carpenter,
I think your point on the impact load point was very interesting.  Since I had
assumed the load to be a static load (that the pin rail held the weight down,
when in reality, it's more of a secondary support), and discovered that the
counter-weight system currently used really doesn't support that theory, I
began to think, "Ah, this is what I'm leaving out of the equation."  However,
in response to your second point, if I'm to believe what their carpenter and
the theater manager have said, that there was at least one show where every
line on the high grid was fully loaded (150# per line, approximately 15,000#
lbs.) and didn't overstress the structure, adding extra lateral support to what
appears to be the weak links will give a factor of safety to the whole system
that it didn't have before.  I realize I sound like I'm being defensive for
dwelling on this, (granted, none of you have seen any of these calculations,
past or present, to give anything more than general advice, but it is helpful
nonetheless) but, some of these other reports have specifically said that
additional lateral bracing was a controlling factor in the capacity.  At the
risk of making Nels feel he was neglected, I'll turn to him now.  I agree you
with on the point about the connections.  I couldn't find one check or split in
any of the original members of the truss I've been discussing.  There are
others on the low grid that are suspect, but I don't want to open a can of
worms.  The truss I'm considering at present has had extensive steel straps,
channels, plates thru-bolted to it over the years.  The bottom chord is
reinforced by a continuous steel plate each side over its entire length.  The
only connections that are suspect are ones which were reinforced with steel
rods, although my analysis indicates the members were compression members (the
rods were loose, consequently).  The aforementioned carpenter mentioned that
one truss vertical member showed signs of movement, from the paint lines.  This
member was added in 1970 and, considering the first thing out of my boss's
mouth regarding this was "shrinkage", and tales (perhaps tall, but they sounded
pretty believable, wait is this a trap?) of early 1800's construction where
timber trusses were allowed to season in place before they were covered with
the permanent roof, I didn't think this was anything that couldn't be fixed
with some steel plates and some thru-bolts.  Anyway, by now you should be
thoroughly exhausted with this and wished you had never replied.  If anyone has
any futher comments or suggestions, let loose.  I will welcome any information
that might help.

Thanks again,
Mark Nowmos
"Nels Roselund, SE" wrote:

>
> Mark,
>
> The Wood Handbook does not seem to address the differences in strength
> between modern lumber and lumber from the old uncut forests.  I would get a
> copy of the grading rules for the species that you have and grade it
> visually myself, or else hire a Grader.
>
> The old lumber often (but not always) looks much better than modern
> lumber -- old, really good looking lumber (straight grain, small or no
> knots, no splits) could probably be graded as Dense Select Structural or
> whatever the Rules call the highest quality timbers.  On the other hand, the
> rules for slope of grain used to be less strict for some species.  I've seen
> old trusses damaged in a way that looked like a sort of tension failure
> across steeply sloped grain in a bottom chord.
>
> The connections are likely to be the weak links in an old wood truss -- be
> sure to check them all.
>
> Nels Roselund
> Structural Engineer
>
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