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help: bowstring truss modification (unbr

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Dennis P.

*If* the splice at the center of the span was/is high-strength bolted, I 
would assume that the truss had been salvaged from another location before 
being erected in the present location.  However, if the bolts are 
square-head, then it is probably just a field splice.

16.5 ft spacing seems to be a rather large spacing, particularly with using 
wood roof joists.  (18' 2 X 10's lapped 1'-6" at each truss?)

That said, the X-brace at the center of the span (each bay? alternate bays?) 
means that you have a 30' unbraced length of the bottom chord.  If conditions 
permit, I would look at running a member under the bottom chord and 
connecting the bottom chords of each truss.  In one or two bays, horizontal 
x-bracing to the plane of the vertical x-bracing should be adequate provided 
the vertical x-bracing is capable of taking the additional loading.

It should be remembered that an advantage of using a bowstring truss is that 
if the depth to span ratio is 1/8, then, under uniform load, there is a 
uniform force in both the top and bottom chords the full length of the truss 
and little, if any, forces in the web members.  Under unbalanced loading, the 
web members come into play.  While I would try to put the longitudinal member 
as close to a panel point as possible, I would not consider offsetting it 
slightly as being a critical consideration.  (If you had a 30' long column in 
a building, braced at 10' in one direction by floor framing, would you have 
any qualms about providing lateral bracing in the other direction that was 
not at the same elevations as the floor framing?)

With the truss being riveted, except at the center splice, the steel would 
probably be A7 [F(y) = 33ksi] which did not have a maximum carbon content 
requirement.  A7 steel was phased out in the early 1960's, being replaced 
with A36, at the same time that HSB's replaced rivets.

Hope this helps.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Dennis Pantazis wrote:

>>i am working on an industrial roof that was damaged (read: blew off) during
a windstorm

the structural system is 60' bowstring trusses, 16' o.c., made up of double
angles gapped at 1/2", and 2x10 joists and 1 by decking. the trusses sit on
a multi wythe brick/clay tile wall.
the truss is spliced at the center with bolts instead of rivets to the
gusset plate. otherwise
riveted everywhere else. the center of the span is braced with 'x' bracing
made up of other double angles.

in my analysis, i have found during MWFRS design windload the bottom chord
goes into compression (obviously) and the unbraced length is too long. the
solution seems to be to brace the quarter points of the span. issue here is
how. i am temped to have a 1/2 tab inserted between the chord members and
bolt X's to the top and bottom of it so it is effectively mechanicaly
connected to the bottom chord member. this would have to occur at about 16"
from the theoretical panel point location. i am uneasy about bracing the
truss at a location off the panel point.

is this a bad detail? alternates?<<

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