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Re: Roof truss connectors

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Looking at ASCE 7-95, Fig. 6-3 or 6-4, you can see there can be an
unbalanced
net suction pressure acting on the windard and leeward sides of the roof.
Therefore, there is in fact a net lateral force on the roof, and this would
all go to the trusses by their trib area if the metal deck had zero
in-plane
shear stiffness. But since it does not, the net lateral force on the roof
is
shared according to the relative stiffness between the in-plane shear
stiffness of the roof deck acting as a diaphragm and the lateral stiffness
of
the wall acting as spring supports against lateral deflection of the truss.
Now without doing the calculation, I supect that this stiffness of the roof
deck (acting like a deep beam) is much greater than the wall, and most of
the
force would not be felt by the wall. But, as a practical matter, I
personally
would probably try to design anchors for the truss assuming all of the
lateral force on the roof went to the truss. Now for the issue of MWFRS vs.
CC for the roof trusses.

Figures 6-3 and 6-4 apply to the Main Wind Force Resisting System (MWFRS)
and
the question arises as to whether these trusses are part of this system or
should they be classified as Components and Cladding ?  I would argue that
they are probably both in the sense that for the roof deck to perform as a
diaphragm it relies on its connection to the trusses and thereby the
structural integrity of the truss. Clearly to me, the truss is part of the
MWFRS. But I would also maintain that any particular truss also has to be
able to resist the (reduced area) pressures in Figure 6-5 for C&C. So they
are C&C elements too. Note that in Figure 6-5, in area 1, there is only max
positive and max negative pressure values given (not symultaneous on WW and
LW), which an individual truss must be designed for. But by considering the
truss to be classified as C&C alone, you loose the physical reality that
when
the wind blows perpendicular the the roof, diferent suctions and pressures
are symultaneously produced and a net lateral load on the roof is too. We
usually take the entire load to the shear walls for their design. But this
load is actually resisted by some combination of the roof deck acting as a
diaphragm spanning between perpendicular shear walls and trusses in between
laterally supported by "springs" where they anchor to the top of the load
bearing wall. And so it's conservative, but not unreasonable, to design the
truss anchors in the wall for all the lateral load and then to ignor this
and
assume for design that the diaphragm is supported only by the perpendicular
shear walls.

I really Wish this were not so long, so I'll just finish by saying that
your
basis for stating there is no lateral force at truss supports on the wall
lies not in the fact that you chose to classify it as a C&C element but
that
the relative stiffness of the diaphragm deck may be so much greater than
the
lateral stiffness provided by the wall at its top that the horizontal
reaction forces at truss supports are very small.


______________________________ Reply Separator
_________________________________
Subject: Roof truss connectors
Author:  <seaint(--nospam--at)seaint.org> ("Imran Khan" <ikhan40(--nospam--at)hotmail.com>) at ROH
Date:    12/12/2001 2:30 PM





I am designing a masonry building with a gable roof made up of cold form
roof trusses. The roof pitch is 5/12 and roof deck acts as a diaphragm to
distribute the loads to the end walls. The code is ASCE7-95.

I am concerned about the connectors between the truss and the wall. The
truss mfr has submitted a load summary sheet from his in-house program
which
shows lateral loads as high as 1500lb from roof trusses. The trusses are
classified as component and cladding.

I believe there should be no lateral loads if the truss is treated as a
component. To obtain lateral load I claculated the pressures using MWRS.
When wind is perpendicular there is suction on both sides (no net lateral
pressure) and only 400lb lateral when wind is parallel.

I expalined this to the truss mfr but he claims that nobody has raised this
issue before.

Also when I claculate the the net lateral from the roof and the walls do I
evenly distribute that load to the connector at each end of truss.





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