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Re:Timber Truss Gusset Plate

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Richard,

I have designed many timber trusses using the method you describe, and I can offer the following advice. Use the results of your frame analysis to help you design the joints. The bolted connection to each member should be designed for the forces transferred to or from that member. To identify the perpendicular to grain bolt forces on the bottom chord, look at the shear diagram of the bottom chord. The CHANGE in shear across the joint is the force that the bolts TO THE BOTTOM CHORD should be designed for. Likewise, the change in axial load is the parallel to grain bolt force. You said that you modeled the bottom chord as a continuous member, which reflects typical construction practice. Now the method of joints that we all learned in our first statics class is no longer valid, because the bottom chord can transfer bending moments across the joint. This means that some of your web and king post reactions can be transferred to adjacent joints by bending in the bottom chord. These forces get into the bottom chord by perpendicular to grain bearing of the bolts.

Several other important points to remember when designing this type of truss. There is never enough room to get all the required bolts in the wood members with the proper clearances. I don't think I have ever designed a timber truss where the member sizes were governed by stress. The size was always dictated by the spacing requirements of the bolts.

The "minor" shear you mention can be a major issue. I have seen a number of truss failures that occurred between offset bolted joints. The truss design penciled easily if it was modeled as the classic design with concentrically pinned members. But if a more realistic model was applied with continuous members and bolts that were offset from the joint centers, shear stress concentrations became apparent that were 2 to 4 times in excess of the wood shear capacity. Naturally, this explained the failure. What it didn't explain was how the remaining trusses of similar design were still in service and undamaged!

One last suggestion. I have had complaints from several contractors when building lightly loaded trusses I designed. The problem was that the stresses were so low, I only used one bolt at each connection (this reduces clearance problems). They said that the truss was not rigid during construction, and the joints kept moving until the truss was fully loaded. Ever since, I always design at least 2 bolts at each connection if I am using gusset plates.

Hope this helps.

Dmitri Wright PE
Portland, OR





From: "Rich Lewis" <sea(--nospam--at)lewisengineering.com>
To: <seaint(--nospam--at)seaint.org>
Subject: Timber Truss Gusset Plate

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I am designing a timber truss using Parallam lumber.  The panel points
are connected with steel side plates and bolts.

I have a question as to how I see the bolted connection designed.  I
analyzed the truss with a frame analysis program and I have all the
member forces for various load combinations.  I modeled the web members
as pinned ends and the top and bottom chords as continuous members.  As
I see the joint connection design I design the number of bolts based on
the member forces.  The forces in the members are primarily axial forces
so the bolts are designed based on parallel to grain values.  The top
and bottom chords have some minor shear due to the continuous modeling.

Let me see if I can explain this further.  I will illustrate this with a
specific joint, a bottom panel joint that has the bottom chord
horizontal on either side, a diagonal going up to the top chord and a
vertical going up to the top chord.  The force from the diagonal goes
into the steel plate.  The vertical component of the force is resisted
by the vertical web and the horizontal component is resisted by the
bottom chords.  The force is distributed to these members through the
gusset plate.  As I see it I don't have any perpendicular to grain
forces in the bottom chord (except the minor shear forces) or the
diagonal or the vertical.

I need a sounding board to bounce my thoughts off of.  Am I looking at
this correctly?  Would this plate put a twisting force in the joint that
will put perpendicular to grain loading in the bottom chord?  Would
joint equilibrium negate this possibility?

Thanks for your help.


Rich Lewis
Lewis Engineering





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