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Gusset plate weld design

Gusset plate weld design

Gusset plate weld design

(OP)
I'm currently looking at a gusset plate with 45 degree diagonals coming into it.  My model is giving me diagonal loads of 66 kips coming into the joint.  I have a bottom chord load that does not equal the difference of the horizontal components of these two forces.  Especially at the center.  The maximum compression in the top chord is 180 kips, but there is a 1 kip load in the braces coming into it.  At the ends, the diagonal forces are higher and chord force lower.  this is expected based on beam design fundamentals.  My question is for weld design at the gusset plate.  The joint are not in equilibrium it seems if the chord force at the center is 180 kips and very small horiz components of the diagonal members.  For the deisgn of the gusset plate weld.  Which horizontal force do you use for the weld design connecting the gusset to the chord, the chord compression or sum of the two diagonal loads coming into the joint?

RE: Gusset plate weld design

Your model is in error, unless the joint is accelerating in the direction of the force.  A free body diagram of structure at rest has equilibrium.

RE: Gusset plate weld design

(OP)
it seems reasonable though.  like a beam shear high at ends and moment high at center

RE: Gusset plate weld design

I am not certain I am interpreting your question correctly, but any chord connections have to develop the chords, and the web member connections must develop the forces in the webs.  The web members near midspan will have only nominal forces, while the web members near the support will have substantially larger forces.  Civilperson is correct, there must be equilibrium.  Just sum the forces at the joint, and the horizontal and vertical components will balance.

RE: Gusset plate weld design

I don't know how you modeled your truss, but the reason why the joint may not apparently balance could be the associated shears in the vertical members.   When combined with the diagonal members, does it sum?

If you modeled it correctly with the vertical members pinned, there should not be contributory lateral shear force, only axial.   

Mike McCann
MMC Engineering

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