truss panel point analysis

[CTW]

This is an existing connection that I'm analyzing for the ability to carry addtional load, but I'm having trouble determining which forces to include in the analysis. The connection has a top chord consisting of (2)C15x33.9 back to back, a vertical web member consisting of (2)L6x4x1/2, and a diagonal web member consisting of (2)L6x4x1/2. The gusset plate is 38" long by 33.5" wide, is sandwiched between the (2)C15x33.9, and extends to the top of the C15's. The plate is attached to the C15's by (14) rivets erratically spaced. The vertical web member is in compression and the diagonal is in tension. Only the neutral axes of the web members intersect.

My question is, what forces should be considered when analyzing the plate's connection to the C15's for additional load? The largest force from the web members, moment (due to both tension & compression being present), or both?

 

[dlew]

CTW,

How are the vertical and diagonal members connected to the gusset plate? Are also riveted? Or are they welded?

I assume that, either riveted or welded, the connections of the vertical and diagonal members are concentric with their respective axes, and that the forces in these two members are transferred to the gusset plate without introducing additional moments. Notice that the vertical components of these two forces cancel each other at the panel point, while the horizontal component of the diagonal is transmitted from the gusset plate to the 2-channel chord.
Notice too, that the axial force in the chord coming from the previous panel point continues through the channels and is not transferred to the gusset plate.

If my assumption is correct, then the erratically spaced rivets that connect the 2-channel chord to the gusset have to transfer the horizontal component of the diagonal (applied at the panel point) to the chord. If the c.g. of the rivets is on the axis of the chord, the force is distributed equally between these rivets, and not moment is introduced into the joint.

If the c.g. of the rivets is not in the axis of the chord, the shear in the rivets varies, and there is a moment introduced into the joint. This moment is resisted by the chord and the web members, and has to be considered.

Hope this will help

AEF

 

[flamby]

CTW,

Before you try to analyze the connection, I feel you may perhaps analyze the truss itself for the loading, you intend to apply. You may run this on any FEM software with joint conditions as Pinned and then again as fixed. Generally, the first case produces heavy axial loads in truss members. The second case gives moments in addition to axial forces. This will give you a clear idea of the axial foreces or moments which can be anticipated if joint is rigid or pinned.

I hope it makes some sense.

flame

 

[CTW]

Thank you both for your comments and suggestions.

Flame-

The first thing I did was run this on the computer as pinned and fixed to get the forces. I just overcomplicated the analysis of the connection as I have never encountered such an odd spacing and arrangement of rivets. I didn't want to miss a force that should have been considered.

dlew-

You cleared that up for me. The connections of the web members to the gusset are riveted. Your assumptions were correct. I now see that the vertical forces cancel each other and I'm left with the horizontal force. The c.g. of the rivets is on the axis of the channels. Thanks for helping me see how simple this situation actually is.