Treatment of Steel Truss Joints

[jheidt2543]

Existing:
I have an existing steel roof truss built in the 1920 - 1930 time period made up of rolled angles, some single, some double, with riveted connections. It spans 44’ and is 6’ deep on one end connected top and bottom chords to a building column and 3’-6” deep on the other end, which is encased in a masonry pier. The truss has a single pitch top chord with 12 joints and a level bottom chord with 9 (5-8’ panels and 1-4’ panel). Roof purlins are spaced at 4' centers, so there is a vertical web member under each purlin. I’ve analyzed the truss in its present state.

Problem:
The existing space is being remodeled and a second floor added to the area. The roof truss depth must be changed to allow the use of the new space. The new dimensions require the 6’ truss end to be 3’ and the 3’-6” end to be 2’-0”. So, I have reanalyzed the truss with a new bottom chord using a rolled A36, channel section, which will be welded in place prior to cutting the existing truss members below it. Now both the top and bottom chords will be pitched. Of, course the web members will no longer meet the new bottom chord at common joints, so the new bottom chord must be checked for bending moment. All seems to work out OK.

Question:
My question has to do with the joint conditions. Since the four corners of the truss are actually connected to the existing building structure, should they can be treaded as fixed or pinned ? (I‘ve actually looked at it both ways). For the panel point joints, since they are rivited at the existing and welded at the new bottom chord member, how should they be treated? Just a loaded joint, a pinned joint or some degree of fixity?

Thanks in advance for your thoughts.

 

[Bannu]

Yes, there will be partial fixity at panel points because of new welding. Also since the supports are firmly connected to the existing structure it may at more like fixed support.

Considering the fact that it is quite old building it would be wise to be conservative in the analysis and design. Two kinds of analysis can be done to figure out the worst possible internal forces. Fistly, assume the truss asction with all joints pinned and design for the axial loads. Second, consider the it as a frame and find out the bending moment resulting and check for combined bending and axial force using the interaction equation given in AISC Manual of Steel Construction.