Support of Existing Steel Trusses at Panel Point?

[grogannc]

I have a residential project where the elevated back patio (~5 ft) is supported by steel bar joists and decking. Due to some waterproofing issues, the ends of the joist (bearing seat on one side) are severely rusted.

The span of the joists is 13.5' and max reaction loads would be ~1000 pounds. The joists are about 12" deep constructed of 1x1 angles for the top/bottom chords.

My idea is to come back from the bearing seat to the first panel point and support it with CMU columns. The is no rust in this location. I still have to support the front 2 ft of floor load but I have another detail for that.

Access is a problem so welding is out (from talking with a few welders that looked at it) and it's residential. Ie. they aren't going to spend 100k ripping everything out and fixing this.

Has anyone ever done this? Short of modeling the joist (which I can do) any suggested methods of analysis? I can't find much about this sort of thing.

 

[Triangled]

sounds reasonable to me.
I am guessing you'll support at 1st bottom chord panel point. should be simple and fine. I would model it myself just because I want to compare the new configuration forces to the original configuration forces. But intuitively, seems right.

if 1st top chord panel point beyond seat, and if modified warren style truss, not good as this web and its connections almost certainly are not strong enough for the new tension force it would be required to handle. But if warren style truss, should be ok.

 

[grogannc]

You are correct-at the first bottom panel point. Thanks for the input. I am in the same camp. Intuitively is seems fine but just wanted a second opinion. I'll probably end up at least modeling half of it to see the behavior change.

 

[DETstru]

You can draw the moment and shear diagrams for the current condition and the new condition then compare them by location on the joist. If at no point does your new shear/moment exceed the old shear/moment (at the same location on the joist) you don't need to check anything.