steel joist reinforcing

[MMOT]

Looking to see if anyone has any guidance...
I have looked through the threads and seen a good bit on reinforcing joists along their length with rod and angle, and have the "Designing with Steel Joists..." book at my disposal, but unfortunately my situation is a little different. I have a situation in which joists span about 22', are 20" on center, can only be accessed by the basement, and there are probably over 100 that need to be reinforced...quickly...
Unfortunately, due to the time constraints, removing and replacing the whole system is not a feasible option. There is not enough time to get new concrete cured and ready to recieve floor coverings. "Fortunately", the owner is willing to tolerate a new intermediate post and girder system slightly off of midspan. The joists are Kalmantruss circa the late 1940's and I was able to get tags off them to use with the SJI load tables.
I know I will need to add vertical reinforcing and cross bracing at the new bearing points, but am not sure how to address the very significant change/reversal in shear diagram with respect to the original envelove due to the new supports. Also not sure how to handle the negative moment or if the chord should be cut to avoid that concern.
Background- this is a revovation under construction and it was discovered during the construction that some significant (undocumented of course) modifications had been made to the floor, including the addition of a topping which renders the floor over-stressed as-is, not including the new work that was scheduled, based on code loads.
Any thoughts on how to handle the design and detailing of this is appreciated.
Sorry for such a long post...

 

[JAE]

On the negative moment issue - you could possibly brace the heck out of the bottom and top chords to ensure that they could enter the inelastic region. When entering the inelastic region, the bottom chord wouldn't buckle - then you could even ignore the negative capacity (assume a pin at the new support) and check for positive moment capacity based on two simple spans, knowing that you really have some negative moment help there.

For shear, you still have a shear envelope capacity across the span and if you over lay the new shear diagram you can see where additional web members would be required... probably some distance either side of the new support.

This requires visual examination of the web members to verify that they are the same size along the span, or do not change size from one panel point to the next (i.e. positive shear and negative shear would have equal capacities at the same point along the span).

 

[jjeng2]

I dont think you are going to get too much design info from the manufacturer. If the joists are mostly the same designation, you could investigate a few of them extensively to get exact sizes of members, welds, etc. Get a test on steel type and yield strength. Map out the whole thing on the computer based on the new conditions. Now you will have a good idea what you are dealing with. See what kind of reinforcing would be required and if its feasible to do it.

 

[jike]

I am sure this has been done before and I think you are headed in the right direction. I would re-analyze the joists as 2 span continuous for LL and add the DL stresses from the original simple span. You are mostly not going to be able to get the DL stresses out. Adequately brace the bottom chord or add reinforcing or both.

If they need to pour topping before the reinforcing gets done, add shoring.

Good luck!

 

[MMOT]

Thank you all for you thoughts, we are still awaiting so additional information before proceeding with the final design.
FYI- I just recieved the 4th quarter 2005 Engineering Journal from AISC, and though is again doesn't specifically address my case, does have an article concerning design methodology of joist strengthening.