Joist deflection due to weld

[WWTEng]

Situation: Existing floor bar joists to be reinforced by welding new plates & bars to chords and webs. The existing form deck does not appear to be connected to the joist in any way other then with a clip as shown in the attached picture. The contractor, in order to weld the top reinforcing bars, had to cut these clips (spaced at about 32” oc each side). The end result was that the mock-up joist with this repair is deflecting downwards, with about a 0.5” max gap between the joist and the deck. The deflection is obviously to due to welding of new member to top and bottom chords.

I have seen and specified joist reinforcements before but so far had not seen this problem. I think part of the reason for the problem is that fact that the joists are only 8” deep.

My initial thought is to forget about reinforcing the joists and just infill with new members in between using a ½” compressible filler over the new members to account for any existing deflection. I also think that the new members don’t need to be directly connected to the existing deck. I think it would be very difficult to weld a new member to a 50 year old existing form deck of God knows what gage.

But for curiosity sake, what would be the fix for the original option. Would shoring the joist and the welding the plates be a solution? Is there a better way to weld the new reinforcement so that temperature differentials do not bend the joist?

 

[slickdeals]

Just a thought : Was the deck acting as a flange to these joists with some horizontal shear transfer happening through the clips? Once the clips were removed the "I" reduced and deflection increased?

It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN

 

[ishvaaag]

As you describe the thing, the original way of support is lost. Now joists float unloaded by anything than the locked stresses and vertical loads from the deck above at undefined but likely close to support points. Any LTB prevention to the top chord of the joist it seems is lost. The concreted (since you say form deck) deck above seems to be bridging between supports, or close to that. This, also, maybe just a device of the minimal intervention till now performed, and such scheme may not stay viable when an intervention of such kind is extended to whole floors, enacting variegated modes of instability affecting from a section of floor to the whole building. In all, the procedure -that may anyway be performed with care (braced shoring) and the prior scheme restored by remaking links- seems unpromising without further measures than unrelented welding to chords.

Since from the test the small joists seem difficult not to be adversely affected by what someone has decided is the weld requirements for the case, one would say some other kind of reinforcement should be investigated as you are doing.

Minimization of deformation from welding may come from minimizing weld procedures; maybe just some small lengths of weld (or lesser than applied) is feasible to reduce deformation for your case. A more difficult set of procedures may involve significative temporary restraint and locked stresses from some scheme that ensures that the locked stresses are to be properly dealt with and with the desired effects once no other restraint than what delivered by the final structure is at work. This may involve some imaginative solutions quite likely improper for one structure like this.

 

[WWTEng]

Slickdeals: I don't believe that this is some sort of a composite deck. The original drawings do indicate the joist type and from the original load tables (circa 1950) its obvious to me that the joists are capable of handling the design loads without any composite action. It may be that they did not weld the deck to the joists and the picture shows the only form of deck-joist connection.

 

[msquared48]

I tend to agree with slickdeals here.

There had to be some kind of composite action that was compromised, possibly a puddle weld, side seam weld, or button punch to the clip from the top of the deck.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com