Open web joists shoring 1

[Heldbaum]

Hi guys,

Contractor has sent me these drawings recently.(see attached some print screens) They are going to replace a roof truss which holds all the roof joists that go perpendicularly and they call out for temporary shoring to support these open web joists. They propose 2 x W beams going parallel to the truss. I am ok with that, but my concern is - shouldn't they support the top flange of those open web joists instead of supporting them at the bottom ? Once the bottom flange is supported, the whole reaction would go to the cross which is in compression instead of going to the first one at the support which would be in tension - and that can cause a problem.. I am not sure though, any advices with an explanation will be much appreciated. Thanks folks!

 

[Heldbaum]

This is the plan of the roof

 

[jjl317]

While I will be interested to see if you get any other feedback, I have completed many of these types of re-shoring jobs in my career, and as a rule, I have always tried to support off the top chord. Two reasons, the first (as you mentioned) is concern about the compression forces in the diagonal members (though that could potentially be addressed through analysis). My second concern is the lateral stability of the joist under load, and the tendency to want to roll. And keep in mind that depending on the work to be completed, lateral loads could inadvertently be added. I had a project once (similar in re-shoring, just not joists) where the lateral impact from a man-bucket rolled the loaded I-beams being used to a temporary support. Not a position you want to be in. I understand that the contractor will want to go off of the bottom chord, as fishing members through the joists can be difficult, but in my mind, it's a better option.

 

[KootK]

I like it the contractor's way. The new support location doesn't change any of the member forces except the end diagonal which sees its force drop to zero. Stability of the shoring in the plane of the beams is an issue as jjl317 mentioned. I typically request that there be some in plane cross bracing in the shoring system to address that. One thing that you have going for you in this regard is that you've usually already got some bridging in the supported joists pretty close to the ends.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[OldBldgGuy]

I don't have any big issue with the proposed method other than I always make sure the support is at a top chord panel point and I add a vertical strut in the joist at the new bearing to ensure that the load is picked up from the top chord. Unless you're looking at very large joists & spans, it can be done with wood: cut a 2x4 or 2x6 marginally overlength and drive it in between the top & bottom chord.