Temporary shoring requirements for gypsum ceiling

[plausibly_civil]

I'm designing a repair procedure for some deteriorated joists in an apartment building, and need to provide a shoring design for construction. It is a wood framed floor and has (2) layers of gypsum ceiling below, which will not be disturbed. The repair procedure is all done from above so that the units and corridor below are undisturbed during construction except for temporary shoring. I haven't done much temp shoring design so I'm trying to get a handle on the design criteria for this task. The current capacity of the joists is unknown since they have some damage to the top of the section, but the construction load case is less than half of the service load case. Should I be concerned about anything other than limiting deflection to L/240 to prevent ceiling cracking? I haven't found many resources for this kind of shoring design, so any tips/design guides would be much appreciated.

 

[Enable]

What does the repair look like? That will dictate what kind of shoring is required. For instance, if we're welding to the joists then we'll definitely need something but if we're just infilling next to them from above shoring is not likely required. Also, if you are staging the repair (even if welding) it may be possible to shore entirely from above if we remove some DL / LL from the joists. Sketches may be helpful here!

You may want to take a look at this thread as well.

Nope. Need not consider any of that since we're not talking steel

 

[plausibly_civil]

The repair is mainly sistering new joists to the old joists. We considered removing and replacing but that adds a lot of cost and disruption to tenants. The floor is all wood framed so no welding required. While shoring might not be strictly required, I want to preclude any ceiling cracking and so does that client. The repair consists of cutting out the damaged top section of the wood joists, then sistering a new joist to match existing. My concern is that as the existing joist sections are reduced, there will be additional deflection that may damage the ceiling.

The design DL/LL is 43psf/100psf for the joists. The DL during construction is 12psf plus 40psf construction load. Now that I'm typing this all out I'm reassessing a bit. I may just analyze the reduced joist sections for the construction/repair loading condition and see if that is greater than the original design deflection.

 

[Enable]

Oops read too quickly, my bad. Thought we were talking open web steel joists.

If excess ceiling deflection during the repair is a concern then you could open the floor, place your joist materials into the cavity (temporarily held or perched), and then run beams overtop running perpendicular to the joist direction. Connect as appropriate (to hang joist to be repaired) and use adjacent joists to spread the load from the one you are repairing. Use design LL for this purpose and restrict the area.

One thing to consider is that this works great for the repair sequence but you will get additional deflection once you return the repaired joist to service as the partial sister begins to take load. It's probably this deflection that you need to be most concerned with unless your sister is full length.