OWSJ Floor Deflection & Jacking

[Eng_Struct]

Hi Group,

For one of my projects, the contractor had gone ahead and placed a 16ft high masonry block wall on a composite metal deck (4" floor) on OWSJ. Note that the floor is not composite with the joist. Based on the analysis, I have determined that the joist will need to be reinforced. The floor is still standing because it has not seen the maximum live load and the new block wall is almost equal to the specified live load. Additional reinforcing is required for combined loading due to the new wall and the live load.

I have already completed the reinforcement design, however, I had to consider destressing the joist for the block wall load to eliminate pre-loading. I am trying to come up with the lift/jacking amount and thought about working with the theoretical deflection of a single joist due to the wall load. Note that the wall sits directly on top of the joist and runs parallel to the span. The theoretical downward deflection for the joist due to wall load is roughly 30mm over a 16m span. To me telling the contractor to lift the joist by 30mm seems a lot and I am thinking if I should be considering diaphragm action from the 4" composite metal deck (again deck is not composite with the joist). The joists are spaced at every 1.320m.

I am not sure how to model the deck stiffness. Based on a few elastic analysis runs, the floor slab seems to be distributing the load significantly if I use the bending stiffnesses of the deck from composite deck catalogs. To give you an idea, I am looking at deflections from 3mm to 10mm depending on how I model the deck in the model.

I am not sure what number to use for jacking at this point. I am concerned about providing a high value and ending up cracking the floor/wall above or potentially reversing the loading on the joist which will be terrible. Nor do I want to give them a smaller value that is not sufficient to fully unload the joists for the wall dead load.

Any thoughts on how to tackle this problem!

Thanks!

 

[phamENG]

Is this new construction?

Plenty of questions about why there's an unexpected 16' tall masonry wall on an elevated floor, but that's immaterial to your request.

If it's new construction and the adjacent joists are not yet loaded, take field measurements.

First, determine how flat the floor is. Then, measure from the floor to the bottom chord of the joist at the point you expect worst case deflection on each joist. That'll give you an idea of the shape of the floor and how you need to jack it. You may find out that the load is spreading to 6 different joists, and you need to carefully jack a much wider area to get it there.

(If you have a rotating laser level, you may be able to set that up and just measure from the bottom chord to the laser.)

 

[SWComposites]

can you measure the deflection of the joist using a laser beam from end to end? and then jack to make the joist straight?

 

[Eng_Struct]

This is an existing floor. I guess I can request the contractor to take some measurements and get an idea.

 

[SteelPE]

Plenty of questions about why there's an unexpected 16' tall masonry wall on an elevated floor, but that's immaterial to your request.

I had something similar happen to me 20 years ago on a new construction project. From what I recall, the original plans called for metal stud partitions.... somewhere along the line the architect decided to change the wall out to masonry. Since our design was 90% complete and we were never told of the change we didn't know about the change until after construction of the wall started. I was only a junior engineer at the time, and my old boss handled the repair.

If I remember correctly, he was able to design the bottom few courses of the masonry to act as it's own supporting beam..... and we ended up beefing up the connections at the end. This only worked because there were two column at each end of the wall (we got lucky on that one).

Modeling the stiffness of the floor is going to be tough as your "model" will only be an estimate of what is actually happening out there. Would it be possible to get the deflection of the joists under the wall (as others have suggested) and also get the deflection of a similar joists (say 5 meters away) to see what that joist is doing.... compare the deflections and then compare to your calculations and come up with a number to give the contractor. Any number you give is going to be a best guess... so having all the possible information is a good idea.

I don't know what else you can do.