Concentrated load on one-way rib-joist concrete slab
Concentrated load on one-way rib-joist concrete slab
(OP)
If anyone could help me on this I would greatly appreciate it. I am currently working with a structure built using a series of ribs spaced at 25" o.c. supported by beams running perpendicular to the ribs. The width of the ribs is 5". Thus the clear span for the slab between the ribs is 20". The thickness of the slab is only 2" with some welded wire fabric for temp. reinforcement. This building was built in the 1920s. Cores have shown a f'c of 4100psi. I need to analyize this system for a concentrated load of 3000 lb over 4.5" by 4.5". The two way shear check is pretty simple, the problem comes when I try to check one way shear. Where should I place the load? What trib. width can I assume to support shear in this case? Any help would be great. Thanks.






RE: Concentrated load on one-way rib-joist concrete slab
RE: Concentrated load on one-way rib-joist concrete slab
On one place, the had to field splice the steel beam so that it can be installed due to present of heavy piping in the way.
I advise you of doing the same, if you can.
Regards,
Lutfi
RE: Concentrated load on one-way rib-joist concrete slab
This is a tough one. One way concrete joists are my favorite structural system. But with the older constructions like the one you are working with, the slab thickness was usually pretty thin and the design was always based on a uniform load situation.
A 3000 lb. concentrated load would really concern me with the slab you describe.
You could check the one-way shear, φVc per the ACI code using between 20" to 40" for the effective width, b, but the thin-ness of the slab, coupled with the age and the fact that these older joist systems rarely had distribution cross-ribs built in them.
Without the cross ribs, this means that you may have longitudinal cracking along the slab/joist interface due to one joist deflecting relative to the other and the cross rib not there to minimize its effects. So this crack may be present to diminish your shear capacity despite your calculations.
The idea Lutfi presented can work, but man, that's a lot of steel to buy to resist the load. If its a moving load, that would tell me that you might have a bit of fatigue factor to consider as well (fatigue in crack development, not in the rebar).
A few other mitigation options:
1. Bolt some angled brackets with one leg attached to the under side of the slab and the vertical leg attached to the joist web side. These angles could run down the length of the slab and provide some additional shear capacity at the slab/joist interface.
2. Add a reinforced topping to add strength to span over the ribs and transfer the concentrated load. Might not be a good option as it raises the floor and adds dead load that may be too much for the existing joists to handle.
3. Load test a few areas with the point load to see what happens (see IBC chapter 17 or ACI Chapter 5?)
4. Tell the owner the 3000 lb load can't be applied to the floor.
RE: Concentrated load on one-way rib-joist concrete slab
This article was given to me by another forum user, SlideRuleEra.
RE: Concentrated load on one-way rib-joist concrete slab
RE: Concentrated load on one-way rib-joist concrete slab
RE: Concentrated load on one-way rib-joist concrete slab