Concentrated load on one-way rib-joist concrete slab

[taldridg]

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.

 

[UcfSE]

If you are using the ACI, try 11.12.1.1 for beam-shear in footings and slabs. That provisions says to use the entire width for one-way shear. If you are uncomfortable with that, maybe try checking the beam-shear with an effective width of maybe the load width plus d or 2d. Be careful with the flexure also. You probably don't have much reinforcement and it is doubtful where the wire is actually located.

 

[Lutfi]

I was tasked on several occasions to review the feasibility of adding high loads on one way concrete joist systems. I opted in same cases to add steel beams directly under the load (placed between the joists) and the steel beam was supported by the concrete beams. The analysis of the concrete beams was easier for me than the struggle of working with joist system that I was not sure of slab thickness, amount of reinforcement and exact location of it.

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

 

[JAE]

taldridg

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, [φ]Vc 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.

 

[MichSt]

The following article doesn’t directly apply to your problem, but you may find it an interesting read ftp://imgs.ebuild.com/woc/C890023.pdf. See the section after the first heading on page 2.

This article was given to me by another forum user, SlideRuleEra.

 

[taldridg]

Thanks everyone for their responses. This building was originally designed for a warehouse type loading and the owner wishes it to be converted into parking. I have to believe that the live load originally designed for is greater than the 40 psf currently required in IBC 2003. The problem araises when I check the 3000 lb concentrated load described above. The original thought was to add a structural topping slab, but that would encompass a total of 150,000 sq.ft. (30,000 ft2 for 5 floors). Combine the cost of this system as well as the fact that if I increase the total dead load by 5% I will have to update the building to comply with current code provisions, I'm not for sure if this is a vaiable solution. Thanks for all the help, and I will appreciate any additional information.

 

[miecz]

3000 lb over 4.5" square works out to 148 psi. Why the high pressure? HS20 loading is only 104 psi, including 30%impact.

 

[taldridg]

Under Note A for Table 1607.1 of the 2003 IBC, it is required that a concentrated load of 3000 lb over an area of 4.5"x4.5" is able to be supported for a vehicular parking garage. After some looking into, I feel that this loading is meant to simulate a jack lifting up a heavy truck. I don't see much leadway with reducing this load per IBC.