Hung Point Load on a Concrete Slab

[AH4]

Hi All,

I'm being asked to design the post installed anchorage connections for a patient lifting system at an assisted living building. The lift system has a main rail that is anchored to the bottom of the concrete slab above (9" thick), at 4ft on center. The loads on these anchors are small (300 lbs), but I'm being asked to confirm the concentrated load from the anchorage will not exceed the floor live load of the slab above (40psf). Is there any generic rule of thumb that can be used to distribute a point load over a certain area of the slab to prove this load will not overstress the slab above?

 

[JAE]

Is there any generic rule of thumb

Not that I'm aware of.

Just have to use engineering judgement based on the type of slab (one-way, two-way, short or long span) and decide.

For a one-way slab I usually like to spread it out over about 2-3 feet of width of a slab strip and then see where I'm at.

 

[To Madeira]

Outside ACI 318, other concrete codes have a methodolgy to determine the width of one-way slabs which support concentrated loads.

 

[Once20036]

Out of curiousity, who's asking you to confirm that the concentrated load from the anchorage will not exceed the floor live load of the slab above (40psf)?
If you find that the concentrated load from the anchorage is equivalent to 39psf, doesn't that mean there's only 1psf left available for users on top of the floor slab?

 

[XR250]

Let's face it, a few 300lb "theoretical" point loads are not going to be the reason a 9" slab fails. More than likely it is due to it not being constructed properly or a file room being built in a 40 psf area. They will get lost in the noise.

 

[hokie66]

I would just say that my engineering judgment leads to the conclusion that the load of the rail does not impair the slab's capacity.

Fastening the rail to the slab is the most important issue. No chemical anchors. Use undercut mechanical anchors.

 

[BridgeSmith]

The floor live load isn't a distributed load, either. It's modeled that way for convenience. That slab and slabs much thinner than 9" support 300 lb point loads (footfalls of 300 lb people) day in and day out.

 

[Tomfh]

300lbs is one heavy person. If the floor is good for normal loads it’ll be fine. I’d personally have no concerns with epoxy fixed anchors (not polyester).

 

[lexpatrie]

Steel Deck Institute has some methods for distributing point loads on concrete/steel deck floors. You could work from there. I've done floor loading analysis for equipment for hospitals, one the older ones, keep in mine moment redistribution was pretty in vogue back when.

 

[structSU10]

its 300 lbs every 4ft, and i doubt the load is always that high. That is 75 plf along the run of supports and if you assume 3-4x slab thickness as effective width you are down to 20-25 psf effective which i think is quite conservative. I wouldn't really be too concerned.