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Point load composite slab 4

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MIStructE_IRE

MIStructE_IRE

Structural
Sep 23, 2018
816
Hi,

I’m wondering if anyone has a reference for point loads on composite slabs. My initial view, as marked up in the below diagram, is that the load is essentially taken by an individual rib. However, if this is the case, then many data centre floor slab designs I’ve seen couldn’t be justified.

Most designs I’ve seen for data halls design a high uniform distributed load, but many fail to consider the fact that the loads are applied via the pedestals of a raised access floor, which if concentrated on a single rib, considerably overload that particular rib.

Any thoughts? Could this justifiably be spread over a number of ribs?

DEDF9749-36B2-4B89-8BB6-0006BF57EC96_vyql67.jpg
 
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I have not seen later research on it, but in the 1990's when I was involved in a project that involved testing and a set of design rules for composite metal deck design, point loads were not allowed for the general design rules for bond development and required special testing. The problem was the linear and then sudden change in stress with a point load compared to the gradual change in stress with UDL loading.

Once you justify this, I would think multiple ribs would be acceptable as long as the slab above the ribs has the stiffness and strenth to carry the load transverse to the ribs to share it with the other ribs.
 
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If punching shear on the upper slab check is fine, then it is fine.
 
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Don't forget it's a two way slab of sorts as rapt eluded to, the load can span across the slab rather than purely relying on a single rib as you've sketched. Check out Eurocodes as they cover this quite well I think.
 
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Get yourself an old United Steel Deck catalog. They have formulas in their for point loads on slabs (which has diagrams that look like the one you provided so..... maybe you already have these formulas?). There was also a spread sheet kicking around for this instance that followed the same formulas I referenced.
 
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SDI (Steel Deck Institute) addresses concentrated loads in their latest design manuals.
 
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If punching shear on the upper slab check is fine, then it is fine.
Click to expand...

I agree with Retired13. As others have said as well, if there is reinforcement in both directions, then the slab stiffness spreads out the load pretty well (in both directions) and engages multiple ribs.
 
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i came here to post haynewp's response. This is the only published design example ive ever seen in 20 years.

Retired13 said:
If punching shear on the upper slab check is fine, then it is fine.
Click to expand...
No, this ignores flexure and shear which is addressed in the haynewp example

JoshPlumSE said:
I agree with Retired13. As others have said as well, if there is reinforcement in both directions, then the slab stiffness spreads out the load pretty well (in both directions) and engages multiple ribs.
Click to expand...
This doesnt sound very scientific to me. The effect is quantified in the example
 
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Yes, faulting my laziness not to write complete procedure in composite deck design. The design flow is like this:

1. Check punching shear. Stop, if no good, use thicker concrete slab. Otherwise, proceed to step 2.
2. Compute one way shear. Stop, if no good, use thicker concrete slab. Otherwise, proceed to step 3.
3. Compute moment and provide reinforcement accordingly. Provide additional bars directly under the concentrate load.

For shear capacity is marginally in deficit, provides the slab is reasonable thick, shear reinforcement can be considered, but usually undesirable.
 
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