SDI - Concentrated Load on Metal Deck

[Nulukkizdin]

I've now run into this issue a few times while looking at checking a concentrated load on a metal (composite) deck using a typical procedure from the design guide (see page 22 of

http://www.vulcraft.com/products/catalogs/designing/document.pdf

for an example).

Based on the style of bending (single span or continuous), the effective slab width (b_e) is determined as a function of the span and the distance from the applied load to support. The concentrated moment is then calculated and divided by the effective width to get a moment per unit of length.

This moment per unit length can then be converted into a equivalent uniform load due to the concentrated load by back solving the equation M = w*L²/8. The equivalent uniform load can then be added to any superimposed loads and self weight to get a total uniform load on the metal deck. This total load can be compared to the listed table values to confirm if the capacity is sufficient for the required design values.

As you decrease your deck span (presumably in hopes to make your deck work better), your concentrated moment goes down by a factor of L (P*L/4). Conversely, your equivalent uniform load increases by a factor of L². This makes sense from a purely numbers point of view, leading to higher equivalent uniform loads as you decrease your span. Again, this makes sense as you have a smaller effective area over which to spread the concentrated load on. The bonus is that as your spans get smaller, your allowable superimposed load from the deck table goes up.

The problem I run into is when you no longer get this bonus for a shorter deck span. For example, a Vulcraft 5.0" composite 1.5VL18 deck has the same allowable live load (400psf) for spans anywhere from 5'-0" to 7'-0". Based on the calcs discussed above, I would be much better off to resist a concentrated load using a 7'-0" span then a 5'-0" span, or even a 2'-0" span. Again, the numbers calc out, but it is hard to convince myself that a longer span is more beneficial in resisting bending in a case like this.

Has anyone else run into this sort of issue? Is there any good way to account for the obvious shorter span strength bonus that goes out the window once the allowable psf values start to level out? Any and all thoughts would be appreciated.

Thanks!

 

[KootK]

As your deck spans get shorter, you may be approaching a condition where shear, rather than moment, governs the design. Once that happens, the allowable load will flatten and the equivalent uniform moment business will give wacky results. The answer, I suspect, is to drop the equivalent uniform load business and start comparing real shear and moment demands to real shear and moment capacities. The equivalent uniform load method is my first stop in a situation like this as it's quick and effective. However, if I don't like the results and I think that there's more fruit to be picked, I switch to more detailed evaluation methods.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[manstrom]

The 400 psf limit is a maximum that they want to safely give out. In reality, the decks can support more, but the loading would be more like point loads.

The 400 psf is conservative in order to keep from people putting too heavy of a load on it and not checking the effects

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[rapt]

The easy solution is not to use load tables. Design it yourself!