multiple concentrated loads on two way spanning slab 1

[cookiem]

I have a two way spanning slab (metal decking), i contacted the metal deck company to ask if they can assess the deck for concentrated loads (the loads are hung from the underside of the deck - lifting eyes).

The metal deck company said they can only assess the slab with udl's and i should design it as a concrete slab with permanent shuttering.

How do i calculate the bending moments and shear in the slab due to the point loads?

sorry if this questions has been asked before

Mandy

 

[ishvaaag]

Include the true solid shape in a 3D model to be analyzed in FEM. As a simplification you may use a ribs and plate system on beams, or conservatively just the plates corresponding to the lesser thickness of concrete above the deck. These won't need a FEM package with brick, tetrahedral and so on elements.

 

[rowingengineer]

Ishvaaag,
I think your suggestion is like using a sledge hammer to nail a painting to the wall. while a finite element analysis could be done, it needs special care and knowledge to interpret the results without getting in trouble.

I would use the equivlent frame method with points loads as addiontal, I sure someone useing the ACI code can point you to the right chapter.

Maybe you culd do both the equivlent frame and FEA to compare.

An expert is a man who has made all the mistakes which can be made in a very narrow field

 

[rscassar]

Being a greenhorn in the design of slabs with metal decks, what type of two-way action is it? Is the slab supported on 4 sides by walls or does the floor system span onto columns.

Treatment of concentrated loads was touched on briefly in this post which came around recently (refer to the comments made by rapt):

thread507-276053

For concentrated loads on two-way slabs supported on 4 sides, I have distributed the proportional of load to each orthogonal direction based on the relative stiffness of that span (i.e. considering span lengths and I values). Being a metal deck, I assume that the slab will be stiffer in one direction than it is in the other?

 

[rapt]

Mandy,

It will depend on the slab/decking depth ratio, but there is not normally a lot of 2 way action in slabs on metal decking due to the stiffness difference. Normally the decking will be placed to run in the short span direction to line supports of some sort, beams or walls.

The problem with point loads is stress concentrations and developemnt of the strength of the decking either side of the point. With UDLs' the stress develops gradually over a long length. Thios is not possible with significant point loads. If the point loads are large, it may be necessary to carry out testing if you want to include the strength of the decking in the reinforcing strength.

 

[cookiem]

Thanks very much for all your help.

The slab is spanning two ways as it is supported on the walls of a square lift shaft. (but yes in essence it will only span one way)

I have told the contractor to use a lifting beam as I had previously specified as the solution to using metal decking wasn't a quick and easy one and they needed a reply asap.

Many thanks, this forum is really valuable

 

[msquared48]

Personally, I would have transposed the point loads to an equivalent uniform load, if possible, then reverted to the tables in the back of the 1963 ACI code where there is a method - actually two I believe - to calculate concrete slab loads with differing edge conditions and aspect ratios. I could then up the loads with current design factors and load combinations to current practice.

Goodness but it's great to be old!

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[SteelPE]

I can’t say that I have ever used a slab on metal deck to span in two directions. It’s always been one way for me.

There used to be a method for calculating point loads on slabs with composite metal decking which was found in the United Steel Deck design manual. I think the procedure came out of the SDI. You used to be able to find the manual online but I can’t seem to find it anymore.

There is this spreadsheet which does go through the procedure… but you would need to verify the procedure yourself which is going to be difficult to do without the information mentioned above.

http://www.calculatoredge.com/structural/deckslab.htm