Slab on Grade - Load Safety Factors

[sgrya1]

I have a slab on grade design program that, by design, does not consider any load safety factors.
It's been around for years and has used on many jobs. Someone told me that no safety factors are required for SoGs.
If country codes have anything to do with it, the program has been extensively used in Australia, New Zealand and South Africa.

Slabs on grade aren't going anywhere, and far safer than suspended slabs, but does anyone have any understanding why the person who created the program might be content in not applying any load safety factors?
Or a reference to what safety factors should be applied to slabs on grade?

It's hard to use the program knowing of a potential shortfall, if it is a shortfall.
Hope someone can help.

 

[txeng91]

Not sure what the program you are using is doing, but for waffle slabs on grade located in regions with expansive sols, you are typically designing to a particular beam depth to resist deflection caused by soil movement. When calculating deflection, you are typically not using safety factors, so maybe that is the reason?. Might be based on WRI slab on grade method or BRAB. That being said, I would think the beam reinforcement should be designed using LRFD strength reduction factor and an appropriate load factor for the anticipated soil pressure.

 

[BAretired]

Safety factors are specified in the concrete code. I am unaware of an exception being made for grade slabs, but I do agree that the consequences of failure are usually less serious for grade slabs as they are for suspended slabs.

BA

 

[dik]

If using Limit States Design, then you have a load factor. If working stress you may have a safety factor, built in.

Dik

 

[txeng91]

Didn't make this clear in the first post but for any components that count on concrete flexure (spread footiongs, strip footings with point loads, or beams subject to flexure from soil movemnt) the reinforcement should be designed using LRFD design which used resistance and load factors. Slabs that aren't only supporting a uniform floor live load are typically reinforced with minimum temperature and shrinkage reinforcement and strip and spread footings are used under high concentrated point and line loads.

 

[Nor Cal SE]

Typically for a slab-on-grade it seems that you wouldn't have a large concern about stresses in the concrete. Do you have some really unusual conditions such as very high point loads in concentrated areas? I could envision checking a localized area for punching shear under a large point load, but I can't envision many scenarios where you would see much flexural stress, outside of specialized conditions such as if you're counting on the slab concrete to resist uplift due to lateral-load hold-downs, etc.

 

[sgrya1]

I'm certain now it's an oversight. I have a SoG design guide which specifically outlines what factors to use. I've recently spoken to one of the users who said he wasn't aware.

It's actually spreadsheet which has calculations hidden behind a user interface.
We discovered it when we started digging into it trying to understand how it worked.
I guess there a few dodgy designs out there.

Thanks all!