Load Combinations and Factors of Safety 1

[UcfSE]

When you use the combinations from the ASCE 7, particularly the 0.6D+W, for footing design, do you use factors of safety of 1.5 for overturning, uplift and shear as well, or is the 0.6 considered the factor of safety? For instance, if I use 0.6D+W and find my uplift or lateral load, will my factor of safety need to be 1.5? Some structural engineers I have asked say use the FS=1.5 in addition to 0.6D and some say use 1.0. I use 1.5 in conjunction with 0.6D to be safe but this seems a bit excessive and I have not located any part of the code that specifically answers my question. Any help or advice would be greatly appreciated.

 

[ajh1]

The 0.6D + W combination is an ASD combination, therefore would be subject to a normal factor of safety. The 0.6 factor is trying to recognize that all of the assumed dead load might not be present in combination with the wind. For example, collateral loads are generally assumed to be dead loads, but just because you plan on 3 psf for a sprinkler system doesn't mean every square foot of the building sees the full 3 psf.
Alternatively, use the Strength Design combination of "0.9D + 1.6W" with an appropriate resistance factor.

 

[DTJ]

You will notice that in the current codes (IBC 2000) the requirement for the 1.5 (using 2/3 of the actual Dead Load) safety factor has been removed. Instead it states that the Minimum Dead loads should be used to resist the overturning loads. If you are looking at 0.6D+W. This is a 1.667 factor of safety using the minimum dead loads, which is larger than the 1.5 you used before.

I typically have two Dead Load cases (Dmin and Dmax). I use Dmin in caclulating my overturning resistance and Dmax to calculate maximum bearing.

I find in a lot of cases that Dmin is quite a bit less than 0.6Dmax

 

[AlohaBob]

I noticed that too DTJ. It appears to have been replaced with the 0.6D + W. I think this load combination is more conservative than the 1.5 FOS as conditions and geometry could at times cause even a 10% increase in dead load to increase the FOS for OT even 50%.

So the FOS with that load combination is 1.0. But in actuality, it practically equates to the old 1.5 FOS.

 

[MotorCity]

Lets not confuse factors of safety with load factors, they are two completely different concepts and should be applied separately. Load factors allow for a certain design load to be exceeded. For example, a load factor of 1.2 implies that there is a 20% chance of a design load being exceeded during the life of a structure. These load factors are the result of much research and statistical analysis. A factor of safety is nothing more than a "fudge factor" to allow for variations in workmanship, material properties, etc. This is a subject that often peaks my interest when these terms are used interchangably. To put it in perspective, it is like a geotechnical engineer referring to soil as "dirt". Thats my 2 cents.

 

[JAE]

Well, a bit more accurately, the load factors are statistical numbers that inflate the load such that there is a 90% probability that the inflated number will NOT be exceeded based on measured variability of the load.

In bit simpler English: When you factor up a typical load, say 100 psf times 1.6 = 160 psf, and this load will vary in magnitude over the life of the structure, you have a 90% probability that these loads won't go over 160 psf.

 

[sme76]

So.....when I'm designing a spread footing under a column and checking for overturning, do I need to assume that only 60% of the weight of footing concrete and overburden soil is available to resist the overturning moment? The load case I'm checking is 0.6D+W per IBC 2000.

 

[AlohaBob]

Yes