0.6D+W Load Comb and 1.0 Factor of Safety 8

[cdi12]

I have designed canopy spread footing using load combination 0.6D+W and 1.0 factor of safety for overturning and uplift but reviewer wants me to use FOS=1.5 It looks very conservative to me and I could not find this in IBC. However PIP STC01015 Structural Design Criteria Section 4.3.7.2 is very specific:
The minimum overturning "stability ratio" for service loads other than earthquake shall be 1.5 For foundation design of building and open frame structure, if the dead load factor is 0.6 in accordance with SEI/ASCE 7, Section 2, the minimum overturning "stability ratio" shall be 1.0
Also thread176-67397 dealt with similar question but no precise answer yet. Please share your thoughts/experiences. Thanks, IV

 

[Lion06]

"...if the dead load factor is 0.6 in accordance with SEI/ASCE 7, Section 2, the minimum overturning "stability ratio" shall be 1.0"

It seems like you answered your own question there. If the dead load factor is 0.6, then the stability ratio shall be 1.0.
This essentially amounts to the same factor of safety as using the 1.5 F.S. with the full dead load. Using the 0.6 DL factor AND a 1.5 S.F. is really overkill.

 

[cdi12]

Thanks StructuralEIT. PIP STC01015 uses FOS=1.0 with 0.6D+W but my question is if IBC allows this. IV

 

[JAE]

For IBC, find WHERE it ever says 1.5 Sf. It doesn't if my memory serves me right....challenge your friend to show you were the 1.5 is in the code. It used to be there prior to the 0.6D+W combination occuring in the code. But now it isn't.

 

[msquared48]

The combo used to be .9D + W. logically, with the factor reduced to .6, then the 1.5 SF factor could be close to being included in the new equation.

Mike McCann
McCann Engineering

 

[WillisV]

This question has come up time and time again, and I believe the confusion stems from legacy codes and the way that certain provisions are currently written. The following is my attempt to explain...

The safety factor against overturning and sliding have traditionally been 1.5. In older codes, such as ASCE 7-95 Section 2.4, the code specifically gave the intent by expressing the load combinations as D+W or D+H (soil) and then stating in a seperate section (2.4.4) that 0.66D should be used in combinations for checking sliding and overturning.

Apparently people missed this caveat, and the code body decided in the newer codes to simply roll the factor into their load combinations. The 0.6D+W combination assumes a 10% over-estimate in gravity load (0.9D) and then a 1.5 safety factor to bring the number to 0.6D+W. So this combination DOES include the 1.5 safety factor.

To a large extent the confusion comes into play in IBC 2006 section 1806.1 which states that for retaining walls, "a safety factor of 1.5 against lateral sliding and overturning" shall be used. It is my belief that this sentence is a nod to the traditional practice of simply using D+H as the load combination for retaining walls, and then making sure that you have a 1.5 safety factor (programs such as RetainPro still use this long standing practice). What the code fails to say, but I believe is the case, is that walls designed using the load combinations in Section 1605.3.1 (0.6D+H) are considered to meet this provision. I believe this is just an oversite in the code and will hopefully be addressed in the future.

Personally I liked the way this issue was handled in the previous codes - stating specifically the failure modes against which a 1.5 safety factor was required rather than having an explicit load combination for it. I say this because the 0.6D+W load combination can have what I consider to be unintended consequences. For example checking a joist for uplift - in the previous codes, this would not have required a safety factor as it is not "overturning or sliding", and in the new code it in essence does have a safety factor because it is within the load combination.

 

[miecz]

WillisV

Thank you for clearing that up.

 

[swearingen]

Where this is especially problematic is with the 0.6D + 1.0E load case. Does your E have to be calculated with 0.6D or full dead load? If you are reducing your dead load, wouldn't the earthquake have less effect on your structure?



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[strucguy]

swearingen -

The load case is supposed to be 0.6D + 0.7E, and, E is based on your full dead load. The 0.7 factor in the combination accouts for reduced earthquake effects to be used in allowable stress design. Please note that "E", as per the definition in ASCE 7, is based on strength principles. Correct me if I am wrong.

Sravan

 

[csd72]

Your factor of safet is NOT 1.0 it is 1/0.6 = 1.67>1.5

Use the correct terminology and you will avoid confusion like this.

Factor of safety is a working stress term not limit states, so it refers to the actual loads not the factored loads.

csd

 

[CJJS]

csd,
According to WillisV, the factor of safety is 1.5. The code assumes there is a 10% overestimation on the dead loads. Therefore .67*.9*D=.6D
1/.67=1.5... which is your factor of safety.