Footing Overturning and IBC 1605.3.2

[SteelPE]

I am currently designing a combined footing to support a steel concentric brace frame above. Although I am not specifically using IBC 2006 I am using an adapted version of the code.

Previously there was a thread, which discussed overturning factor of safety that can be seen here.

http://www.eng-tips.com/viewthread.cfm?qid=183966&page=1

Basically it states that the factor of safety against overturning is already figured into the load combinations of 0.6D + 1.0W and 0.6D + 0.7E. Therefore, when I use these combinations with my loads I then conclude (resisting moments)/(overturning forces) > 1.0, (the FOS is already built into the combination when I take 0.6x(dead loads)).

To check my calculations I have decided to use the new combined footing module inside of EnerCALC. However, when calculating the resistance to overturning they are using the alternative load combinations of 1605.3.2 specifically equation 16-17 minus the live load component of this equation (0.66D +1.3W) and then asking requiring a factor of safety of 1.5 against this combination.

I don’t understand why they are using the alternative load combinations and I don’t understand why they are developing a FOS of 1.5 on top of this combination. Have I been doing this wrong or is EnerCALC incorrect with their caclulation?

 

[JAE]

EnerCALC is simply wrong if they are requiring a 1.5 SF on top of a 0.66D combination in my view.

 

[SteelPE]

Oops sorry, they give you the ability to vary the FOS for overturning and sliding inside of the program. I don’t know why they would set the default to 1.5 when they are using these combinations by default.

 

[JAE]

SteelPE - I noted on our RISA Foot program that they have a SF entry. The overturning checks are performed based upon the load combinations entered into the program and the 1.5 (or 1.0) SF that you enter is only for reporting purposes.

In other words, if you had a 0.6D + W combination that you have entered in the software, and entered 1.0 as the OT safety factor, the program would provide an automatic warning if you go over 1.0 on the overturing check.

So the SF in RISA, at least, is just a red-flag check and doesn't really affect the OTurning calculations directly.

Enercalc might be like that too.

 

[abusementpark]

I don't trust anything from Enercalc anymore. That program has more bugs than an insectarium.

 

[SteelPE]

Thank you for your responses.

I assume that they are using the alternate load combinations of 1605.3.2 because you benefit form the addition of the live load being included in the combination?

 

[vandede427]

I've warped my brain thinking about this before. If you just put all the load factors and safety factors aside and just back calculate a required dead load, then the 0.6D / 1.0 SF and the 1.0D / 1.5 SF are the same.

 

[MDBROOKS]

Gentlemen,

We have been aware of this for some time and have been collecting the opinions of many users on the topic. There are many preferred load combinations to use for reactions and deflections because the code is not specific on this (although I believe IBC 2009 has some more clarifying items).

The issue with stability combinations has various viewpoints among users nationwide.

Our intent is to always follow the code, but also to provide users with additional ability to specify controlling items.

We will most likely be adding a load combination database for reactions/deflections/stability. It will be preloaded with load combinations that the comments we have received suggest are most widely preferred and then the user can modify it from there (similar to the programs ability to allow you to specify load combinations for stress analysis).

Please always contact ENERCALC first with your questions as we have probably heard them before and can be more proactive with responses than this list (which we do not monitor).

If "abusementpark" can please contact us and let us know your concerns we can look into them.

Respectfully,

Michael D. Brooks
for
ENERCALC, INC