Alternative Basic Load Combinations 1

[WARose]

I am working with the Florida Building Code [FBC 07]. And I had a uplift situation (for some anchor bolts) I am trying to address with the Alternative Basic Load Combinations [sect. 1605.3.2]. One thing that I don’t get here is the absence of a load combination that has just dead load and wind. Granted, the introductory paragraph tells you to have a 0.66 factor times the dead load (where it counteracts the wind) and the load factor on the wind is 1.3......but this still seems to me that it could possibly underestimate the uplift. Anyone know the reason this combination isn’t included?

 

[firai]

The alternative combinations are taken from the 1997 UBC and definitely don't reflect the latest knowledge. I would suggest at least using the basic ASD combos instead to capture all the effects that you pointed out.

Structural Design Engineer
New York, NY

 

[NITTANYRAY]

I would use the ASD load combinations in ASCE 7 which include both .6D + W and D + W. The reason for checking both is that tension are usually governed by .6 D and compression members when checking overturning are usually governed by D + W or .75 L + .75 W.

 

[WARose]

Thanks for the replies. Trust me: I'd love to use the basic ASD combinations [that include a 0.6D+W combo]. Only problem is: that's killing me for uplift. That's what got me looking at the alternative combos.

Since my original post, I found this [on this topic]:

Section 1605.3 Load Combinations Using Allowable Stress Design

--------------------------------------------------------------------------------

June 2007 » Columns

The purpose of this "Code Simple" is to shed some light on the load combinations used to check overturning and sliding for allowable stress design (ASD).

By S.K. Ghosh, Ph.D., and Susan Dowty, S.E.

e-Mail Print

The purpose of this "Code Simple" is to shed some light on the load combinations used to check overturning and sliding for allowable stress design (ASD). First, a review of the two sets of load combinations used for ASD will be provided. The "basic" load combinations shown below set forth in International Code Council’s 2006 International Building Code (IBC) Section 1605.3.1 are taken directly from the American Society for Civil Engineers’ Minimum Design Loads for Buildings and Other Structures (ASCE 7-05), Section 2.4:

Equation 16-14: 0.6D + W + H
Equation 16-15: 0.6D + 0.7E + H

The code-prescribed earthquake load effect, E, is multiplied by 0.7 to align allowable stress design with earthquake effects set forth in the code, which are based on strength design.

The "alternative basic" load combinations set forth in 2006 IBC Section 1605.3.2 are taken from the 1997 Uniform Building Code. The following equations address the situations where the effects of lateral or uplift forces counteract the effects of gravity loads:

Equation 16-17: D + L + (?W)
Equation 16-18: D + L + (?W) + S/2
Equation 16-21: 0.9D + E/1.4

These two sets of ASD load combinations are based on different philosophies and are not specifically intended to be equivalent to each other. The "basic" set of ASD load combinations adopted from ASCE 7 is based on the premise that the design strength resulting from the allowable stress method should, in general, not be less than that resulting from the basic strength design method. The alternative basic set of ASD load combinations is based on the premise that the designs should be about the same as those resulting from the Uniform Building Code.

Answers to FAQ’s:

Q: Why does Equation 16-14 have a load factor of 0.6 on the dead load, D, but Equations 16-17 and 16-18 do not? Also, aren’t Equations 16-17 and 16-18 much less conservative than Equation 16-14 because they do not have a load factor on D and include live load, L, and snow load, S?

A: In 2006 IBC Section 1605.3.2 (Alternative basic load combinations), there is a statement that reads: "For load combinations that include the counteracting effects of dead and wind loads, only two-thirds of the minimum dead load likely to be in place during a design wind event shall be used." This is obviously the same as multiplying the dead load by 0.67, which is not very different from 0.6. The live load and snow load effects will help in counteracting the wind load effect when using the "alternative basic" ASD load combinations, and as a result, they are somewhat less conservative than the "basic" load combinations. When it comes to sliding, as opposed to overturning, Equations 16-17 and 16-18 are indeed much less conservative than Equation 16-14 with its 0.6 factor on D.

Q: Why would anyone want to use the "basic" ASD load combinations for seismic design? The reason I ask this is because for seismic deign, you are allowed to use 0.9D for overturning checks with the "alternative basic" ASD load combinations, whereas with the "basic" ASD load combinations, you can only use 0.6D.

A: It is true that in seismic design, the alternative basic load combinations are likely to result in a more economical structure, because 90 percent, rather than 60 percent, of the design dead load effects can be counted upon to counteract service-level earthquake effects. In that sense, there is an incentive to using "alternative basic" ASD load combinations, which are in fact used more often than the "basic" load combinations. An increase in allowable stresses in masonry design that is permitted in conjunction with Equation 16-21 adds to this incentive.

S.K. Ghosh Associates Inc., is a structural, seismic, and code consulting firm located in Palatine, Ill., and Laguna Niguel, Calif. President S.K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the development and interpretation of national structural code provisions. They can be contacted at skghosh@aol.com and dowtyskga@cox.net, respectively, or at www.skghoshassociates.co

So it appears you can use those combinations even though they are much less conservative than the afore mentioned “basic” combos. As a sanity check: I also checked with a realistic minimum dead load and full wind load……not necessarily as part of my official calcs but just so I could get that warm fuzzy feeling about it.

 

[firai]

The next logical step would then be the same legal/ethical dilemma all over again: should you use the code-allowable combinations even though they're probably against your better judgment? I personally wouldn't feel comfortable doing so in this case.

Structural Design Engineer
New York, NY

 

[3doorsdwn]

The next logical step would then be the same legal/ethical dilemma all over again: should you use the code-allowable combinations even though they're probably against your better judgment? I personally wouldn't feel comfortable doing so in this case.

That was the whole point of doing the "sanity check" I mentioned. [I.e. the likely Dead load + the full wind load.] If the Alternative combos were giving me something outrageous I wouldn't use them.....but it doesn't seem so in this case.

And that sort of prompted my question as to the mentality of having no combination without live load [other than the seismic case]. It doesn't let you completely off the hook: you do have to use a factor of 1.3 on the wind and 0.66 times the dead.....but with all that, I didn't follow including the live load.

 

[firai]

My best guess would be that they were trying to keep the number of combinations down and weren't as concerned about capturing every possibility.

Structural Design Engineer
New York, NY