ASCE 7-98 Load Combination 1

[nype]

Under ASCE 7-98 Load Applications 2.4 and 2.4.3 (pg5,6) and Commentary C2.4.3 (pg 221). Is it correct in applying 1/3 Allowable Stress Increase (ASD Steel Design) to the load combination: Dead Load + 3/4(Snow load + Wind Load)?

The Comnmentary clearly cautions against double reduction, but it also provides that if the reduced (3/4) loads are transient they double reduction is acceptable?
Am I reading this correctly?

 

[rholder98]

Be sure and verify that 7-98 is your governing code. The 1/3 stress increase is not allowed under 7-02. (2.4.1)

 

[DaveAtkins]

My understanding is that when using 3/4 times transient loads, you cannot use the 1/3 stress increase for masonry or steel design (but you can still use the 1.6 increase for wood design).

DaveAtkins

 

[chichuck]

nype,

I think you are not reading that commentary section correctly. The ASCE 7-98 commentary refers to two different "types" of allowable stress reductions that are in various material standards:

1) ... a one-third increase in allowable stress for certain load combinations have justified that increase by this same concept.... The 1/3 stress increase in the 1989 ASD is in this category, and should not be used with your load combination.

2) ... allowable stress increases that are based upon duration of load or loading rate effects.... This one refers to such things as load duration factor in wood design, and also I think to situations where dynamic load effects are being considered. I am not certain if fatigue falls into this category or not.

But, your load case DL + 075(Snow + Wind) falls into category 1.

I do wonder though, if it is necessary to combine snow + wind. I usually do not, my assumption is that if the design wind event ever occurs (90 mph +), this will blow the snow off of the roof. (I realize that this may generate a whole bunch of replies concerning drifting, shadow from other nearby windows, direction of wind eg blowing toward an upward sloping roof surface etc. But bring those on, I'd like very much to get the insight that such an exchange would provide. I'd probably learn something from it.

Regards,


chichuck

 

[mrMikee]

I think that the 1/3 stress increase was removed from the steel ASD by one of the later supplements. Also the IBC specifies two different set of load combination equations depending on whether or not the 1.33 is used.

-Mike

 

[boo1]

The current edition of ASCE 7 standard is ASCE/SEI 7-05.

 

[rholder98]

Good point boo1. What does the 7-05 have to say about it (we don't have our copy yet)?

IBC 2003 references the 7-02. Are there any codes in force already that reference 7-05?

 

[UcfSE]

In the 2001 supplement to the AISC ASD 9th edition, the 1/3 increase is removed, as Mike pointed out. Either way, you cannot combine a 1/3 increase with a 0.75 load reduction. Double dipping is not permitted.

 

[boo1]

ASCE 7-05 is the current revision to the ASCE 7 standard
2003 IBC references ASCE 7-02
2006 IBC references ASCE 7-05

ASCE 7-05 Chapter 2.4.1 COMBINING NOMINAL LOADS USING ALLOWABLE STRESS DESIGN

1. D+F
2. D+H+F+L+T
3. D+H+F+(Lr or S OR R)
4. D+H+F+0.75(L+T) + 0.75(Lr or S or R)
5. D+H+F+(W or 0.7E)
6. D+H+F+0.75(W or 0.7E) +0.75L +0.75(Lr or S or R)
7. 0.6D+W+H
8. 0.6D+0.7E+H

 

[rholder98]

boo1: I should have phrased my question differently. What I meant was, have any jurisdictions already adopted the 06 IBC?

But this brings up a good question (perhaps I should go to a different thread). If the current ASCE 7 standard (05 in this case) is more stringent than the one referenced by the governing code, which should be used? For example: the Florida Building Code does not require the additional parapet wind load prescribed by 7-02 and later. In hurricane country, that makes a difference.

chichuck: Wind and Snow loads are required by code to be considered acting simultaneously, though they may be reduced when taken together (as you pointed out in your own response). You pointed out several reasons why, yet you said you usually omit that combination. Generally, if it's in the code, I figure there's a reason for it. I see your point, but I know there are tons of scenarios I couldn't begin to imagine.

 

[boo1]

That was my question too. What code body sould you use?
The IBC 06 is not addoped yet
The ASCE 7-05 is current
The FBC 05 recognizes ASCE 7-02

ASCE 7-05 (and 02) includes parapet design provisions.

 

[rholder98]

My mistake. I thought the current FBC still referenced the 7-98. Bad example, then, but the question remains. In a jurisdiction that references 7-98, should you design by 7-05? I would really like to hear everyone's thoughts on this, as it came up at my last job, and nobody could agree.

 

[haynewp]

There have been a couple of threads about this whether to use the designated Code or the most up to date one.

My thought on it is that if you use something other than what is required in a jurisdiction, then you need to make sure you are at least as conservative as what the jurisdiction requires. Or you can be held responsible if something happens.

However, since we are also obligated to protect the public and use engineering judgment, then if there is something that is in an older Code required by a jurisdiction you feel is not adequate when compared with an updated version of the Code(latest research information), then it may be seen as you should have used your judgment and applied the latest information from the new code if a catastrophe happens.

I would meet the jurisdiction requirements and use judgment based on the latest research as to whether I need to go beyond what the jurisdiction requires (as my responsibility as an engineer).

 

[rholder98]

Good point, haynewp, and one I tend to agree with. The argument of my former supervisor was that if we design it stronger than required by code, we make it needlessly more expensive, and the client will go to someone willing to design it to the less stringent code. It just comes down to your own judgment, like you say--whatever lets you sleep at night.

 

[WillisV]

As an aside to rholder 98 - the original 2005 FBC did in fact reference ASCE 7-98 (though in almost most other areas it is identical to IBC 2003 which references ASCE 7-02 - go figure). The reference was just recently updated to ASCE 7-02 in the FBC supplement dated Nov. 21, 2005.