ASCE 7-98 Load Combination
ASCE 7-98 Load Combination
(OP)
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?
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?






RE: ASCE 7-98 Load Combination
RE: ASCE 7-98 Load Combination
DaveAtkins
RE: ASCE 7-98 Load Combination
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
RE: ASCE 7-98 Load Combination
-Mike
RE: ASCE 7-98 Load Combination
RE: ASCE 7-98 Load Combination
IBC 2003 references the 7-02. Are there any codes in force already that reference 7-05?
RE: ASCE 7-98 Load Combination
RE: ASCE 7-98 Load Combination
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
RE: ASCE 7-98 Load Combination
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.
RE: ASCE 7-98 Load Combination
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.
RE: ASCE 7-98 Load Combination
RE: ASCE 7-98 Load Combination
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).
RE: ASCE 7-98 Load Combination
RE: ASCE 7-98 Load Combination