ASCE 7-98 Allowable Stress Increase 2

[haynewp]

Have others been using a 1/3 increase in allowable stress for dead plus wind or dead plus seismic load when using ASCE 7-98? It seems that IBC takes it's wind and load combinations essentially from ASCE 7-98 and it does not permit a 1/3 increase in the basic load combinations. ASCE 7-98 is so poorly worded for the dead plus ONE transient load case that I'm not really sure what to do.

 

[Ron]

haynewp...
I agree that ASCE7-98 is poorly worded in this respect. I am using the 1/3 stress increase for transient loads, since, in my humble opinion, it can be interpreted either way under this document.

This trend appears to be a "not so gentle" push toward LRFD.

 

[measure]

See Sections 2.4.1, 2.4.2 Basic Combinations, and 2.4.3 Load Reductions. I hope this helps

 

[dougantholz]

Okay lets get this one right. When you have two transient loads (Wind, Live, Live Roof, Snow) AND Dead load you take 0.75 times the transient loads. This is equivilent to your 4/3 stress increase because it is a 3/4 load reduction. Do not multiply 0.75 times dead because it is there to stay. As always watch out that 0.75 *( W+Lr) is not less than W.

 

[dlew]

haynewp,

For many years the 30% increase in allowable stress for load combinations that include wind was allowed, independently of the number of loads in the combination. That increase was based on the fact that wind loads were transitory loads of short duration. High wind gusts are measured in seconds.

The ASCE 7-92 took the approach that any decrease in load shall be based on the probability that two or more loads in addition to dead load, happen at the same time. As Dougantholz said, the decrease in the total design load is equivalent to the decrease in allowable stress.

It is my understanding that there were a lot of discussions about the approval of this change by the commitee members, and as a compromise some loopholes were introduced in the final version.

For some unexplicable reason, this change affected only the Allowable Stress Design. For steel design, the LRFD's load factors remained as they were before the change. The load factors for concrete design were not changed either.

Also, the Commentary of ASCE 7 allowed certain industries to continue the use of the 1/3 increase in the allowable stresses. I believe steel decks and siding could still be designed increasing the allowable stresses.

AEF

 

[haynewp]

I talked to an AISC engineer today and he said that the 1/3 increase in allowable stress for a wind load only (not 2 transient loads) combination with dead no longer exists using the new codes (ASCE and IBC) and the upcoming ASD manual will reflect this. He said the ASD manual will reference ASCE 7 load cases.

 

[onlyp]

A very good resource for this 1/3 stress increase is "The Mysterious 1/3 Stress Increse" by Duane S. Ellifritt, Engineering Journal, AISC; Fourth Quarter 1977.

In which he quotes " ..I can see no valid reason why modern wind standards with gust factors and mean recurrence intervals and peak coefficients should not continue to permit the designer to use the 1/3 stress increse." But, remember it was almost 30 years ago.

I think we really need a furthur research whether all the new wind standards should allow that 1/3 stress increse.

 

[dougantholz]

ASCE 7-98 Section 2.4.3
"When structural effects due to two or more loads in combination with dead load, but excluding earthquake load, are investigated in load combinations of Sections 2.4.1 and 2.4.2, the combined effects due to the two or more loads multiplied by 0.75 plus effects due to dead loads shall not be less than the effects from the load combination of the dead load plus the load producing the large effects."

This is ASD design. This means that you may not, not, not also include the 4/3 stress increase.

 

[haynewp]

dougan...right but..
Right below that, "Increases in allowable stress shall not be used with these load combinations unless it can be demonstrated that such and increase is justified by structural behavior caused by rate or duration of load."
Isn't that just a little misleading??????

 

[dougantholz]

Haynewp,
What I believe ASCE is doing is puting the onus on the engineer if they have multiple transient loads to come up with a reasonable statistical or logical proof that would allow a further reduction; especially a reduction in dead load (or an increase in the allowable stress of dead load). I believe that in 99% of applications the .75 times multiple transient loads would govern.
The only instance I could think of where you might argue for further reductions would be if you had like a traffic barrier live load in combination with the maximum wind event - very short durations for each occurence; then I might wish to further reduce my loads.