wind directionality factor

[bjb]

In ASCE 7-98, they have the wind directionality factor, and in the LRFD load combos they changed the factor on W to 1.6 from 1.3 to compensate. How come in the allowable stress combos of ASCE 7-98, and the basic ASD combos of the IBC 2000 there is no apparent accounting for the directionality factor? In the commentary in ASCE 7-98, they only talk about the load factor, so maybe they were only thinking about LRFD and neglected ASD.

When I use the basic ASD load combos, I take the directionality factor equal to 1, because if they adjusted the LRFD load combos, I think there should be a similar adjustment to the ASD basic combos. Otherwise my wind loads end up being smaller than those calculated with ASCE 7-95

My apologies if this has been dealt with in the latter IBC and ASCE 7 versions, but here in New York State we have to keep on using the 2000 IBC. We're supposed to move on to the 2003 IBC sometime in 2006.

 

[JAE]

Could I be wrong or didn't the IBC also take out the 1/3 stress increase which "accounts" for the difference?

 

[JAE]

Also - read the ASCE 7-98 commentary - C2.3 as they discuss this a bit.

 

[mrMikee]

Alternate load combos in section 1695.3.2 do still allow increased stresses (where permitted) which I think could be confusing with ASD and LRFD. It seems to me to be too easy to make a significant error in the basic setup of your calcs.

I second JAEs suggestion to look into the ASCE commentary C2.0 for a good discussion of this. Also chapter 2 in "Loads and Load Paths in Buildings" by Narenda Taly has an extensive discussion of the loads, combos, and the codes.

Regards,
-Mike

 

[DaveAtkins]

Basically, I think, JAE is correct. When you use the wind directionality factor, you cannot use the 1/3 stress increase. I use the wind directionality factor with ASD, and never use the 1/3 stress increase anymore.

DaveAtkins

 

[bjb]

I have looked at this commentary, and it seems focused on lrfd combinations, and only says that directionality effect was not taken into effect in ASD, which I assume means the ASD combinations. In my "thesis" below, I use LRFD instead of the term "strength design".

In the commentary on the directionality factor, it says that the directionality factor was hidden in the 1.3 load factor for wind according to the 1995 version of ASCE 7. Now they have broken this factor out, and incorporate it into the equation for velocity pressure. This lowers the velocity pressure, and to compensate, they increased the load factor to 1.6 for W. At least that was one of the reasons they increased the load factor. The same calculation for velocity pressure is used if you are using ASD combinations or LRFD calculations. Therefore, in ASCE 7-95 directionality was taken into account only in the load factor for W when using strength design. It was not in the equation for velocity pressure, so directionality was not taken into account for ASD design. Now that directionality is in the velocity pressure calc in 7-98, directionality has now become incorporated into ASD designs, but in contrast to the LRFD combinations, there is no corresponding increase to the wind load in the load combinations.

For LRFD, your wind pressure W is less with ASCE 7-98 than if using 7-95, but the load factor in 7-98 increased to compensate, so you have the same wind effect with LRFD combinations when comparing 7-98 to 7-95. (This ignores changes to GCpi and some other tweaks in the code). For ASD, this is not the case because there is no compensation in the ASD basic load combinations.

If the wind effect between 7-95 and 7-98 when using LRFD combinations is the same, how can the wind effect when using basic ASD combinations per 7-98 be less than the wind effect with 7-95? I think that there is a contradiction here, and that is why I use Kd=1 when using basic ASD load combinations. I'm not seeing the same reasoning behind the treatment of the load combinations for ASD or LRFD, and I don't see anything in the commentary that addresses this. Maybe I am missing something here and being too conservative, but I have a hard time rationalizing the treatment of the directionality factor with ASD combinations.

 

[aggman]

I think you answered your own question in the first paragraph of your second post and you are just not seeing it. According to the commentary, in ASCE 7-95 the wind directionality factor was only accounted for in the LRFD load combinations and was neglected in the ASD equations. In the 98 edition they adjusted the wind directionality factor into the velocity pressure equation so that it could be accounted for when using ASD or LRFD. The LRFD equations where then adjusted to 1.6 from 1.3 to take the directionality increse out.

(1.3 / .85) = 1.53 - Approx = 1.6

This is why you have the difference. By definition in the commentary your ASD wind loads should be lower when comparing 7-95 - 7-98. Your LRFD loads are the same under factored loads.

 

[aggman]

To continue on my above post.

In essence in the ASCE 7-95 you were penalized if you were using ASD. I don't think the allowable stress increase has anything to do with it. We are still using allowable stress increases now with the 7-98 and 7-02. It is only wrapped up in the 0.75 multipliers.

 

[bjb]

I don't doubt that you may be right. I would have liked to see something in the commentary. In my opinion, going from no consideration of directionality to considering it with ASD should have been worth of an explanation. They should be saying something to the effect that "in the past we didn't account for directionality with ASD, but now because we know so much more we will allow its consideration with ASD". I find both the IBC and ASCE 7 to be confusing and not always well written.

 

[aggman]

I guess I am seeing them say it in the commentary. On page 219 (7-98), right hand side, in the second sentence of the last paragraph. "This directionality effect was not taken into account in ASD."

I agree that they should have explained why they didn't include it in the ASD but I would assume it had to do with the fact that in the late 1990's code writers believed (or wanted to believe) that no-one should or would be using ASD very much longer so why incorporate it into the load combinations. It would only confuse the issue more.

 

[bjb]

Thanks to all for the thoughtful replies.

At the time that ASCE 7-98 and the 2000 IBC were being developed, I don't believe that there was a published or code accpeted method for using LRFD in wood design, and I think that today most (but not all) engineers are still using ASD for wood design, and that many still use ASD for steel and masonry. The code writers have done the design community a disservice in my opinion by almost ignoring ASD when a large percentage of engineers still use it. If the future of structural engineering is really strength design based for all materials, then I think it should be realized that there will be a transitional period where both methodologies need to be adequately supported by the code.

Out of curiosity aggman, do you use ASD or LRFD for steel, wood, and masonry? I use ASD for these materials, but not for concrete. However, in my area concrete is mostly only used in foundations, not much in superstructure framing, and I haven't found the use of LRFD in concrete to be that cumbersome. That may just be because most of my concrete structures are very simple. I do know engineers that still use allowable stress for concrete, they argue that it's so much easier to use, and if you're just doing a lot of footings with concrete the conservatism and extra rebar are not a bad thing.

 

[aggman]

I use ASD for everything except concrete. LRFD makes sense to me for concrete because it’s a non-linear material anyway. Interesting enough I learned only LRFD in college but now I use only ASD. I work in an industrial market and I like being able to "beef" up my structures by just looking at lower stresses. It does not create the uniform strength across the structure that the LRFD approach gives but my customers don't care about that. What they want is something that can survive in a harsh environment for long periods of time.