Wind Load Reduction 2

[EngineerofSteel]

Hmmm..

I designed a CMU wall using UBC 1612.3.2 alternate load combinations and took a .75 reduction to calculate my WL of 12.96 psf.
Using Enercalc, I took a 1.33 short-term load reduction for the steel reinforcement. The reviewer kicked my design back, saying I have "effectively doubled up" on the .75 discount. He noted I used the 1612.3.1 load combinations (not correct).

I believe I am correct in reducing my WL and also increasing the short term strength of my steel. I can't remember where the section for the short term increase in steel strength is, any help?

Any input?

Thanks, Dairydesigner.

 

[JAE]

Why did you take a 0.75 factor on the wind under 1612.3.2? I don't see that in the UBC.

 

[EngineerofSteel]

Ah.

JAE,
In seeking to answer your question, I found mine. page 2-240 Section 2209 A5.2 "Wind and Seismic stresses" Allowable stresses may be increased for load combinations, including wind and seismic, as permitted by Section 1612.3.2. No increase in allowable stress is permitted for Section 1612.3.1"

This was the source of my reduction, in both cases. It's the reason I could not find (an imagined) second source for a short term increase for the steel. This increase originates from here, in 2209.

So the reviewer was right. I hate it when that happens!

Thanks, DD

 

[Lutfi]

It sounds to me that you double dipped!!

Although I do not design to the UBC, it does not sound logical and conservative, to take a reduction of 0.75 in the combination and use the 33% stress increase. This is what I call double dipping.

From the sounds of things, I must agree with the building official.

The 33% stress increase gets engineers in so many heated debates. I never liked it. AISC allows the 33% stress increase under ASD even for DL + WL combination! I do not follow that rule.

For the origins of the 33% stress increase, I suggest that you read a paper published by DR. Duane Ellifritt.

Here are some links to some valuable readings as well:

http://www.aisc.org/MSCTemplate.cfm...Management/ContentDisplay.cfm&ContentID=26812

Regards,
Lutfi

 

[mrMikee]

Someone mentioned in a thread about two months ago which supplement withdrew the 33% stress increase, so it's not actually part of ASD anymore.

About 15 years ago I designed silos and bins and seldom used the extra 33% which fit in with companies position of building stronger products. Recently I got back into the business and found that I really have to use the reductions in the new load combinations (IBC) to get a competitive design. I'm still not real comfortable with this yet. But at least the reduction for wind only (or overstress depending on how you look at it) is gone.

Regards,
-Mike

 

[DaveAtkins]

I don't know about UBC, but my understanding of the IBC is that for designing CMU, you can't multiply WL by 0.75 (unless you are designing for DL + LL + WL), and you can't use the 1.33 increase either (unless you are using the Alternate Load Combinations, which increase WL by 1.3 anyway, thus eliminating any advantage of using the 1.33).

DaveAtkins

 

[akastud]

Dairy Designer,

You didn't specify what the wall was for, but section 1624 allows you to multiply your wind load by .75 for certain structures, and I still think you get the 1/3 increase.

akastud

 

[EngineerofSteel]

akastud,

I do use the qs reduction allowance for agricultural buildings. These must, by definition be unoccupied ag. Milking parlors, as in this design, are occupied a significant portion of the day, during the milking cycles. Thanks, DD

 

[EngineerofSteel]

Ah, I found the reason I made the discount. UBC 1621.1 allows for a 1/3 reduction in the combined effects of uplift and overturning when the height to width ratio is less than .5, and my building height is only 27 feet at the ridgeline, under the required 60 foot max for the allowance.

My h/w ratio is only .34

Does anyone know why they allow this discount? The building is simply stronger with these proportions? Wind behaves differently over a structure like this?

Any insight is appreciated. There may be a conversation with the county office there.

-DD