Wind: ASCE 7-10 versus 7-05 2

[TomBarsh]

ASCE 7-10 introduced a new load combination reduction factor of 0.6 for wind in paragraph 2.4 (ASD) versus a factor of "1.0" used in 7-05.

The Commentary provides some explanation of the basis for this change. But the end result seems to be that for a nonbuilding structure designed to paragraph 29.5 the base shear is reduced by 40% from same structure analyzed under 7-05 rules. I wonder if this was fully intended by the Committee. Any comments? Thank you.

 

[WillisV]

Huh? The velocity "V" which is used in the determination of qz that is part of equation 29.5-1 has been increased from ASCE 7-05 to ASCE 7-10 such that the force calculated by this is already at strength level and a 1.0 load factor is now used with LRFD.

Th base shear is approximately equal to the previous strength level of 1.6 times the force calculated using the slower velocities from ASCE 7-05.

 

[SAIL3]

So the wind velocity shown now has no meaning in real life?..a doctored, ficticious number!. At least it is in keeping with the spirit of these new revised codes.

 

[WillisV]

@SAIL3 - No. The target has always been to design for a mean recurrence interval (MRI - i.e. return period) of about 700 years at strength level for wind. This was accomplished by provided 50 year MRI velocity maps in ASCE 7-05 and then increasing by a 1.6 load factor which had the effect of increasing the MRI to 700 years. ASCE 7-10 did away with that smokescreen and just gives you 700 year MRI wind speeds to use with a 1.0 load factor.

It is actually a bit more complicated than that in hurricane regions. in ASCE 7-05 the wind speed maps were fudged to represent MRIs between 50 and 100 years so that when multiplied by the 1.6 load factor they would still get an ultimate MRI of about 700 years (because hurricane recurrence intervals are calculated differently than inland storm recurrence intervals). The new maps even the playing field by just using the 700 year recurrence interval wind velocities in both non-hurricanes and non-hurricanes areas with a 1.0 L.F. so that no wind speed fudging is necessary.

In summary, though different than ASCE 7-05 (and nobody likes change), what is presented in ASCE 7-10 actually makes MORE sense and is MORE transparent than what it used to be.

 

[Lemony]

I don't know about that Willis.

-The question was about ASD
-In most places there is 50 years of data (where is there 700?)
-This change introduces the possibility (yes it would be a bad, stupid mistake) of mixing and matching load factors and maps.

Just my opinion.

 

[TomBarsh]

Okay, mystery is solved for me. Bear in mind that my work deals with nonbuilding structures. In this work (ASME pressure vesselS) the wind speed and other site conditions are normally specified by the purchaser, and are not the responsibility of the vessel manufacturer. So the selection of the wind speed is outside our purview (whereas determination of the wind pressure is).

Calculation of the velocity pressure (qz) is identical between ASCE 7-05 and ASCE 7-10, at least for a rigid nonbuilding structure. Thus for a "given wind speed" the calculated pressures will be identical. The application of the load combination factor 0.6 for wind will result in lower base shear under the new Edition.

However, the wind speed maps have been changed in the new Edition, increasing the standard wind speed for the same location. Thus entering the new, higher wind speed in the calculation for factored wind load should yield near to the same load as the older Edition.

I have to wonder if this has really been picked up by the engineers who specify the purchase of the pressure vessels. Yesterday a customer's consulting engineer (PE) wanted to apply the "importance factor" to the ASCE 7-10 wind load, as in ASCE 7-05. ...but the new Edition treats this quite differently.

 

[WillisV]

@Lemony - return periods are extrapolated not based on an actual set of data lasting that long - the return period for the maximum considered earthquake used in ASCE 7 is approximately 2,500 years, do you think we have 2,500 years of good seismic data? The wind provisions are just catching up with the seismic provisions with the use of ultimate values and 1.0 load factors. ASD works the exact same way, you are just putting a reduced load factor in combination with an increased safety factor.

@TomBarsh - correct. The wind speeds used with ASCE 7-10 equations need to be based on the new maps. That coordination certainly needs to take place between purchaser and manufacturer.

 

[SAIL3]

this is getting more opaque by the minute....so the new speed map
showing 150mph is actually approx. 90mph in the real world....not to worry...these are ultimate speed levels!!...this is fine if your goal is to invent a new language or logic....maybe we should apply the same logic to our engineering income...so I just got a hefty virtual raise in income...feeling better already.

 

[JLNJ]

The is like going from Seismic Zone information in the UBC to the seismic contours we now all use. Equipment vendors to this day are still asking, "What seismic zone are you in?".

I can only imagine how long it will take non-building equipment guys to get up to speed on this new-fangled "ultimate strength" wind.

 

[TXStructural]

I think the -10 MRI is 1700 years. The change was simply to remove the load factor and importance factor. Now there are different maps for different risk categories. So much simpler. (wink)

Also, the wind speed shown in FEMA 361 for tornados are 100,000 year recurrence interval.

 

[WillisV]

@TXStructural - I was basing my discussion on standard Occupancy Category II Buildings (I=1.0 in ASCE 7-05). ASCE 7-10 uses an MRI of about 700 yrs for those as noted.

For buildings in Occupancy Categories III and IV (corresponding to old higher importance factors) the MRI is 1700 as you suggest.

 

[steellion]

Don't mix and match! If you use ASCE7-10 wind speeds with ASCE7-10 load combinations, you will end up with approximately the same wind pressure as with ASCE7-05.

 

[Lion06]

I'd like to know how the extrapolation was made to a 1700 yr return period with 50 years of data. Does anyone know?

 

[JStephen]

TomB, I was just at an ASCE seminar on ASCE 7-10; I thought it was well done and recommend it if you have a chance.

Lion06, that's just statistical manipulation. If you have a given probability distribution, whatever level gives you a 1 in 1000 chance of occurring in a year has a 1,000 year return period, etc. It doesn't mean the wind will blow that strong once and only once every 1,000 years, or that you had to record wind speeds for 1,000 years to get that data.

 

[Lion06]

JStephen, that's what I thought, but when I do that manipulation, I don't get the same ratio from a 50-year wind to a 10-yr wind (per the table from the commentary ASCE 7-95).

 

[Longweekend]

This is all great dialogue. I would like to hear peoples thoughts on presenting this to a Client or Owner who does not understand ultimate loads.

If I put 160 mph on the note sheet of my building drawings the average non-engineer will get a false sense that their building can handle a hurricane of up to 160 mph. When actuality it is designed more to a sustained wind of 115 mph.

Does anyone have a suggestion of wording or language that can be put on the drawings to demonstrate the sustained or the more realistic design wind speed for a building?

 

[dcarr82775]

Oh great, another I hate LRFD threat. Can we just incorporate it into the thread form a month or two ago?

 

[dcarr82775]

thread of course, not threat

Longweekend,

Nope, your building is designed to resist the wind speed of 160 mph.

 

[Hacksaw82]

Funny this should come up. I just had to deal with this issue a few weeks ago.

We did a wind calc for a architect client doing an interior reno of an existing tall hotel in Texas. No structures on the job, but the window guy needed design pressures. The owner wanted us to use the 2010 loads.

I did the calcs, based on the 'new' speed maps. Usually in a case like this we just give the design pressures, unfactored. It would be up to the contractors engineer to apply the correct combo factors.

When we sent the loads, they freaked out. They said they couldn't design to such high loads. Even though we told them, these were based on the new methods, they were not yet familiar with the new methods.

We needed to walk them through it, and do the calcs again, with 2005 methods so they could understand.

I fear this won't be the last time we run into this kind of confusion with this new method. I respect the science and reason behind it. Unfortunately our clients, owners and contractors are not scientists, stats professors, or academics. It's going to take a while to get them to understand, "my building is designed for 160mph wind, but safe for 90mph."

 

[WillisV]

Hacksawy, I think you have it backwards, previously it was your building is designed for 90mph, but can withstand 160mph without collapse. Now it is simply your building was designed for 160mph without collapse.