Basic Wind Speed - 90 mph or 115 mph

[JohnnyB_]

Hello -

I am trying to calculate wind pressures and I am confused on the basic wind speed as noted in ASCE 7-10 Chpt 26. Per Figure 26.5-1A for Risk category II (residential) structures the design 3-second gust is 115 mph. This is self explanatory to this point.

Where it gets confusing is the local county code states, "Wind Load: The minimum basic wind speed for determining design wind pressure at any site shall not be less than 90 miles per hour, 3 second gusts sustained (115 ultimate)."

Per the county's direction am I to use 90 mph or 115 mph for the basic wind speed, when calculating wind pressure per Chpt 27 of ASCE 7-10?

Thanks in advance.

 

[driftLimiter]

Per your quote it says ....(115 Ultimate). Why not just use that seeing at is the highest of what you have determined. It isn't drastically higher either so I doubt it would have a large impact on your design. Generally speaking the local jurisdiction has the most ability to modify or override the model codes. In my area they scoff at the ASCE7 tabulated wind and snow loads and increase them significantly. Usually if the county wants to say that you can go lower than the model code it is cause there is a long history of doing that in the area.

 

[ChorasDen]

You should be using the ultimate design value, in this case 115 mph, per the local jurisdiction.

 

[phamENG]

Is this a residential building designed to an old version of the IRC (or a local code modeled from an old version of the IRC)? If so, that's the source of your confusion.

Look up the difference between wind load calculations based on ASCE 7-05 and ASCE 7-10. There was a shift from using a single wind speed for all buildings based on the 50 year mean recurrence interval (MRI) with an importance factor to using different wind speeds for each based on a variable MRI and no importance factor. This shift also took the wind speeds from 'ASD level' loading to LRFD/Ultimate level loading.

In practical terms:

A house designed for ASCE 7-05 might have design wind speed V=90mph (50 year MRI), I_W=1.0, and used in ASD load combination as 0.6D+1.0W.
The same house designed for ASCE 7-10 would then have a design wind speed V=115mph (700 year MRI), no I_W, and used in ASD load combination as 0.6D+0.6W.

The resulting loads calculated these two ways are generally within 2-5% of each other. (Not the case for ASCE 7-16 - 5 and 10 used roughly the same data sets to determine the wind maps, but 16 uses expanded data and the contours on the maps shifted.)

Though the IBC picked up the change immediately, the IRC was a little slow. There was at least one version after 7-10 came out (2012, I think) that still used the older 50-year MRI as the design basis. It has since changed and the newer versions of the IRC reference the new maps and load calculations.

 

[JohnnyB_]

Per Figure 26.5-1A for Risk category II in ASCE 7-10 the 115 is a 3-second gust. The county states a 90 mph 3-second gust.

The reason I am trying dial this in is that, based on the ASCE 7-10 Chpt 27 calculations the wind pressure drops 15 psf using 90 mph as opposed to using 115 mph. this is significant because calculating the (Lateral Force Resisting System) LFRS using perforated shear wall calculations shows that traditional sheathing and fastening schedules fail to provide the lateral resisting force necessary to resist the 115 mph wind loads but not the 90 mph wind loads.

I am designing a light framed structure and have an issue recommending a metal frame LFRS, especially when there are structures in the area with standard OSB sheathing and fastening schedules still standing. I am trying to find the balance between the calculated results and the actual data from existing structures.

 

[JohnnyB_]

Thanks phamENG.

 

[driftLimiter]

Either way, you must understand the difference that @phamENG is pointing out when taking either of these loads you must know if it is ASD or Ultimate then apply the correct load combinations to get a good result. Very little of the map in Table 26.5-1B is 90mph Ultimate.

 

[WinelandV]

JohnnyB,

In ASCE 7-05 and earlier, the entire middle of the country had a published wind speed of 90 mph. In ASCE 7-10, the entire middle of the country now had a published wind speed of 115 mph (Cat II buildings).

What changed?

There was a shift from using a single wind speed for all buildings based on the 50 year mean recurrence interval (MRI) with an importance factor to using different wind speeds for each based on a variable MRI and no importance factor. This shift also took the wind speeds from 'ASD level' loading to LRFD/Ultimate level loading.

In ASCE 7-05 and earlier, LRFD load combinations used a 1.6 factor, and ASD load combinations used a 1.0 factor for wind.
In ASCE 7-10, LRFD load combinations now use a 1.0 factor, and ASD load combinations now use a 0.6 factor for wind.

tl;dr - the 90 mph and the 115 mph give the same wind pressure once you account for the load combination load factors that go with each wind speed.

 

[ChorasDen]

That's a hole you want to be careful going down. I image most of the homes were designed per provisions for braced wall lines found in the local edition of the IRC. You will not be able to assign a lateral resistance design value to IRC braced wall provisions that will align with engineered solutions that are published in the methodology found in SDPWS, they are incompatible.

I do not use ASCE 7-10 very routinely, at a quick glance, looks like Figure 26.1-5A is for exposure category C. Is that what your project exposure is, or does ASCE 7-10 combine exposures B and C for wind speeds?

Curious how your structure is failing when using light wood framing, care to post a sketch and loads?

 

[JohnnyB_]

winelandv / phamENG -

I should have been more direct with saying, I do understand what phamENG is saying with the shift from ASCE 7 version 05 being directed towards an ASD approach and the 10 version shifting to the LRFD approach with a higher wind speed and a less conservative design factor in the load combinations.

I will ask the county if they are trying to de-rate the wind speed per the latest ASCE7 or if the 90mph 3-second gust was just a legacy notation from teh ASCE7 version more directed towards ASD design calculations.

 

[JohnnyB_]

ChorasDen -

Attached are my calculations. I will attach the sketch in a follow up post. IBC 2006 section 2305.3.8.2.1 also states that the allowable shear resistance shall not exceed 490 plf, for some reason??

 

[JohnnyB_]

ChorasDen -

Attached is the sketch of the wall.

 

[phamENG]

JohnnyB - based on your initial post, the county's intentions are quite clear.

V_asd=sqrt(0.6)*V_ult

If V_ult=115mph as your initial post indicates, then V_asd=89.1mph (about 90mph).

The V_asd equation in the code is a means of converting between wind speeds, primarily for apples to apples comparisons of test reports. It's clear that the county is saying 90mph (Vasd) and 115mph (Vult). If your code dictates a load combination of 0.6*W for ASD, then you should be using 115mph. If your code dictates a combination of 1.0*W for ASD, you should be using 90mph.

ChorasDen brings up a good point - that guidance is likely directed at builders and unlicensed designers using braced wall tables, not engineers designing diaphragm and shear wall systems. (I'm assuming you're doing the latter since you're calculating wind pressure - a step that is completely unnecessary in braced wall table usage)

 

[JohnnyB_]

Thanks Pham, that is straightforward. You are correct I am the engineer providing the stamped structural design.

I will use the 115 mph and try to figure out how to get around the steel frame that may be required.

 

[phamENG]

If it's required, it's required. If you want large, open floor plans and beautiful views, you have to pay for them.

I just did a 3 story house with a 3 story steel moment frame in the back and a 2 story steel frame in the front. Next week I'll be doing one with a 20' tall great room with a steel frame in the wall. If it needs it, it needs it. My career and my family's security isn't worth saving someone a couple thousand dollars on his house.

 

[JohnnyB_]

Thanks Pham. I will have to agree and adopt your position.

 

[Aesur]

I noticed a reference to 2006 IBC in your calculations - is this just a reference that needs to be updated, or are you using the older 2006 IBC? 2006 IBC still used ASCE7-05 hence why I'm asking. Also was the spreadsheet updated to use the correct load factors for 7-10?

 

[BridgeSmith]

Another way to look at it, that may alleviate some of your confusion over the county code's presentation, is that for a 90mph "fastest mile" wind speed (the basis of design under the older specs), the 3-second gust speed producing the pressure you actually design for, is 115mph. AASHTO (which mostly follows ASCE for wind loads) used to handle it by calculating the wind pressure using the "fastest mile" wind speed (90mph for most of the country) multiplied by a 1.3 "gust factor" (90mph*1.3 = 117mph). Now the AASHTO spec has maps showing directly the 3-second gust speed (115mph for those same areas), and has eliminated the gust factor, so you get the same answer; it's just calculated differently. All that to say, it seems to me that they want you to design for the wind pressure of the 115mph 3-second gust, which if I understand correctly, is the same as ASCE.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[JLNJ]

If you go with ASCE 7-16, you might be able to skinny the wind loads down a little. In many areas of the country you can interpolate between contours and use less than the 115 nominal ultimate speed.

 

[JohnnyB_]

Aesur -

The 2006 IBC reference was due to the current IBC not having detailed calculations for shear wall design. Since I made these calcs I have found the 2015 version of the AWC SDPWS.