Question on wind load provisions of IBC 2000 1

[jheidt2543]

I put forth the following examples regarding the wind load charts in the IBC 2000 Building Code. In Table 1609.6.2.1 (2) Component and Cladding Loads for a Building with a Mean Roof Height of 30 Feet Located in Exposure B:

Example 1: If I have a wall system with girts at 6’ center-to-center and column spacing of 25’ center-to-center, the contributing area (effective wind area) for a girt would be 6’ x 25’ = 150 sf. If I’m in a wall Zone 5, with a basic wind speed of 90 mph, then Table 1609.6.2.1 (2) gives wind load values of +12.4 psf and -15.1 psf, based on Note a.

Example 2: If in the same wall system as Example 1, I install 6” metal studs at 16” on center x 8’ high, below the first girt, then the contributing area (effective wind area) for the stud would be 1.33’ x 8.0’ = 10.64 sf. For the same conditions as in Example 1, the wind load values would be +14.6 psf and -19.5 psf.

Example 3: For the same conditions above, with a building eave height of 20’, the building columns would be sized based on a contributing area (effective wind area) of 25’ x 20’ = 500 sf. From Table 1609.6.2.1 (1) Main Wind Force-Resisting System Loads for a Building with a Mean Roof Height of 30 Feet Located in Exposure B, the wind loads transferred to the columns from a wall End Zone with a Roof Angle of 0 to 5 deg., the wind load would be 12.8 psf.

So for the same wall surface with the same wind blowing, there are three different positive wind loadings, 12.4, 14.6 and 12.8 AND two negative loads -15.1 and -19.5. Is this correct? Also, am I interpreting the column under the C&C Table labeled Effective Wind Area correctly?

Are you all actually refining your designs this much, or do you just take the highest positive value and the highest negative value and use them through out? You don’t change girt sizes between the end panel and the middle of a building side wall do you?

 

[JedClampett]

I, like you, am trying to get familiar with this code, so I'm figuring out all the cases. I'm hoping something will click and it will all start to make sense.
I was talking to another engineer in my office and we suspect it's just a scheme to sell wind load software.

 

[JAE]

Your numbers look right (although I'm away from my book/spreadsheet right now) - but yes, each "piece" of the structure has different tributary areas and result in different wind pressures, with the smaller areas resulting in larger pressures due to localized spikes in the pressures.

I usually do use the different At values and wind pressure values when designing girts vs. columns. But usually the end zone values aren't that much higher and we do sometimes simplify by using the larger value. But rarely do we use the larger value of pressure due to a different At value. Since you design the girts separately from the columns, etc. we just use the applicable value.

I'm not sure of your last numbers (using the MWFRS for the column - I believe you still have to use the C&C value for the individual column design if the At is less than 1000 sf. Once over 1000 sf you can then go to the MWFRS numbers.

 

[jheidt2543]

JAE,

Now you REALLY confused me or I missed a few paragraphs in the code. Where did you find the 1,000 sf limitation? My thinking is that you decide if a member is a MWFRS or a C&C and then use the appropriate table and effective area size. The effective area numbers are 10, 20, 50 and 100 so, I don't see where the 1,000 sf comes from.

JedClampett,

Yes, it is a despicable plot too. I think you can add seminar givers and thesis writers to the list of culprits.

What happened to the days of 20 psf wind load for everything (at least in the midwest)? Maybe the price of steel has gone up so much that we can now do are part in reducing the price of buildings, cutting down on pollution from steel mills, saving the enviornment, apple pie and motherhood. Well, I do like apple pie and motherhood!

I'm all for economy in building design, but the codes are getting way too complex. Just because our computer programs can finite element a building down to the Nth degree, doesn't mean we should. We should remember that complexity is the borderline between simplicity and chaos.

 

[JAE]

Under 1609.6 - the simplified version in the IBC 2000, you are essentially correct - that you "decide" whether or not a given element is a component or part of the MWFRS.

My 1000 sf limit was based on ASCE 7 where they also give C&C pressures up to 100 sf and then provide a footnote saying that for areas between 100 and 1000 sf use the 100 sf area in the calcs. Then it goes on to say that elements with areas > 1000 sf should use the primary MWFRS pressures.

This is all based on the concept that the higher pressures are due to small area localized spikes in the wind pressure that would affect a small element, while an element taking wind from a larger area would have those spikes in pressure averaged out more and result in a lower pressure overall. As your element gets bigger, with very large tributary areas, they actually approach the primary pressures and ASCE 7 allows you to utilize them.

So I guess that if you use the IBC 2000 simplified, technically the 1000 sf concept won't apply.

 

[denoid]

I recently attended a local monthly CASE, (Council of American Structural Engineers), meeting here in Minnesota where a number of my colleagues have become incensed about the complexities of the IBC wind load tables. They formed an Ad Hoc committee and have developed a "simplified" alternative MWFRS wind load table which has been proposed to the MN State Bldg Code Advisory Committee. (The jury is still out whether it will be accepted to our state bldg code amendments to the IBC, but it was well received in the Advisory Committee, I am told).

Esssentially, they developed a single horizontal and vertical pressure for each case of Exposure B, C, and D, as well as a single Overhang vertical pressure for each exposure case, based on our state wind speed and common sense usage. I was not on the Ad Hoc committee, so I don't know further details of the numbers they generated for use.

In the presentation to the CASE meeting, the idea is "simple is better", and due to general confusion over how the IBC tables are to be applied, simple values using common sense will better serve our profession better than getting 20 different interpretations of IBC table usage by 20 different SE's.

I like it! I hope our state accepts the amendments, even though I'm told it will take more than a year to go thru the legal and political process.

 

[CTSeng]

The new wind load criteria can seem difficult to manage but like any new systems after you learn them and work with them you will see some benefits. The ranges of Component and Cladding pressures based on Tributary Areas probably do more accurately depict the real pressures but not a lot has actually changed in design. For members with small Trib Areas the pressures are high but the spans are low so that traditional member sizes still work fine while members with large Trib Areas have significantly smaller pressures. Spend a little time creating a spreadsheet and you’ll find that it’s not as difficult as it seems.

The good and bad thing about complicating the wind loads is that there seem to be unlimited unique conditions that do not easily fit the code criteria. This leaves a lot of room for an individual’s judgment. This is what I like to call engineers job security. Since the code changes I’ve been asked many times by unknowing architects ‘how do I find the wind loads for this wall. I used to be able to get the wind loads and design this myself.’

Here in south Florida the new wind loads have taken a while to get a handle on but the savings would be significant compared with using a uniform worst case pressure.

PS JAE: ASCE7-98 section 6.5.12.1.3 Tributary areas greater than 700ft^2: Component and Cladding elements with tributary areas greater than 700ft^2 shall be permitted to be designed using the provisions for main wind force resisting systems.

 

[JAE]

CTSeng - sorry - you're correct - 700 sf is the limit - I have a bit of the UBC disease in me still.(UBC has the 1000 sf limit)

 

[jheidt2543]

Thanks for all the REALLY helpful comments. Not only did I learn something, but it is nice to know others have struggled with the same questions - missery likes company?