Int'l Bldg Code Wind Loads 2

[papaaj]

In table 1609.6.2.1(1) of the 2000 IBC, there are columns for "Horizontal Loads" on walls in the "End Zone" and the "Interior Zone", and there are also columns for "Maximum Horizontal Wall Loads" in Zones 1E, 4E, 1 and 4. Zones 1E and 4E are the same as the "End Zones", and Zones 1 and 4 are the same as the "Interior Zones", so it appears the different sets of columns are giving loads for the same portions of the walls.

It seems that the loads presented in the columns entitled "Maximum Horizontal Wall Loads" will always govern over those in the corresponding columns entitled "Horizontal Loads". Is this correct? If so, what is the purpose of including the columns for the "Horizontal Loads" on the walls, if the "Maximum Horizontal Wall Loads" always govern? In what circumstances would one use the loads in the columns entitled simply "Horizontal Loads"?

Thank you very much for your assistance.

Tony

 

[JAE]

Tony:
Good question - I've tried to come up with an answer and the only thing that I thought was that the far right column uses the phrase "Maximum" and goes to the trouble of separating the windward from the leeward.

The values in the left columns have to be multiplied by the Height and Exposure coefficients found in Table 1609.6.2.1(4) (this is in accordance with 1609.6.2.1). Thus, the values to the left may actually get higher than the far right table values.

Perhaps the far right column values are a ceiling (thus the term "maximum&quot that limit how high the wind loads get.

For a "simplified" method we sure have to struggle here, eh?

 

[papaaj]

Thanks for your response, JAE. Simplified indeed. I kept going back to confirm that I was actually using the simplified provisions.

What you said about the values in the "maximum" columns on the far right being upper limits makes sense. However, the way I interpret section 1609.6.2.1, it seems that the factors from table 1609.6.2.1(4) get multiplied by any value of pressure selected from table 1609.6.2.1(1), including those in the "maximum" columns on the far right. If this is the case, then those numbers in the "maximum" columns are not true upper limits. Furthermore, as you pointed out, the values in the "maximum" columns are broken out into windward and leeward pressures, and these components can't be compared directly with the values in the columns entitled "Horizontal Loads", as the latter are combined windward and leeward pressures.

Very confusing. I'm inclined to just use the "maximum" loads and multiply those by the factors in table 1609.6.2.1(4). It may be a bit conservative, but that's not a big deal; I'm more concerned with understanding what the code is trying to say.

Tony

 

[JAE]

Tony,
Perhaps other Eng-Tippers out there will take a stab at it. I agree that it may be the far right column is also multiplied by the gust factors....

Maybe its more simple to go back to the main ASCE 7 procedure - We rarely use these simplified versions as we always develop spreadsheet templates based on the ASCE 7 and just use them - its easier.

 

[DaveAtkins]

I attended an IBC Code Seminar last winter, and the presenter was an engineer from BOCA. He said that the column entitled "Maximum Horizontal Wall Loads" is the wind load on the exterior wall, INCLUDING INTERNAL PRESSURE. When using the "Horizontal Loads" column, the internal pressures on the windward and leeward walls cancel each other out, so the overall wind load is lower. The "Horizontal Loads" column, with the appropriate multipliers, should be used for lateral analysis.

 

[JAE]

Thanks for clearing that up Dave...very good!!!

 

[haynewp]

I looked into this on a previous occasion and this is what I had found:

They are using something very close to ASCE7-98's coefficients for case"A" from fig. 6-4 of ASCE7. For example, 0 to 5 degrees gives a 0.4 wall "1" and a 0.29 wall "4" coefficient under ASCE. Add each one with 0.18 internal pressure from table 6-7 and you get 0.58 and 0.47 total windward and leeward coefficients.

Take the chart from wall 1 max horizontal of 6.4psf and wall 4 max horiz of -5.2psf under 85mph IBC table.
P=(6.4/(0.58))x0.4-(-5.2/(0.47))x0.29=(about 7.6 psf) as listed for total windward + leeward.

I checked this with other angles and wind speeds also, the values were approximate.

Interestingly, it seems that case "A" of ASCE7 is disregarded in the IBC tables, which would give a higher negative coefficient:
=-0.45-0.18=-0.63

 

[papaaj]

Thank you all for the informative responses.

To haynewp - in your last paragraph I think you meant to say case "B" is disregarded in the IBC, correct?

Regarding what Dave said, it makes sense that the loads in the "Horizonal Loads" column of IBC table 1609.6.2.1(1) (with multipliers) are to be used for the lateral analysis of the building. But then in what situation would the "Maximum Horizontal Wall Loads" be used? I don't think they'd be used for the design of each individual wall for out-of-plane bending - I think that table 1609.6.2.1(2) would be used for that design (in other words, an exterior masonry wall would be designed for out-of-plane bending using the "Component and Cladding" loads, which are actually greater than the "Maximum Horizontal Wall Loads&quot.

Tony

 

[haynewp]

Yes, I meant case "B"

 

[StephenEd]

It doesn't seem right to me to be able to use just a net of 8.5 psf for the HORIZONAL LOAD on the MWFRS at an INTERIOR WALL ZONE at 90 mph. Anyone else feel this way? (I know there is the 10 psf minimum that will control.) It's hard to believe that the windward + leeward force is so small. Heck, the code says you need to use 5 psf on INTERNAL walls that are not exposed to ANY wind.

If you use the sum of Zones 1 and 4 under the MAXIMUM HORIZONTAL WALL LOADS you get (7.2 - (-5.8)) = 13.3 psf. This is an increase of > 50% simply for the addition of the internal pressures. This is what the previous codes had called for and seems reasonable.

Maybe I've been designing too conservatively for too long!