IBC 2003/ASCE 7-02 wind loads vs. UBC

[RobLauder223]

I'm looking for feedback about the 2 codes. For the simplified procedure in 2003 IBC/ASCE 7-02, it appears the loads you come up with (using table 1609.6.2.1(1) and 1609.6.2.1(4))are significantly less than what you get using UBC. comments?

 

[JedClampett]

Make sure you're comparing apples to apples. The simplified tables assume Exposure B, where typically we assumed Exposure C. It might be all right, since the definition of Expsoure B is a little more liberal.

 

[RobLauder223]

I think I'm ok because I multiplied the table (1609.6.2.1(1)) values by the factor from Table 109.6.2.1(4), which for this Exposure C case is 1.21 (structure is only 14' high). It seems odd that the Zone B and D values are negative - I'm sure that's why my overall windload is less than UBC, which didn't recognize the negative pressures in the projected area method.

 

[SacreBleu]

The UBC Method 1, which follows more closely the original ANSI and ASCE research, has negative pressures for the windward surface of a sloped roof.
The IBC is based on the same research, however, the pressure coefficients are revised somewhat, and the code-writers attempted to "simplify" the procedure in tables presented in the IBC2000. The end result was very confusing, and did not lend itself well to automated calculation.

 

[RobLauder223]

Your last sentence said it all - IBC is WAY too comprehensive and laborious for most purposes. And I think you undestood my point - if you look at UBC Method 1 vs. Method 2, Method 2 is very conservative at least as far as horizontal oading goes.

 

[SacreBleu]

One reason the UBC code-writers made Method 2 conservative for sloped roof is because in a case of a complex roof which has more than 2 ridge or hip lines in any given direction, the engineer would treat that essentially one ridge line (projected area on a vertical plane), rather than treating it as 2 or more projected areas, all added up. This is very typical of custom homes.
A good software or spreadsheet makes it easy (practical) to input the various roof configurations, so the UBC Method 1 was very easy to automate. Later, when the IBC came, all I had to do in my spreadsheet was to convert to the 3-second gust velocity and adjust some coefficients, and re-write the basic pressure formula. That way, I avoided all the confusion with the IBC tables, and came up with the same result as ASCE-7.

 

[RobLauder223]

Hmm... are you willing to share your spreadsheet?

 

[SacreBleu]

Rob,
I would - but only if you have a spreadsheet to trade (sorry, I don't like to hand the farm over to strangers). <g>

It doesn't do the full IBC overcomplicated analysis (doesn't recognize higher pressure at end zone, wind direction at 45 degrees off principal axis, etc) but it may give you some ideas to incorporate in your own spreadsheet projects.

 

[RobLauder223]

Understood. Don't blame you one bit. Do you work in Excel and/or Mathcad? Most of the stuff I do is pretty unsophisticated, i.e., I wouldn't call myself a programmer. What I would have to offer is pretty tame stuff - beams, footings, reinf. concrete, columns, etc. that I expect you already have some version of already if you've been practicing for any length of time. The little bit of feedback really helped, though. Thank you.

 

[SacreBleu]

Rob,
I actually don't have anything recent for concrete, except I recently did a cantilever retaining wall and a concrete basement wall spreadsheet.
I like to look at other's spreadsheets because it gives me ideas to incorporate. I only use Excel; Mathcad is not an option.