confused in ASCE7-10 wind calcs for R-1 structure 4

[chrislaope]

I am still using ASCE7-05, but am trying to begin to shift to ASCE7-10. today I just tested to use ASCE7-10 to calculate wind load on a Risk Category-1 structure. What makes me confused is I found using ASCE7-10 result in a much larger wind load because I have to use ultimate design wind speed (105mph) in Figure 26.5-1C (which vs. 90mph wind speed in ASCE7-05). While for Risk Category-2 and 3 building, ASCE7-10 allows to use nominal design wind speed(refer to IBC-2012, section 1609), i.e. for a 120mph ultimate design wind speed, it use 93mph nominal design wind speed in calculation. Thus, wind load for Risk Category-2 and 3 building becomes much smaller than Risk Category-1 building/structure. This appears not right. Am I missing anything?

Can anyone shed some light on it?

Thanks a lot.

 

[JAE]

I don't have IBC 2012 - can you quote the section your refer to?

Per ASCE 7-10, there is no "nominal" wind speed different for Cat 2 or 3 buildings. I'm not sure that the IBC would mess with the ASCE 7 wind requirements like that.

 

[steellion]

I'm not sure what you mean by "nominal wind speed". There are wind speed maps for each Risk Category. The wind speeds increase as the risk increases. The design wind speed in ASCE 7-10 is higher than ASCE 7-05 because wind has been converted to a strength-level criteria (notice the coefficients in the load combinations have changed from 1.6 to 1.0). When the calculations all shake out, you should end up with approximately the same design wind pressure for strength in ASCE 7-10 as ASCE 7-05.

 

[ron9876]

ASCE7-10 uses ultimate wind speeds vs.allowable for 05. You calculate the ultimate pressure then multiply x 0.6 to get allowable--which is what you would use to compare to 05.

 

[JAE]

chrisalope seems to be implying that the IBC 2012 somehow alters the wind provisions of ASCE 7-10 and that is what is confusing.

I thought that the IBC was moving towards almost a complete external reference to ASCE 7 without including provisions within the IBC itself.

 

[woodman88]

Per the 2012 IBC

Section 1609.1.1
"...Exceptions:
1. Subject to the limitations of Section 1609.1.1.1, the provisions of ICC 600 shall be permitted for applicable Group R-2 and R-3 buildings.
2. Subject to the limitations of Section 1609.1.1.1, residential structures using the provisions of AF&PA WFCM.
3. Subject to the limitations of Section 1609.1.1.1, residential structures using the provisions of AISI S230.
4. Designs using NAAMM FP 1001.
5. Designs using TIA-222 for antenna-supporting structures and antennas, provided the horizontal extent of Topographic Category 2 escarpments in Section 2.6.6.2 of TIA-222 shall be 16 times the height of the escarpment.
6. Wind tunnel tests in accordance with Chapter 31 of ASCE 7.
The wind speeds in Figures 1609A, 1609B and 1609C are ultimate design wind speeds, Vult, and shall be converted in accordance with Section 1609.3.1 to nominal design wind speeds, Vasd, when the provisions of the standards referenced in Exceptions 1 through 5 are used."

From TABLE 1609.3.1 WIND SPEED CONVERSIONS footnotes
"b. Vasd = nominal design wind speed applicable to methods specified in Exceptions 1 through 5 of Section 1609.1.1.
c. Vult = ultimate design wind speeds determined from Figures 1609A, 1609B, or 1609C."

The ASCE 7-10 is not in the 2012 IBC exceptions that allow the Vasd to be used.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[chrislaope]

Thank you very much guys. Now things get very clear to me.

 

[MJB315]

This may go without saying, but be sure to still calculate your forces based on the governing code. If you're in a region that still references ASCE7-05, it may not be prudent to shift away.

"We shape our buildings, thereafter they shape us." -WSC