IBC 2000 Code Question

[SmithJ]

Hello All,

In Section 1605.3.2 of IBC 2000, Alternate Load combinations are listed. Formula's 16-14, 16-15 and 16-16 require that wind loads calculated using ASCE 7 or IBC section 1609.6 be multiplied by a factor of 1.3. I have two questions here:

1) Does anyone know the background reasoning for this factor? Is it because the ASCE 7 method of calculating wind loads yields less conservative results than existing UBC Codes?

2) When reporting building reactions and brace forces to the foundation Engineer, we typically like to report unfactored loads. Is the 1.3W considered a factored load or is this just an internal correction applied to ASCE 7 and IBC 2000 wind loads alone?

Any help you can give in this issue will be highly appreciated. Also, does anyone know where I can get some research material on this issue.

Thanks in advance.

JS.

 

[jheidt2543]

This question has been asked and answered a few times on this forum, so you might try a "key word search" for other replies.

My understanding of 1.3 factor is that it is a recognition of the short term duration of a wind loading, so the code authorities allow for an increase in allowable load or stress due to wind.


Yes, the use of the 1.3 multiplier means that it would be a "factored" load.

 

[SmithJ]

Jheidt,

Thanks for your reply. I tried the keyword search but could not come up with suitable matches. Could you please post any links to where this question has been previously addressed in this forum.

Regarding your comment on short term duration, my belief is that the short term duration is handled by the 33% allowable stress increase allowed for combinations that contain wind or seismic loads. However, this is different from the 1.3 factor (symbol: Omega) that is applied only to wind loads derived from IBC section 1609.6 or ASCE 7. From the wording in Section 1605.3.2, I take it to mean that the allowable increase and 1.3 (Omega) factor are additive.

All thoughts on this would be well appreciated.

Thanks.
JS.

 

[peinapod]

here's my guess
IBC 2000 decreased their wind loads, they also got rid of the 1/3 allowable stress increase. Kind of an even trade.

So, I'm thinking that they are saying, "Don't use our smaller wind loads with load combinations from other codes." The load combinations in this section are from UBC.

 

[SmithJ]

Peinapod,

Thanks for your input. I also suspected that this was the case and that the 1.3 factor was only used to bring the IBC Wind Loads back up to par with UBC wind loads. In my opinion, the 1.3 is just a correction applied to the IBC loads.

JS.

 

[etompos]

Wind loads are at stress level (ASD) not strength level (LRFD), therefore they must be increased when used in strength design. Conversely, seismic loads are at strength level and must be reduced for stress design.

 

[PhDE]

This is due to directionality factor imposed in the ASCE-98 code for both ASD and LRFD.

ASD
IBC2000 refers to ASCE-98. The 1.3 factor is applied to the wind to account for the -15% directionality factor in the ASCE-98 code. (1/0.85 = 1.18 rounded way up to 1.3) The previous ASCE-95 code did not include this 15% directionality reduction and therefore codes did not have to increase the load factor in the combinations to account for this change.

LRFD
Same as above but the numbers are a bit different. The wind load factor increased for 1.3 to 1.6 (1.3/0.85 = 1.53 rounded up to 1.6).