Design Hurricane Zone Wind Pressure :: 2007 FBC Section 1620.1

[strl99]

2007 Florida Building Code Section 1620 in entitled HIGH-VELOCITY HURRICANE ZONE--WIND LOADS.

Section 1620.1 states in full:

"Buildings and structures, and every portion thereof, shall be designed and constructed to meet the requirements of Section 6 of ASCE 7, as more specifically defined in this section, based on a 50-year mean recurrence interval."

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I am looking for some opinions, perhaps from the Florida P.E.'s out there in eng-tips-ville.

What is the intent of the phrase "based on a 50-year mean recurrence interval"? Does this code section stipulate that the Wind Importance Factor cannot be set at 0.77 for a Structure that has been classified as Occupancy Category I?

Thanks much.

 

[slickdeals]

Nope, the importance factor is independent of the wind speed. The wind speeds given in ASCE are based on a 50 year mean recurrence interval. Structures of lesser importance can be designed for an importance factor of 0.77, but the wind speed shall still be 146 mph if you are in Dade County.

 

[slickdeals]

I should have said "pressures based on a wind velocity of 146mph"

 

[Ron]

slickdeals is correct except for one consideration....the importance factor of 0.77 only applies if the wind speed is 100 mph or less...that only applies to a small area of Florida.

The wind speed is as stated. The importance factor is applied separately. Please note that the importance factor in ASCE 7-02 and 7-05 is applied differently than in previous versions.

 

[strl99]

Thanks for the input.

I understand that the Basic Wind Speed is the Basic Wind Speed. That value can be taken from the published map or is set by the jursidiction. In this subject case, it is set by 2007 FBC Section 1620.2--that is: 146 m.p.h. (3-second gust) for Structures located in Miami-Dade County.

I also understand that the Wind Importance Factor is the Wind Importance Factor. That value is set via class of Structure and can be taken from ASCE 7-05 Table 6-1. This subject Structure is classified as Occupancy Category I.

However, what prompted my posting of this topic is the concept of mean recurrence internal (i.e., MRI).

Reading the ASCE 7-05 Commentary, it is my understanding that the mapped Basic Wind Speed ASCE 7-05 Figure 6-1 is the corresponds to a 50-year MRI. And also that the Wind Importance Factor is the parameter that is applied to alter the MRI.

This concept is nicely presented in ASCE 7-05 Table C6-7.

For example, naturally: if the 50-year MRI is needed, use a Wind Importance Factor of 1.0.

So: it would appear that an application of a Wind Importance Factor of 0.77 for this subject Structure under a 146 m.p.h. design wind speed--that is: the square of the table value of 0.88--would result in the Design Wind Pressure being one that has 25-year MRI.

And it would appear that because the Design Wind Pressure has a 25-year MRI, this Design Wind Pressure wouuld not meet the requirememt of 50-year MRI as coded in 2007 FBC Section 1620.1.

I feel I have corrnered myself with my thinking. Any further thoughts?

Thanks much.

 

[slickdeals]

Ron,
I am not sure when in the FBC it states that you cannot use a 0.77 Importance factor for Occ. Cat. I in HVHZ zones. It looks like ASCE permits that per Table 6-1.

Strl99,
Importance factor and wind speed are independent quantities. By using a 0.77 factor, you are essentially designing for a 25 year MRI, but you are using 50 year MRI wind speeds to calculate your wind pressures, which you are subsequently reducing per the importance factor.

In other words, if you used a 25 year MRI, then your importance factor would be 1.0. You cannot use a 25 year MRI and then use the Importance factor on top of it.

Thoughts?

 

[strl99]

Yes, slickdeals.

In the calculation of the Design Wind Pressure, the Basic Wind Speed is squared and the Wind Importance Factor is not. And it appears that ASCE 7-05 Commentary Table C6-7 presents values for the square root of the Wind Importance Factor--that is: [I,W]^(0.5).

So--albeit, in a roundabout way--couldn't the part of the Design Wind Pressure equation actually be represented as the [Design Wind Speed squared] times [the required value presented in ASCE 7-05 Commmentary C6-7]. That is:

[V,3s]^2 * table value of [I,W]^(0.5)

in lieu of the more standard-looking

[V,3s]^2 * I,W


So: in the corner I am in, it could be read that the row for 50-year MRI, the Design Wind Pressure would use

[146]^2 * table value of [1.0]^(0.5) = 21300 = 50-yr MRI

or the more standard-looking

[146]^2 * [1.0] = 21300 = 50-yr MRI

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Likewise, the same calculation using a Wind Importance Factor of 0.77 is as follows

[146]^2 * table value of [0.88]^(0.5) = 16500 = 25-yr MRI

or the more standard-looking

[146]^2 * [0.77] = 16400 = 25-yr MRI


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So to have a design 50-year MRI, wouldn't the resulting product of 21300 need to be used, not the 16400???

Any thoughts?