2017 Florida Building Code - Hurricane Zone - Wind Load Factor for Deflections

[bones206]

Table 1604.3 of the IBC lists allowable deflections for structural elements with the following footnote:

The wind load is permitted to be taken as 0.42 times the “component and cladding” loads for the purpose of determining deflection limits herein.

There is another list of allowable deflections specifically for High-Velocity Hurricane Zones (HVHZ) in Section 1616.3, but there is no similar footnote or clause explaining what load factor should be used for the wind when checking those deflections.

Does anybody here know the intent of the HVHZ code requirements for deflection under wind load? If it's left open to engineering judgment, I plan to use 0.6 times the component and cladding loads. It seems appropriate to drop the extra 0.7 factor since there is a higher probability of the design wind force occurring in this part of the country. But I'm interested to hear how others approach this.

 

[merrittEIT]

I have seen this go both ways depending on the EOR and how comfortable they are with location and what is being constructed. There is generally something in the specification for what I am designing as well that delegates this and further ASTM testing requirements for C+C materials help support as well. Generally if it is left up to me, I take the .6 or .45 reduction on my ASD design load but disregard the additional allowable 30% reduction. I feel it's double dipping for Miami-Dade HVHZ.

 

[SteelPE]

I thought the .42 was a probability based factor? The current wind maps are based upon a 700 year MRI for category II buildings and a 1,700 year MRI for category III and IV buildings. Wind deflections are typically calculated for either a 10 year or 25 year MRI. Hence the 0.42 factor (conversion from a 700 year MRI to a 10 year MRI). Although not the exact situation you are dealing with, the attached paper give some insight on how some of the factors were created.

I was only forced to work in Miami-Dade once, so I don't quite remember what I used at the time.

 

[bones206]

I understand that the 0.6 and 0.7 factors are probability adjustment factors, but I'm not clear on what the target probability is for these special HVHZ areas. The code is explicitly clear that 0.6 x 0.7 = 0.42 times the wind load is acceptable for typical structures, but is silent in the HVHZ section of the code.

It makes sense that an area with higher probability of high wind forces would use a higher load factor, but that just isn't stated anywhere explicitly as far as I can tell. I'm designing CMU infill walls with stucco coating, so I want to make sure I'm designing to the correct wind force for deflection control. If there's a hurricane and the stucco cracks all over this building, there's a pretty good chance someone is going to be looking closely at my calcs when the owner is searching for someone to blame and pay for the damage. But at the same time, I don't want to be overly conservative here and check deflection under the full 186 mph C&C forces, unless that's what I have to do to meet HVHZ code.