Importance Factor used in serviceablity design?

[joder]

If you have a builidng that requires an Importance Factor of 1.15 for wind (per ASCE-7), can this factor be waived in non-life safety situations? For example, it is neccesary to design metal stud backup of brick veneer for a deflection criteria of L/600? I don't believe this is addressed in ASCE-7. I would still design for strength because a veneer failure could be a life safety issue. But it seems reasonable to me to waive the 1.15 factor in calculating cladding deflections since no one is going to get hurt if a mortar joint happens to open up during a 3 second gust.

 

[abusementpark]

We generally leave the importance factor in place even for the purposes of serviceability checks, but you raise an interesting point.

You are allowed to multiply the load by 0.7 to roughly convert to a 10-year wind for serviceability checks.

BTW, I don't think the code dictates an L/600 deflection criteria for brick. L/240 is what's listed for brittle finishes.

 

[rscassar]

The importance factor (I) adjusts the wind speed for an event with a 50-yr mean recurrence interval (MRI) to other probabilities (say 25-yr MRI for a serviceability wind).

In my country, the serviceability relates to a 25-yr recurrence event whereas the ultimate wind event (LRFD) would be a 500-yr recurrence interval.

This typically means that the serviceability wind pressures will be 47% of the design wind pressure.

I also think that a L/600 deflection criteria for a metal stud wall supporting a brink veneer is too stringent. Agree with abusementpark in using L/240.

 

[Lion06]

Are you guys really using L/240 for brick backup deflection? We always use L/600 for backup supporting brick. Won't you get cracking all over the place using L/240?

As far as I goes in the serviceability check goes, I use it. I back off from the fifty year wind to the ten year wind, typically, but I've never come across anything that says you can ignore the importance factor for serviceability checks. I think that would open a door to ignoring it for drift, and even lateral accelerations at the top of tall buildings, which I don't believe is prudent.

 

[WillisV]

Importance factor should be used even for servicability design. This is perhaps more apparent in ASCE 7-10 where they give maps for different return intervals for EACH occupancy category, so the "I" is build in for the 10 year return serviceability return period map depending on which Occupancy/Risk category you are in.

 

[SteveGregory]

I use the importance factor and the 0.7 factor to reduce the wind to a 10-yr number.

The L/600 deflection is required by ACI 530-05 for beams and lintels supporting masonry and you could argue that a stud is a vertical beam. The deflection of brick veneer w/ stud backup is not specifically defined by ACI. However, here is a quote from the ACI 530 commentary "The Brick Industry Association has held that an appropriate deflection limit should be in the range of stud span length divided by 600 to 720.

 

[joder]

Thanks for all your input. I do use the 0.7 factor for wind deflections, which effectively reduces L/600 to L/420. And I do think that is too stringent, but the Brick Institute is clear about their recommendation and I don't know any way out of it. But that's another subject for discussion. I did once review cracking of a brick veneer building that had cracks and shifting bricks where the lintel met the jamb over a door because when the door slammed, the wall shook. The studs were 3-5/8" and about 10 ft. tall, which I know could not have met the L/600 requirement. So, a stiff backup for masonry veneer is a good thing.

My situation is this: its an outdoor stadium. I = 1.15 because it holds more than 300 people. I think even the 1.15 for MWFRS of the structure is ridiculous because no one is going to occupying an outdoor stadium during a 140 mph hurricane. And I think its even more ridiculous to have to beef the studs up because of that 1.15 factor. It's already conservative enough with the L/600 criteria in my opinion. But I don't see any way out of it.

Which brings up another subject: Can my engineering judgement override the code?

 

[joder]

I also note that I posted this message in the wrong place..