Importance Factor used in serviceablity design? 2

I believe you should use the importance factor for serviceability checks.

I also don't think L/600 is terribly conservative for deflection of studs backing up brick. We would never use anything less.

I think that allowing your engineering judgment to override the code sets you up for a lot of liability down the road. I also think allowing too much deflection for studs backing up brick does the same thing. Who do you think the owner is going to call when mortar joints start cracking all over the place because you decided to use L/240 with a 10-year wind and no importance factor?

Just out of curiosity, what's your rub with the deflection and importance factor? Most engineers I know (unless they're working for a contractor) tend to prefer the more conservative route - I certainly do.

SEIT has it exactly right. Brick mortar cracking is a serviceability issue...it allows more water intrusion which in turn deteriorates the studs, degrading the integrity of the facade structural system.

I am not suggesting using L/240 for brick deflection. I am satisfied with the L/0.7(600)=L/420. But I'm not sure that even L/240 would result in "mortar joints cracking all over the place". We're designing for a once in 10 years event. Because stud strength is satisfied, the studs will remain elastic and snap right back into place after a horizontal mortar joint opens at midspan of the wall. The mortar joint would then remain closed until the next ten year storm.

My rub is this. Our job as structural engineers is to design safe and ECONOMICAL structures. The wind speed in our area is 140 mph which results in high wind pressures which I believe are correct to conservative (wind tunnel tests generally result in lower pressures). It is an unneccesary expense to use a 1.15 factor on top of a perfectly good design simply to prevent mortar cracking. The reasoning behind the 1.15 factor is the Occupancy Category III, which is defined as:

Buildings and other structures that represent a substantial hazard to human life in the event of failure, including, but not limited to:
• Buildings and other structures where more than 300 people congregate in one area....

Mortar cracking is not a hazard to human life.

Fair enough, but I still don't understand why you would want to assume the risk and liability for designing to something less than code and standard recommendations.

Also, I'm not sure I believe that just because the studs remain elastic that the joints only crack momentarily and close right back up. That assumes there is no crushing of the mortar. I also think that once the joint has cracked it still makes for easier infiltration of water just because there is a clear and defined crack (even if it is smaller and closed from the studs rebounding). I could easily be wrong on that and would dedi itely lime some other opinions.
I just come back to the fact that nothing in the code allows you to neglect the importance factor for serviceability checks. Are you getting push back from a contractor or architect on sizes or do you yourself thinks it's overkill?

joder...think about your comment...

The mortar joint would then remain closed until the next ten year storm.

Click to expand...

Not really. Once the joint cracks, it remains cracked. Then wet/dry cycling from your everyday moisture changes and afternoon thunderstorms will exacerbate the cracks. This allows more water intrusion and correspondingly more structural deterioration. Brick ties tend to corrode and brick will then fall off the wall.

No, not necessarily a hazard to human life at the time of the event, but can become so when least expected.

Not sure if you've ever seen a brick facade fail and fall off a building during a wind event. I have. The last one I saw was in Altamonte Springs, Florida. That one happened during a severe thunderstorm. Subsequent investigation found deterioration from water intrusion and brick ties that had completely corroded.

Another point to think about is that just because a structure would be empty during the design wind event doesn't mean it should be designed for less. Failure of your structure (even though empty) could have catastrophic downwind implications.

I agree with Ron and SEIT that the backup studs should be stiff, but no matter how stiff you make them (within practical limits), the brick will still be stiffer, and will crack in extreme events. Water will enter the cavity of any brick veneer structure, so the design should anticipate that.

I'm going to put a different opinion here, but I want to stress that I'm not familiar with ASCE-7. If what I say is inconsistent with any of the code provisions, then follow the code. You can (and should) use engineering judgement to interpret a code, but not to ignore specific code provisions.

Importance factors in limit state design are related to the consequences of collapse. If the collapse of a structure will result in many deaths, or in the removal of facility important for post-event recovery, then it is given an increased importance factor, to further reduce the probability of collapse. In my opinion it has nothing to do with serviceability, and indeed it is expected that in these extreme events structures may exceed the serviceability limit state. Applying an importance factor to serviceability limit state loads therfefore seems illogical to me.

Doug Jenkins
Interactive Design Services

The argument that cracked mortar will lead to water intrusion and hence corrosion of the ties and studs seems a bit weak to me.
Masonry should be expected to leak in wind-driven rain, and the building should be detailed to cope.
I'm not familiar with ASCE-7 so I won't comment on that.

It's not pushback from an architect or contractor; it's me. I'm just trying to be responsible with the owner's money. I have full confidence of success using an Importance Factor of 1.0. Every building I've designed using I=1.0 in my 28 years of practice that has gone through hurricanes has functioned exactly as they should.

apsix is correct: brick veneers always leak, that is why there is a cavity with weep holes at the bottom and the moisture barrier is against the sheathing. The Brick Institute tested full scale walls built in laboratory conditions, subjected them to simulated rain, and every one of them leaked. It's expected.

I have seen numerous brick veneers walls come off buildings after hurricanes. I have a photo library full of them. I'll try to attach one. Corroded or non-existant brick ties were the culprit. Stainless steel is recommended in coastal towns. They will get wet regardless.

loose bricks from serviceability failure are a life safety hazard.

It is quite common to hear of veneer walls that are 'sucked' off the backing frame during minor events (say 2-5 year recurrence). When the brick ties corrode, it is not just the wind events which cause the veneer walls to collapse but seismic events can also initiate this.

Nice photo.

The L/600 limit for a 10-yr event seems stringent to me. For a 10' wall, we are limiting deflection of the stud backing frame to 2". I may be doing my calculations wrong but this would require a 6" deep metal stud at 12"c/c to satisfy this requirement. If I saw this on-site I would initially thick that the stud walls appear oversized.

I'm sure that everyone has seen this article posted before.

Kikflip: good article. I found the following very interesting:

“Is L/600 too stringent for out-of-plane bending for a serviceability issue? Canadian research titled Technics Steel Stud / Brick Veneer Walls, by Trestain and Rousseau drew from earlier McMaster University studies. The McMaster studies actually constructed veneer stud walls and tested with wind pressure and simulated rain.
The result was that there was no increased system vulnerability due to excessive leakage from flexural cracking. The L/720, L/600, or L/360 deflection limits do not eliminate flexural cracking. The deflection limit is intended to reduce the flexural cracking size. But as the McMaster study indicated, the size of the flexural cracking did not increase system vulnerability.”

Joder:

Have you considered the information in the AISC Design Guide 3? It provides serviceability requirements for a variety of cladding types, and it can be downloaded for free if you are an AISC member. Nearly all of the suggested deflection limits are based on a 10 year wind.

It seems to me that this is a discussion we should have with our clients. What do they expect of the building if a large wind or seismic event occurs?

For what it’s worth I would not use the 1.15 factor with the 50 year wind, and a L/600 deflection limit. I would use the 10 year wind, a 1.0 IF and the L/600 deflection limit. I would also communicate this to the client.

D

I'd say keep the importance factor in the calculation, but take advantage of the 30% reduction in going from a 50-year to a 10-year recurrence interval. Follow the code, IT'S THE LAW. Don't take on added liability to save a couple bucks on steel studs. If it bothers you, follow up with the code council to attempt to get it changed in the next cycle. In the StructureMag link, it looks like Canada has recently backed way off their L/720 deflection limit and changed it to L/360.

Thanks for all your responses. I'm going to keep the Importance Factor (IT'S THE LAW), and use L/360 like the Canadians (its not the law).

It is interesting that most codes are very prescriptive on the strength design requirements but a little gray on the serviceability aspects.

Yet MOST problems (and lawsuits) in buildings come from serviceability issues.

We are Virginia Tech
Go HOKIES

I find the US design approach to be very stringent.

ASCE-7
Ultimate Design: 50-yr recurrence
Ultimate Load Multiplier: 1.6
Service Design: 10-yr recurrence (70% of ultimate design)
Service Criteria: L/600

AS1170.2
Ultimate Design: 500-yr recurrence
Ultimate Load Multiplier: 1.0
Service Design: 25-yr recurrence (40-50% of ultimate design load)
Service Criteria: L/300 (As stipulated in the Australian Masonry Code AS3700)

Section 7.7.2(b)

Serviceability limit state

For the serviceability limit state, the deflection of the structural backing under the design serviceability wind load shall be not greater than its span divided by 300. Earthquake loads do not need to be considered for the serviceability limit state.

This is from the Australian masonry code AS3700

Section 7.7.2(b)

Serviceability limit state

For the serviceability limit state, the deflection of the structural backing under the design serviceability wind load shall be not greater than its span divided by 300. Earthquake loads do not need to be considered for the serviceability limit state.

@kikflip:
Where in ASCE does it talk about the L/600 limit. I don't recall seeing any in the body of the code.

We are Virginia Tech
Go HOKIES

I didn't mean to link L/600 directly to ASCE, but from this thread it seems that L/600 is the US design philosophy??