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Curtain Wall / Storefront Wall Design

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Loui1

Structural
Apr 25, 2006
102
I'm designing a 33'tall x 180'long glass storefront wall. Mullions every 5' or so. 90mph wind zone. The design/build contractor opted to have the structural engineer design the steel backup support for the curtainwall and have the main steel fabricator provide it. This is instead of the curtainwall manufacturer providing it.

The curtainwall manufacturer is telling me their deflection limit is L/240+0.25".

The architect is limiting me to vertical members every 15' and two lines of horizontals. They're crying me a river that the verticals need to be 10" deep wide flanges and the horizontals 10" deep HSS to achieve the deflection requirements. They claim they've seen much smaller steel on the same exact design. I'm catching flack from my project manager, and I want to see if anyone else has designed anything like this.

They're architects so I take it with a grain of salt, but on the same note sometimes make me wonder.

I've wind loaded the thing per ASCE 7-02 and broke down the velocity pressures based on each height gradient. Roughly a 25psf C&C load.
 
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Oh yea, Use group II also if that matters.
 
Have you taken advantage of the allowable 30% reduction in component and cladding wind loads (see IBC Table 1604.3, footnote f)? I believe the intent is to check deflections for a "service level" wind instead of the 50 year storm used for strength design.
 
Hmm, good info. I see that footnote, but does it only apply to the limits set forth in that table. If the curtainwall people say they cant take more than l/240+0.25, I doubt they intend for you to take only 70% of the design wind pressure. What do you think.
 
Loui1:

I have done this type of thing before. Your W10 vertical and TS10x horizontal seem reasonable.

I have heard the same whinning from architects.

The only helpful idea I have is if you have some way of making a fixxed connection at your base? You can cut the deflection down significantly this way, and possibly get down to a 8" deep vertical member.

I would limit my maximum deflection to the l/240+.25" that the mullion suppier says their system can take. I would tend not to take advantage of the 30% reduction.

FWIW: I would tend to use a TS10 ( or TS8) vertical, as this eliminates the unbraced length problem for a W-section compression flange. But this is just my idea.
 
I agree with lkjh345 - the sizes seam reasonable. We typically don't take advantage of the redution either. When 'discussing' with an architect, we usually just smile and tell them how good verticals look when spaced at 5'-0" o.c.
 
One other thought to add, if I might... If you don't want to go fully-fixed at the base connection, you could use a proportion of your EI, solve for the load attracted and design your base connection for this level of partial fixity. It will still bring your deflections down. Other than that, I agree with the group... Particularly skier1578's smile and nod approach...

Good luck,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
 
Loui1...looks like you're in the right ballpark. Those are large areas for glazing. They'll have a heck of a time doing the glazing design on this one. Will likely have a glass thickness of about an inch.

I would also go with tube sections (HSS). Will look a lot better and easier to fabricate the interfaces.

Glazing and framing for glazing are typically limited to L/175 or L/240+ 1/4 for spans over 13 feet, or up to 0.75".

Wait 'till they run their storefront numbers! Their mullions will be in the 10" deep range with internal reinforcement, and still have large deflections.
 
I agree with taro--use the 30% reduction for checking deflection. And are you using the 0.85 Kd factor?

DaveAtkins
 
I don't think that you are required to use C&C wind pressure for the steel backing up the curt. wall. The mullions need to be designed for the C&C pressure, but for the steel design, use MWFRS design pressure. Also, when you use MWFRS, you won't be able to use the 70% wind pressure for the deflection check.
 
ntpe,

I disagree. MWFRS loads are for braced frames, moment frames, shear walls, diaphragms--those elements that stabilize the structure.

DaveAtkins
 
Dave,

By definition of the MWFRS is made up of members generally loaded by more than one surface. The kicker is "generally". The intent of the higher wind pressure for C&C is to account smaller trib areas that may be subjected to wind "hot spots". You can see these on a wind study (wind tunnel analysis). The larger the trib area, the more diluted the effects of these hot spots become. The vertical members of Loui1's steel back-up have a trib area exceeding 400 ft^2, and are loaded by components (the mulls), which, in my opinion, eliminates the need to apply C&C wind pressure to the steel verticals. In the curtain wall industry, it is standard practice to design a steel back-up system this way.

BTW Loui1, you will never get storefront to span that far; 12' to 13' is about all you can usually hope for. They will have to use curtain wall.
 
I agree with Dave on using the C&C pressures. Let's keep that to it's own thread. It's been talked about before in this forum and obviously no conclusion was reached.

Loui1, I would request from your client to ask the EOR and architect to work out some bracing for the wall. Thrity-three feet is unreasonable and they are getting exactly what they should: 10-inch members.
 
Thanks for the replies guys.

My C&C pressures include the 0.85 Kd factor.

Just to verify, some of you said the mullions would be 10" deep. I am providing 2 lines of horizontal steel tubes that will make the max vertical mullion span 12'. With that being said, is the 10" still correct?

Definitely going with C&C pressures unless ASCE7 changes their wording of MWFRS.
 
If the vertical mullions are divided into three spans, the problem becomes making the horizontal mullions span the full width of the opening.

DaveAtkins
 
The horizontal mullions only span between vertical mullions, or so the manufacturer says. Plus with a 5'wide x 12'tall window, most of the load is going to the vertical mullions. They said connecting horizontals to any backup steel is uncommon and expensive because they have to make special clips for some reason.
 
Please explain your concept for backup steel framing supporting the curtainwall (I don't care what the curtainwall mullions do or don't do). You can't have 180' long horizontal members, so you must have 33' high vertical steel framing BEHIND the curtainwall (and I ALWAYS see steel framing behind vast expanses of curtainwall as you are describing--aluminum mullions simply cannot span these kinds of distances).

DaveAtkins
 
Ahh, I see the problem here. You think I dont have any horizontal backup steel. There are 2)lines of 10" deep HSS horizontals spanning between the backup steel verticals. These support the vertical mullions.
 
Are you designing the mullions or the backup steel?

DaveAtkins
 
Only the backup steel.

The system in front of the steel I'm designing is someone elses baby.
 
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