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Curtain Wall / Storefront Wall Design

Curtain Wall / Storefront Wall Design

Curtain Wall / Storefront Wall Design

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.

RE: Curtain Wall / Storefront Wall Design

Oh yea, Use group II also if that matters.

RE: Curtain Wall / Storefront Wall Design

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.

RE: Curtain Wall / Storefront Wall 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.

RE: Curtain Wall / Storefront Wall Design


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.

RE: Curtain Wall / Storefront Wall Design

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.

RE: Curtain Wall / Storefront Wall Design

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,


B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

RE: Curtain Wall / Storefront Wall Design

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.

RE: Curtain Wall / Storefront Wall Design

I agree with taro--use the 30% reduction for checking deflection.  And are you using the 0.85 Kd factor?


RE: Curtain Wall / Storefront Wall Design

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.

RE: Curtain Wall / Storefront Wall Design


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


RE: Curtain Wall / Storefront Wall Design


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.

RE: Curtain Wall / Storefront Wall Design

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.  

RE: Curtain Wall / Storefront Wall Design

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.

RE: Curtain Wall / Storefront Wall Design

If the vertical mullions are divided into three spans, the problem becomes making the horizontal mullions span the full width of the opening.


RE: Curtain Wall / Storefront Wall Design

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.

RE: Curtain Wall / Storefront Wall Design

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).


RE: Curtain Wall / Storefront Wall Design

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.

RE: Curtain Wall / Storefront Wall Design

Are you designing the mullions or the backup steel?


RE: Curtain Wall / Storefront Wall Design

Only the backup steel.

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

RE: Curtain Wall / Storefront Wall Design

OK, so the backup steel spans 33' vertically, and 15' horizontally.


RE: Curtain Wall / Storefront Wall Design

I love how you gentlemen talk about architects.  I am one -I am sure it is all in fun. I wrote civil because the list of professionals did not include architects - perhaps we are not professional.

It would help if engineers as a whole travel aboard - specifically Europe - and observed all of the elegant engineering solutions that many architects have seen in person - or in magazines. I know that engineers travel also (especially since you are well compensated) but I am not sure if you observe your built surrounds the way an architect does.

In any case, architects are very ignorant at times.  I found this web site because I was researching Exposure B and C criteria.  It is so hard to find this kind of information. If we could have a good source; it would allow us to have solutions in place anticipating your engineering requirements. Can you help me with a book, an article or magazine?

I have asked this question of many engineers and their response it to refer to the code.  This answer is as cryptic as the code.

RE: Curtain Wall / Storefront Wall Design

Do you really expect anyone to help you when you make acerbic remarks to the engineering profession in a thread that has nothing to do with your subject?

No, its not in fun when an architect threatens my name and livlihood because he/she doesnt understand the correlation between deflection and moment of inertia.  

I'd recommend reading ASCE7-02.  If then you have a specific question regarding the descriptions, come on back. If you still have no clue whatsoever, then I recommend purchasing a structural engineering consultant so you dont kill somebody.  

RE: Curtain Wall / Storefront Wall Design

Damian, the best thing to do is to start a new thread.  Tacking on to the end of this one not only disrupts the flow of this thread, but it hides your question from others who aren't looking in this location.

RE: Curtain Wall / Storefront Wall Design

Back to the original thread, according to ASCE 7-02, Seciton CB, "Serviceability Considerations", the 30% decrease in the wind loading for deflections is for the assembly as a whole taking into account the transient nature fo the loading and the inherent stiffness in non structural elements.  It is not intended to be a reduction for component loading, it is intended to be a reduction for system drift loading.

RE: Curtain Wall / Storefront Wall Design

My quick take:

Your moment on the 33' vert. steel is:
w = (25 psf) (15') = 375 lb/ft
L = 33'

M = 51 k-ft

Required Moment of inertia for L/240 + 0.25" deflection (not using wind reduction) = 182 in4

There are several W10x shapes that work.  It may be possible to use a lighter, narrower (flang width) W12x instead of the W10x, though the architect may blow a gasket if you even mention the option.  A tube may be another option.

I have no problem working with people who don't know anything about engineering principles or practices, whether they're an architect or contractor, so long as they know they don't know anything, or at least realize when they have a limited understanding of something.  It's the people who question me without knowing what they're talking about that get to me.  Just yesterday, I had a client want to move a foundation 5 feet off the select fill base, with the contractor arguing with me about why that's fine, but that is a whole other story.

In your case though, I'd be a little frustrated.  I'd probably ask the architect for a project I could compare this one to, as "he has seen this done several times with much smaller steel".  When he gives you a building, and you go out and see a 25' tall, vertical backup steel at 10 o.c. building, it'd be satisfying to explain the difference.

Damian1, steel is steel, and concrete is conrete.  Engineers in Europe and Asia come up with the same thing we come up with in the USA.  There is no getting around having a stiff enough section for a backup steel in this case.  I think you're mistaking good planning by architect and coordination of architect with engineer for "elegant engineering solutions".

I looked up "airports" looking at the window backup systems:

Airport in Stockholm: http://www.photo.net/photo/pcd0510/stockholm-airport-hopskotch-102.4.jpg
Vertical mullion spacing much tighter, floor to floor distance approx 15 feet?

Airport in Austria:
Appears to be a high ceiling, perhaps 50 feet?  There are building columns which span from floor to roof.  There are encased horizontal members spanning from building column to building column, at an elevation of approx. midheight of the window.  There are tube vertical members which span from floor to midheight horizontal brace to roof.  These tube vertical memebers may actually be encased wide flange members.  There are also "sub" horizontal braces which brace the vertical mullions.  The main thing to consider is that not only are there vertical braces, but the vertical span is "only" around 25 feet.  I'd guess they're using an 8" section.  Is this an "elegant engineering solution", or is it a clear case of good planning by the architect along with good coordination with the engineer.

RE: Curtain Wall / Storefront Wall Design

Your sizes seem reasonable to me for the spans you indicate.

Ask the architect to send you some drawings for the "exact same" design.  I would be curious to see how "exact" exact is.

My understanding is that wind deflections can be checked for a 10 year wind, which is about 70% of the 50 year wind used to check strength.  I typically check for deflections at the 10 year wind level.

Damian1, your comment about engineers being so well compensated is very amusing!

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