Curtain Wall / Storefront Wall Design

[Loui1]

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.

 

[DaveAtkins]

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

DaveAtkins

 

[Damian1]

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.

 

[Loui1]

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.

 

[UcfSE]

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.

 

[VTPE]

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.

 

[AggieYank]

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:

http://www.wausauwindow.com/images/galleryLg/Austi-Berg-Airport.jpg

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.

 

[bjb]

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!