Open front steel deck diaphragm

[txeng91]

Let's say I have a commercial box building, 55' wide x 150' long x 20' high. Steel frame, bar joists with metal deck clear spanning the 55' with wide flange girders and 6" tube columns and light gauge infill. I have chevron bracing on both 55' walls and on one of the 150' walls. The front end is all storefront. The chevron bracing and diaphragm are stiff enough so that any sort of moment frames I put on along the storefront side take little to no load and my chevron frames can take the torsional diaphragm forces no problem. My question is should I still include some moment frames along the storefront end for general stability purposes, or just go straight pinned with all the connections? I'm not completely sure what the general practice is here. Also I'm not in a zone with no seismic and low wind.

 

[KootK]

It's a judgment call but I believe that most of your respondents here will recommend the moment frames with additional redundancy under seismic being the main sell. I agree with that but am personally willing to let three sided just be three sided as long as I've dotted all my eyes and crossed all my tees with regard to the diaphragm analysis and torsional/irregularity accounting. Sometimes I'll throw in a bolted moment frame for 25% of the longitudinal lateral if I feel as though I can really get something meaningful out of it. Occasionally, with wide flange columns and fairly deep trusses acting as the moment frame beams above the openings, I feel that I can milk it for some stiffness without too much of a cost penalty.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

any chance you can dedicate any portion of the store front as 'filled' and use conventional X-bracing? Likely less costly than a moment frame and 150' isn't too long. 6" tube columns for a 20' high wall may be a tad light... can you use W8's? Deeper and connections easy...

Dik

 

[hokie66]

I always use moment frames in a case like yours. I don't really believe in carrying torsion on C sections.

 

[KootK]

Least of your worries I suppose but I'd be inclined towards a second brace on the long side at 150'.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[thaidavid40]

I recommend against any X-bracing in the front, as the owner's building use requirements are subject to change in the future, and that could add quite an expensive obstacle to those.
Dave

Thaidavid

 

[vmirat]

With that much storefront, I would be concerned about serviceabilty. Lateral drift from wind could cause a problem. Your diaphram would have to be pretty stiff. If the owner or architect insists on full-length storefront, then I would make the front columns really beefy inverted pendulums to resist moment. I might also consider making a truss frame across the top of the storefront to help the columns.

 

[XR250]

Seems to me that with a 150' long rear wall and only 55' side walls, the three sided building would be alot stiffer than a moment frame up front (like the OP suggested)
With a 15 psf wind load, the shear (wind blowing on the side) is only about 55 plf is I am doing my math correctly. I would go three-sided and make sure your I's and T's are dotted like KootK suggested.

 

[dik]

Thaidavid... if X-bracing can be used at the ends or in the middle, it is so much stiffer than a rigid frame... shouldn't be a problem, the building is relatively small. If all open storefront, then rigid frame it is...

Dik

 

[DaveAtkins]

I am in favor of the three sided diaphragm. I don't think there will be enough lateral drift along the open side to be a concern.

DaveAtkins

 

[txeng91]

Thanks for the replies.

I have about 4-5 feet in between storefront so I could theoretically fit some frames in if the architect will go for it, but I think thaiDavids point is valid about leaving room for possible future renovations. My girders along the front end are supporting brick above the storefronts so they are fairly deep due to deflection restrictions, W27x84 at 25' bays. So I will probably go with rigid frames and use a W8 or W10 column as Dik suggested. I'm thinking of going with end plate bolted moment connections, it seems like a simple connection and I feel like the fabrication/erection costs wouldn't be much higher than a simple shear connection.

Anyway, to me it feels like there's not really any way to justify putting lateral resistance along the line other than "it feels right", and it's easy to agree with even though the numbers say otherwise. The stresses in the diaphragm are going to be much high when it's in a simple span condition resisting wind forces perpendicular to the 150' dimension, and no one's going to bat an eye at that. I get that the open front diaphragm is relying on torsional force and is going to twist on my frame, but I have 1 story Chevron frames with 0.8:1 aspect ratios so they're not going anywhere. Ultimately I know the rigid frames im putting in won't have the stiffness to actually attract any significant load but my boss wasn't going for the 3 sided diaphragm anyway. I have a tendency to where if more than one person is telling me not to do something, even if I disagree, it's hard for me to ignore it.

 

[txeng91]

Least of your worries I suppose but I'd be inclined towards a second brace on the long side at 150'.

I'm assuming you are referring to adding another brace on the backside where I already have one? I was considering that just for redundancy since it's a fairly long stretch. Is that your reasoning as well?

 

[KootK]

That is what I had in mind. Things I like about it:

- redundancy
- if that line is to do all the work, I like it wicked stiff.
- if that line is to do all the work, I like to avoid long collector slip accumulation.
- back when machines were dumb and judgment reigned supreme, common wisdom was a brace every fourth bay or so. Guessing 25-30 ft bays.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.