Tall Light Gage Jamb Question

[XR250]

I'm designing the light gage steel for an apartment complex renovation. On one floor, the studs span 22'-4" and have a 12' wide window. I am calculating I need (4) 6", 12ga studs with 2 1/2" flanges to make this work. So my jamb ends up being 10" wide. Seems kind of ridiculous and I end up having to worry about torsion etc. due to the window header and sill only framing into one side of the jamb. At what point should the EOR have specified a tube steel jamb for this condition?
I get this all the time and it drive me nuts. It seems it is done out of ignorance for the limitations of light gage steel and the fact that they want to keep their fee low by delegating the designs to the light gage subcontractor.

 

[DaveAtkins]

I agree with you 100%. Unfortunately, you are probably designing the cold formed steel after the bids have already been awarded, so it is too late to switch to hot rolled framing.

I would stick with your design (don't forget to include the contribution of the nested tracks), and don't worry too much about the torsion--there is sheathing on both faces of the wall, as well as bridging within the wall, to resist the twist.

Also, studs with wider flanges (I think as much as 3.5") are fairly common now--you may want to check into that.

DaveAtkins

 

[XR250]

I agree with you 100%. Unfortunately, you are probably designing the cold formed steel after the bids have already been awarded, so it is too late to switch to hot rolled framing.

Probably, but I have been successful in the past by complaining loudly enough.
I usually do not count the nested tracks unless I really need it due to the fact that they are usually spliced in a jamb of this length. I guess i could specify that they custom order cont. ones.

A piece of HSS6x6x1/4 would work nicely.

The other issue is the use of L/360 for the cladding deflection (composite metal panels). It was originally spec'ed at L/600 and I spoke to the EOR and he changed it to L/360. Seems I should be able to use L/240 for this material.

 

[KootK]

Ignorance and fee presevstion do both enter into it. I'd say that it's mostly the ignorance however. Most EOR's want to fo a good job. I think that the CFM industry would do themselves a service if they provided EOR's with some simple "not to exceed unless you know your stuff" guidelines for the situations that routinely cause problems.

As EOR's, we struggle with two opposing constraints:

1) Gratuitous use of red iron adds project cost so CFM use should be maximized.
2) Our fees, dictated largely by our competitors, assume that the detailed design of CFM will be delegated.

It's a set up guaranteed to encourage exactly the problem that you've encountered.

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.

 

[XR250]

As EOR's, we struggle with two opposing constraints:

1) Gratuitous use of red iron adds project cost so CFM use should be maximized.
2) Our fees, dictated largely by our competitors, assume that the detailed design of CFM will be delegated.

It's a set up guaranteed to encourage exactly the problem that you've encountered.

Exactly and it likely ends up costing the owner more overall or the light gage sub just takes it on the chin. Seems if the EOR included light gage in their fee and subbed it out at least everything could be coordinated prior to the bid set. That way everyone is bidding the same thing and they don't have to excessively pad their bids for engineering or other unknowns that may occur after engineering. Cost saving there for sure overall. Now convince your client of that :>

 

[KootK]

So what's the standard, available, spliced length of nested track? I wasn't aware of that limitation.

Seems if the EOR included light gage in their fee and subbed it out at least everything could be coordinated prior to the bid set

Part and parcel with the ignorance bit is the fact that most EORs' egos won't allow them to admit to sucking at CFM. It's just sticks, right?

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.

 

[XR250]

So what's the standard, available, spliced length of nested track? I wasn't aware of that limitation.

Track can be ordered in any reasonable length but is usually stocked in 10' lengths

Part and parcel with the ignorance bit is the fact that most EORs' egos won't allow them to admit to sucking at CFM. It's just sticks, right?

Most EOR's suck at a lot of things such as Geotechnical engineering but the owner still separately hires a geotech that the EOR coordinates with. I think if EOR's were on the receiving end of what I typically get, they might change.

 

[Teguci]

In our case (Strut), the EOR's often try to detail our systems and wind up with imaginary connections that can't be built as well as details that are unnecessarily expensive. I review the non-typical proposals in order to catch cost problems. We also include a "performance design" statement in our proposals and reserve the right to re-price if the EOR enforces his design. When a situation suggests it, we have no problems providing structural steel as part of a solution. At some point I'm planning on putting together a paper for EOR's to implement strut designs properly, but I have a lot of important but not urgent plans.

 

[XR250]

You are my hero :>

I think most EOR's don't want to add steel later as it gives the perception that they were negligent.

 

[StructuralEd]

I often see jamb designs come through using a stud capped with heavy track to make boxed tubelike section. Sometimes two such assemblies but never more.
That would save some width.

 

[Ron]

Don't forget that your "stud pack" will create an area of greater stiffness....areas away from there will deflect more, so pay attention to the wall covering (finish). If it is brittle (stucco/brick) then check deflection in both the vertical and horizontal spans! While this is a serviceability issue, it can have significant repercussion down the road due to water intrusion.

 

[XR250]

I often see jamb designs come through using a stud capped with heavy track to make boxed tubelike section. Sometimes two such assemblies but never more.

I am looking into that with my client. I am also trying to get the EOR to let me use L/240 instead of L/360. It is composite metal panels so L/240 should be fine.

Don't forget that your "stud pack" will create an area of greater stiffness....areas away from there will deflect more, so pay attention to the wall covering (finish). If it is brittle (stucco/brick) then check deflection in both the vertical and horizontal spans! While this is a serviceability issue, it can have significant repercussion down the road due to water intrusion.

I guess the only difference between the jambs and a common stud is that I use a lower wind pressure due to their larger tributary area. Also, I usually do not have to push a common stud to the limit so it generally has more relative stiffness than the jamb. I am going to let the arch worry about water issues.