Steel strap lightgage shear walls
Steel strap lightgage shear walls
(OP)
Getting into more serious lightgage design than I have done in the past. Attached is a sketch that shows tension straps welded to the shear wall double end posts (in reality the straps are configured as x-bracing, 4 straps total). I am concerned about a few things.
1. With a single-sided angle, the vertical component of the strap force has to pass through the inside stud to get to the angle. To me, the allowable tension capacity of the end post is limited to one stud. Our shear wall spreadsheet does not check this. Maybe there is good reason not to?
2. Our spreadsheet relies on the horiz. component of the strap force to be resisted by the track, but doesn't explicitly check it (!). My question is what unbraced length lengths should be used for this compression member with PAF's at some interval. Also, there has to be an AISI limit on the number of fasteners over a given length that can resist shear.
This single-sided angle detail seems to be the standard at our office, but my feeling is that two angles would greatly help out with my question number 1, in additional to prying on the embed (which also seems to be ignored). Part of me also, wants to provide standard holes in the angles (or weld the washers) and transmit the shear directly to the anchors, and ignore the track for in-plane loads.
1. With a single-sided angle, the vertical component of the strap force has to pass through the inside stud to get to the angle. To me, the allowable tension capacity of the end post is limited to one stud. Our shear wall spreadsheet does not check this. Maybe there is good reason not to?
2. Our spreadsheet relies on the horiz. component of the strap force to be resisted by the track, but doesn't explicitly check it (!). My question is what unbraced length lengths should be used for this compression member with PAF's at some interval. Also, there has to be an AISI limit on the number of fasteners over a given length that can resist shear.
This single-sided angle detail seems to be the standard at our office, but my feeling is that two angles would greatly help out with my question number 1, in additional to prying on the embed (which also seems to be ignored). Part of me also, wants to provide standard holes in the angles (or weld the washers) and transmit the shear directly to the anchors, and ignore the track for in-plane loads.
"It is imperative Cunth doesn't get his hands on those codes."






RE: Steel strap lightgage shear walls
2) I can see an argument for the distance between PAF's but initiating some kind of fastener pull through failure concerns me. I'd be inclined to use the stud spacing. I would expect a limit on the number of effective shear fasteners. Particularly given that the track won't be a single piece forever (10?).
These details address some of your issues: Link
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.
RE: Steel strap lightgage shear walls
Something about the design example doesn't make sense to me. The straps are tension only members. That should make the tension chord a zero force member from top to the location where the strap is connected.
"It is imperative Cunth doesn't get his hands on those codes."
RE: Steel strap lightgage shear walls
Agreed. That doesn't change the design tension demand on the stud group however.
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.
RE: Steel strap lightgage shear walls
You are correct about the double stud--it has zero force. But the double stud at the other end of the shear wall will have a large compression force. Since lateral load can occur in either direction, a double stud must be provided at each end of the shear wall.
DaveAtkins
RE: Steel strap lightgage shear walls
I quite agree with this. Frankly, I have a hard time seeing the proposed detail passing muster save for the lightest of loadings. A partial list of issues would include:
- those already mentioned.
- weak axis bending of the stud pack.
- fastener tear out across the flanges of the stud pack members.
- stud flange crippling where the stud pack pushes against the angle.
- angle prying on the anchor bolt (common to many versions of the detail really).
- as far as I can see, no mechanism for transmitting brace tension to track/PAF shear.
While I appreciate the impetus to not instigate a bunch of fee burning change, this might represent an important opportunity to help your firm improve their product. Some of the details that are much better mechanically should be no less costly to construct in my estimation.
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.
RE: Steel strap lightgage shear walls
That takes all that nonsense out of the equation. Still got to check bolt pullout, shear and prying force, however.
RE: Steel strap lightgage shear walls
That is a major problem. I played around with the effects of the strap eccentricity by analyzing the real geometry in a general FEM program that can code check cold-formed sections. As expected, the minor axis bending prohibited a reasonable quantity/gage of studs.
Another major problem, other than two screws per stud to the the track, which would never do the work.
I also agree with the other items in your list. I also asked around the office about some of these things, and the typical answer was "I never checked that, I don't know, *shrug shoulders*".
I tend to try to do this when I notice something, but the resistance is always high and chock full of "we have always done it thing way". It has pushed me to just doing what is right for my projects and not worry about others.
I follow for attaching the straps to the channel flanges, but I don't see how that works with the chord studs. Sketch, maybe?
"It is imperative Cunth doesn't get his hands on those codes."
RE: Steel strap lightgage shear walls
Just wait until you're management and you actually find yourself part of the inertial mass of mediocrity that keeps things from improving. It's the bestest.
I believe that this is similar to your own point. For one story, there is not tension chord with XR's detail. Not even for the bottom 6" or whatever. Just diagonal tension strap and compression strut.
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.
RE: Steel strap lightgage shear walls
I hear you.
I got it - for some reason I was imagining the studs nestled in the channel
I forgot to mention that I like that simple argument.
"It is imperative Cunth doesn't get his hands on those codes."
RE: Steel strap lightgage shear walls
RE: Steel strap lightgage shear walls
The track, as shown in the example, is easy to design for tension, as if the track 'behind' the shear wall grabs the shear force down its length. This is difficult to have in reality, and the track is very hard to design for the shear in compression, even assuming the distance from PAF to PAF is the unbraced length.
RE: Steel strap lightgage shear walls
structSU10 - yes, our office has reference jobs where HSS posts were uses. In designing my first wall, I have noticed that the stud-to-track connection in compression is a problem even when uplift is taken by a hold-down device. From a double stud chord, 600S250-68 [50], and 68 mil track, I can only get 2.6 kips per stud in LRFD compression. That seems very limiting when trying to develop the full axial capacity of a double 600S250-68 stud weak-axis braced at 48" o/c.
I am assuming your increase in studs was because of the track compression connection?
If there is not a practical way to increase the stud-to-track crippling strength, adding more studs to the post group seems like a waste of time in a hurry. Maybe I need to add more shear wall segments or switch to tube posts.
A sketch in progress attached based on XR250's recommendation. As David mentioned, I am working on making a concentric detail for the top of the wall with the straps welded to a 3" deep distribution tube (drag strut).
"It is imperative Cunth doesn't get his hands on those codes."
RE: Steel strap lightgage shear walls
RE: Steel strap lightgage shear walls
You are proposing welding a channel section to the underside of the distribution tube too?
I am already using a 6x3 distribution tube for all of the load-bearing walls, so I was just going to replicate that at all shear walls and weld the straps directly to the tube. It gives me enough weld length, and it seems concentric enough - the center of the weld group aligns about at the inside face of the end post stud. I feel good about that tube uniformly compressing the post studs.
I am still mulling over what appears to be the Achilles heel of the system, the stud-to-track bearing.
"It is imperative Cunth doesn't get his hands on those codes."
RE: Steel strap lightgage shear walls
What I typically do is a 6x2 tube on top of the wall, an MC6x18 section at the bottom of the wall and a strap on each side. The double strap keeps from having out-of plane eccentric loads on the studs.
Stud to track bearing - are you talking about the gap that always exists between the two? Go do a few inspections and you will see how big it can really get.
I have stopped doing most light gage load bearing jobs. I just never see it getting built correctly. I imagine the interior and exterior sheetrock are what really brace these walls.
RE: Steel strap lightgage shear walls
Not exactly. I can imagine the problems with the construction, particularly if special inspections are not employed (I feel they need to be for the lateral force resisting systems). The AISI standards seem fairly clear about the problems when the gap is excessive, but I am really looking for guidance on how to get the end bearing capacity of the double 600S250-68 post to be much closer to the braced compression capacity of the stud group. Basically, with my current shear wall configuration, I need 28 kips of post-to-track bearing (or some other force transfer mechanism). I am leaning to "impossible/impractical". But again, I am not overly experienced in light gage design, so I may be overlooking at provision for built-up posts that significantly increases the groups bearing crippling capacity.
See the output below, it seems I am maxed out at 2.6 kips per stud (software doesn't have a module for built-up posts).
Currently, I am thinking about:
1. Switch back to a post hold-down that could also transfer compression? I looked at the Strong-tie hold-downs, but they are not rated for compression. Further, if I go back to that configuration, the shear needs to be resisted at the hold-down too. Not good.
2. Sketching to HSS chords
3. Reconfiguration my rigid diaphragm lateral load distribution in order to reduce the max chord compression to no more than 2x 2.6 kips.
Am I missing something here?
"It is imperative Cunth doesn't get his hands on those codes."
RE: Steel strap lightgage shear walls
RE: Steel strap lightgage shear walls
I believe you are right! So does that mean there is no end of stud bearing check?
"It is imperative Cunth doesn't get his hands on those codes."
RE: Steel strap lightgage shear walls
You actually can check a built-up stud in AISIWIN, but only two back to back or boxed.
I have never checked end bearing. The stud will fail in many other ways before that occurs.
The module you are using is for checking out-of-plane loads between the stud and the track, not end bearing.
RE: Steel strap lightgage shear walls
"It is imperative Cunth doesn't get his hands on those codes."
RE: Steel strap lightgage shear walls
RE: Steel strap lightgage shear walls
RE: Steel strap lightgage shear walls
In the "original" poor detail, the studs are not web-to-web, rather flange lip-to-lip. They were welding to the web of the inside stud, which creates a bunch of problems at as discussed above. I have thrown that detail in the garbage and have since then expressed my concerns with my firm.
Not sure what you mean. Make the studs composite together??
"It is imperative Cunth doesn't get his hands on those codes."