Lateral resistance of cold formed steel holddowns 1

[milkshakelake]

If I use a holddown with cold formed steel diagonal straps, there will be uplift and shear. The holddown schedules from Simpson only show tension/uplift capacity, not shear. How do I know if the shear is okay?

Wood is different because the plywood shear wall will take the shear and transfer it into the bottom plate, which can transfer it into the foundation with anchor bolts. I called Simpson about this but got bounced around between different "experts" so I'd rather ask here.

 

[XR250]

There is no lateral capacity. You have to get the load into the bottom track somehow. I usually use an MC6x18 or an L6x4x5/16 with (2) anchor bolts to resist both components of the strap tension. Welding is required.
If the loads are not high, the Simpson hold-down and a heavy gage, deep leg bottom track can also work.
Check out The Steel Network. They likely have a better solution than Simpson.

 

[milkshakelake]

@XR250 Thanks! I will figure out a way to transfer the load into the bottom track. And I have a bad history with The Steel Network due to price, but I'll definitely check it out.

 

[milkshakelake]

@XR250 Again thanks for your response. Sorry if I'm being rude but can you send a picture of your detail? I've been trying to figure out how to get high loading into the track with an MC6 or L6x4 like you mentioned but I'm stuck. I can't figure out the load path because first the load goes into the post.

 

[phamENG]

milkshakelake - I typically detail a plate or heavy sheet to accomplish it. It acts like a gusset in structural steel, but because the members tend to be so much more flexible the horizontal reaction at the stud approaches zero pretty quick.

 

[milkshakelake]

@phamENG I try to avoid a gusset plate sheet and instead, I go into the stud/post directly. But if this is the only way to transfer the horizontal reaction, then it has to be done. Thanks for the sketch!

 

[dold]

Our firm has moved towards steel sheet (sheathing) instead of straps. I know that doesn't answer your question, OP, but this is one of the reasons. I've never seen a blockout in the foundation wall to embed the post into. The guys above are right - any regular hold down isn't rated for shear. TSN has a solution (photo 1) that's essentially a baseplate that is stiff enough to load the anchor bolts in shear and tension, which you'd then design the anchorage just like any braced frame column anchorage. The method you're proposing would put the holddown-to-stud screws in tension and shear depending on the orientation of the hold down relative to the stud/strap, which I wouldn't count on the contractor to get right. Not sure I'd try that. You really need to get the shear into the track like Pham is talking about. I think XR's angle would be installed with a couple bolts into the foundation and the strap fastened to the angle and not into the stud (photo 2).

 

[milkshakelake]

@dold Nice! Thanks for sharing; I'll probably move to this detail. And I agree that I've never seen a blockout. Simpson shouldn't be showing the flat strap with holddown detail in their catalog since it doesn't work.

Using steel sheets is an interesting idea. I've never done it before because it's not clear to me how to model the stiffness, but I'll definitely look into it.

 

[KootK]

I was able to coax XR's clever detail out of him some time ago. I very much favor the symmetry of the channel version shown below.

Our firm has moved towards steel sheet (sheathing) instead of straps.

I'd love to hear more about this:

1) We're talking about using steel sheet to sheath entire walls, right? Kinda like steel plate shear walls?

2) Do you know of any design guides etc that would elucidate the important concepts in detailing and design?

3) What has motivated your firm's choice with this? How do you see the construction economics playing out for your clients?

It acts like a gusset in structural steel, but because the members tend to be so much more flexible the horizontal reaction at the stud approaches zero pretty quick

I've always viewed this as a double edge sword. If the brace load is delivered concentrically then, yes, the fact that none of the members involved are worth a damn for resisting bending will have the advantage of steering the load components where we want them to go. On the other hand, if the load delivery is not concentric, then you're stuck with a moment that has to be resisted by at least one of the members which is worthless for bending. I'd be interested to hear others' feelings on whether or not it's reasonable to expect the gussets to produce a concentric load delivery. I'd imagine that we'd all feel more comfortable with it when the assembly is done pre-fab in a shop environment.

This is another thing that I view as a benefit of XR's detail: it's quite insensitive to where the brace work point lands.

 

[dold]

Another thing about using a gusset between track and stud - this creates a fixity and induces weak axis bending in the chord studs as story drift occurs.

Regarding the steel sheet sheathing:
Yes, we're talking about sheathing the wall with 22/20 gauge steel sheet. Not necessarily the entire wall, but just as much as youd need for your lateral system. It is essentially an alternative to wood sheathing. There are values for this in the AISI documents immediately below the values for wood sheathing. As far as detailing goes, you should be able to use all the same details you would for wood sheathing - there are no special considerations that i'm aware of. Obviously you'd want to ensure proper edge distance, etc for your fasteners.

As for the motivating factors: First would be R = 6.5 for steel sheet sheathing, as opposed to R = 4.0 for flat straps. So, that helps. Second is constructability (according to the guys who are in charge of making these decisions...but i tend to agree). Steel sheet and flat strap are both sensitive to temperature (of course, since theyre the same thing) but the mechanism by which force is transferred from above to below is different. Say you build your flat strap wall when it's cold out. Then the next day is hot - the straps expand and bow out. Now your flat strap wall is free to wobble around until the straps go tight. It'd be pretty quick to calc the story drift due to slop in the straps. Also it's a pain in the butt to get the straps taught in the first place - some CFS suppliers (TSN) sell tensioning devices (pretty much just a clamp and angle brackets) for this reason. Anyway, looking at a steel sheet, once the top and bottom of the panel are screwed into the top and bottom tracks, the only 'slop' that can develop is (guessing here) screw slip and maybe some sort of hard-to-describe deformation as the tension fields develop and maybe flatten out any bulges from the thermal expansion(?).

It also ends up looking cleaner than most flat strap installs i've seen. Flat strap usually ends up with a huge cluster of screws and crooked warped gusset plates and it looks like hammered dog s***. Steel sheet is pretty much idiot proof.

That's my takeaway from discussions and presentations in the office.

Here's a solid overview of CFS lateral systems: [URL unfurl="true"]http://curee.org/projects/nehrp-jv/publications/nistgcr16-917-38.pdf[/url]

 

[KootK]

Thanks for taking the time to share that with us dold.

 

[milkshakelake]

@dold Thanks! I think that'll help a lot. I didn't think about the tensioning effect and temperature before; I assumed it would be properly tensioned, and I always calculated the stiffness that way. But now that I think about it, it should be like a rod and turnbuckle situation when installed.

@KootK About your points, I've never considered steel sheets because I didn't know that literature existed. Thanks dold for bringing up a new field of inquiry. I knew there were nominal capacities in AISI but that's about it. I read the link dold sent a few years ago but I didn't take it seriously and took it as a research article. I didn't know that real firms are using it. I'll have to check up all the referenced codes.

The connection between the strap and angle doesn't necessarily have to be welds for smaller projects. Screws are okay according to the Marinoware (cold formed steel supplier) and PAFs are okay according to Hilti catalog.

For your detail, how come you clipped the bottom track instead of extending it? That way, we can transfer the shear into the bottom track as XR250 suggested.

 

[KootK]

For your detail, how come you clipped the bottom track instead of extending it? That way, we can transfer the shear into the bottom track as XR250 suggested.

1) I was just trying to acknowledge that, for some channel / track combos, the track might have to be clipped to accomplish the welding or screwing AND;

2) One of the prime benefits of the detail is that it delivers the base shear straight into the foundation element without involving the track. Shoving monster amounts of thrust into bent tin isn't the greatest of load path arrangements in my opinion.

 

[milkshakelake]

@KootK Thanks! I totally agree. If we can get the shear done in that channel or angle, then it's a done deal. No need to transfer it into the bottom track as others have suggested.

 

[r13]

If I use a holddown with cold formed steel diagonal straps, there will be uplift and shear. The holddown schedules from Simpson only show tension/uplift capacity, not shear. How do I know if the shear is okay?

Both system can deliver, only a matter of preference.

 

[milkshakelake]

Thanks r13 for the new opinion. I think I'll stick to the angle idea on the right side. I don't trust contractors to do a good sheet steel gusset plate like on the left side.

 

[XR250]

My issue with the gusset plate is that you are assuming that the vertical force component is managed by jamb and the horizontal force component is managed by the track. I think that if it were a very stiff gusset plate compared to the resisting members, that may be the case. However, I beieive in reality, it acts more like a membrane and will pull the jamb sideways and the track upwards - In which case the weak axis strength of these members would come into question. My experience is that it would be difficult for these to pass. If someone can disprove this, please do.

 

[r13]

However, I beieive in reality, it acts more like a membrane and will pull the jamb sideways and the track upwards

The track and the jamb shall be fastened to the foundation, though it is not shown on the sketch. The right hand side method seems more strong by fasten the strap directly to the foundation, but in doing so, you need to enlarge the footing, and deal with the eccentricity between the strap connection and the jamb. So it is a choice of preference rather than other matters.

 

[milkshakelake]

@r13 I don't think I'll enlarge the footing that much. Let's say I have the ASD maximum capacity uplift. For a 6" wide 12 gage strap on one side only, that is approximately 0.6 x 50 ksi x 0.097 = 2.91 kips tension. If I have straps on both sides, it'll be 6 kips. That's not a large load, even for cold formed steel. It will affect the connections more than the footing.

 

[r13]

milkshakelake,

Yes, both works, it is your preference, no objection here.