Steel to Wood or Wood To Steel? 2

[BSVBD]

I'm reinforcing an existing 2nd floor office, assumed previously designed for (less than) Office Live Load of 50 PSF.

The proposed facility will support a Fitness Center that I will design and reinforce for 100 PSF.

I will add 10" CFS (Cold-Formed Steel) joists immediately adjacent to the existing 2x10 DFL's.

The CFS joists ALONE, at the 15' span, can support the newly imposed load.

To make them all work together, do I:
1. Pre-drill CFS and fasten to wood with wood screws
2. Fasten CFS to wood with wood screws only for lateral bracing
3. Fasten wood to CFS with self-drill TEKs
4. None of the above

I'm opting for #2, but, i don't want to cause the additional process of pre-drilled holes... BUT... I'm not convinced that fastening the wood to the "thin" CF steel will provide any benefit in this particular application.

Please advise and comment...

Thank you!

 

[jayrod12]

Doubling up with wood won't do what you need?

 

[KootK]

If sticking with CFS, I'd be inclined to:

1) Use 8" CFS centered on the existing joists vertically.

2) Fasten to wood with TEKS designed to transmit load share from wood to CFS.

3) Use two rows of fasteners spread out but centered on the common center line of both members. that way you're good for LTB but there's no VQ/It demand.

My recommendation is partially informed by the special character of fitness facilities. Your most likely call back issue will be vibration. As such, I'd definitely want all of the pieces working together predictably on both the up and down cycle.

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]

Seems like sistering with wood or LVL - even if double sided, would be easier.
If you go with CFS, use 16ga min, as the wood to metal screws require 16ga min to reliably break off the reeamer wings unless you wanna pre-drill.

.... and what KootK said

 

[BSVBD]

jayrod & XR... Wood / LVL was the logical first choice, but, the existing is a IIB Metal Building which requires non-flammable construction, which would require either the more-expensive fire-treated lumber versus the less-expensive CFS. I didn't research any further (with Hoover Treated Wood Products, for example), if LVL can even be fire-treated, since the contractor client would rather pursue the CFS in this case.

Koot... I don't think TEKs will do anything but drill a hole into wood. I believe the threads are too fine to effectively engage the wood to transfer any load. Further, i believe that any "light-gage" CF (of 12 gage or less) is too thin to prevent the TEK from teetering / wobbling. Am I exaggerating in my thinking?

Any further thoughts?

 

[KootK]

See if you can't find a workable fastener in here: Link. It's predominantly a dowel shear connection so all you need from the threads, head, and washer is some nominal pullout resistance. Install from the wood side with washers would get the capacity. Might be an easier install from the steel side though. An install test in the garage at home might be fun.


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.

 

[jayrod12]

I guess the main point is, as you alluded to, if the CFS is adequate all alone, do you need the connection between the two at all?

I would say no, provided you can adequately connect the CFS to the diaphragm to brace the top chord. Otherwise you'd have to find some other way to brace the top chord.

How tricky is the connection at the ends of the CFS to the support structure? Were you planning on relying on the wood to take the shear load into the support?

 

[BSVBD]

Koot... thank you for you input. Very helpful.

Koot... I've considered your 8" CFS suggestion. However, since, at this time, I've decided to place a 10" CFS next to the existing is 2x10(9.25") and shim the existing 2x10 with PL 3/4" steel, I prefer the idea of the live load fitness floor bearing equally on both the DFL & CFS, thereby imposing the equal unity, thereby eliminating the shear effect.

Based on the above:

To make them all work together, even better, do I:
1. Pre-drill CFS and fasten to wood with wood screws
2. Fasten CFS to wood with wood screws only for lateral bracing
3. Fasten wood to CFS with self-drill TEKs
4. None of the above

I'm opting for #2, but, i don't want to cause the additional process of pre-drilled holes... BUT... I'm not convinced that fastening the wood to the "thin" CF steel will provide any benefit in this particular application.

Also... Koot... I don't think TEKs will do anything but drill a hole into wood. I believe the threads are too fine to effectively engage the wood to transfer any load. Further, i believe that any "light-gage" CF (of 12 gage or less) is too thin to prevent the TEK from teetering / wobbling. Am I exaggerating in my thinking?

Do any of you feel that TEKs in wood are actually effective in any way?

 

[BSVBD]

jayrod... Thank you! I didn't realize you replied before I sent my last reply.

Right now, I'm leaning toward asking the contractor client which method he would prefer - #2 or #3 - since, in my opinion, either method would provide sufficient top chord brace of the CFS which is my main concern at this point.

jayrod... Regarding the end connection, we are demo-ing the wood bearing walls and installing a new wide flange.

jayrod... if you are referring to wind shear, this is an existing metal building which has the wind shear taken care of through the shell with typical diagonal bracing.

Still looking for an opinion on the TEK screws in wood - are they effective at all? I say no! - "Challenge me - change or adopt my opinion!" Thank you!

Thank you all!

 

[jayrod12]

I would be using pan head wood screws, not teks. and depending on the gauge of CFS, they may be able to self drill anyway.

 

[KootK]

I prefer the idea of the live load fitness floor bearing equally on both the DFL & CFS, thereby imposing the equal unity, thereby eliminating the shear effect.

I've got one more arrow in my persuasion quiver. After vibration, maybe ahead of it really, your biggest risk for performance issues is creating a wildly squeaky floor.

Firstly, the wood floor has likely deflected and crept a bit so getting the CFM snug to the sheathing along its entire length is unlikely. Secondly, the wood will creep in the future and perhaps grow and shrink a bit with humidity changes. Thirdly, any load applied to the side of the wood will rotate the flooring such that only the wood joist is loaded directly.

All of these things add up to there being a shear transfer demand whether you like it or not. If the fateners are unable to handle that demand, they'll plow slots in the CFM that may squeak as the fasters move up and down the slots with load.

Also... Koot... I don't think TEKs will do anything but drill a hole into wood.

If you install the screws as I've recommended above, all they have to do is drill a hole through the wood. It's a dowel shear connection. For all the difference that it would make, the length of screw within the wood could be smooth shank. Truly, if you could predrill or otherwise get it through the CFM, you could use clinched nails here.

Further, i believe that any "light-gage" CF (of 12 gage or less) is too thin to prevent the TEK from teetering / wobbling

Not sure I understand. Why should it teeter or wobble? Embedment in the wood should be plenty adequate to stabilize the fasteners.

I don't know if TEK screws are getting best fastener for a wood/steel connection but they are absolutely a common fastener for that application. Just look at that hybrid doc that I linked. They give examples right in there.

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]

I always spec something similar to this...

http://www.screwsolutions.com/ReamerTek-Wood-To-Steel_c_15.html

 

[XR250]

...although, pre-drilling the CFS and installing a wood screw may work better (as was mentioned by others) as it may have less slop and prevent those KootK squeaks.

 

[KootK]

Those look perfect. Install always from the wood side, right? Maybe that isn't as difficult as I've been imagining.

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.

 

[jayrod12]

Teks has the same ones. Or at least similar, but also installed from the wood side first.

 

[RobertHale]

I will chime in with an endorsement of the self-drilling, self-tapping screws with wings. I use them for wood top plates on beams all the time. One word of warning, make sure you know the product they are going to use and make sure the product is compatible with the steel thickness. I typically specify the Simpson product, and you have to have at least 16ga metal behind the wood.

Robert Hale, PE