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Wood Knee Braces - Carport

engineerED930

engineerED930

Structural
Jan 24, 2024
18
Can anyone point me in right direction for a procedure to design knee braces for a carport?
 
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I’ve found that quarter-point knee braces @ 45s work great. Check the braces for axial and the beam and columns for bending and buckling. Simpson KBS1Z connectors. Refer to the AWS deck guide.
 
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ANE91 said:
I’ve found that quarter-point knee braces @ 45s work great. Check the braces for axial and the beam and columns for bending and buckling. Simpson KBS1Z connectors. Refer to the AWS deck guide.
Click to expand...
If I bolt the knee brace into the beam and column, the weakest point is the shear of bolts correct?
 
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Sorry, I don’t understand the question. Every element in the load path needs to be able to withstand the loads that they are subjected to. Critical sections for gravity are different than critical sections for lateral.

Nails are “weak” in pullout; screws are “weak” in shear, etc. It’s all relative.
 
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I have actually never considered these but the capacity seems reasonable enough to use in some situations.
Make sure you check drift and also combined bending and axial in the columns/beams.
 
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I don't use knee braces very often, but I posted here a little over a year ago about their design: https://www.eng-tips.com/threads/pre-fab-steel-knee-brackets-for-6x6-deck-posts.516562/post-8969757.

It's a bit of a rambling comment, but I concluded that for most wood knee brace connections, the weak link is likely the bending capacity of the post (or rather the combined bending and axial capacity). So, with that in mind, before you go crazy designing the knee brace connection to the post and beam, I'd start by checking the post.

Assuming the post is good for the load, then I would check:
  1. The bending capacity of the beam (or horizontal member at the top) due to the vertical component of the load from the knee brace (probably ok).
  2. The knee brace capacity presumably in both tension and compression, unless you're basing the design on it only working in compression or tension (in which case an opposing side knee brace would be taking the force).
  3. The connection between the knee brace and the beam when the knee brace is in compression (if applicable).
  4. The connection between the knee brace and the post when the knee brace is in compression (if applicable).
  5. The connection between the knee brace and the beam when the knee brace is in tension (if applicable).
  6. The connection between the knee brace and the post when the knee brace is in tension (if applicable).
  7. The connection between the beam and post to resist the horizontal load between them.
  8. And finally the horizontal and uplift (if applicable) forces at the connection to the foundation.
I'm probably forgetting something, but that should cover most of it.

If you happen to notice knee braces in existing construction, most connections you'll see won't meet code. (Yeah, I know it hasn't fallen over, yet).

Hope this helps!
 
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Around here on decks, most knee braces are lagged into the groove between the plies of the beam with a 3/4"Ø lag. And, yea, they usually don't fall down. Probably more to do with the decking acting as a diaphragm than anything else. As Eng16080 stated, lots of things to check. Drift seems to be an important one though.
 
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When I do these, I notch the column and beam to allow a bearing connection for the brace. And I also only design them for compression. When in tension I ignore the contribution.
 
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I've always had difficulty with the connection of the brace to the column and the P/A+Mc/I of the column. Also, when seismic governs, in my case, seismic always governs, there's that penalty of R = 1.5, instead of 2.5 for steel special cantilever column systems (i.e., flagpole HSS columns). I've never used the KBS connector, but the capacities seem lower than what I would have needed on my previous projects. It certainly simplifies the most difficult to design and detail connection of the entire system.
 
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I use the KBS connectors a lot on knee braces and haven't heard any complaints. But we are mostly using them for raised A/C platforms, short roof eave overhangs, things like that. I can't tell you the last time we specified knee braces for primary support on a deck or structure like a carport. No one wants to look at those. Eng & XR have the key items to check.
 
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Eng16080 said:
I don't use knee braces very often, but I posted here a little over a year ago about their design: https://www.eng-tips.com/threads/pre-fab-steel-knee-brackets-for-6x6-deck-posts.516562/post-8969757.

It's a bit of a rambling comment, but I concluded that for most wood knee brace connections, the weak link is likely the bending capacity of the post (or rather the combined bending and axial capacity). So, with that in mind, before you go crazy designing the knee brace connection to the post and beam, I'd start by checking the post.

Assuming the post is good for the load, then I would check:
  1. The bending capacity of the beam (or horizontal member at the top) due to the vertical component of the load from the knee brace (probably ok).
  2. The knee brace capacity presumably in both tension and compression, unless you're basing the design on it only working in compression or tension (in which case an opposing side knee brace would be taking the force).
  3. The connection between the knee brace and the beam when the knee brace is in compression (if applicable).
  4. The connection between the knee brace and the post when the knee brace is in compression (if applicable).
  5. The connection between the knee brace and the beam when the knee brace is in tension (if applicable).
  6. The connection between the knee brace and the post when the knee brace is in tension (if applicable).
  7. The connection between the beam and post to resist the horizontal load between them.
  8. And finally the horizontal and uplift (if applicable) forces at the connection to the foundation.
I'm probably forgetting something, but that should cover most of it.

If you happen to notice knee braces in existing construction, most connections you'll see won't meet code. (Yeah, I know it hasn't fallen over, yet).

Hope this helps!
Click to expand...
Hi Group,
We often review Knee Braced Portal Frames as well. Jerseyshore did a fine job covering what needs to be reviewed. As a visual learner and someone who has always wanted to cross reference the statics and FBDs we've looked extensively for a write up. Attached are two that AISC published long ago. As expected, anyone can figure the statics out for a Knee Braced Portal Frame under lateral loading. It's takes something extra to figure out the vertical loading which this paper explains in greater detail where stiffness and deflection are equated. Also, like Jerseyshore sums up we find the connections are usually suspect. The knee braces suck up a lot of vertical dead load. Hope this helps those involved.
 

Attachments

  • Analysis of Knee-Braced Portal Frames for Vertical Loading (1983).pdf
    292.2 KB · Views: 10
  • Analysis of Knee-Braced Portal Frames for Vertical Loading Part (2) - Columns of Unequal Stiff...pdf
    312 KB · Views: 9
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