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More Knee braces #4227 02/22/99 08:01 PM
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clark_bremer Offline OP
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I've got several Timber Framing books, and I've noticed something about many of the plans in these books. On almost every interior post of a bent section, both braces are present (in the plane of the bent). But the same is not true about braces perpendicular to the bent. They always seem to be there in exterior walls, but in the interior, they are often omitted. Is there a reason for this? Thanks, CB.

Re: More Knee braces #4228 02/22/99 10:53 PM
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Joel McCarty Offline
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Sure, there's a reason for it.

Symmetry is one.

Frames see stresses on raising day that they will never see again (hopefully). Some folks just stick braces in everywhere, figuring that more than a few of them will be left out or cut out by the people who actually have to use the building, and are not interested in having a lobotomy every time they use the stairs.

There is also the common engineering phenomena known variously as "I never met a brace I didn't like", or as "Floor to ceiling x-braces right across these windows."

Timber frames, even when poorly designed, usually include sufficient redundancy to permit the removal of a brace or two without catastrophic result. Of couse, in these litigious times, you didn't hear this from me.

Re: More Knee braces #4229 02/23/99 02:15 AM
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clark_bremer Offline OP
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OK. Let me be more specific. Let's say I'm planning a small house with 4 bents, each with 3 posts. That's 12 posts, where only 2 of them are 'interior'. All of the possible knee braces are in place, except for the ones on these 2 interior posts. On these 2 interior posts, there are 8 possible knee braces. I'd like to omit 2 of them, one on each post, perpendicular to the bent plane. Does that seem reasonable? Thanks, CB.

Re: More Knee braces #4230 02/23/99 01:56 PM
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Rudy R Christian Offline
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It's worth noting that braces are usually placed in pairs. Basically, the purpose of bracing is to withstand intermitant transverse loading. The stress of raising is one example. Wind and snow are others.

Wind loads are hard to predict, both in strenth and direction. By placing braces in opposite "pairs", one works to resist the wind from one direction, while it's pair rests. When the wind switches direction the opposite occurs.

In many early barns, braces were not pegged. This lack of pegs indicates the early framer's understanding of the work a brace does being resisting a force by going into compression.

In a nutshell, moving a brace to keep it paired is the best solution. If you have to remove one, it's likely you should remove it's pair also.

Having an engineer look at the drawings is of course the most dependable solution.

Re: More Knee braces #4231 03/04/99 04:27 AM
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Chris Hoppe Offline
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Knee braces are not often present perpendicular to the bent at the center post for the following reasons: 1. Framing is based largely on tradition. The old hay barns did not have a timber running perpendicular to the bent at the top of the center post. The center post usually terminates at the bottom of the tie beam. 2. In new frames where a second floor is present, often the joists run perpendicular to the bents. It is easier to avoid having a joist at the same location as the center post to avoid many timbers comming together at one joint. In this situation there may be no member or only a joist to set the brace into, where at the eaves there is usually a heavier plate to let the brace into.
An engineer can help you make an informed decision on the number and location of braces required. Good Luck!

Re: More Knee braces #4232 03/04/99 12:44 PM
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Grigg Mullen Offline
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One of the main purposes of the braces is to resist racking of the frame due to wind loads. Parallel to the bents, there is a much larger surface area of exterior wall to catch wind like a sail. Perpendicular to the bents, there is only the end wall to catch wind, and the braces in several bents to reisit that load. The braces are generally not as heavily loaded and it is possible that one set can be omitted without compromising the stability of the frame. This can be checked with an engineer's analysis.

Re: More Knee braces #4233 03/12/99 02:47 AM
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Matthew_Marino Offline
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Framing must have been much simpler before the advent of the Lawyer.

It's hard to say, without knowing the overall floor plan, what options seem likely. Taking into consideration what we know about braces in compression being the strength, I have installed braces in opposition at the floor level. If the bent has continuous timbers to transfer the stress it could be anywhere in the bent and still be a benefit. For what it's worth. Not all engineers are brilliant and they can be sued as well.

Re: More Knee braces #4234 03/12/99 03:08 AM
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Rudy R Christian Offline
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I have trouble believing timber framing predates lawyers.

The concept that bracing "anywhere in the bent" creates the same result, sounds like something a lawyer would argue. Maybe that's one way of telling the lawyers from the engineers (in case we ever need to).

It's hard to say, without knowing the overall floor plan......

Re: More Knee braces #4235 03/12/99 08:26 AM
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Bill Keir Offline
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This is very interesting stuff.


Bill

The pessimist sees difficulty in every opportunity

While

The Optimist sees Opportunity in every difficulty
Re: More Knee braces #4236 03/12/99 10:01 PM
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Paul Freeman Offline
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Perhaps lawyers are the second oldest profession *wink*

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