Knee Brace Analysis (not a repost) without a computer program?

[b2frison]

Hi everybody,

Sorry if this looks like a repost, but I have searched the forum and I'm not quite satisfied with some of the previous threads on knee-brace design for my specific inquiry.

Lets say we have a basic wood frame with pinned connections at the two supports, and questionable moment carrying capacity of the hinges. It will take both lateral loads, and uniform downward loading. The solution to the lateral problem is undoubtedly x bracing or some eccentric bracing. I've chosen K-bracing because I have limited ability for my specific project to connect at the supports.

For even just a simple lateral point load, I would assume that the amount of force induced in the knee-brace is highly affected by the initial stiffness of the 90 degree connection -- it doesn't take force until deflection of the frame. Lets just say for simplicity that there is no stiffness in the 90 degree connection, so as soon as the structure is laterally loaded there is force in the knee brace.

How does this analysis actually work?

 

[b2frison]

If a picture needed:

 

[msquared48]

Depending on the depth of the knee braces, for one, they will set up a reverse curvature condition in each column, with the triangle created by each brace, serving to effectively create a moment at each corner of the frame.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[OldPaperMaker]

b2frison,
One way to solve for the loads on the frame shown is to:
1. Assume that the horizontal load H is spread equally between the two columns.
2. Sum the moments about the top "pin" which will give you the horizontal component of the knee brace. Depending on the slope of the knee brace you can find the vertical component and resultant load in that knee brace.
3. Sum the forces horizontally to find the reaction at the top of the column.
4. Now design the beam, column and knee brace for the loads that you just calculated.

I don't understand your question about "...the initial stiffness at the 90 degree connection". There is no "stiffness" if there is a "pin" at that joint.

 

[Robvh1984]

Oldpapermaker,

I think it was just a misuse of semantics in regard to those joints. It seems like he was thinking they would be partially restrained. It may be slightly true in many cases but if you are building this from scratch, you can design it with no moment capacity. AITC has a wealth of details for various connections to ensure the type of full moment or force release you desire.

-Robert Miller, E.I.T.
KPA Structural Engineers