Knee Braces pro's and con's & Timber Pile Bending Stresses 1

[eng003]

I am trying to evaluate the pros and cons of knee type braces for use in a marine boathouse type structure. It is an "open" structure with roof and wood deck. Supporting timber piles may or may not be braced below the deck elevation. Knee braces would be constructed near the top of the timber pile connected to supporting roof beam. Instinct tells me knee braces are good for resisting lateral wind forces but I believe they actually end up increasing the bending stresses in the piles. The knee braced structure is indeterminate and will be influenced by the stiffness of the knee bracing and bracing below the deck elevation (if it exists), the pile will be deflected in double curvature; is there a simple way to calculate the bending stresses of the pile to compare with and without? Even if the knee brace increases bending stresses doesn't it enchance buckling resistance by reducing the "k" value? Otherwise I guess all they do is stiffen the structure against sway deflection?

 

[msquared48]

Knee braces actually decrease the bending stresses in the pile / column as it throws the pile into reverse curvature if the knee brace is far enough down from the top of the pile. This is done in pole buildings all the time to control the size of the columns.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[eng003]

Thanks for the response Mike. That is what I thought but I had another engineer question me on it recently on how the addition of knee braces would help reduce bending stresses in the pile at the elevation of the deck and I just was reading a fema doc (see page 6-18

http://www.fema.gov/library/file?ty...&fileid=9f8ddad0-4b7e-11de-8732-001185636a87)

which says that knee braces actually can increase the bending stresses in the pile?? Please see the attached sketch, without doing a frame analysis on the knee braced structure is there simple hand calculation method to estimate the moment diagram of this situation?

 

[dhengr]

Your sketch looks about right, seems you have the right idea. But note, that in wood and timber construction the braces that are in compression (two in either direction of loading) are most effective, while the other two in tension are less effective. You can notch or mortise and tendon the joints so they act in bearing and through the bolts, when in compression. While the tension joints see some movement and are dependant on bolt tension only. While all four of the braces could be brought into play in a steel structure.

 

[BAretired]

A knee brace can add to the bending in a pile because it carries gravity load in addition to lateral load. The brace is attached to a beam which is carrying gravity load. In effect, part of the gravity load is transferred from the pile to the knee brace.

BA