Notched timber beam capacity
Notched timber beam capacity
(OP)
I am checking the design of a simply-supported, notched timber purlin and need some advice. Based on the NDS section 3.4.3.2, I have calculated the shear capacity of the notch, which is on the tension side on each end of the beam. This calculation yields of capacity of 1402 pounds. If I determine the max. beam length allowed based on shear, and excluding the load a distance "d" from the ends, the max length is 12.24 ft.
When I calcalculate the max. moment capacity of the beam, I have end/support reactions of 1567 pounds and a max length of 9.50 ft.
Am I correct in thinking that moment controls even though the reaction exceeds that of the allowable shear?
Thanks for your advice, Alex
Section 3.4.3.1 of the NDS says I do not have to include the load
When I calcalculate the max. moment capacity of the beam, I have end/support reactions of 1567 pounds and a max length of 9.50 ft.
Am I correct in thinking that moment controls even though the reaction exceeds that of the allowable shear?
Thanks for your advice, Alex
Section 3.4.3.1 of the NDS says I do not have to include the load






RE: Notched timber beam capacity
If you can discount load for calculating shear, do so. I think you still have to use the full load for moment. I would check the full loading for bearing at the reactions as well as deflection. I would then check the appropriate Vr' in 3.4.3.2 for shear. The important thing is to be consistent and not apply reductions where they don't make sense, such as using the reduced design shear load to check the end bearing.
RE: Notched timber beam capacity
The 1567 includes the full load across the beam, as do the bearing calcs. The only place I have taken a reduction is according to 3.4.3 for the shear loading. It all seems correct, but it just seems odd to have a reaction load higher than the shear capacity. When you do a shear and moment diagram, the load covers the entire beam...as there isn't any reduction for shear. I know I could be conservative and assume the shear load goes to the beam ends, but I would like to know if my initial approach was correct.
Thanks again.
RE: Notched timber beam capacity
RE: Notched timber beam capacity
With an un-notched beam when dealing with uniform shear loads you are check the shear capacity at a distance d from the face of the support. The reaction is higher than the shear load because you are not checking the shear at the support. I always found it easier to check the stress so what I would do is calculate the shear a distance d from the support and then determine the shear stress at that point.
Neglecting the load a distance d from the face of the support is similar to the way stirrups were design for concrete when I was designing concrete beams 10 years ago. At that time you were allowed to base the strirrup spacing from the face of the support out to a distance of d on the shear at distance d.
To be quiet honest I am not sure that I ever understood why. I was always told that the load between d and the support face was tranmitted to the support by diagonal tension.
Returning to the 2001 NDS, there now are restrictions regarding the neglect of concentrated load between the face of the support.
One final item it is my understanding that when checking the shear in a notched beam you are not allowed to neglect loads a distance d from the support. The NDS indicates for notched beams the shear should be calculated based on engineering mechanics.
You could consult the NDS commentary or AFPA for more information.
RE: Notched timber beam capacity