Timber Design

[CalvinD82]

I have a situation with a timber lintel, simply supported (framed into and resting on masonry walls). The lintel 10" x 6". I have been asked if i can cut 4" off the bottom of the lintel. The lintel is supporting a giant window and has been installed for many years and no one knows what type of wood it is. I feel ok with calculating the stress, the only thing i don't know is what would be a good value to assume for the allowable stress of the lintel?

 

[COEngineeer]

Usually older woods are stronger (slower growth, smaller rings, a lot denser). But to be on the safe side, always assume it is D-F #2. It is 10" depth right? cutting 4" is going to make it 6" deep. That is a lot of cut. It is probably not strong enough.

 

[CJJS]

I believe you mean older trees produce stronger wood. Old wood is not necessarily stronger.

 

[COEngineeer]

yes...thats what i meant . But I heard its not true on some type of trees (oak maybe?)

 

[structuralaggie]

I agree with COEngineer that older wood is stronger than todays stuff. If you are in the south, maybe look at SP #2 instead of DF, but will be close to the same value anyway. Because of it likely being better than #2, you might venture using higher values, like #1 or even higher.

If you can see all 4 sides of the timber, a forester could tell you what grade the timber is. On a job some time back I had an old timber truss with only only had 3 sides visible, and a forester told me it was at least a #1, but since he couldn't see the other side that was as high as he could go. He could also tell you what species it is too if you don't know that.

I don't know exactly how they grade timbers, but I know it has something to do with the size of knots and how many, and I would imagine thier location in the section. If the cut you are proposing is going to put a knot on the edge, that will certanily reduce the allowable bending stress.

 

[dik]

Some codes have a limitation on notching... you might check with the local building official.

Dik

 

[structuralaggie]

The age of wood does not make it stronger, no. Wood used in construciton 40+ years ago was made from higher quality timbers. Statistically speaking then, older wood is stronger.

 

[CalvinD82]

gentleman i appreciate the discussion about the age of wood. It is helpful. Regardless of the age, what would be a good allowable stress to assume?

 

[structuralaggie]

#2 DF beams and stringers is 875 psi and #2 SP 5x5 and larger is 850 psi accoding to the 2001 edition of the NDS. If you don't intend to persue nailing down the species or grade, I certainly wouldn't exceed these values. Adjusment factors are not applied to these numbers.

 

[structuralaggie]

The stress I gave you was bending stress. Also, you could also look at older editions of the NDS. I can't say for sure because I don't have one handy, but it is likely the values would be higher.

 

[dik]

You might consider using the allowable stresses for the material from that time period... and if you can get it graded, DF (dense) Sel 2250 psi, DF (dense) Const 1920 psi, DF (dense) Industrial 1500f 1920. fv 130 psi for all... from CITC Timber Construction Manual 1963 and may not be applicable to your area. Stresses for DF-Western Larch 2290, 1650, and 1650 respectively and fv 130 psi... at the time, 1500f was commonly specd.

Dik

 

[CalvinD82]

thanks guys that helps out

 

[miecz]

Are you taking 4 inches off the entire lintel or are you notching the ends? If you are taking 4 inches off the entire lintel, making it a 6x6, and you are carrying glass, then you should consider the deflection as well as the stress.