bending stress in compression

[Triangled]

I am reinforcing a 4x roof beam, under a nailed diaphragm, by sistering a couple of microlams, designing it as a composite section such that the neutral axis is lowered, etcetera...
The result will be such that my extreme fiber bending stress in compression is greater than my extreme fiber bending stress in tension.

Given typical values such as for DF-Larch#1, Fb=1000, Ft=875 and Fc||=1500, I understand limiting the extreme fiber bending stress in tension to Fb (with all the factors), but do you think there is margin to allow the extreme fiber bending stress in compression to exceed Fb?

 

[KootK]

but do you think there is margin to allow the extreme fiber bending stress in compression to exceed Fb?

I certainly do. The trick, however, will be determining just what that margin is. I don't know the answer to that. And, of course, you'll need to satisfy yourself that you've jacked or considered locked in stresses etc.

I wonder if there might be some way to adapt the beam-column provisions to your problem. One could treat the reinforced member on its own as a beam-column loaded:

1) Transversely by whatever share of the transverse load belongs with the original member and;
2) Axially by the horizontal shears that the reinforcing members will impose along the length of the original member.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Triangled]

I'd like to use the full value of Fc|| with all the factors.

 

[KootK]

I bet you would. And I say go for it! The tension numbers seem to imply that you get a break on the stresses when there's a stress gradient involved (flexure) compared to the uniform axial stress case. As such, I suspect that you could probably mobilize a flexural compressive stress greater than fc_11 here.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[wannabeSE]

I'd just design the members as non-composite and distribute the load to the members based on stiffness.. But, I like KootK's suggestion to treat it as a beam column. If you treat it is as a compression member, won't the column stability factor drop the allowable stress down closer to or below the allowable fb.

 

[KootK]

If you treat it is as a compression member, won't the column stability factor drop the allowable stress down closer to or below the allowable fb.

I'm curious about the same thing. Not curious enough to prosecute if myself though. My suspicion is that the central member will be immune to axial load buckling concerns. Kinda like how axial prestressing of a precast column doesn't contribute to buckling.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Triangled]

in the case I am considering, the 4x is a beam in a panelized roof system and is supporting 2x6 flush-framed subpurlins @24" each side, all directly attached to a wood diaphragm. compression, as I see it, is induced solely through bending, such that it's not apparent to me how to introduce a beam-column concept. sure appreciate your thoughts.

 

[wannabeSE]

If you are you having problems finding he equivalent compression stress and bending stress in the member, then
1. Find the stress in the top and bottom of the 4x.
2. See attached for example on calculating the equivalent bending stress and axial stress.

 

[Triangled]

ok, I think I see what you're saying.... like looking through the same window from a different direction....my composite beam under bending only is mathematically equivalent, at least at a particular cross section, to a column under compression load with a somewhat less bending moment.... interesting. thank you for that line of thought.

 

[Triangled]

curious as to what length you'd consider using for the 'column', since the compression component is changing along the length of the beam.... maybe just a 2' section, the distance between my subpurlins?

 

[KootK]

I'd consider it to be zero. Like a prestressed concrete column, there's still no net compression on the combined section. I feel that whatever you're doing for whole a section LTB check covers you for instability. The 4X is really only a beam-column in s pure, combined stress check sort of way. At least that's my take.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Triangled]

thanks for the thoughts. Instinctively, I feel that the bending compression value is in actuality larger than the bending tension allowable in typical wood beams, but, obviously, in standard construction the case would be rare that such a condition would be encountered and, likely therefore I suppose, values don't seem to be readily available. It's well known that published Ft values used to be essentially the same as published Fb values, until we learned better. Ft was radically reduced, but I don't see any similar reduction in Fc||....suggesting to me that it's the tension side in flexure that is governing the value of Fb. So now I'm ransacking the internet for data developed by the guys using FRP to reinforce wood beams, in which cases, I am sure they would have encountered the same question. In the meantime, I think I'll stay close to the published Fb.

 

[KootK]

It's absolutely the tension value that governs fb. In tension, you get brittle fracture at some manner of flaw or stress concentration, similar to glass. In compression, you get to more or less crush the material.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.