Wood Design Question

[structural-eng]

Do the values listed in the NDS account for the amount of defects allowed for each grade of wood? Specially I'm wondering about waning. I have some 2x6 studs that were near capacity for the code required loading and one of the studs has substantial waning (section is missing from where bark was before the stud was made). The studs are stud grade so I'm wondering if the reduces allowable stresses in the NDS account for the amount of waning or if I need to look at the reduced section and consider the reduced allowable stress.

 

[Eng16080]

Well, in general, a graded piece of lumber is required to meet certain rules, and these rules allow lumber defects to a certain extent. I can't speak specifically to the requirements for wane. It's probably a judgement call here, but if the stud/s are critical in this location, I would probably take some additional measures to strengthen the questionable pieces considering that you're aware of the defect.

 

[dauwerda]

Do the values listed in the NDS account for the amount of defects allowed for each grade of wood?

Yes.

As long as you are designing using the values for stud grade, you do not need to account for a reduced section.

 

[Lomarandil]

This is one of those cases where the code is the worst (stud) you are legally allowed to design. Agreed that if this came to your attention somehow, while technically fine, it's probably worth doubling the stud.

 

[Althalus]

For the most part I agree with the concensus here. The grading system does take into account the level of defects. That's a major part of the system.

However, if you as an engineer see something that is so bad (like waning at an end that cuts out 50% of the cross-section) you might want to use engineering judgement and say,"You can't use that piece!"

 

[structural-eng]

This particular case is actually at my house. I'm a structural engineer and we are remodeling and they had to reinstall a couple of windows because the bond beam lintel over them was failing so it was replaced with an LVL lintel. There is only room for a double 2x between the windows and the contractor installed 2 stud grade 2x6 when I thought he was going to be using #2 2x6. One of the 2 studs has a significant amount of waning. The waning is towards the joint between the studs so I can't get a good look at it but there's considerable section loss. If I look at the stud grade values in the NDS assuming full 2x6 section the studs just barely checks (interaction = 0.99) for DL+0.75*SL+0.75*LL if they're properly nailed together which they currently are not. The contractor said he would replace the studs with 2x8 but he hasn't done it yet. I'm trying to decide if it would be better to remove the windows and nailed the 2x6's together sufficient to meet built up column criteria or have him just replace them. I'm alright pushing the interaction because I know the load case is conservative for residential (although we can get a lot of snow in MN). My reluctance to replacing the studs is that they already took the shoring down that was supporting the loads on both sides of the lintel and I can about imagine the pounding and hacking that will happen to get the new 2x8s in.

 

[jerseyshore]

Wood design values are based on the 5th percentile, so most wood used in practice should be better than their book values.

Your stud might be that 95th out of 100 studs that would pass grading. But in design, as long as it's graded, we treat them all the same.