Wood fracture shear resistance at notch

[STpipe]

I'm evaluating repairs being done to some residential roof trusses. At the dormer locations, the top chord (2x12) of the truss is notched to allow it to sit on the 2ply - 2x8 header. I'm estimating the depth of the notch to be 5.3in which would exceed the maximum 0.25d allowed in CSA 086. My forces are relatively small, and if I check the fracture shear resistance due to the notch and the shear resistance due to the reduced cross-section, they are both fine.

Anyone know why there is a limit of 0.25d was established? Since the calculations show that it's fine, would it still need to be reinforced?

 

[SWComposites]

How are you calculating the "fracture shear resistance"? Some sort of fractural mechanics approach? Using what material properties?

 

[STpipe]

The code provides an equation for the notch stress. It's dependent on the ratio of the notch depth to the overall depth of the member, and the distance between the point of bearing to the edge of the notch. By the looks of it, I would guess it's an empirical or semi-empirical equation.

 

[SWComposites]

Using a "stress" value to predict the horizontal crack propagation in a wood member is at best inaccurate; fracture mechanics methods are required for an accurate prediction. The code may have a "stress" value that is backed out of some sort of test data or refined analysis done separately. The code value should not be extrapolated. (I have lots of similar experience with interlaminar fracture in composite materials).

 

[ChorasDen]

SWComposites is correct here, unless you have confirmation from the code writing body, or additional non-public test data that has followed a robust and thorough analysis, extrapolating any equation that has specific defined boundaries is at best misguided, and potentially negligent.

To answer your initial question regarding where the limits come from, I do not deal directly with Canadian wood code nor their councils (I'm primarily work with AWC and ASCE in the US), my expectation would be that this limit of 0.25d was found to be proven through empirical testing, and if that value is exceeded (if such a scenario was tested), that the equations quickly diverge and are no longer valid.

 

[ChorasDen]

To add, smaller notches tend to focus on a local shear failure, larger notches tend to exhibit a local tension perp failure. It may be that CSA defined their notch limits to prevent design considerations for wood tension perp, which is not a design value that is published for use in any North American code that I am aware of.

 

[ChorasDen]

Back to your initial question, do you have a detail of the condition that you can show? I generally prefer a posted detail rather than a written description. You note your reactions are relatively small, how small exactly? How is the notch detailed? If you determine that reinforcement is required, how would you get load out of the stiffer truss chord and into repair material?

 

[STpipe]

Hey everyone,

Thank you very much for your insights. It definitely makes sense to stay within the bounds of the code provisions.

For additional clarification I've included a sketch, although based on what I've seen in this thread, I should be looking at connecting this member to the header through other means.

 

[STpipe]

Just wanted to bump this back up. In terms of potential solution, would a joist hanger do the trick? I haven't looked at specific products to see whether the top chord of the truss protruding significantly beyond the header.

 

[Eng16080]

Per your sketch, I would consider using plywood gusset plates along both sides plus a top mount joist hanger to the header to be an adequate solution. I might also extend the plywood plates to the end, so they are also notched around the header. That should further reduce the splitting potential of the 2x12.

 

[STpipe]

Hi Eng16080,

Thanks for the quick response. Is it something like the attached sketch that you had in mind?

 

[Eng16080]

STpipe,

Yes, I'd feel comfortable with a detail like that. I'd connect the gussets with nails or woods screws spaced on a 3"x3" grid or something like that (with the fasteners in accordance with the recommended spacings of the NDS commentary, or whatever your code is). In particular, try to get a reasonable amount of fasteners between the gusset and 2x12 to either side of the location where the 2x12 would split. The notched plywood and fastening is debatably unnecessary with the top mount hanger, but I think it's worth the small extra effort.

 

[STpipe]

Eng16080,

Thank you for confirming that I interpreted your suggestion correctly.

With this solution, I'm trying to understand what the plywood gussets are doing. Since it too is notched, doesn't that just shift the problem to it rather than the 2"x12"?

 

[Eng16080]

The plywood is just adding redundancy. It's perhaps not really needed considering that the splitting risk should be minimal now with the joist hanger supporting the bottom of the 2x12.

But to answer your question: The plywood will be much less likely to split than the 2x12 due to the fact that there is not a uniform grain direction between the individual plies of the plywood. I want to say that it's even less likely with OSB (Oriented Strand Board).

If some of the 2x12s have already started to split, I would expect the notched plywood detail to minimize the risk of the split worsening. Again, is it really necessary? That's probably debatable.

 

[STpipe]

Eng16080,

That makes perfect sense - it completely escaped me that plywood grains would not be uni-directional. I think this is the solution I will move forward with, thanks!