Allowance/Guidance to Exceed 1/4D Notch in Timber Floor Joist at Support 2

[briceandon]

For a timber floor joist, is there any allowance/guidance if you want to exceed the 1/4D maximum notch at the end of a joist, which is specified by NDS/IRC? NDS 2015 is the applicable spec for the project, but I could only find a copy of the 2005 NDS commentary. In this commentary, Article C4.4.3 indicates the 1/4D notch is good practice and recommended partially because of the stress concentration at the notch corner and the potential for splits at the corner. My thought is that if the joist is tapered at the ends, rather than notched, the stress concentration would be greatly reduced and exceeding the 1/4D may be possible, provided the remaining sections meet the allowable stresses.

For some background information, my project is to strengthen/stiffen an existing floor in a house built in 1897, and my plan is to install a sister joist at each existing joist. The existing joists are 2"x8" rough cut lumber (measured dimension) spaced at 24" and they are notched 2" at the sill beam.

Based on the current live loads and span (14.7ft), the sister joists (SPF #2) need to be 2x10s; however, to maintain the same floor elevation, a notch >1/4D would be required. If I use a 1/4D notch, the floor will be raised and there will be an ~1” difference between floors at the room threshold. I tried to stop the sister joist short of the supports and just use the sister joist to stiffen/strength the existing joists but the calculated shear stress in the existing beams is too high.

I’m looking for a section in a code/spec that discusses using notches/tapers >1/4D. Any insight would be greatly appreciated.

 

[Agent666]

Those types of recommendations within codes for notching are primarily based on the assumption of unreinforced notches. However, there is plenty of literature around related to strengthening these type of notches, usually the least labour intensive methods involve utilising fully threaded screws driven from the underside as shown below. Effectively the screws intercept the horizontal crack that naturally wants to form at the root of the notch, with the screws getting engaged in tension across this potential crack to hold things together and drag load into the reduced section at the notch.

So, strengthening usually involves simply justify a screw anchorage either side of the expected crack location at the notch, and justification of the shear and bending capacity of the reduced depth section.

Locally here in NZ we have this recently published guide for the reinforcement of timber members which might get you started, the concepts involved are relatively universal and I believe most of the recommendations are based on European guidance where they seem to embrace the mass timber concept.

I'm sure AWC has similar publications/recommendations if you're after something north american.

https://engineervsheep.com

 

[briceandon]

Agent666,
Thank you for the reference. I quickly looked through the guide and it looks like it will really help me out!! I'll also check if I can find a similar AWC publication.

 

[SWComposites]

You could also nail plywood or steel doubler plates on the sides of the joints to reinforce the notches.

 

[phamENG]

Agent666 - sadly we do not. Plenty of research on the engineered wood products side, but very little on sawn lumber and nothing codified. Just vague "principles of engineering mechanics" requirements for analysis. The guide you linked to has come in handy for me.

 

[XR250]

Can you install a ledger strip at the sill beam?