Calcs and approach for sistering wood members

[mfstructural]

I'm working on a project where there are existing 2x12 roof joists on a low sloped roof. The building is about 100 years old, and the city is requiring the capacity of the joists to be brought up to code. Based on what the architect is telling me, the joists are spanning between bearing walls and/or wood headers. The roof is not currently open so we can't see anything at this point. The architect was on site when the roof was opened up in a localized area and that's what the current information is based on.

My question is regarding sistering another 2x12 joist to an existing 2x12 joist? When calculating the capacity of the two joists of the same size together, do you use the equations in NDS for a beam and go from there? what about for joists of different sizes? Could you check span tables for a single 2x12 and double that capacity?

If sistering with a 2x12 doesn't work, I may have to go with something else, which is different in size from the 2x12. In that case, how would you approach that? assume equal deflections in each member and calculate the stiffness (EI) ratio between the two members?

Another thing is that the architects pictures show that the 2x12 is notched where it passes over a perpendicular joist/beam. From the pictures it looks like the notch is less than the code of D/3 for depth and D/6 for length. as long as the notch is less than the code allowed the capacity doesn't have to be reduced correct?

The current scope is to determine the maximum span of two 2x12s and also the maximum span of the headers. It's unclear exactly how the headers are constructed so that will likely be addressed once the roof is opened and everything is visible. The calculations I'm preparing now may have to be completely revised we open and we find another existing condition.

 

[JAE]

do you use the equations in NDS for a beam and go from there?

Yes, you would analyze the combined section and design per NDS requirements (assuming in the US - you didn't say).

assume equal deflections in each member and calculate the stiffness (EI) ratio between the two members?

Yes

Could you check span tables for a single 2x12 and double that capacity?

I wouldn't use span tables.

as long as the notch is less than the code allowed the capacity doesn't have to be reduced correct?

Notching is allowed but just because it is allowed doesn't necessarily mean you can ignore the effects of the notching.
I'd check the member for shear/moment at the notch for the reduced section based on the forces directly at the notch.

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[pmtrevisan1]

depending on span and loading you want to sister an 11 7/8 lvl and ignore the existing system.

using span tables on old growth isnt really applicable as the old material is quite a bit stronger than todays.

 

[RWW0002]

As far as sistering or adding members, I think you have the general approach down. For allowable stresses in historic wood, using "modern" code prescribed values is likely conservative. Approaches I have seen used or used to justify higher allowable stresses in historic lumber include the following:

-Use the timber design information in historic publications from around the time of construction (Carnegie Pocket Companion, Bethlehem Steel Manual, etc..) to back into allowable stresses used at the time.

-Field-grade the timber and have a sample tested to determine species. The national park services have some useful tech notes on historic structures and field-grading of historic lumber. (

https://www.ncptt.nps.gov/blog/a-gr...-and-timber-in-historic-structures-2009-01/).

At one time the USDA would test a sample for free for species determination. Once you know species and approximate grade, you can use "modern" or "historic" allowable stresses based on NDS or other resources.

-Use #1 grade and modern NDS allowable stresses justified by old-growth harvesting without field grading.

You may still end up sistering, but I think it is worth taking advantage of the inherent strength of the existing roof when analyzing the roof/sizing the new members.