Bending Capacity and E of Existing Wood Joists in Historic Building 5

[mfstructural]

Hey everyone -
I'm interested in getting people's thoughts on the strength of existing wood joists in an existing building built in 1909. My scope was related to evaluating the roof framing for new RTUs and the 2nd and 3rd floors for some new mechanicals, but we came across an issue where I ended up evaluating the existing joists. The building is 32' wide x 155' long. The joists on the 2nd and 3rd floor span from front to back between 15'9" and 17' (span lengths), between steel beams. The building was renovated in 2007 and the drawings from that renovation list the LL as 100 psf and the joists as 1.75"x13". I performed a check on these joists and they were not working for a Fb of 1,000psi. I informed my client and they were not happy. In fact, they found another engineer to run some numbers and see if it works to verify my opinions. A calc I received from a supposedly licensed engineer uses a Fb of 1750psi for Douglas Fir Larch #1, with a repetitive use factor included. I feel like this is somewhat unconservative, but the drawings from 2008 also listed these joists for 100 psf LL capacity. I wanted to get people's thoughts on using 1500 psi for existing wood rough sawn joists. I've included some photos. They are not happy with me and my findings and I've been asked to sharpen my pencil, but I'm not sure how far I want to go. Reinforcing all the joists would be a serious undertaking of costs and time, but I can't let that dictate my decision.

 

[lexpatrie]

AWC is a lot more gunshy than, say, AISC's solutions center.

Checking an existing structure based on the code and values at the time makes more sense to me, provided.......

And incidentally, people in Florida have gotten spanked for that approach that AWC advocates, where they then find a "defect". (It's not "clean" in that disciplinary ruling, because there were a number of misrepresentations in the report as well, however. The guy got in trouble for evaluating a window based on the current wind load (and missing a duration of load), calling it defective, claiming to have witnessed three window disassemblies (actually saw one), and didn't run the calculations based on the code in force at the time of original construction.

https://fbpe.org/cardona-pe-alberto/

 

[ChorasDen]

Late to this discussion, oh well.

[Wood] is a natural and highly variable material. Reference design values are set at the 5th percentile - roughly 95% of all visually graded lumber used will be slightly to significantly stronger than we give it credit for.

This is mostly correct, we do set design values based on a 5th percentile. However, there is more to establishing design properties than a standard normal curve. If there wasn't, we would have 5% of design members below the characteristic design strength. Maybe not such a big deal for wall studs or floor joists, were we can rely on some form of basic composite action and stiffness adjustments to hopefully transfer load between members, but if we are dealing with say a cantilever deck or roof beam, something without any form of redundancy, we definitely do not want a contractor installing a beam that's in the lower 5% tail of design strength.

We take the 5% percentile number, reduce value by all adjustments in NDS (including duration of load, tests usually are performed between 1min - 5min, so reduced by Cd to represent 100% load duration). We also reduce by a mandated minimum value per the appropriate standard (ASTM for engineered wood, grading standards for visual grade), you could possibly call this reduction a 'safety factor', although I loathe that term in wood design. these factors vary based on what design value is being established, whether it's shear, tension, bending, etc, similar to how phi reduction factors vary in concrete design. The combination of these factors give us a design value that is significantly lower than the tested 5% value, because of adjustments for load duration and strength reduction factors. This nearly guarantees that any individual piece's strength exceeds the design strength published within the applicable design standard.