A question about vigas

[Augustina]

Vigas are traditionally used in support of flat roof construction in the southwest. The NM residential building code was recently revised to include a span chart specifying the number and span of vigas based on the nominal diameter mid-span with a minimum of 5 in diameter. The code refers to the Section R802.1.3 Structural log members, which states that the log members must comply with the provisions of ICC 400, and adds the span chart.
It seems that complying with the ICC 400 would require the viga to be graded especially as
the span of vigas serve as the main structural member that support the roof and transfer its weight to the exterior wall.
Table R802.1.3.1. The New Mexico Viga Span Chart includes, “Vigas shall have the bark removed to visually inspect for straightness, bending, splitting, loose knots or rot which would reduce strength.” However, it does not say who is responsible for inspecting for these defects or to what degree the presence of any of these defects would deem the log (viga) unsuitable.
I am being told that vigas do not have to be graded as long as they are installed according to the span chat.
I am not an expert, my roll is in communicating this span chart in a newsletter and other communications. I am uncomfortable reporting that vigas do not have to be graded but I do not have the expertise to argue the point. I have attached the revised code, if anyone is interested in offering an opinion or perspective. Thanks!

 

[Yankee Chronicler]

Well, that's what happens when politicians meddle in codes. Not only does this new provision not require grading, it requires "inspection" for "straightness, bending, splitting, loose knots or rot which would reduce strength" -- without establishing any criteria by which to assess how straight is straight, how bent is bent, how split is split, how loose is loose, or how much rot is allowed before it reduces strength. Reduces strength from what? What's the baseline?

It also doesn't say by whom this inspection shall be performed.

Wearing my architect hat for the moment, as a designer I wouldn't touch this with a ten-foot ... viga. I would want a structural engineer to sign off on the intengrity of any vigas, and I very much doubt that any responsible structural engineer would undertake such a determination in the absence of any established parameters.

 

[Yikes]

Good question! The code talks about inspecting for items that would reduce strength, but it does not say how to grade, inspect, or even describe what to do once you have concluded that it might reduce strength (e.g. does not tell you to reject use of the timber as a viga). Very subjective!

Table R802.1.3.1. The New Mexico Viga Span Chart includes, “Vigas shall have the bark removed to visually inspect for straightness, bending, splitting, loose knots or rot which would reduce strength.” However, it does not say who is responsible for inspecting for these defects or to what degree the presence of any of these defects would deem the log (viga) unsuitable.

It appears the building code rules were passed at the 5/17/23 meeting of the CID, and the minutes approved at the subsequent 7/14/23. Their online portal does not have the agenda packet related to that meeting.

They have another meeting coming up on 9/18. My suggestion is that you email the contact listed on this notice:
https://www.rld.nm.gov/wp-content/uploads/2024/08/CIC-9-18-2024-Mtg-Notice.pdf
Either ask Eliza to have the CDI post an interpretation on their website, or ask her to have your email read aloud in the public comment section of the meeting requesting the posting of an interpretation.

 

[Yikes]

Wearing my architect hat for the moment, as a designer I wouldn't touch this with a ten-foot ... viga. I would want a structural engineer to sign off on the integrity of any vigas, and I very much doubt that any responsible structural engineer would undertake such a determination in the absence of any established parameters.

Many years ago the state of CA once had a historic native American lodge house reconstructed within a state park. The native construction technology consisted of binding the roof logs together via wet leather straps that tightened up as they dried, cinching everything together. As you might suspect, they had real trouble finding an engineer who would stamp it. They finally got a historic preservation engineer, a one-man office, and the guy was near retirement. He believed that the leather straps had previously worked for hundreds of years, so he put his stamp on it. They did some field testing/loading during construction just to be sure, and set up a post-construction regular inspection regimen.
No problems for decades since.

 

[jar546]

Well, that's what happens when politicians meddle in codes

They only meddle in the codes when they are lobbied by an influential group.

 

[Augustina]

Well, that's what happens when politicians meddle in codes. Not only does this new provision not require grading, it requires "inspection" for "straightness, bending, splitting, loose knots or rot which would reduce strength" -- without establishing any criteria by which to assess how straight is straight, how bent is bent, how split is split, how loose is loose, or how much rot is allowed before it reduces strength. Reduces strength from what? What's the baseline?

It also doesn't say by whom this inspection shall be performed.

Wearing my architect hat for the moment, as a designer I wouldn't touch this with a ten-foot ... viga. I would want a structural engineer to sign off on the intengrity of any vigas, and I very much doubt that any responsible structural engineer would undertake such a determination in the absence of any established parameters.

Thank you so much for taking the time to consider my question and provide such a thorough, expert opinion!

 

[Augustina]

Good question! The code talks about inspecting for items that would reduce strength, but it does not say how to grade, inspect, or even describe what to do once you have concluded that it might reduce strength (e.g. does not tell you to reject use of the timber as a viga). Very subjective!


It appears the building code rules were passed at the 5/17/23 meeting of the CID, and the minutes approved at the subsequent 7/14/23. Their online portal does not have the agenda packet related to that meeting.

They have another meeting coming up on 9/18. My suggestion is that you email the contact listed on this notice:
https://www.rld.nm.gov/wp-content/uploads/2024/08/CIC-9-18-2024-Mtg-Notice.pdf
Either ask Eliza to have the CDI post an interpretation on their website, or ask her to have your email read aloud in the public comment section of the meeting requesting the posting of an interpretation.

Excellent advice, thank you so much!