Old Architectural Design Standards tables 2

[Iasonasx]

I got a hold of an old edition of the AGS and in there I found tables that were evidently used to specify joist spans and distances O.C. I looked at the case of 2x12 Southern Pine or Douglas Fir spanning more than 20 feet. How is that possible? Was wood so much better back then?

 

[phamENG]

That's not all that far. Current code (post 2013 downgrade of southern pine) puts No.1 Southern Pine 2x12 at 16" o/c at 19'1" for max span. Consider that the lumber above was actually larger (1/8" wider and 1/4" deeper), and you're really not that far off. Going back to pre-2013 span tables out of the 2012 IRC, and SP No.1 at 16" o/c is spanning 20'4" - only 7" less than the table above. Again, the lumber in the 2012 IRC (1.5"x11.25") is slightly shallower, so the allowable stress is probably about the same.

Now, there's essentially no reason to use 2x12's spanning that far. To get a piece of lumber that large in cross section and that long in length without some significant defect makes it very expensive - even before lumber prices in general shot up. An I-Joist can give you comparable or better performance at a comparable or lower cost, and it comes off the truck straight and true.

 

[dik]

Yes... and the wood was a lot better... closer grained and less knots... We built my dad's house out of salvaged 2x12s that were 100 years old (my dad, my brother and myself took down an old lumberyard warehouse that was given to us to remove- the developer want to use the property for a large precast apartment bldg). You couldn't drive a nail into the material or pull one out. We had to use air nailers.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[skeletron]

I don't know if vibration & deflection were accurately captured in the dinosaur days.

 

[phamENG]

skeletron - Deflection is captured. See the note at the very bottom. Vibration, not so much - but then it isn't captured today, either. At least not for solid sawn lumber. I found a Master's thesis from 2019 that addressed it, but nothing even close to the scale of Dr. Murray's work out of Virginia Tech for steel.

 

[phamENG]

dik - I won't argue with you about the decline in wood quality. That's why the allowable stresses for wood members were downgraded a few years ago for southern pine - the material showing up at the mills had gone down enough to cause a shift in the statistical model large enough to justify a downgrade. But don't use the fact that you couldn't hand drive a nail as justification. Wood - particularly pines and spruce - tend to get harder with age. That hardness goes up without any real gain in strength - and some loss as the fracture toughness drops and the tensile capacity of the wood also degrades over time. After all, somebody had to drive a nail in without an air nailer 100 years ago. So it was a lot softer when it was first put up.

 

[skeletron]

@phamENG: Part 9 span tables (NBC) have added consideration for deflectionvibration based on a 2000 paper on the serviceability of light-frame wood structures and experience to that point. If you go into Forte/WoodWorks, the option for floor/ceiling attachments will affect this calculation along with the span. Before that point, I don't think it was a quantified element of the calculation other than "that worked good before".

 

[dik]

It wasn't a justification... it was a simple fact about the effect of ageing. The old growth with fewer knots and close grain is what provided the strength. Many of these 20' Doug Fir 2x12 had no knots in their length... just the higher quality of lumber, and seasoning. I had a problem at the Gimli Hospital several decades back... the 2x6 stud walls had shrunk and the silicone caulking to the wall at one set of counters had separated at the wall and had torn the wallboard... I mentioned to the contractor that the problem had been caused by shrinkage... and he replied that it couldn't be... the lumber was so green it was still growing.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[STrctPono]

dik,

Agreed. I think we would all love it if old growth trees were abundantly available, but for many reasons (some good) that just isn't the case, anymore. Even though the newer growth trees are not as dimensionally stable and are more prone to swelling/shrinkage. Lumber should still be kiln dried which should still help a lot of those problems, but many times when I go to the hardware store, their lumber is improperly cared for and stored. It's saturated with water and is noticeably heavier.

 

[phamENG]

skeletron - do you mean deflection, or vibration? I don't think I've ever seen a span table that did not have deflection considered - usually L/360 for live load - even going back several decades. Vibration on the other hand - never seen it for solid sawn wood. The forte software settings only work for Weyerhauser TJIs - not for solid sawn 2x lumber. If the NBC has considerations for vibration in its span tables, I'd love to see them. Do they account for flexibility in supports, too?

 

[skeletron]

@phamENG: Typo fixed. Vibration (and deflection) is captured in the span tables. Woodworks has the option for solid sawn lumber. That follows the NBC criteria that is based on floor construction (how thick the sheathing is? how is the floor system attached?). I think it's more a weight-based function vs. flexibility based on end conditions which is appropriate if your joist is supported on a wall.

 

[phamENG]

Good to know. Thanks. I'll have to see if I can dig up a copy of the Canadian code and see what the background is. With more and more renovations of older houses with walls removed, vibration issues seem to be more prevalent now. It's a consideration I've been wanting to incorporate but haven't had a lot of luck with reliable references and/or actionable results.

 

[dik]

Every jurisdiction needs to adopt a code portion similar to the Ontario Building Code Part 11 for renovations. It sort of codes 'grandfathering' and does an excellent job.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[phamENG]

dik - is that the one that looks at the past performance of the building and the reliability implications for the allowable stresses? I think KootK brought something like that up in a previous discussion. It sounds like a fantastic idea to me.

 

[Enable]

Never thought I'd see the day where the OBC was referenced as the ideal...what kind of codes are you folks working with that our crummy, inconsistent, erroneously padded excuse for a code is the preferred template

BTW phamENG our code is free and online: Ontario Building Code

 

[dik]

It codifies renovation work on renovations and old buildings really well, and takes a lot of the discretion away from building officials. It's a great part of the OBC and should be present in all codes.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik