span table for old ACTUAL dimension floor joists?

[Arsenalfan]

Has anyone ever seen a span table for old ACTUAL dimension floor joists? Ie for 2X4 which are actually 2x4, not nominal 1.5 x 3.5. I realize that without a structural stamp, its hard to tell the exact attributes of such wood. Still, an actual dimension lumber span table would be a useful starting place.

 

[BadgerPE]

I haven't seen span table for nominal sizes before, but I would think it would be quicker and easier to jut write a quick spreadsheet on your own. Then you can modify for material properties, sizes, etc. and see how it affects your load capacity. Just my $0.02.

 

[msquared48]

I agree. Even my old Douglas-Fir Use Book from 1958 has Nominal sizes for its tables.

Mike McCann
MMC Engineering

 

[dhengr]

Arsenalfan:
Crackerjack pretty well sums it up for me. Since you can’t buy it, without getting a small mill to specially cut and dry it for you; and since you then have to have it graded, if you are going to sue it for anything serious; I think a table for a bunch of different grades and sizes would not get much use. On the rare occasion that you do run into this situation, you calc. the section props. based on what you’ve measured; then you take an educated and experienced guess at the species and the grade and go from there. You can have a good wood lab determine the species, and a grader come in and make some grading judgements on site, if needs be.

 

[Arsenalfan]

Thanks to all for input. I created the attached table showing factor to apply to span table max span values to determine max span for true size lumber. Factors are calculated based on the fact that bending stiffness, dictated by moment if inertia, depends on cube of rafter/joist height. Thus, for a true 8” rafter/joist, max allowable span is 1.34 times that for 2x8 dimensional lumber with an actual depth of 7.25 in. This assumes that the wood material characteristics (modulus etc) are consistent. From what I understand, older lumber is of better quality, tighter rings etc and hardened, so assuming strength and bending characteristics for SPF No.1/2 would be conservative.

 

[BAretired]

I may not be entirely clear on how you intend to use your table, but it addresses only deflection, not bending or shear strength. So if a member is governed by shear or bending, the table is not valid.

BA

 

[Arsenalfan]

Noted. because span tables are for service level deflections (L/180 to L/360), I figured the tables indicate spans than are well clear of max bending stress or shear. assumption is that deflection for service governs. Make sense?

I'd like to learn how to identify species and grade, in lieu of grade stamp, that is. Any pointers/references on this would be welcomed.

thanks in advance

 

[msquared48]

Not for high loads and shorter spans...

Mike McCann
MMC Engineering

 

[hokie66]

You have to check the stresses as well as the deflections.

 

[woodman88]

I have always used the nominal sizes span charts to size rough lumber. Due to the lack (or at least limited) of testing and research on rough size lumber grades/values, I prefer to have this added safety factor.
As for "...how to identify species and grade, in lieu of grade stamp..." a grading book from WWPA or other grading association will have information on what is required for each grade of lumber. The species?
But I do think you can get graded rough sawn lumber or are you looking at reusing lumber?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Arsenalfan]

Thanks for the replies, this is an incredibly useful forum. My questions relate to assessment of existing roof framing (for solar panel install). I live in an area with many century-old homes that have rough sawn lumber roof framing. Analyzing using dimensional lumber could be 50% over-conservative and could make/break the feasibility of adding solar panels. I think I've got enough to go on now. thx!

 

[dhengr]

Arsenalfan:
Write your own spread sheet to do the required calcs. as a function of field measure sizes and spans and assumed species and grade. As suggested you can study the grading rules over the years, there are classes for becoming a lumber grader, which you could take. Hundred year old houses might have been made of any good local wood, ask some old contractors, restoration people, old lumber guys, not the kid at the bog-box. To do what you are talking about the right way, you have to do an inspection of the/each roof system and measure sizes and spans. Probably, more importantly, look at the entire roof and all the important details for condition or defects and deficiencies. Is the roof sway-backed from years of permanent set and deflection, indicating too flexible a roof for the existing loads? Are the joints intact, not pulling apart, crushing or splitting, enough bearing area, properly cut and fitted, no rafter thrust problems at their bearings, etc. Now, you can start to make a current condition judgement, and start thinking about any excess rafter cap’y., or how to add some reinforcements, or how to distribute your new point loads to several rafters to meet your needs. With time you may gain some experience and efficiencies at doing the analysis and making the judgements about possible options, but I think you pretty much have to do the inspection, etc. to arrive at an educated and engineered sale. One size does not fit all, and one simple table will not do it for you either.

 

[msquared48]

Thirty years ago, I wrote my own program in basic for joists and wall studs, and still use it today.

Mike McCann
MMC Engineering