green vs. kiln dried on strength?????

bikedude73 · September 25, 2011, 04:05:00 PM

I am working on a floor joist project how much does kiln drying change the strength of poplar ?

bikedude73 · September 25, 2011, 04:32:47 PM

Working on a project replacing some floor joists 2" x 10" x 13' ideas on strength difference green vs. kiln dried?? Thanks for any input........

beenthere · September 25, 2011, 04:47:53 PM

Asking about yellow poplar?

Strength increases as the wood dries.
So not sure if you need the green strength or want to calculate the strength when the wood dries out (as it eventually will).

Are you concerned that the yellow poplar doesn't have enough strength for the floor joist?

ladylake · September 25, 2011, 05:36:41 PM

No need to kiln dry but lumber needs to be dry for full strength. Steve

bikedude73 · September 25, 2011, 05:38:37 PM

I was asking because we have cut joists and 4 of five have broken as we tried to put them in. We bought kiln dried pine and have had no problem......

SwampDonkey · September 25, 2011, 06:06:10 PM

Even if it is kilned, the wood will go to equilibrium within the climate it's used.

sigidi · September 25, 2011, 06:29:13 PM

Down here, as mentioned, timber just needs to be seasoned, how that happens - kiln dry or air dry doesn't matter. Having said that, I'd say you are just looking to know the strength difference between green timber and 'dry' timber.

Down here the timber increases one strength group - so a green F17 stick of hardwood goes up to a F22 when seasoned. Not sure if you guys use the same kind of grading system as us though.

beenthere · September 25, 2011, 09:49:03 PM

I see you started two threads with this question.
Some more explanation of how/why they broke when installing would help.

A floor joist usually is there to hold up the flooring above.

What caused the extra weight to break the floor joists? i.e. how was it loaded?

Jeff · September 25, 2011, 10:22:49 PM

These two topics have been merged.

Ianab · September 25, 2011, 10:25:24 PM

A board like that should not break, (green or dry) unless it's way overstressed, like parking a car on it? Or it has some sort of defect like large knots or a pre-existing crack etc? What sort of stress where they under as you tried to put them into place?

The commercial pine would would have been graded for structural strength, either visually or by machine, and any rejects sold for "non-structural" purposes.

I can't see green or dry making THAT big a difference in strength. Green boards straight off the mill don't just fall apart unless there is some sort of defect that comes from the log structure itself, then they can indeed snap in 1/2 when you pick them up. They would still be almost as weak when dry, so would be rejected during grading, or the sawyer would spot the problem in the log, and saw a different product or sawing pattern.

Ian

Tree Feller · September 26, 2011, 12:11:41 PM

Lumber strength is tripled by going from green to oven dry state. That's one of the reasons for drying lumber. While construction lumber is generally kiln dried to only 15% - 20% MC, the strength increase from a green state is substantial. The EMC for your area is in the 12% MC range so some additional drying will still occur after installation.

To get an idea of green vs dry strength, place a weight (like a stapler) on a sheet of copy paper held by the ends. Now, wet the sheet of paper and try to hold the stapler with it. Get the idea?

bikedude73 · September 26, 2011, 03:03:15 PM

Tree Feller thanks so much it does make a lot of sense when put into those terms........ Thanks for all the help from FF

dovetails · September 26, 2011, 03:49:56 PM

Quote from: bikedude73 on September 25, 2011, 05:38:37 PM
I was asking because we have cut joists and 4 of five have broken as we tried to put them in. We bought kiln dried pine and have had no problem......

I just want to know HOW you broke them??? A joist is installed on edge,
and you said they were 2x10x13 foot. I use 2 - 2x8x6's to load cars on my trailer
and have not broken one yet,and that is being used as ramps,laying flat.

sigidi · September 27, 2011, 06:35:17 AM

If the boards where cut from the heart of the log, I can believe they snapped.

I'm not quite sure a green stick versus a dried stick triples in strength though?!?!

bandmiller2 · September 27, 2011, 07:21:07 AM

Usally green will tend to deform under load think wet and supple wile dry is more brittle/hard and resists deformation.Probibly you twisted the ones that broke trying to install them as it can be real tough replacing floor joists.I like to reduce their depth so they can be installed easier then shim the ends up, and make them thicker for stregnth or double up. Frank C.

beenthere · September 27, 2011, 10:45:40 AM

Quote from: sigidi on September 27, 2011, 06:35:17 AM
If the boards where cut from the heart of the log, I can believe they snapped.

I'm not quite sure a green stick versus a dried stick triples in strength though?!?!

Doesn't sound right, but he did say triple when going to oven dry.
Can roughly figure about (almost) double the modulus of rupture for green to 12% moisture content.

The OP isn't telling us anything about the conditions that caused the 5 out of 6 "poplar" joists breaking, but doubt it had anything to do with them being green. The mystery continues.