Using a Green Timber as a Girder
Issues to be aware of when using green or partially-green softwood timbers as structural beams. November 28, 2006
Question
We want to use a timber about 10"X16" and 22 ft long, which we will saw from a Douglas fir log (minimum diameter about 20 inches) using a chainsaw mill. The log is green and we don't have time to dry more than two months with bark on. We would like to saw the timber and put it up green. It will be anchored at either end by "hangers" fixed to a concrete wall and at two points down its length where it sits on posts and on top by log joists and with subfloor (1-1/8" T/G plywood). Should we be concerned that as the timber dries, it could twist and, at the ends, pull on the hangers enough to damage our concrete wall (8" thick wall)? We are not concerned about aesthetics of the timber and some twisting and checking is not a problem.
Forum Resposnes
(Sawing and Drying Forum)
From contributor D:
The wood won't shrink much lengthwise as it dries, so I'd assume that the concrete moorings at either end will be safe. I'd go ahead and use it.
From contributor P:
I'd think more about the height shrinkage of the timber with the joist on top.
From contributor B:
Yes, the main shrinkage issue will be away from the second floor. Many builders build in a screw jack (hidden behind removable trim boards) to adjust the top of the timber back to the original level. Sounds like you would need to add a post against the wall to do this. Consult the shrinkage calculator on this site to determine the amount of shrinkage so you can decide if you are comfortable with what will happen. As far as twist goes, you should be fine unless you have a timber with the grain running to a left-handed twist. To figure that out, stand at the base flare end of the log with hands out in front of you and your thumbs touching each other, fingers pointing away from each other. If the grain runs from thumb to fingers on the left hand, then it is left handed twist and should be avoided.
From contributor W:
Yes, you should be concerned with twisting! In the timber frame business, it is common that timber frames are built with green timbers and they then will be at risk of twisting if not joined properly. As a result, you need to have the ends of the timber secured to the concrete with a hanger that not only supports the beam on the bottom but wraps up the sides so you can thru-bolt the beam to the hangers. I would have assumed that your architect or engineer would have designed the hanger for you. If not, I would not move ahead with installing it without getting an engineer's approval of what you intend to use. When you are working with a beam as big as this, the stress that beam can exert is enormous! Without the proper joint design, you are looking for trouble! Checking is expected, as with all timber frames, and has no effect on its structural integrity, but twisting must be prevented, as it can throw off balance the strength of the beam if it twists too much, not to mention change its physical relationship to the joists.
From contributor T:
I have sawn a lot of Doug fir and used the timbers in buildings. The wood tends to dry well and quickly and I like to get big beams in place as soon as possible and anchor the ends to try and prevent twisting. Lock down the ends as tight as you can. Next time, you can actually put knockouts in the concrete wall that will leave a slot after the pour and really lock it in.
