Green or Kiln Dried Heavy Timbers?

[marinaman]

I have a project this is a heavy timber framed picnic shelter. It will be comprised of heavy timber trusses supported by heavy timber columns. The lateral stability is provided by diagonal bracing.

I am trying to decide if I need to call for these timbers to be kiln dried or green. The column members are fairly large (8x8) and some of the truss members are also fairly large (8x8's or 10x6's) to accommodate the architect's desired appearance.

As I understand it, in air drying pieces of this size, it takes about a year to bring 1" of the material down to 19% or so moisture content. As I understand it, kiln drying does it faster, but the inside may be still wet while the outside is relatively dry.

I know that historically, these structures are built with green materials....not kiln dried and not air dried. Cut, brought to the site, and then erected.

Has anyone on this board worked with these materials and has a preference one way or the other?

By the way, I am requiring the columns and diagonal brace materials be pressure treated, as they will likely be exposed to rain. Thank you for your consideration.

 

[BUGGAR]

I remember a ceremony we used to have when we completed a timber structure. After the Contractor made his mandatory final tightening of all the bolts once again at the completion of construction, we gave the owner a gold plated (not real gold) wrench that would fit all the bolts and told him he had to use it at least once a year or the gods of timber structures would smite his project.

 

[Canuck67]

The heavy timber framers we work with exclusively use green material and most often white pine. It is easier for them to work with when green and cheaper to purchase green vs. kiln dried.

Any suggestion that they use kiln dried material to fabricate their heavy timber framing was met with flat rejection. We have not detected any significant problems with the process of using green timbers.

 

[Tmoose]

We built our garage from rough cut 2X lumber from a local saw mill. The lightly loaded floor joists have developed a noticeable bit of midspan sag, and the the joist tops are lower than when first installed (they are in joist hangers nailed to a old, dry, reclaimed beam), so the floor has a high spot over the central beam. When the garage was first finished the floor was nice and flat.

Literature suggests shrinkage when drying is greatest in the crosswise direction, which I guess explains why the joist are less tall now.

http://www.awc.org/helpoutreach/faq/faqFiles/Shrinkage.php

http://www.motherearthnews.com/green-homes/green-wood-zmaz76mjztak.aspx?PageId=3

Also that initial deflection and "creep" of wet/green lumber is much greater than for seasoned lumber.
file:///C:/Users/Robyn%20Timberlake/Downloads/TB-101.pdf

 

[Triangled]

If you're sizing for architectural purposes such that your members are significantly understressed, that's one thing.....

I like designing heavy trusses with glulam. Architect had some on a church sand-blasted and it gave a great look. The glulam of course can be rendered dry relatively easily. The glulam can even be made out of decay resistant lumber if that interests you.

 

[Sawbux]

Timbers are typically not available kiln dried. That is limited to 2x up to 4x materials. 6x are too thick to get dried in a timely manner and demand is lower, so economically not worth it to the producers. If you want really dry timbers, use glulam or paralam or other engineered wood.
If you are using pressure treated, they will be wetter than original lumber unless air dried for quite awhile.