huangyhg

timber post moment connection
i am designing a section of a house that has a custom truss roof system supported on heavy timber posts that were gravity designed by a timber post and beam company. the owner wants to keep the aesthetics and the design. i ran a quick lateral design only to find it didnt work. i added a two struts and was able to make it work as long as i can achieve some kind of fixity in the bottom of the post to the foundation wall. i realize that the post may have as much as 16% water content and could shrink 4% or more in dimension. what type of connection would be the best solution? i thought of designing a moment bootmade out of steel plated welded to a baseplated designed for moment but realized the shrinkage in the wood will change it's fixity. the post is 7 1/2 by 7 1/2 inches in dimension and 12 feet high.

for light moments, i have ran plates up each side and designed bolts through the assembly to act as a couple. the wood will not shrink much in the longitudinal direction so the bolts can be spaced far enough to get a good couple if the owner is okay with the appearance. if you try to use 2 bolts top and bottom, keep them as tight as possible to avoid splitting due to the transverse shrinkage. there is a provision in nds that governs this.
watch out for fastener slip. if your anchorage needs to give 1/8" before the fasteners take the load you might run into some pretty high deflections due to joint rotation.
if its a pole barn its probably just fine, but if its a nice big window wall you might crack the glass in a high wind event.
you could consider using steel, concrete, masonry, or have the architect add some wall length somewhere for you.
see the teco catalog & design manual for manufactured products that may work for you application. here is a link
engineer817...if you use a boot connection, the ultimate fixity can be achieved assuming you can accept the lateral drift due to shrinkage. assuming a 5% shrinkage and an 8" timber, your drift would be 0.2" either side under full load.
the strap approach at noted by mwpc is more adjustable and can be adjusted after the initial shrinkage takes place (which will be most of the shrinkage that occurs)
thanks for the info. im going to look into the design of the couple formed by the bolts.
i don't exactly follow what you are designing, you are trying to provide a moment resisting connection at the base of a wood column?
our place is using a booted type connection but i'm not planning on any moment resistance, only lateral restraint.
a couple from the bolts may work but wouldn't be my choice as when the timber dries the end checks might line up with the bolt closest to the bottom and cause a split the first time a moment is applied.
as for better ideas? don't know if its better but a steel strap up each face could move the concentrated loads away from the end. it could be lagged to reduce the need to retighten the frame as it dries.
there are also connectors around that you cut a slot in the column and insert the "blade" of the connector into it and bolt through it. these can be designed to carry some moment.
i would really prefer to stay away from trying to resist a moment at the end though.
we have convinced the owner to go with a steel moment frame to take the lateral in that part of the building.
thanks for your replies!

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