Pre Tension Bolts in Wood Connection 1

[canwesteng]

I'm designing a connection to wood that will be carrying tension into a glulam member. I know if this was steel, these bolts need to be pre tensioned, but my intuition says to me that pre tensioning here might damage the wood. I've checked bearing on the back plate against the actual tension loads though and it has adequate capacity, but I'm not sure what load pre tension will impart and if it should be additive to the tension load. See sketch attached

 

[racookpe1978]

Pre-tension?

You have a load pulling to the right of your plate in your sketch, right? This force is (somehow) distributed to the right-side plate, right?

Is that force what you are calling "tension"?

Then, you must clamp (tighten) the two plate together across the wood with the 4x bolts (or 2x?) so each bolt needs to carry 1/4 of the original load/effective area of each bolt.

With wood, you MUST "clamp" the two plates together during assembly, but - over time - the wood both chrinks (as it dries) AND "compresses" away from the loaded area so this original clamping force (which is added to the original load) will trend towards zero. Maintenance is required to periodically re-tighten the clamping bolts.

 

[canwesteng]

The load pulls on the plate to the right, this load is split between the bolts which transfer the load to the plate on the left, which bears on the glulam beam. Because these bolts are in tension, they should be pre tensioned as per AISC/CISC. In the relevant steel design specifications, it says to neglect the pre tension force in designing the bolts, but I'm not sure how I feel about applying this to the wood. As per S16 table 7, required pre tension applies many times the design load in clamping force to the member.

Bolt pre tension refers to pre loading of the bolts by tightening them to a point where they yield or a certain tension is achieved.

 

[Tmoose]

This starts to talk about bolting glulam beams in Section 5, and provides references to timber construction manuals.

http://www.aitc-glulam.org/shopcart/Pdf/aitc_119-96.pdf

"National Design Specification® for Wood Construction"

"AITC Timber Construction Manual to determine the capacity of shear plates and split rings"

"See AITC 104-84, Typical Construction Details, (Ref. 4), for additional information on connections."

 

[dhengr]

Canwesteng:
Pre tensioning bolts in a steel to steel connection (AISC, etc.) and tightening bolts in wood/timber framing are two completely different animals. And, gluelams which are typically made from well/properly dried lumber don’t shrink cross grain nearly as much as some almost green dimensional framing lumber does. The upshot is that you tighten your connection to draw everything tight and that’s about it. You do not particularly want a washer to dig-in (crush) much lumber in the tightening process or in the load application, and you might use a larger/thicker washer or pl. to prevent this. You might also check the connection six months or a year after it is under cover if it is important that it stay tight. The tension is taken by the right pl. in bending to the bolts. The bolts need sufficient diameter/area for the tension load (AISC rules here, or your ON/CA steel codes) and this load is taken to the gluelam bm. in bearing (crushing perpendicular to the grain) by the left pl. In shear the bolt acts as a cantilever bm. loaded by the right pl. and in bearing in the gluelam, and again, pretensioning is not that critical.

 

[canwesteng]

Thanks dhengr that's what my intuition was telling me just wanted to make sure I was on the right track.