Change in timber bolt clamping force over time? 1

[bwehmeier1]

I am trying to find any information on whether the clamping force (actual tension in the bolt) will change over time in a timber connection and if so by how much. I suspect that factors such as moisture gain or loss, addition and removal of load, and temperature change may cause the clamping force to vary over the life of a connection. My internet search thus far has turned up nothing in regards to this subject. Does anyone have any information on this topic or ideas of where to direct my search?

Thanks in advance.

 

[hokie66]

The primary reason for loss of tension in bolted connections is cross grain shrinkage of the timber. The amount of the shrinkage varies with the species and the moisture content when erected. The Timber Construction Manual of the American Institute of Timber Construction gives guidance on the dimensional stability of a number of species, and similar resources should exist in other countries.

 

[msquared48]

And that's the reason why some holddown systems have automatic tensioning devices worked into their system - just because of the shrinkage issue with time.

Mike McCann
McCann Engineering

 

[RARWOOD]

What is your appliction? I saw one example of an attachment of a ledger board to a beam where the engineer added friction to the bolt shear. As indicated above, as the wood dries it would shrink releliving the friction that the designer was using to justify their design of the ledger attachment.

I'm curious about your application because I don't know how much tension you could develop in a bolt in a typical wood connection. For example if you bolt two glulam beams together, at some point you will start crushing the wood, which I think would limit the amount of tension you could develop in the bolt.

 

[hokie66]

RARSWC is correct. I would never count on friction in a bolted timber connection, as the clamping force is unreliable and can go to zero.

 

[graybeach]

bwehmeier1,
Stress laminated bridge decks rely on transverse tension rods at regular spacing to keep all the members acting as a unit. Periodic retensioning is required.

On another note, would spike grids help increase the shear capacity in your application?