Mixed fasteners in wood

[kareng]

Hi all -

C11.1.4 of the NDS states "Allowable loads for connections employing one type or size of fastener shall be based on analyses that account for different connection stiffnesses, on test results, or on field experience."

So if you have a wood joint with multiple types of fasteners that you need to analyze, is there a procedure anyone knows of?

Thanks in advance,
Karen

 

[molibden]

Eurocode does not allow this. You could divide forces based on stiffnesses but it is only a guess how will really behave.

 

[dhengr]

Kareng:
This mixing of fasteners in wood connections typically does not work very well, because the various different fasteners react and perform so radically differently during the loading and failure of the joint. One type of fastener might be on the verge of failing before there is enough movement, in the joint, for another type of fastener to really start to come into play. One type of fastener may be so strong and stiff that it really takes all of the load until it is severely overloaded, at which point the other type of fastener comes into play, but at a small percentage of the total demand of the joint. Alternatively, the weaker, tighter fitting fastener (nails) may prevent the slippage needed to bring bolts/dowels (the stiffer/stronger fastener) into play until the nails are on the verge of failure. Thus, summing for a total cap’y. is suspect.

While the cap’ys. of several different fasteners probably (most likely) can/do combine/sum-up for the total strength of a connection, actually analyzing this, and making some reasonable calcs. and engineering judgements to put a total value on this would be a very dicey proposition. The cap’y. values that we use for various fasteners are based on testing, long history of usage and performance, etc. of each type of fastener, and groups thereof, in various joint conditions. The bolt guys don’t much care about how nails work, their strength values, etc., so it is unlikely that you’ll find much mixed fastener testing, except maybe for some proprietary condition. Take a look (and actually study) the connection and fastener sections of the NDS and some good Wood Design textbooks to start to understand how various fasteners function in use, and what issues are considered in a total connection design. Finally, because of the kinda random nature of wood, mechanical properties, non-homogeneity, non-isotropic, etc. etc., grading rules are somewhat subjective, and factors of safety are high to account for these. And, a number of these have experience/history and engineering judgement adjustment factors thrown in for good measure, so a good clean analysis is unlikely. I think you would have to test a particular detail, many times, to start to put some numbers on what’s actually going on.

 

[kareng]

Thanks dhengr and molibden. I kind of figured as much. I'm pretty familiar with the fastener section of the NDS, but was hoping for a packaged analysis method. Not sure if it makes sense to just take the lowest fastener capacity x the number of fasteners or not. But sounds as if we're better off starting a new connection and ignoring or removing the old. Appreciate the input!