Carried And Through: ……Bolts Out Of The Blue
Jay Markanich’s recent blog raised a debate about the use of carriage bolts in outdoor wood structures. Some appear to claim that they are an inferior fastening technology. I say consider the source.
Some facts: Carriage bolts were designed to be drawn into the wood to lock the head in place and allow tightening of the nut. That system works fine as long as the bolt holes were not too big in the first place and as long as the heads don’t slip when torque is applied.
This is ‘the carriage bolt school technology‘ and it is quite sufficient for the weight and stresses that these structures are subjected too. A little maintenance is required over time to retighten the bolts and keep water out of the head area. Water entry at the head area can degrade the wood there, but it would have to be extreme for the connection to fail.
Simpson Strong-Tie people are selling a product. From a strictly engineering standpoint more torque can be applied to a properly washered through bolt. In theory the joint can be clamped tighter, but both bolts have the same load carrying strength for their size.
So through bolts can be made tighter and as their installation does not involve crushing wood fibres they have lesser incidence of wood deterioration risk around the heads (Note that they do have some, just not as much risk).
Technically the Strong-Tie sales peoples claim that their opinion about through bolts being better is supportable. But stating that carriage bolts are categorically inferior is pushing it. Remember they are selling a product they believe in so a little bit of overstatement is forgivable.
Carriage bolts properly installed and maintained are fine. Suitably large bore washers can be installed under the heads if the wood species is problematically soft or constantly wet.
Carriage bolts have the advantage in that the heads are smooth low profile forms that do not protrude to skin shins or knuckles and catch on ropes.
Carriage Bolts….carry on!
This article was written by Robert Butler and was inspired by the original blog article mentioned above by Jay Markanich and written in part as a comment on this follow-up blog. Link through to these blogs for other opinions and more discussion on the subject.
N.B. This might seem as a serious debate and it is to a point. Both products and systems are structurally functioning methods. The debate is really about degree and longevity.
Contrast that with the frequently discovery of structures built with out benefit of either fastening system. This is where Jay’s original blog and indeed most blogs on the subject stem from.
Original blog post on ActiveRain: Link to Blog Post