Strength of bolted timber joints

[Dippy]

I have a project where I need to build a simple wooden frame to support a static load of no more than about 150kg. I'm not great at woodworking so I plan to just bolt lengths together to make the structure. I am trying to avoid over-engineering it (as I tend to do) but am starting to doubt my design. I am using C16 45mm x 95mm timbers and plan to make the joints using a single M10 carriage bolt with a double-sided connector plate between the pieces. There will be 6 'legs' to the structure, so assuming the structure speads the load reasonably, the maximum force on each joint should be no more than about 30N (allowing for the weight of the structure itself)?

Surely I am worrying needlessly? The OCD part of my brain is telling me to use 2 bolts to be safe and I am trying to resist! What sort of load can such a joint really hold?

 

[jeds]

A single M10 bolt will take many times more than 150kg. 50mm square plate washers will add considerable strength to the joint. But, having said that, you should triangulate the design to prevent rotation rather than relying on joint friction.

 

[Dippy]

Thanks for the quick reply John. I didn't want to go into the detail of the structure, but yes I have arranged bracing to prevent rotation.

I'm interested in your comment about plate washers. The carriage bolt heads are mushroom with a square section below so are clearly designed to sink into the wood and couldn't be used with a washer. So I could only use plate washers on the nut side. Would this still help? I wasn't planning to tighten the nuts too much so that the bolts would not be under much stress.

 

[jeds]

That sounds ok. 150kg isn't that much. Not that I'd drop it on my toe to test it out but you know what I mean.

 

[big-all]

toothed dogs will transfer far more than 2bolts on one bolt[dog- toothed timber connectors]

 

[jeds]

Specially a couple of toothed rotweilers.

 

[IJWS15]

BTW 150kg (Mass?) = 1500 N (Force) so surely 300 (ish) newtons per leg.

Well within an M10 bolt

 

[RonnyRaygun]

BTW 150kg (Mass?) = 1500 N (Force) so surely 300 (ish) newtons per leg.

Well within an M10 bolt

Yes, although the bolt strength isn't the issue in timber. It's the shear and bearing capacity of the members which is important.

Anyway, BS5268 gives a capacity of 1.54kN (perpendicular to grain) and 2.25kN (parallel to grain) in C16 thicker than 25mm with an M10 bolt and a 38mm TPC on one side.

1.54kN is approximately equal to 150kg so way over the 25kg being supported.