On Tuesday, 3 September 2013 14:51:43 UTC+1, Rick Hughes wrote:
On 03/09/2013 12:02, wrote:



All traditional woodworking joints significantly weaken the wood due to the

amount of timber that has to be cut away.

Mortice/bridle/halving joints are the very worst for this. Dovetails are

only slightly better.



Bollox again. If the joints are glued as the OP stated, the joints will

be stronger than the timber.



This is becoming rather contentious, but it's not true to say a glued joint will be stronger.



A glued joint *can be* stronger, if the joint is well designed, has very close tolerances, the glue is used exactly as the manufacturer recommends, and a high clamping force is used. That's pretty much what engineered timber does, but requires carefully controlled conditions to achieve it.



Any significant notch in machine strength graded timber has to be regarded as considerably reducing its strength. The effects of a deep notch aren't very predictable.







AS my description may have caused this 'debate' .... here is a sketch of

the joint I will use.



http://tinyurl.com/odmhukq



This is a Pergola project ... a number of 100x100 posts fitted to ground

brackets, running along top edge of posts on each longitudinal side is a

200x47 beam.



At 90 deg to this beam are the 100x47 'joists' which I intend notching

out to the 30mm shown.

These will be glue fitted ... probably use Polyurethane, as it's foaming

will also seal any gaps form water ingress.

What span on the 100*47 timbers?

Notching them to a third of their depth will considerably weaken them.