How many acrows do i need?

[joe-90]

In the OP's case, there is no height above the opening for the magic triangle to form - it's truncated so there is no top point to equalise the forces ... so its effectively a flat arch ... or just a course of unsupported bricks relying on friction

If the joints are soft, then they crumble.

And whatever can drop vertically will tend to do just that

Yeah. See what you mean now.

 

[jabuzzard]

When it comes to openings, if you draw an equilateral triangle above the opening with a bit of chalk - only the complete bricks within that triangle are able to fall. All the others are locked in and cannot fall even if you wanted them to.

An equilateral triangle has three internal angles of 60 degrees. I would expect it to be self supporting at 45 degrees. The state of maths education these days

 

[joe-90]

Yeah that's true. But you know what I mean.

 

[geraint]

By chopping in a strongboy over a doorway,,, you will be disturbing whats above...

...yes, if the o.p. goes at it with a sledgehammer.

Alternatively, you can stitch drill a slit with a 10mm masonry bit and only damage the bed joint....

Why the negativity Grrrrr?

So the vibration in stitch drilling it is not disturbing the bricks above....

 

[geraint]

When it comes to openings, if you draw an equilateral triangle above the opening with a bit of chalk - only the complete bricks within that triangle are able to fall. All the others are locked in and cannot fall even if you wanted them to.

An equilateral triangle has three internal angles of 60 degrees. I would expect it to be self supporting at 45 degrees. The state of maths education these days

So tell me at what angle the bricks are self supporting.