Improved model

Engineers make models to represent

The things they build which are meant

To help them more fully comprehend

How things will strain, stress, break or bend.

In the engineer’s model of tradition

A masonry arch is viewed in partition

A cross-sectional view works just fine

To show stress as a curved thrust line.

A catenary line like a hanging chain

Turned upside down in your brain

Represents the lines of thrust

In the arch’s thickness it must-

Be constrained within that wall

Or a hinge will form and it will fall

This is the old traditional way:

To show an arch with this display.

But now there is a newer block

Whose edges each interlock

Hinges will no longer suffice

To show how well and truly nice

And stronger too these arches are

Conjugate shearing is the star

Of this newer model which is much better

Made of improved block that lock together

They do not hinge or buckle wide

Against each other they simply slide

And if you get this simple point

It is a better masonry joint.