Double Scaffold Boards

Question: What is the strength of two scaffold boards simply supported, laid one on top of the other?

Answer: Two times the strength of one board... right? Well we think so.

On a recent design project we had a large birdcage scaffold with double scaffold boards laid on the top lift. The load case we applied involved the application of a series of point loads on the boards from mobile aluminium towers and small independents erected off the deck.

In the design we assumed the worst load case for the boards in bending is a centrally located point load, the boards were spanning between transoms in the usual way.

The permissible bending capacity of a single 38mm thick timber scaffold board is 0.486kNm. We then take the capacity of two boards, one on top of the other as 2 x 0.486kNm = 0.936kNm. The calculation behind this is simple enough:

For a single scaffold board: Mc = 0.486kNm = f x z = f (bd2/6)

We were quizzed on this assumption by checking engineers who stated that the bending capacity of the double boards could not be increased unless the two boards were bonded together.

Our theory is:

For a double scaffold board: Mc = 2 x 0.486kNm = 2 (f x z) = 2 x (f (bd2/6))

For a bonded double scaffold board: Mc = f x z = f (b(2d)2/6) = 4 x 0.486kNm

This is based on the assumption that as the boards have the same stiffness (EI) they will carry an equal share of the applied load.

We proceeded to ask fellow scaffolding designers and structural engineers their opinion on this matter, we had a mixed response, some agreed, some disagreed.

By far and away the most comprehensive and detailed assessment came from our friend Alwyn Richards, at Alwyn Richards Temporary Works Design (www.alwynrichards.com). He sent us this fantastic video, I hope you enjoy this as much as we did!