BEAMS OR JOISTS LIKENED TO

SHELVES

Because the structural stability of suspended wooden

beams or joists is mostly dependant on their cross-

sectional depth, suspended shelves – by comparison –

are like ignorantly- positioned, suspended wooden

beams of rectangular cross- section, lying wrongly

on their widest sides – and yet they have to support

so- called live loads (be it only of books, etc) similar to

floor joists that carry a live load of people and fur-

niture, etc. As beams or joists, therefore, shelves are

structurally weak and – to avoid excessive deflection

under load – must be limited in span in relation to

their thickness and their intended load. All relatively

thin, loaded wooden shelves will suffer a downwards

deflection to some extent, but, with calculated design,

this deflection should not exceed (in my opinion) 1 in

300 (i.e. a shelf with a 900mm span should not have

more than a 3mm mid- area deflection under load).

UNDERSTANDING BASIC

MECHANICS

Figures 7.1(a)(b)(c): The fibres of loaded beams or

shelves, as illustrated below at 1(a) and (b), are subject

to a crushing effect referred to as compressive stress

(compression) across their horizontal width below the

topside – and to being torn apart (stretched) by tensile

INTRODUCTION

Although designing is not the designated domain of a
joiner, a love of the craft and an acquired understand-
ing of wood and wood- derived products can inspire
many of us to create our own designs. However, even
if our designs are not revolutionary, we do need to
consider shelf thicknesses in relation to the weight of
the books or other items to be placed upon them; and
in so doing, we become involved with the principles
of structural mechanics. This is because the shelves,
usually required aesthetically to be kept thin in depth
in relation to their span, are likely to bend (deflect)
in their middle area under their load. This is rarely of
concern regarding the actual shelves reaching a point
of collapse, but it must be realized that bending is the
most severe form of stress in a shelf. And to avoid
the unacceptable appearance of excessive deflection,
we can work out a shelf ’s required mechanical thick-
ness by applying simple or complex mathematical
formulae – in a similar way to working out the depth
of timber beams or floor joists. Not being a structural
engineer myself, but having designed and built many
successful shelf arrangements over the years, the fol-
lowing notes and illustrations are based on practical
intelligence and common- sense mechanics related to
simple formulas.

SHELF MATERIAL

Although this chapter will refer to engineered- wood
products such as plywood and blockboard, and
wood- derived products such as chipboard and MDF,
etc, which could be used for shelves, the calculations
given here for the critical shelf- thickness (depth) are
mainly based on the use of good quality, solid redwood
or a hardwood species of at least equal density and
strength.

7

Designing and making

shelving arrangements

Compression

Cross

section

Tension

Figure 7.1 (a) Exaggerated deflection of a simply sup-

ported beam, with arrowed indication of the compressive

and tensile stresses; the broken line through the centre

represents the neutral layer. The extreme vertical arrows

depict the bearing points.