132 Designing and making shelving arrangements
referred to as bearings and – depending upon which
of the three usual kinds are used – it has to be real-
ized that each of them affect the degree of stiffness or
elasticity of the shelf and its ability to resist deflection
under load – as mentioned above.SHELF SUPPORTS
Built- in end- bearings
Figure 7.2 (a) Part of a side cheek showing a stopped-
housing and part of a shelf showing the recess- shouldered
front- end positioned for assembly.Figure 7.2(a): There are traditionally three differ-
ent ways to support wooden shelves – apart from
modern innovations by manufacturers and individual
designers. The first, illustrated at 7.2(a), is where the
ends of shelves are housed (usually stop- housed)
into the cheeks (sides) of a shelf unit by one- third
the cheek’s thickness – the housing- stop distance
equals the thickness of the shelf away from the edge.
Note that if this is done neatly – i.e. the shelves
fit tightly in the housings – you will achieve more
mechanical support by having altered the tensile and
compressive stresses, as indicated in Figure 7.1(b)
above. Another advantage is that the restraint
created by the housings will restrict warping or
cupping of the shelves.Unfixed end- bearings
Figures 7.2(b)(c)(d): The second traditional support,
illustrated at 7.2(b), is where the shelves rest on – or
can be fixed to – wooden end- bearers. However, it must
be realized that shelves simply- supported in this way
(whether fixed or not) are entirely or mostly subject
to full- span stresses of compression and tension, as
described above and indicated in Figure 7.1(a).stress (tension) across their horizontal width above the
underside. Being at maximum stress on the top and
bottom outer- surfaces, where these two theoretical
forces meet in the horizontal middle area of a beam
or shelf, they neutralize and have no stress. This area is
referred to as the neutral layer, or neutral axis. Because
the compressive stress in the layer of wood- fibres
above the neutral axis causes a limiting- or balancing-
effect on the tensile stress in the layer of wood- fibres
below the neutral axis, it follows that the greater the
thickness of the wood- fibres above and below the
neutral layer, the less bending (deflection) will occur –
even though the weight of the beam or shelf will
increase.
Another way to appreciate the validity of the limit-
ing- or balancing- effect of the opposing compression
and tension forces in a rectangular beam is to study
the purposely- exaggerated concaved beam- shape at
7.1(a) and question its ability to actually (physically)
compress and shorten its length on top and stretch and
expand its length on the bottom (as, geometrically, any
such concentric curvature demands) without fractur-
ing (rupturing) the beam on the underside. In theory,
of course, an excessive dead load on a simply- supported
beam (the ends of the beam resting on supports, but
not built- in) can also cause horizontal shear, as illus-
trated at 7.1(c). This causes a separation of the beam’s
horizontal fibres (to a greater or lesser extent), which
tend to slide over each other as the top of the beam is
shortened and the bottom lengthened by bending.
Understanding these simplified explanations of
the theory of basic beam- mechanics should help
in visualizing its modifying effect on the different
shelf- supports described below. The supports are also
TensionTensionTensionCompressionCompressionCompressionFigure 7.1 (b) Exaggerated serpentine- shape deflection
highlighting the changes in the compressive and tensile
stresses on a beam with built- in end- bearings.
Figure 7.1 (c) A simply supported, overloaded beam dis-
playing exaggerated deflection and a theoretical impres-
sion of the effects of horizontal shear.