AW Extra – Stronger Shelves


Stronger Shelves

Designing sag-proof shelves.

By Tom Caspar

 

Have you ever heard a shelf groan?
Well, maybe not, but some shelves
look like they would if they could. So
much stuff gets piled on them that they
end up sagging like a limp noodle. It’s
not a pretty sight. A span that holds up
weight should look strong and sturdy.
Even a slight sag sends an unappealing
visual message.

A shelf sags in two stages. There’s a
small sag when you first put weight
on the shelf. This sag, or deflection,
increases as more stuff is loaded on
the shelf over the years. The shelf keeps
on sagging, a little more each year,
because wood slowly but surely
changes shape under a load. This characteristic
of wood is called “creep.”

Arm yourself for the fight against
creep with common sense engineering
knowledge about how each dimension
of a shelf affects its strength, or rigidity
(Fig. A).

The most important dimensional factor
is a shelf’s thickness. The effect of the
other two dimensions, width and length,
is straightforward; add 10 percent to
the width of a shelf, and it’s 10-percent
more rigid; add 10 percent to the length,
and it’s 10-percent less rigid. However,
add 10 percent to the thickness of a
shelf and it’s 21-percent stronger!
Strength increases at an exponential rate
as you add thickness. That’s why
wooden joists and steel beams stand on
edge. A shelf that’s 7⁄8-in. thick is about 36-percent stronger than a 3⁄4-in. thick shelf. If you’re careful, you should be
able to get 7⁄8-in. thick boards from 4/4
lumber. With such a substantial increase
in rigidity, it’s definitely worth the effort.

Shelving Standards

Let’s put our common sense engineering
to the test. How wide or long does
a cherry shelf have to be to hold up, say,
a set of encyclopedias? We can use
standard dimensions that have proven
themselves reliable over the years to
answer this question (Fig. B).

Encyclopedias are large books, so we
need a shelf that’s at least 12-in. deep.
(Books look best when they sit
1-in. or so back from the front edge of a
shelf.) How heavy are they? It wouldn’t
be a bad idea to place them on a bathroom
scale and find out. Books and
magazines can be surprisingly heavy.
These standards suggest they’ll weigh up
to 50 lbs. per running foot.

How long should a 3⁄4-in. cherry shelf
be to hold up these books? The standards
tell us that a wide hardwood shelf
that carries a heavy load should be no
more than 36-in. long. But this doesn’t
tell us the whole story, and that’s why we
have to look at one more variable before
we can build this shelf with confidence.

How Strong is the Wood?

Some species of wood are much more
rigid than others. In terms of deflection
under a load, hickory is about twice as
strong as butternut. If we make similar
shelves out of hickory and butternut,
and apply the same load, the butternut
shelf will sag twice as much. If we cut
the butternut shelf in half, it will sag the
same amount as the hickory shelf.

The shelving standards are based on
averages. Is our cherry encyclopedia
shelf average? No, cherry is more than
10 percent weaker than an average
wood like walnut or soft maple (Fig.
C). Thus, the shelf has to be shorter or
thicker.

Plywood and Composite Woods

Manufactured wood products are not as
strong as solid wood. Plywood is only
about half as rigid as the average hardwood because it’s made of alternating
layers of thick veneer. Wood is not as
rigid across the grain as along the grain.
In a shelf, the grain of some of the
veneer layers runs the long way, but
almost half runs the short way.

Particleboard and MDF (medium
density fiberboard), collectively called
composite woods, do not have the grain
structure of solid wood or of plywood veneers. These products have
a hard time holding up their
own weight. They have about
one-quarter of the strength of an
average wood. Nevertheless, composite
wood is widely used for shelving.
Comprehensive information on
sizing composite wood shelves is available
from an industry trade association.

The rigidity of plywood and composite
wood can be improved by gluing
on solid wood edges or plastic laminate
faces (Fig. D.)

How to Strengthen a Shelf

After weighing the encyclopedias, you may find
that they exceed the limits of the standards. In
that case, we need to make a more rigid shelf. We
could widen the shelf from 12-in. to 16-in., an
increase of 25 percent. That would make the shelf 25-
percent stronger. It’s an option, but for the sake of
argument let’s say that a 16-in. shelf is too deep for our
design. What else could we do?

We could shorten the shelf. Like width, it’s a straight percentage reduction. A shelf that is
25 percent shorter will also hold up
25 percent more weight.

A shorter shelf for our encyclopedias
isn’t going to work, however.
With all the supplements added to it,
there’s more than three feet of books!
How about making the shelf thicker?
If we’ve already purchased
3⁄4-in. stock, this option is out.

It’s time to be creative about building
a shelf that will be stronger than
just one solid board. The simplest
solution is to add a lip or two to
the shelf. Rip some of the
3⁄4-in. stock into 11⁄2-in. wide strips,
turn them on edge, and glue them
onto the front and back edges of the
shelf (Fig. E). If you need considerably
more strength, make the rear
strip several inches deep.

Another way to add thickness to a
shelf without using expensive thick
lumber is to make a hollow shelf
1- to 11⁄2-in. thick from thin plywood and strips of solid wood (Fig.
F). This type of shelf is based on the
engineering principles of a torsion box.
It’s light in weight but very strong. Use
it to carry very heavy loads.

Housing a shelf into the sides of a
case will enable the shelf to carry more
weight. A fixed shelf is more rigid than
a loose shelf because its ends are joined
to the sides at a stiff right angle (Fig.
G). Some joints are stronger than others,
so choosing one kind over another
can also effect the strength of a shelf. A
plain dado works fine, but a sliding
dovetail is stronger because it has more
mechanical strength and a larger glue
surface area.

Supporting a shelf in the middle strengthens a shelf more than you might
think (Fig. H). Weight on one side of a
middle support helps hold up weight on
the other side, like kids on a see-saw.
The net effect is that one long shelf
with a center support can hold up more
weight than two shorter shelves.

For More Information

On dimensions for composite woods:

Particleboard and MDF For Shelving
Technical Bulletin

Composite Panel Association

18928 Premiere Court
Gaithersburg, MD 20879
301-670-0604
pbmdf.com

On wood’s strength:

The Wood Handbook

Forest Products Laboratory
Madison, WI
608-231-9200
You can download it from the Web at www.fpl.fs.fed.us/

This story originally appeared in American Woodworker October 1999, Issue #75.

Click on any image to see a larger version.

Fig. A: The ABCs of Holding Up Weight

Fig. B: Standard Shelving Dimensions

Fig. C: Rigidity of Common Wood Species

Fig. D: Strengthening Plywood and Composite Shelves

Fig. E

Fig. F

Fig. G

Fig. H

6 Tips for Stronger Shelves