To sum
up, the strength of a simply supported solid beam to resist bending due to
a load |
applied
to its middle (the simplest case) is a function of its: |
- height squared - e.g. height doubled, strength quadrupled,
stresses reduced by four; |
- span - e.g. span doubled strength halved; |
- width - e.g. width doubled strength doubled; and |
- modulus of elasticity - e.g. modulus doubled strength doubled. |
. |
Post
and lintel construction |
. |
Wood has
been a preferred material for making beams because of its good compressive |
and
tensile strength, provided that the beam is cut parallel to the wood's
grain. However, |
wood can
rot, burn, and be attacked by insects. Ancient builders preferred to use stone
for |
building
structures that were meant to last for the ages. A popular design
for masonry |
structures comprised of simply supported, hewn stone beams is called post
and lintel |
construction. Ancient Egyptian, Greek, and Roman architects made
extensive use of this |
. |
 |
|
 |
Fig. 145 - Post and |
lintel construction |
(scale visualization model) |
click image to enlarge |
|
|
. |
type of
construction to support the roofs of temples and public places.
However, while |
stone
has high compressive strength, it is comparatively weak in tension.
The fact that the |
greatest
tensile stresses are concentrated in the midpoint of the bottom edge of a
beam |
causes a
disproportionate reliance on the tensile strength of this small area.
This grossly |
underutilizes most of the beam's other mass. As a result the lintels
had to be made |
especially thick and wide in order to increase the amount of mass they had
to resist these |
tensile
stresses. This increased their own dead weight, which significantly
reduced the |
distance
they could reliably span to about eight feet on average. As a
consequence, Greek |
temples,
like the Parthenon, were so crowded with columns that there was not much
room |
for
crowds to gather inside. One solution to using stone beams to span
longer distances |
was the
to arch them. |
. |
Arched beams |
 |
. |
The Romans used the stone arch extensively to provide for open |
areas in buildings as well as to bridge obstructions and channel |
water across valleys. An arch can be imagined as being |
assembled from wedge shaped sections of stone, called |
voussoirs, that are cut out of a beam and then rearranged into |
the shape of an arch. This reduces the distance any one
section |
click to enlarge |
|
. |
Back to
Knowhere |
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Page 96 -
Building stability - Lintels and arches |
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