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re: precast concrete
26 apr 1996
darryl parker wrote:
>now lets say that pvc pipe was manufactured in the shape of an arch so that
>none of the the molecules were being stretched to maintain that shape. now
>what would happen if you cut that pipe? would it spring outwardly? no...
the cut pipe might not move at all, unless pushed sideways. recall gaudi's
sagrada familia church made with large rocks in non-vertical columns, with
no mortar, because the angles and weights were correct. he made upside-down
catenary-shaped models with weights and string to find the correct angles.
an arch needs to develop at least 4 hinge points before it can collapse. many
modern arches are deliberately built with 3. see the delightful $14.95 da capo
paperback book, _structures: why things don't fall down_, by j e gordon
("when we play tennis or walk down stairs, we are actually solving whole pages
of differential equations, quickly, easily, and without thinking about it,
using the analogue computer which we keep in our minds. what we find difficult
about mathematics is the formal, symbolic presentation of the subject by
pedagogues with a taste for dogma, sadism and incomprehensible squiggles.")
>i agree, the base of a dome is indeed under tension... engineers... do
>concentrate the bulk of their reinforcement in the "base ring"... they
>seem to view this ring as receiving the bulk of the tensile forces.
if the dome is a hemisphere, there is no tension or compression in the circle
on the ground, just downward force. a shallower dome pushes outward at its
base, so it needs some sort of ring to contain the dome at the base, like
a rotated arch. this could be a tension ring, or it could be a collection
of buttresslike foundations, like the ends of an arched bridge.
nick
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