Sliding metal plate on a slanting roof

jumanicus

It is said that a metal plate kept on a slanting roof (say made of
concrete) slides down (although slightly) over a period of many days
due to repeated heating and cooling during the day and night. Ofcourse
it is assumed that friction is sufficient to prevent sliding due to
gravity alone. Somehow repeated expansion and contraction of the plate
during the day and night causes this phenemonon. Can anyone expalin
what is happening?

James Dolan

in article <1160011858.467520.295640@c28g2000cwb.googlegroups.com>,
jumanicus <vishwesh.muzumdar@rediffmail.com> wrote:

|It is said that a metal plate kept on a slanting roof (say made of
|concrete) slides down (although slightly) over a period of many days
|due to repeated heating and cooling during the day and
|night. Ofcourse it is assumed that friction is sufficient to prevent
|sliding due to gravity alone. Somehow repeated expansion and
|contraction of the plate during the day and night causes this
|phenemonon. Can anyone expalin what is happening?

it seems quite plausible that even when friction is sufficient to
prevent sliding-due-to-gravity-alone, it may be insufficient to
prevent expansion-and-contraction-of-the-plate-due-to-heating-and-
cooling. so as the plate expands and contracts (while the concrete
_doesn't_ expand and contract anywhere nearly as much), something's
got to give- either the higher edge of the plate or the lower edge of
the plate must slip against the concrete. during expansion, it's the
slipping downwards of the lower edge that's more energetically
favorable than the slipping upwards of the upper edge, and during
contraction it's the slipping downwards of the top edge that's
energetically more favorable than the slipping upwards of the lower
edge. (that's the way gravity works: going downwards is generally
energetically more favorable than going upwards.)

thus the plate inches down the roof like an inchworm, expanding and
contracting while preferentially moving downwards rather than upwards.

thus even when gravity isn't sufficient to _cause_ slippage all by
itself, it can be sufficient to determine the _direction_ of slippage
caused by something else.

--

jdolan@math.ucr.edu

Oz

James Dolan <jdolan@math.ucr.edu> writes
>thus the plate inches down the roof like an inchworm, expanding and
>contracting while preferentially moving downwards rather than upwards.

This is the reason given for cellular polycarbonate roofing sheets
(found on conservatories, typically) for working their way out despite
well clamped edges. Its easily fixed with one screw at the bottom end as
per the installation instructions.

--
Oz
This post is worth absolutely nothing and is probably fallacious.