tip outwards. This in turn causes the thrusting force to move |
 |
farther off center towards the outside of the wall so that the |
wall tips outwards some more, and so on. If the wall is not
|
thick enough, and the thrust moves to the outer one-third |
section of the wall, the compressive stress on the inside of the |
wall will become zero. The inside of the wall will then begin |
pull apart from tension thereby tipping the wall outwards even |
more. If the thrust moves so far outside that it acts
completely |
beyond the wall's edge,
the wall will tip over and fall down |
Fig. 217 - Wall tipping over |
as
shown by the animation to the right. |
(training aid) |
|
. |
One of
the ways medieval cathedral builders overcame this problem was by propping
up |
the
walls from the outside with an inward pushing masonry brace, called a
buttress. The |
buttress
resisted the outward thrust of the roof. This caused its load to
shift
more towards |
the
centerline of the wall which helped stabilize the structure.
Additional weights, such as |
statues,
were often placed on top of the buttress to help direct the outward
thrusting forces |
downwards nearer the
centerline. |
. |
 |
Fig. 218 - Buttressing |
 |
 |
walls against |
tipping over |
|
using weights to |
direct
the thrust ► |
(visualization model) |
click image to enlarge |
|
. |
However,
the height attainable by masonry wall bearing construction was limited by
the |
sheer
massiveness of the structure. As land values in the hearts of the
cities increased and |
steel
prices dropped around the start of the last century, the desire to build
up rather than |
out
caused engineers to turn to steel skeleton structure for a solution. |
. |
Skeleton construction |
. |
In
skeleton construction the live and dead loads are supported by a
structural framework |
skeleton. Unlike wall bearing construction, the exterior walls do
not bear any of the load. |
Instead
the walls are hung from the skeleton. Therefore, they are called
curtain walls. The |
floor and roof
loads are dissipated to the girders, beams, and columns of the skeleton.
The |
structure is comprised of right-angled boxes interconnected with each
other. Since the box |
lattice
structure is inherently unstable, the joints of the skeleton framework
must be rigidly |
fixed. In steel
Ι-beam
construction this is done by riveting, bolting, or welding the joints. |
. |
Back
to Knowhere |
 |
Page 129
- Building stability - Skeleton construction |
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