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.

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Page 129 - Building stability - Skeleton construction

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