Thus far we have discussed primarily how structures like beams,...

columns, and bridges support downward acting loads applied in

the same plane as the structure.  That is, basically parallel to the

two dimensions of height and span.  Buildings, however, can

experience significant sideways, or lateral, loads also due to

two-dimensional loading

winds blowing against the exposed walls, snowdrifts, earthquakes, etc.  This is in addition to

vertical acting external and internal loads such as snow, and loads borne by the floors above

ground.  In the following section you will see how structural engineers stabilize buildings,

spaceframes, and towers to withstand forces in three-dimensions.


Three dimensional stability


Building designs vary widely to serve many different purposes.  In simplest terms, a building

can be considered to be a three-dimensional box-like framework designed to envelop an

interior space.  Attached to this framework is a floor upon which rests the people, animals,

and equipment being housed; a roof that shelters the interior from rain, snow, and wind;

and walls that support the roof and supply shelter.  The skeletal framework is created by

joining beams and columns end to end at right angles to each other like the edges of a cube.

When the sides of this box are viewed face-on the inherent instability of each of the square-

shaped faces is self-evident.  Recall the previous discussion of stable polygons. 



Fig. 194 - Instability of the


square faces of a cube


◄  4 < 2 ( 4 ) - 3


therefore unstable


(visualization model)





There are four basic ways to stabilize the structural members framing the faces of a cube.


a) single cross brace

b) double cross brace

c) diaphragm

d) rigid joints


Fig. 195 - Four basic ways of stabilizing the faces of a cube   (visualization models)


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Page 117 - Building stability - Buildings

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