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Architectural Design

In order to design buildings an architect must understand the manner in which stresses flow through structures. In figure 1 below we see a cable suspended from its two ends. The gravitational force acting on the cable is shown by the cyan arrows in the diagram while the stresses within the cable are approximately indicated by the degree to which each connector is tinted maroon. In the next series of pictures these stresses are shown in detail. The magenta vectors are the forces exerted by the connectors and the upward dark blue arrows show the vector sum of these forces.

Figure 1.
Figure 1a.
Figure 1c.
Figure 1b.

In each case we see that the connectors stretch until the force they exert on each mass exactly counters the gravitational force. The cable is more sharply pitched at the ends because a greater vertical component to the connector forces is needed there to support the entire weight of the cable.




In the following diagrams the cable has been inverted to form an arch. Because the connectors are aligned exactly as before, as the gravitational forces compress the arch rather than stretch it they will create a net force on each mass in a direction opposite to that for the cable. The arch stands for the same reason that the cable sags. Note in the last diagram that the force exerted by the base of the arch into the ground is co-axial with the lowest connector. Consequently steeply-pitched arches exert less lateral thrust than do shallow ones. Shallow arches are usually seen only in bridges where these lateral thrusts can be easily contained.

Figure 2a.
Figure 2b.
Figure 2.
Figure 2c.