Because the cables have a constant dimension for ease of construction and economy, they had to be designed to resist the maximum tensile force, which occurs in the side spans at the top of the towers. The maximum force is 262,000 kips and the maximum stress is 81.9 ksi. The allowable stress for the steel used in this bridge is 82 ksi. The resulting efficiency is:
While this analysis is only an estimate, an efficiency value of almost 1.0 indicates quite an efficient design. This is especially important in bridges because the dead load makes up a majority of the weight on a bridge , and because the more material a designer uses, the stronger the bridge must be. Therefore, a bridge designer searches for an optimum of minimum material (and dead weight) to provide the necessary safety.
Overall bridge efficiency should
be considered also. A suspension bridge
is inherently efficient because the flexible cables change shape to take the
forces most efficiently. Also, the sag
to span ratio is important. A deeper sag
reduces cable force, but increases the height of the towers and makes them more
susceptible to large forces. A generally
accepted optimum is a 10:1 span to sag ratio.