Epic structural failures
By - Indrajeet Chauhan
What are structures?
for maintaining the shape of the building under the influence of the
forces, loads and other environmental factors to which it is subjected. It
is important that the structure as a whole (or any part of it) does not fall
down, break or deform to an unacceptable degree when subjected to such
forces or loads.
A little background information.
Tacoma Narrows Bridge
A little background Info.
(1) The principal cause of the 1940 Narrows Bridge's failure was its "excessive flexibility;"
(2) the solid plate girder and deck acted like an aerofoil, creating "drag" and "lift;"
(3) aerodynamic forces were little understood, and engineers needed to test suspension bridge designs using models in a wind tunnel.
The failure of the bridge occurred when a never-before-seen twisting mode occurred, from winds at a mild 40 miles per hour (64 km/h). This is a so-called torsional vibration mode (which is different from the transversal or longitudinal vibration mode), whereby when the left side of the roadway went down, the right side would rise, and vice versa, with the center line of the road remaining still. Specifically, it was the "second" torsional mode, in which the midpoint of the bridge remained motionless while the two halves of the bridge twisted in opposite directions. Two men proved this point by walking along the center line, unaffected by the flapping of the roadway rising and falling to each side. This vibration was caused by aeroelastic fluttering
Fluttering is a physical phenomenon in which several degrees of freedom of a structure become coupled in an unstable oscillation driven by the wind. This movement inserts energy to the bridge during each cycle so that it neutralizes the natural damping of the structure; the composed system (bridge-fluid) therefore behaves as if it had an effective negative damping, leading to an exponentially growing response. In other words, the oscillations increase in amplitude with each cycle because the wind pumps in more energy than the flexing of the structure can dissipate, and finally drives the bridge toward failure due to excessive deflection and stress. The wind speed that causes the beginning of the fluttering phenomenon (when the effective damping becomes zero) is known as the flutter velocity. Fluttering occurs even in low-velocity winds with steady flow. Hence, bridge design must ensure that flutter velocity will be higher than the maximum mean wind speed present at the site.
Because of materials and labor shortages as a result of the involvement of the United States in World War II, it took 10 years before a replacement bridge was opened to traffic. This replacement bridge is 5,979 feet (1,822 m) long—40 feet (12 m) longer than Galloping Gertie. The replacement bridge also has more lanes than the original bridge, which only had two traffic lanes, plus shoulders on both sides.Half a century later, the rebuilt bridge that was completed in 1950 was exceeding its traffic capacity, and a second, parallel suspension bridge was constructed to carry eastbound traffic. The suspension bridge that was completed in 1950 was reconfigured to solely carry westbound traffic. The new parallel bridge opened to traffic in July 2007.
The Tacoma Narrows bridge failure has given us invaluable information...It has shown that every new structure that projects into new fields of magnitude involves new problems for the solution of which neither theory nor practical experience furnish an adequate guide. It is then that we must rely largely on judgement and if, as a result, errors, or failures occur, we must accept them as a price for human progress.