tacoma bridge collapse

November 7, 1940. You could never have stated if Tacoma Narrows was actually a Bridge or a pretty wobbly Roller Coaster. Why is that? If you have never heard of this, I suggest watching a Youtube video and then following up. (Disclaimer: Just make sure your jaw lands on something soft)

This is a really interesting classic case study of engineering failure! We’d love wind for many reasons and on many occasions, but one that dismantles an entire bridge is definitely not getting there. Hey! It seems unfair to blame the wind completely. Is the wind even the one to be accused of? After all, all the bridges that have ever collapsed were not due to wind. Damn, this thing is conspiring against my ability to judge. Why don’t we just see what actually happened and then build on from there? Yeah? Let’s do it!

The Case

The Tacoma Narrows Bridge that connected Tacoma and the Kitsap Peninsula collapsed on November 7, 1940, at 11 a.m. It was barely four months since it was opened for public use. The constructions started in 1938 and on July 1, 1940, the bridge was opened to the public. It was the third-longest suspension bridge in the world at that time with a center span of 2800 feet.

Interestingly, our wobbliest bridge has had (or even now “has”) a nickname for itself even before the collapse. The workers noticed the sensitivity of the bridge towards even light winds and hence called it the “Galloping Gertie”. And true to its name, the bridge deck did develop rippling vertical waves. Measures were taken. Nevertheless, those weren’t enough!

On the morning of the collapse, the bridge started to undulate as the wind speed reached 42 kmph and the traffic was closed. During the course of time, one of the cable bands slipped and the bridge started twisting. Unexpected stresses-worse isn’t it! And the bridge collapsed after a fretful battle for life!

The Bridge Design

The Tacoma Narrows is a Suspension Bridge.

Gravity was the major load concern before suspension bridges came into existence. It was challenging for engineers to hold up the bridge and the traffic against the gravitational forces. Also, these bridges(arch bridges, beam bridges) had a great appetite and would consume lots of materials, were rigid, uneconomical, and couldn’t span over large distances.

Over the years, knowledge in the field of structural engineering and the understanding of force transfer led to better shapes and designs. And a Suspension Bridge is one among them.

Suspension Bridges and their mechanism of load transfer

Okay. So if the suspension bridges are more efficient in shape and design, why did the Tacoma Narrows Bridge Collapse? Well, suspension bridges had to take care of wind loads in addition to gravity loads(the gravitational force was not much of a concern since suspension bridges have low self-weight). Also, the bridge wasn’t rigid enough. Let’s see the components of a suspension bridge and the load transfer mechanism first and later discuss the flaw in the Tacoma Narrows Bridge Design.

Parts of a Suspension Bridge

  1. Main Cables
  2. Towers
  3. Deck
  4. Suspenders
How do they work?

I came across a quote by Mehmet Murat Tidan in google which says, ” While sleeping in a hammock, with the touch of a warm wind we remember why we are in love with the life!” Wow. That was beautiful. But next time, try remembering the suspension bridges when you find a hammock. It’s exactly how a suspension bridge works. Imagine that you are lying on a hammock whose both ends are tied to two trees with the help of ropes. What are the forces? Your own self-weight and the tension in the ropes yeah?

Now, due to the self-weight the hammock sags down and the ropes have tension in them. The tensile force on the ropes has two components: Vertical and Horizontal. Whatever may be the change in the hammock’s shape(whether it sags more or sags less) the vertical component of force remains the same. Whereas the horizontal force changes accordingly i.e., decreases with more sagging. Because, the height at which you want to tie the rope is the same but the span over which the hammock is to be stretched(the distance between the trees) change. So in order to tie the hammock to trees that are far, it needs to be stretched further and hence the larger horizontal component of tension in the ropes.

Now that’s exactly how the main cables in a suspension bridge work.

Load Transfer

  1.  The weight of the deck and of the traffic above causes the deck to sag or deform a little(excessive deformations can be prevented by the use of less flexible decks)
  2.  The forces get transferred from the bridge decks to the Transverse beams and finally to the suspenders.
  3.  The suspenders transfer the load to the main cables and it’s similar to the hammock’s story hereafter.
  4. The main cables experience tension along the ends
  5. The vertical forces get transferred to the foundations through the tower.

NOTE: The horizontal forces are important. Because thin towers might buckle due to large horizontal forces. In that case, the tower has to be strengthened enough.

Tacoma Narrows Bridge

Now, what went wrong in the Tacoma Suspension Bridge?

Due to funding issues, the bridge used two narrow plate girders instead of the originally proposed trusses in order to stiffen the deck.

The very slender deck and a general lack of knowledge of aerodynamic forces on such a structure resulted in the dramatic failure of the bridge.

How/Why did it collapse?

As I had stated in one of the earlier posts- Earthquakes and Buildings- resonance is the reason. Resonance occurs when the natural frequency of a material or a structure matches that of the excitation(driving) frequency- in this case, the wind’s frequency. In the case of winds, the periodic driving force occurs due to vortex shedding.

Vortex shedding

When a fluid flows past an unstreamlined object, the flow breaks and vortices are formed behind the object. The vortex formation and shedding occur alternately in the wake of the object.

On the day of the collapse, the Tacoma Narrows bridge experienced resonance from vortex shedding. Apart from its usual vertical deflections, the bridge experienced a torsional motion and started twisting before 45 minutes of its collapse. Normally in bridges, the wind would pass through the trusses. But the replacement of trusses with narrow H-shaped girders led to flow separation and hence the formation of a vortex. Another possible debate is that the collapse might be caused due to aeroelastic flutter. It is an unstable, self-excited, structural oscillation at a definite frequency and the energy is derived from the motion of the structure itself.

Ultimately, the bridge’s slender design could no longer take any more stress. And the bridge collapsed.

A lesson from the past

The failure could have been prevented if a less flexible deck had been used by the addition of deck panels. But, on a brighter side, this failure paved the way for increased research and understanding about the effects of wind and the need for aerodynamically stable structures. Today, we build suspension bridges with ease and do even more. We have advanced a lot. Suspension bridges and the lessons they taught us has led to many different and efficient types of cable-supported structures.

Happy learning 🙂