Silver Bridge

What materials and systems were used in the construction of this structure?

The bridge was designed as a parallel-wire-cable suspension bridge. With an alternate design that had a heat treated steel eyebar suspension system. The latter would only be used if it met with certain performance criteria. The financial statement of the final cost of the bridge was put out by the American Bridge Company and show the cost of construction to be approximately $900,000, of which the general contractor received $805,000. For the chain-link design, which was ultimately used, the heat treated steel had an ultimate strength of 100,000 psi; a yield strength of 75,000 psi; and an allowable working stress of 50,000psi. The bridge was painted aluminum which is why it was called the “Silver” Bridge. The bridges roadway was made using a steel grid and asphalt. The Bridge itself was made with heat treated steel and is a suspension structure using eyebars. Two eyebars where used for each chain. It used Warren trusses with subverticals. Total length is 528m. Also, towers were supported on rockers to allow their movement under different positions of the live load on the bridge and expansion or contraction that may happen to the bridge caused by temperature changes. Reference Figure 1 for a drawing of the Silver Bridge. The bridge was designed using the AASHO-H-15 loading, which corresponds to a 133 kN (15 ton) truck with a rear axle load of 107 kN and a front-axle load of 27 kN.

Figure 1. Side View of Silver Bridge also shows the location of the failed eyebar

Figure 2. Structure of standard eyebar of the Silver Bridge

 What were the causes of the structural failure?

The Bridge failed because of stress corrosion in one of the eyebars which made both eyebars collapse and made the bridge completely collapse. Some structural there was an air pocket in the design of the eyebars that could have been susceptible to air corrosion which leads to stress corrosion which was the ultimate failure of the Bridge.

Figure 3. Close up view of fracture of the failed eyebar

The Bridge’s stress distribution was very disproportional. The stress in the eyebar heads was three times higher than the rest of the bridges components.  In the case of the Silver bridge the way it was designed if one eyebar fails then the other does too ultimately resulting in an instantaneous collapse of the entire bridge. Next, the eyebar failed due to stress corrosion in the ‘head’ of the eyebar were it is connected to the joint. The eyebars designed for the Silver Bridge had the space where the pin fit was elongated by 3mm in a horizontal direction for ease of erection. But this design is flawed because that space creates and air pocket for corrosion to occur undetected. It made inspection of the eyebar head impossible without having to dismantle the entire joint which is irrational. Since the St. Mary’s Bridge was designed using this eyebar design it had to be taken down to prevent another Silver Bridge calamity.

What were the consequences of the failure?

As a result of the Silver Bridge collapsing; 46 lives were lost with nine people injured. Essentially the entire bridge collapsed and fell into the Ohio River causing millions of dollars in damage.

What technical lessons were learned from the failure?

The design of this bridge was interesting and had a lot of good ideas and implications on it. But, some of these ideas were not executed to an optimal effectiveness. For example, the Silver Bridge used eyebars in its design like many other bridges in that time such as the 6^th Street, or Roberto Clemente, Bridge and the Florianopolis Bridge in Brazil. The difference between these bridges’ eyebars and the Silver Bridge’s eyebars is the fact that the Silver Bridge design only incorporated 2 eyebars per panel instead of several eyebars in the 6^th Street and Florianopolis Bridges’ cases. If one eyebars fails when there are several eyebars surrounding it the bridge can continue to operate fairly well until the eyebar can be replaced or repaired. The practice of using eyebars was looked into more because of the collapse of the bridge and bridges using the eyebar construction were inspected for safety.

What ethical lessons were learned from the failure? 

Some ethical lessons that were attained from this were to have bridges regularly inspected because the tragedy that was the collapse of this bridge could have been prevented if the problem was caught before the collapse of the bridge. First off, regular inspection of bridges was nonexistent before the collapse of this bridge and because of this the country created the National Bridge Inspection Standards (NBIS). This requires states to periodically inspect all highway bridges that are on public roads. The main method of inspection for these bridges is visual inspection. But other tools are used to inspect these bridges to make sure they are in good form to stay in commission. Such as ultrasonic probes which check for levels of decay using ultrasonic wave velocity measurements. Because of these inspection standards a lot of bridges were found to be in need of some maintenance and the repairs were mad before anything bad could happen like the collapse of the Silver Bridge.

Was there any way these failures could have been avoided?

These failures could have been avoided for a couple reasons. The most important is if the bridge was inspected and then repaired the whole problem could have been avoided. If the bridge was inspected the fracture would have been most likely noticed. The design and implementation of the eyebars were very poor. Supporting the system with more the then two eyebars on each joint would increase the stability of the structure also removing the air pocket design flaw in the eyebars. The bridge was also very disproportional in that the stress on the eyebars was much higher than that of the rest of the bridge. Thus, relieving stress of the eyebars by adding more support to the bridge would also increase its stability.

What are you personal take-away from the failure?

This failure, although very tragic, was detrimental to the well-being of this country’s infrastructure. The start of a Bridge Inspection program had to happen at some point to insure the safety of the bridges that the public is using so nothing like this happens. It is unfortunate that the collapse of the Silver Bridge had to happen before these National Bridge Inspection Standards were created. The Silver Bridge could have definitely benefited from these standards considering the flaw was a fracture caused by stress corrosion. I feel as if this bridge collapse was one of the more influential in coherence with progress towards bridge maintenance and inspection. The Silver Bridge was one of the first bridges to use this eyebar design and was the first bridge to used heat-treated steel eyebars for its construction. Considering the uncertainty behind the heat-treated steels performance and longevity I feel as if this type of steel should have been tested thorough before using it. Or use it on a bridge that doesn’t receive as a much traffic as the Silver Bridge did. The prevention of this collapse was something well within the reach of people in that time and it is a shame that it had to happen.