Importance of Bridge Monitoring: A Case study of Morbi Bridge Collapse

The suspension bridge tragedy in Gujarat in October 2022 has brought the question of durability of key civil engineering structures into the limelight yet again. In this article, Soumya. N, Communication Engineer, FGS Engineers & Innovators, highlights the importance of Structural Health Monitoring (SHM), considering the case of the Morbi suspension bridge collapse.

Recently, Jhulto Pul, a pedestrian suspension bridge, in?Morbi, Gujarat, India, across the Machchhu River?collapsed. The bridge was 230m long and 1.25m wide. After 7 months of repair, this bridge was reopened for tourists a few days before the collapse. On 30th October 2022, there were more than 400 tourists on the bridge and the bridge collapsed, which led to the loss of 140 lives.

Morbi Bridge Collapse (Source: The Hindu)

Loss of more than 100 lives in an incident is really scary! Families were waiting at homes expecting the return of their loved ones. When an engineer hears such a news, the first question that pops up in mind would be the technical reason behind the structural failure. This is a journey of the thoughts of an engineer looking into the possible causes of such failures.

Let’s find out what are suspension bridges. ?

Suspension Bridges

A?suspension bridge?is a type of?bridge?in which the deck, the part on which pedestrians and vehicles can move, is hung using?suspension?cables. The cables will be in tension, and therefore materials such as rope, bamboo?or steel wire with a high resistance in tension will be used as suspenders.

Suspension Bridge (Source: Dream Civil)

Forces on a suspension bridge (Source: engineeringdiscoveries.com)

The different forces acting on the main suspension cable, suspender cables, and the towers which are considered for design are shown in the figure above. The tension forces are acting on all suspension cables and the towers are under compression force due to both gravity and pull from the cables.

In addition to the vertical forces mentioned above, lateral forces like earthquake load, wind load, etc. are also important factors that are considered during design.

Cable suspension bridge collapses in world history

There are many examples of similar collapses of suspension bridges in world history.

A major suspension bridge mishap had happened in 1850, the Angers bridge collapse over the Maine river in France, which killed 226 people. The suspected reasons for the collapse were wind and resonance of soldiers marching on the bridge. The wind caused additional horizontal load and soldiers marching induced vibrations.

Angers Bridge Collapse (Source: Wikipedia)

Another example is the Tacoma Narrows bridge, a suspension bridge which was built in Washington, US, between Tacoma?and the Kitsap Peninsula. It was opened in 1940 and from then the deck was moving vertically during windy conditions, and it collapsed during a 64km/hr wind in the same year itself. The deck was undergoing oscillation with alternating twisting motion of itself. It was found to be a design issue where relevant forces like wind load were neglected during design. The bridge was drawn to self-induced rocking. This is termed as aerodynamic flutter where the high energy formed was not easy to be dissipated. Thus, the twisting motion continued which led to the collapse.

Tacoma Narrows Bridge Collapse (Source: Britannica)

Another similar incident that happened was in The United States - the Silver Bridge collapse in which 46 people were killed. The bridge was named after its colour of aluminium paint. The bridge was over the Ohio River from Point Pleasure, West Virginia to Gallipolis, Ohio. The bridge collapsed due to heavy load and poor maintenance.

The Silver Bridge Collapse (Source: The Structural Engineer)

POSSIBLE REASONS FOR FAILURE

The most common reasons for a bridge collapse are overload, design & construction errors, hydraulics and collisions, lack of maintenance and repair works etc. (Zhang et al. 2022)

In the case of a suspension bridge, there are high chances of failure due to damping, fatigue, and external vibrations. Examples of external vibration include earthquake, wind, etc.

It is known that army troops are not allowed to march in steps on suspension bridges as this may cause the failure of the bridge. The reason is that the frequency of the footsteps will match with natural (resonant) frequency of the bridge, and it may end up in a high amplitude motion of the bridge leading to its failure.

The resonance can be avoided such that the forced frequency should not attain the value equal to or near to the natural frequency. If the forced frequency cannot be changed, the natural frequency of the structure needs to be modified. This will be achieved by changing the mass or stiffness. By increasing the stiffness to maintain natural frequency above the forced frequency helps in reducing vibration.

The guitar can be said as a good example for this. The strings of guitar with lower notes are thick, that is with larger mass and those with higher notes are thin, that is with lower mass. The natural frequency decreases as the mass of string increases. The stiffness can be increased by tightening the string which increases the natural frequency.

THE MORBI BRIDGE CASE

The Morbi bridge was over 143 years old. Witness accounts state that the bridge was overcrowded. Adding to it, some people were found to be jumping, shaking and swinging on the bridge.?Reaching the ultimate load is a reason for the failure of any structure which could lead to catastrophic events.

Main suspension cable, Morbi bridge collapse (Source: India Today)

As per the video of the tragedy available on the internet, it is clear that the main reason was the breakage of main suspension cable. It can be seen that the main suspension cable was highly corroded from the figure shown above.

How to AVOID such TRAGEDIES?

These kinds of tragedies can be avoided by the introduction of bridge monitoring (Structural Health Monitoring) system.

Structural Health Monitoring (SHM) is a method of observation and analysis in which a group of sophisticated sensors are installed on a civil structure for monitoring its different components and assessing the actual condition of the structure. The SHM methodology is depicted in the following figure.

SHM Methodology

The main parameters to be determined on a bridge are environmental parameters, operational and environmental loads, and response parameters.

The different environmental parameters such as wind load, weather and visibility, atmospheric pressure, temperature, relative humidity, rainfall, etc. can be monitored by installing sensors on the structure.

The responses of live loads like stress, vibrations, displacement, inclinations of different parts, etc. can be monitored by placing sensors for the same on the corresponding components of the structure.

Corrosion is another important parameter to be monitored. The sensors for measuring the level of corrosion can be installed on the structural components which are likely to be corroded.

These sensors can be implemented in all civil structures including bridges so that such mishappenings can be avoided. The measured values of the parameters from each sensor at a specific time will be recorded and saved offline and/or online. The input from the sensors will be provided to the computers and mobile phones of concerned personnel, along with alarms when the monitored parameters exceed their threshold values. The structural elements can be repaired or replaced at that stage to avoid the loss and damage to life and property.

In India, as per 2019 data, there are about around 39000 railway bridges which are more than 100 years old. The Indian Railway confirms that a systematic process of inspection every year for all these bridges is ongoing. This is a welcome measure.

By introducing structural health monitoring to all key civil structures including bridges, the reliable information about the health of the structures can be provided quickly to the concerned team for taking precautionary measures.

In the case of SHM, there will be a large, saved data available for each monitoring parameter of the structure. From the analysis of this data, the health of the structure can be predicted for any later stage by introducing Artificial intelligence.