It is believed that angels use bridges and floating walkways in the heavenly space to move around. This mythical imagination of sorts have descended into many forms and shapes in the human endeavour to connect between people, places and beyond.

Bridges of various forms and utilities were in use as early as in the 13th century, the Neolithic people built boardwalk bridges across marshland for commuting between patches of lands to find their livelihood.

The19th century witnessed a notable change in the design, construction and usage of multiple materials for bridge construction. The change was driven by industrialisation and need for longer, stronger and enduring infrastructure to last for extended periods. The use of Steel and RCC became the default standard for construction combined with a variety of design novelties.

Since most bridges are for human usage in some form or the other, the design considerations shall take into account both the static and dynamic loads that the structures has to undergo.

Typical human foot fall induced forces and vibration

The integration of modern day architecture combined with acute focus on aesthetics and visual appeal have lead to considering many new materials and highly optimum designs for bridges, skywalks and other similar applications. However, this in many instances has lead to the following consequences.

- Structural Instability and integrity related issues.

- Discomfort for Human usage, motion sickness caused by oscillations

- Structural failures and consequential collapse

It is important to look at the root cause/s of such resulting phenomenon:

While most of the structural design considerations would have taken into account the direct static load + the factor of safety that the bridge has to bear during its operational usage, the dynamic load consideration are either not accounted for correctly or, are hypothetically assumed.

The term "dynamic" typically implies to the repetitive and / or the cyclic load that the structure goes through due to the input forces acting on it by its users, such as walking, vehicular traffic or the natural excitations like wind loads.

Unlike the static load which is typically defined by the effective mass over area, the dynamic load essentially adds few more dimensions to the way the structure responds, these are:

- Forcing loads (wind, human, vehicular) and their frequencies

- Structural response in terms of acceleration, velocity and displacement

- Natural frequency of the structure, inherent structural damping

Typical vibration assessment scenario on a bridge

Each of the above has its specific contribution to the behaviour of the bridge in its designated usage - all going back to the basics of mass, viscous damping, stiffness and natural frequency / resonance in the system.

The typical natural frequency of the bridge in consideration in its predominant degree of freedom plays a critical role is the stability, operational usability and overall endurance of the structure. Most large civil structures possess natural frequencies of lower order (ranging from 2Hz to 15Hz) and when excited with forces from various inputs such as foot fall, vehicular movement or natural excitations, tend to set the bridge into a resonant condition that makes the structure to go through unstable condition based on the mass, stiffness and the damping characteristics.

Compounding the problem of natural frequency is the mode shape or the typical physical deformation shape of structural components during their movements. Depending on various factors of construction configuration, the operational part of the structure attains shapes of bending, skew and torsional modes; each with varying severity of structural instability and/or its failure.

Typical bride modes and operational deflections

Historically, there are multiple instances wherein both the newly commissioned and some of the in-use bridges have shown attributes of instability, resulting in either abandoning their usage or failure of their structural components in parts or full. The incidence of Tacoma Bridge is one such well known reference of such a behaviour and consequent failure.

 

Tacoma bridge in one of its critical modes, bridge failure due to structural resonance and aero elasticity

As indicated earlier, the modern day design architecture and its over optimisation, preference to visual appeal over the operational considerations do lead to such situations. There are even instances of industrial walk bridges along the conveyer belt lines that have gotten into structural instability due to constrained design inputs combined with space limitations.

NV Dynamics has and is working on many of such 'post mortem' scenarios; from assessing the performance of skywalks in high rise buildings to industrial worker walk bridges to boardwalks in IT parks, we have developed an adaptive approach to include and assess all variables, system responses and to arrive at highly viable solutions plans.

The cumulative knowledge and expertise is being back integrated to design process by working with civil structural engineers and architects, who are actively considering the factors of dynamic loads as a mandatory parameter to be included for effective performance of the bridges and structures of similar usage.