Jun 17, 2021

Why engineers must always consider human-induced vibration

Vibrations
Engineering
design
Structuralintegrity
Dominic Ellis
3 min
Human-induced vibration can lead to a number of effects upon the structure and its users

Human induced vibration, or more accurately vibrations caused by human footfall, often conjures images of Millennium Bridge-style swaying or collapsing buildings.

But in reality, the ‘damage’ caused by human-induced vibrations is less likely to ruin a structure and more likely to cause discomfort in people. Though not as dramatic as a structural failure, any good engineer wants to make sure the people using their structures, be it bridges or buildings or anything in between, can do so safely and comfortably. This is why human-induced vibration must be considered within the design process.

Resonance v Impulse

There are two ways that human-induced vibrations affect structures: resonant, and impulse or transient response. Put simply, resonance occurs when Object A vibrates at the same natural frequency as Object B.

Object B resonates and begins to vibrate too. Think singing to break a wine glass! Although the person singing isn’t touching the glass, the vibrations of their voice are resonating with the glass’s natural frequency, causing this vibration to get stronger and stronger and eventually, break the glass. In the case of a structure, resonance occurs when the pedestrian’s feet land in time with the vibration.

On the other hand, impulse or transient vibration responses can be a problem on structures where its natural frequencies are too high for resonance to occur, such as where the structure is light or stiff. Here the discomfort is caused by the initial “bounce” of the structure caused by the footstep and is a concern on light or stiff structures.

Engineers must, of course, design to reduce the vibration effects caused by either impulse or resonance.

Potential impacts from human induced vibration

Human induced vibration can lead to a number of effects upon the structure and its users. These include:

  • Interfering with sensitive equipment Depending on the building’s purpose, what it houses can be affected by the vibrations of people using the building. Universities and laboratories, for example, may have sensitive equipment whose accuracy and performance could be damaged by vibrations. Even in ordinary offices the footfall vibration can wobble computer screens, upsetting the workers.
     
  • Swaying bridges One of the most famous examples of human-induced resonance impacting a structure occurred with the Millennium Bridge. As people walked across the bridge, the footsteps caused the bridge to sway, and everybody had to walk in time with the sway because it was difficult not to. Thankfully, this feedback can only occur with horizontal vibrations so building floors are safe from it, but footbridges need careful checking to prevent it.
     
  • Human discomfort According to research, vibrations in buildings and structures can cause depression and even motion sickness in inhabitants. Tall buildings sway in the wind and footsteps can be felt, even subconsciously by the occupants. It has been argued that modern efficient designs featuring thinner floor slabs and wider spacing in column design mean that these new builds are not as effective at dampening vibrations as older buildings are.
     
  • Jeopardising structural integrity The build-up of constant vibrations on a structure can, eventually, lead to structural integrity being compromised. A worse-case scenario would be the complete collapse of the structure and is the reason some bridges insist that marching troops break step before crossing. Crowds jumping in time to music or in response to a goal in a stadium are also dynamic loads that might damage an under-designed structure.

How to avoid it

As mentioned, modern designs that favour thinner slabs and wider column spacing are particularly susceptible to all forms of vibration, human-induced or otherwise, but short spans can also suffer due to their low mass. Using sophisticated structural engineering software is an effective method for engineers to test for and mitigate footfall and other vibrations at the design stage.

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Jul 26, 2021

Collaborate to avoid commercial risks and systemic failures

construction
collaboration
COVID19
Leadership
Dominic Ellis
3 min
Construction Leadership Council publishes guidance on NEC4 contracts and wants 'pingdemic' date to come forward to prevent industry grinding to a halt

Clients should work collaboratively to minimise commercial risks and avoid inappropriate risk transfer as this will not only lead to a negative outcome on individual contracts but also systemic failure in a fragile market, the Construction Leadership Council has warned.

The CLC, in collaboration with NEC, has today published joint guidance to industry and clients on dealing with and accommodating the impact of Covid-19 on work under NEC4 contracts.

The guidance adds to the suite of outputs from the CLC’s Business Models: Contractual Best Practice group which has routinely called for strategic collaboration between clients and the supply chain to avoid systemic market failures and compromised project delivery.

The guidance focuses on the NEC4 Engineering and Construction Contract (ECC), although it can also be applied to the NEC4 Engineering and Construction Subcontract (ECS), NEC3 ECC and ECS, subject to some amends which are outlined within the guidance.

To help clients and the supply chain to collaborate, the joint guidance offers support in navigating a number of circumstances within the context of the Covid-19 pandemic, including: Act of prevention; Project Manager’s instructions; Compensation events; Evaluation of a COVID-19 Related Compensation Event; Working Areas; Resource Utilisation; and Dealing with risk on future contracts.

Steve Bratt, Chair of the CLC’s Business Models Workstream said this guidance was developed in response to a series of questions which were raised with regard to projects impacted by Covid-19 operating under NEC contracts.

“As industry continues to manage the challenges of Covid-19, we are becoming increasingly concerned that many outstanding disputes remain unresolved and much uncertainty exists with regard to future contracts," he said. "We are therefore keen to do all we can to ensure clients work with their supply chains to fairly and collaboratively manage the commercial risks caused by Covid-19. Safety is paramount, but collaborative risk sharing will ensure secure project delivery and a long-term sustainable industry.”

Covid-19, safe working procedures and wider disruption has presented all parties with unquantifiable and unmanageable risks and costs, he added. Traditional behaviours such as inappropriate risk transfer will not only lead to a negative outcome on individual contracts but will almost certainly lead to systemic failure in a fragile market seeking to build back greener and better.

Peter Higgins, Chairman of the NEC4 Contract Board said NEC is pleased to have worked with the Construction Leadership Council in preparing this advice on dealing with covid-related issues under NEC contracts. "NEC has always been a contract focusing on the parties working together to achieve a successful contract, and this guidance will help in managing collaboratively the risks which have arisen from COVID-19," he said.

Industry leaders have called for acceleration of rules relaxing requirements for COVID-19 self-isolation for double-vaccinated workers. Currently the rules will only be relaxed on August 16.

CLC co-chair Andy Mitchell said it has received reports from across the industry of plants, sites and offices having to wind down activities as staff have been asked to isolate.

"This is putting very significant pressure on the sector, risking project delivery and even the viability of some firms. Where staff are already fully vaccinated, and recognising that such people will be free to work from 16 August anyway, we are asking the Government to bring forward this date for essential industries like construction, ensuring that the industry doesn’t grind to a halt."

An RICS survey of the global construction sector found over 40% of professionals reporting an increase in disputes since the onset of the COVID-19 crisis. By contrast, fewer than 3% of respondents noted a fall in disputes over the same time, suggesting that the pandemic is exerting further pressure on an already stressed industry.

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