Structural vibration

[edit] Introduction

The design of modern lightweight structures and buildings in busy and congested cities requires careful consideration of the effects of vibration due to people, traffic and wind.

More than ever before, it is important for civil and structural engineers to manage structural vibration such that occupant comfort, structural durability and performance of specialist equipment can be maintained. Indeed, vibration-related criteria are increasingly found to govern structural design.

In August 2016, the Institution of Civil Engineers (ICE) has therefore published a themed issue (169 SB8) of its Structures and Buildings journal on the latest research and practice related to reducing unwanted structural vibrations.

[edit] Bouncy people

Footfall-induced floor vibration is one of the more commonly encountered sources of structural vibration, particularly where modern lightweight floor structures are used. Zhang et al present the result of an extensive experimental study of lightweight floors constructed using metal-web timber joists to improve vibration performance.

In the design of stadia and public assembly buildings, engineers need to make a robust assessment of the dynamic performance of the structure under crowd loading. With reference to a series of experimental studies conducted at the University of Bath, UK, Browning challenges the Institution of Structural Engineers' 2008 dynamic design criteria and proposes a new approach to specifying stadium vibration requirements. These YouTube videos show Frankfurt stadium here, and here. Concrete can flex and bend quite a bit before cracking.

The design of lightweight pedestrian bridges is also often governed by the performance under crowd loading. Brownjohn et al experimentally investigate the dynamic performance of The Helix bridge in Singapore (pictured top), which unusually also serves as a viewing platform. The London Millenium bridge in aluminium famously vibrated due to resonance wave produced by people walking, it was closed for 2 years from its opening day for repairs.

[edit] Shaky ground

As cities densify and transport networks expand, the problem of rail-induced ground-borne vibration is becoming increasingly significant for new and existing developments. Gjelstrup et al. have developed a novel probabilistic approach to the assessment of structural vibration and structure-borne noise in buildings close to railways.

The provision of base isolation may be used to mitigate the effect of ground-born vibration and re-radiated noise on building occupants. Talbot has undertaken a review of current practice in the design of base isolation and highlights challenges that need to be addressed to move towards a performance-based design approach.

Medical and research facilities containing precision equipment are particularly sensitive to the effects of ground-borne vibration. Brownjohn et al present a case study of the Orion laser facility at the UK Atomic Weapon Establishment, detailing the experimental and numerical studies undertaken to guide its design and to assess performance prior to and during construction.

Damping and base isolation can also be used to mitigate the effects of seismic events on buildings. Kasinos et al investigate the response of building subsystems subject to seismic vibration events and present a new methodology giving improved predictions for irregular structures.

Jonathan Glancy's book Lost buildings talks of the old Baltic Exchange, shaking due to an IRA terrorist bomb outside, which made the traders scared so they had to move to a new building. Fosters Gherkin was build in its place following demolition.

[edit] Buffeting breeze

In taller buildings, wind-induced lateral vibration frequently governs the serviceability performance of the structure. Criteria used to set lateral acceleration limits are typically based on the concept of avoiding occupant complaint.

However, Lamb et al present new multidisciplinary research that suggests significant vibration serviceability issues can arise at a level of vibration well below that which might give rise to complaint, and indeed at a level that may not even be perceptible to occupants.

Where the level of lateral vibration in a tall building has been identified as excessive, there is often scope for designers to control acceleration levels by introducing additional structural damping.

For many buildings, the use of a tuned liquid column damper is an efficient solution. Cammelli and Li present a case study demonstrating the use of experimental and numerical studies to guide design of a large liquid damper in a high-rise structure on the US east coast.

In conclusion, the breadth of papers on the issue shows the level of interest in the dynamic performance of structures and its relevance to civil and structural engineers. As buildings and structures become lighter, urban transport networks expand and brownfield development is pursued, the ability of practising engineers to design out unwanted vibration will become ever more important to satisfy the needs and expectations of society.

This article was originally created by ICE based on 'Engineering out bad vibes'. The original article was written by John Ward and published on 1 Aug 2016.

--The Institution of Civil Engineers

[edit] Find out more

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• Vibrations.