Movers and shakers

The possibility of human footfall loading leading to excessive vibration of structures has long been recognised. Soldiers marching across a cast iron bridge in 1831 generated vibrations that caused the bridge to collapse

The introduction of lightweight long span composite construction and open plan offices in North America in the 1960’s led to concerns not over safety, but that normal walking caused uncomfortable vibrations for occupants of the buildings.

There are numerous market forces causing clients to insist on floor vibration checks:

- Hospitals - operating theatres require the utmost stability for delicate operations, while night wards are nearly as onerous
- Laboratories - equipment, such as optical and electron microscopes to laser research systems, are very sensitive to vibrations. Such floors must comply with the BBN or Ashrae standards
- Airports - Airport owners require maximum response values for the passenger waiting areas as floor vibrations can upset nervous travellers
- Retail - many major retailers require a maximum liveliness for their display floors, such as where they are displaying glasses on glass shelves: if the floor is too lively then the glasses will rattle
- Commercial - on lively floors, computer users complain because their screens wobble, making it difficult to work

The vibration problem
For many years now, serviceability requirements have been a part of structural design. Initially, these were just deflection limits to prevent finishes from cracking and building occupants noticing the floors sagging. These proved adequate for decades, until advances began to be made into more efficient, lighter structures, such as composite beam or post-tensioned slab floors. Unfortunately, the users of these buildings found that the floors could be rather lively.
Industry experts recognised that what was needed was not a measure of the floor frequency, but of how much the floor responds to the footsteps of a person walking over it: a footfall response calculation.

Measuring vibration
There are a number of sources of vibration, or rather force excitation, in structures. Examples include foot falls from walking individuals and groups, rhythmic people inputs, such as dancing and aerobics, machinery, transportation (roads and railways – sometimes integrated with the structure) and temporary inputs such as from demolition and construction.

The three general characteristics of the way the structure responds to the force inputs are Impulsive or Transient Response, Resonant Response, and Radiation as an audible sound.

Design criteria generally considers human comfort (perceptible or tactile vibration or noise), interference with equipment or processes, and structural damage.

Oasys GSA footfall
For many years, GSA has been one of the leading packages for structural analysis. Developed by Oasys to meet the demanding and diverse requirements of Arup, one of the world's leading firms of international consulting engineers, GSA's capabilities are proven on thousands of complex and prestigious projects worldwide.

GSA Footfall is for structural engineers who need to accurately predict the response of a structure to momentary or vibrational loading. It is a finite element analysis program that provides the ability to analyse any structure for footfall response, whether steel or concrete, bridge, floor or stair.

GSA Footfall analyses structures to the Arup method (as adopted by The Concrete Centre), The Steel Construction Institute specification, and more. Not only that, but because it analyses which floor areas have high and low responses; it enables consultants to position sensitive equipment and services or to improve problem areas in a cost-effective way.

GSA analysis enables you to locate regions of high and low response to enable sensible locations of sensitive equipment or activities and to assess localised modifications to floor structures for sensitive equipment or activities to minimise cost to the client.

For more information and to download a free trial of GSA, go to:
www.oasys-software.com/GSA