Balfour Beatty Awarded £25 million London Primary Schools Contract
Balfour Beatty has been awarded a £25 million contract to extend and upgrade six primary schools for the London Borough of Merton as part of the £1 billion, four-year iESE (Improvement and Efficiency South East).
The contract includes a £5.1 million three-storey extension to Pelham Primary School; a £4.4 million new school hall, main entrance and teaching block at Hillcross primary school; a £4 million extension, including a new hall and teaching block, at Poplar Primary School; a £2.7 million teaching block, library extension and link at St Mary’s Catholic Primary School; a £3.9 million new teaching block at Singlegate Primary School and a new £1.6 million two-storeyteaching block at Merton Abbey School.
The six-school contract is the latest to result from Balfour Beatty’s appointment to the iESE framework in 2011, through which the company has been awarded in excess of £100 million worth of work, including two recently completed primary schools in Winchester in Hampshire.
Much of the work is on school buildings dating back to the 1930s and will incorporate updates to the latest energy efficiency standards, including innovations such as photovoltaic panels and low energy lighting.
Nick Kent, Balfour Beatty iESE Senior Contract Manager, said: “These six school projects for Merton Council further strengthen our position as a leading construction partner in the education sector and build on our good working relationship with the London Borough of Merton.”
The schools projects are scheduled for completion in September 2015.
Balfour’s framework portfolio currently includes the £4 billion Education Funding Agency Framework, the £400 million National Capital Works Framework and the £250 million Regional Defence Framework for Capital Works Projects for East Midlands and Eastern England for the Defence Infrastructure Organisation (DIO).
Why engineers must always consider human-induced vibration
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.