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Thermal Mass

Thermal Mass

As the importance of sustainability has increased, attention has focused on ways that a design can reduce a building’s carbon emissions. For framing and flooring systems, the choice between steel and concrete solutions has provided limited opportunities to influence operational carbon. There is, however, one exception – thermal mass.

Thermal mass acts as a heat sink, tempering the internal environment by reducing and delaying the onset of peak temperatures.

Benefits of thermal mass:

Keeps the building cooler in summer and warmer in winter
Helps to avoid the use of air conditioning, leading to significant reductions in carbon emissions.

How much mass is required for the best result?

Research has proven that, for a 24-hour cycle of heating and cooling, only 100mm of mass is needed to absorb excess heat – providing additional concrete mass over 100mm will not increase the amount of excess heat that the floor is able to absorb. It may, however, result in larger foundations and additional building costs.

Composite floors are generally 130-150mm thick with 70-90mm of concrete above the ribs; precast concrete units, on the other hand, are generally 200-250mm thick. As such, composite flooring solutions provide optimum thermal mass for lightweight construction. Clearly, the choice of structural system will make a difference to the weight of the floor, the size of foundations and the overall building costs.

FLOOR VIBRATION

In recent years there has been increased demand for office buildings that are fast to construct, have large uninterrupted floor spans and offer flexibility in their final use. This, along with the obvious benefits of lighter superstructures in a seismically active country, has made the dynamic performance of floors an important consideration in building design.

While there is a subjective element to perceptions of flooring vibration induced by human activity, there are now internationally recognised performance criteria included in standards.

Until recently a lack of robust methods for assessing the vibration performance of floors has made it difficult to accurately predict their dynamic performance. A new method – based on extensive European research that includes both in situ measurements of the vibration performance of a number of buildings and computer simulations – is now available.

Design of Floor for Vibration: A New Approach, developed by the UK’s Steel Construction Institute (SCI), uses a finite element approach to determine the accelerations at various points on the floor due to walking activity. These accelerations are then compared with the limits cited in the international standard ISO 10137 to determine if the response of the entire floor is acceptable. Software is commercially available to implement this procedure; it produces contour plots of floor response to allow designers to easily identify areas of the floor with excessive vibration. Designers can easily modify the structure locally to reduce the floor vibration to acceptable limits.

This approach has been used in the UK since 2004, including some very exacting applications. Previously there was a perception that only very heavy floors could guarantee adequate vibration performance of suspended floors. In the UK this perception was limiting the use of structural steel-concrete floor solutions in vibration-sensitive areas such as hospital operating theatres and test laboratories. With the advent of a reliable design method, structural steel now enjoys a high market share in the health sector.

In New Zealand, SCNZ has assisted a number of consulting engineers to assess the vibration performance of local projects using the SCI’s methodology (the Finite Element Analysis package) and Oasys GSA Footfall analysis.

LOW-DAMAGE SOLUTIONS

Exciting new technologies will not only improve the seismic performance of steel construction, they will enable structural elements damaged in a severe earthquake to be easily and affordably replaced.

The Canterbury earthquakes highlighted the importance of seismically resilient building construction. Steel-framed buildings, on the whole, bore the earthquakes very well, even though the shaking was significantly greater than the design level – they not only satisfied their mandate to protect lives, but were also back in service shortly after the earthquakes. The HSBC Tower is an exemplar of this.

Engineers learned a great deal from the performance of these structural steel buildings, and continue to improve their designs to ensure new buildings are not only safer for their occupants, but also avoid severe and expensive damage.

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