Construction World July 2015

St Mary Axe, better known by its nickname Gherkin, in London.

‘height-to-base ratio’, being the height of the building divided by its narrowest plan dimension. Essentially, higher height-to-base ratios and lower natural frequencies increase the dynamic component of the response to wind. Wind engineering of high rise buildings Wind loads affecting the design and construction of high-rise build- ings are intrinsically dynamic and random in nature (in both time and space). Wind speed can be described as a mean value upon which random fluctuations or gusts are superimposed. The wind loads arising from the mean and gust wind speeds are called Mean and Background components, respectively. For slender tall buildings, there is a third component of wind load namely the resonant component that dominates the structural behav- iour. The resonant wind load is the result of the fluctuating frequency of wind effects matching the natural frequency of the building structure. When designing a building, the mean, background, and resonant wind loads need to be considered: • Mean wind load : Steady drag and lift forces due to dynamic pressures arising from the mean wind speed. • Background wind load : Non-steady drag/lift forces to due additional dynamic pressures arising from gust wind speed. • Resonant wind load : Transient inertial forces generated by the dynamic responses of the structure to wind (both direct and interference effects), which is negligible for low-rise buildings but dominant in tall buildings. Wind has always been an important consideration when erecting tall buildings and it becomes more important and complex as the height increases. Seismic engineering of high rise buildings The prime objective of seismic design is clearly to provide life safety. The common practice to achieve economic and safe design is to dissi- pate seismic energy in the structure during an earthquake by forming controlled and stable ‘damages’ in the structure (in the so called ‘plastic hinges’). To ensure that damage is distributed rather uniformly among floors and that the gravity load path is not compromised, engineers often use what is called a ‘strong column/weak beam’ design philosophy, which stipulates that the columns of a joint needs to be at least 20% stronger than the beams framing the same joint. While this philosophy provides life safety, the implication of widespread plastic hinges is extensive damage throughout the structure to the extent that the building might be damaged beyond repair due to an earthquake. Christchurch’s central business district saw the devastating impact that an earthquake can have on a city after the 6.3 magnitude earthquake of 22 February 2011 led to the death of 183 people in New Zealand’s second-largest city. The heavy financial cost of the Christchurch earthquake demon- strated the value that a ‘low-damage’ design philosophy could have offered. As Aurecon, we are a world leader in low damage design solutions for buildings, aimed at reducing primary structural damage in bracing systems. By concentrating and dissipating seismic energy in predefined parts of the building, a low damage design philosophy creates a more resilient system that increases post event operability for owners and

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Structural material systems for high-rise buildings should be chosen by carefully considering architectural, economical and site factors. Preferences and the economic viability of the different structural materials that are used in tall buildings’ construction are also changing. In 1970, 90% of the 100 world tallest buildings were all-steel build- ings. Today, all-steel buildings account for less than 15% in favour of concrete or composite structures. Driving economic design in the construction of high-rise buildings isn’t the same all over the world. What is cost-effective in one country won’t necessarily be cost-effective half way around the world. Mate- rials, labour cost, the value of time and the value of space all need to be carefully weighed in order to drive economic design. ‘Tall behaviour’ From a structural engineering point of view, as high-rise buildings get taller and more slender, their design becomes increasingly (and funda- mentally) influenced by specific behavioural factors that are much less significant for shorter buildings. These factors include the dynamic response of tall buildings to wind loads both in the ultimate and serviceability limit states, and the differential axial shortenings of the vertical elements of tall buildings under gravity load effects. As far as these factors are concerned, the absolute height of the building is not necessarily the best measure for ‘tall behaviour’. In particular, the magnitude of the dynamic wind response is more significantly influenced by the overall slenderness of the building and the natural frequencies of its fundamental modes than its absolute height. The overall slenderness of a tall building is usually defined by its

CONSTRUCTION WORLD JULY 2015