Creating a whirlwind of new knowledge about extreme wind events and their effects on built structures
Extreme wind events such as tornados and thunderstorms commonly hit the headlines. Also newsworthy is the cutting-edge scientific research going on behind the scenes to make buildings more robust in the face of twisters and violent wind bursts.
Dr Mike Jesson, a Research Fellow in the School of Civil Engineering, is part of Birmingham’s wind engineering research team – thought to be the UK’s leading research team in this subject – which conducts a range of groundbreaking studies to address the many and complex challenges extreme wind events pose for engineers.
One of these is a novel set of experiments examining the effects of wild winds on buildings, by simulating thunderstorm downbursts in a lab and measuring their effects on building models of different heights– the first time this has been done .
The three-year, EPSRC-funded project has thrown up important results that Mike and his colleagues hope will lead eventually to changes in European and US building design codes.
These results are included in a paper entitled ‘Aerodynamic forces on the roofs of low-, mid- and high-rise buildings subject to transient winds’ that was recently published in the peer-reviewed Journal of Wind Engineering and Industrial Aerodynamics.
‘When you design a building, structurally it has to be sound, and wind loading is one of the forces on the building that it needs to withstand,’ explains Mike, the paper’s lead author. ‘Various design codes cover this, but most are designed around what are called boundary layer or synoptic winds, which occur due to large-scale weather systems such as the high and low pressure regions seen on weather maps, and cover hundreds or thousands of miles and last hours or days.’
But there are also non-synoptic winds, the most extreme example being tornados, but which also include thunderstorm downbursts (downward-rushing currents of air that can be caused by thunderstorms). Although small-scale and transient – affecting areas perhaps just 10km across and lasting only a few minutes – these weather events can see wind speeds of up to 130mph. What’s more, unlike synoptic winds, downburst winds reach a maximum close to the ground, due to the combination of outflow from where the downward air meets the ground, and a a ring vortex which is formed by the flow.
This means non-synoptic or transient winds pose a particular threat to built structures; they are the cause of design-load wind speeds in many countries.
‘So potentially you have very different forces on a building than you would normally see,’ explains Mike. ‘There have been efforts made to change design codes, but little more has happened than scaling factors being included in tornado zones.’
Although extreme wind events are becoming more prevalent, thunderstorm downbursts are still relatively rare and unpredictable. This makes it very difficult to measure the wind loading on a building in a full-scale such weather event – still more difficult to compile a series of data. Yet an understanding of the loading experienced by buildings during a downburst is important to allow the construction of well-designed and engineered buildings. Hence the necessity of simulated laboratory experiments.
Mike and his team designed the University of Birmingham Transient Wind Simulator to mimic a downburst striking buildings of different heights. Using a 1m diameter impinging jet with aperture control, fans suck in the air, which builds up the pressure behind closed flaps. The flaps then open, causing a rapid acceleration of the air.
‘No one has done this in a lab before,’ says Mike. ‘In order to get a ring vortex, you have to have transient flow, and research [to measure loading on buildings] carried out previously hasn’t simulated the vortex.’
Two forms of building were used – a square-plan, flat-roofed structure, and a rectangular, portal-frame – at three angles (0°, 45° and 90°) relative to the radial wind direction. The model buildings were built on to a platform, which was raised or lowered to simulate buildings of different heights.
We discovered new things from these experiments, such as differences between the two different types of building, and confirmed things we thought we knew, such as that very low buildings have different pressure fields than high buildings.
By rotating the buildings, the researchers were also able to learn more about what happens when winds hit from different directions.
‘We rotated the building so you had wind coming from the front face and also from the corner. With a cornering wind, a synoptic wind causes suction on the roof and loads forces trying to lift the roof away. What we saw with a thunderstorm downburst wind was a small vortex appearing, which was something we hadn’t entirely expected.’
Pressure transducers converted the pressure changes – sampled at a rate of 500 times a second – into digital signals that could be recorded and computer-analysed. In this, Mike’s software skills – he spent ten years in industry as a software engineer – have proved invaluable.
‘It’s been a three-year project, 18 months of which has been spent just getting the data,’ he explains. ‘For every measurement, you have to repeat it ten times in order to get a generic effect and then average it out.'
We’ve concluded that the flow field and forces on the building are different with the different winds. The next step of the project is to take the data and work on how we can modify design codes. Those modifications might, in some cases, allow buildings to be less robust. ‘It costs a lot of money to build structures, and it might be that you don’t have to built them as strongly.
Mike adds: ‘At the end of the day, you’re never going to design a building that can withstand anything, so it’s about making judgements that are reasonable and rational. This isn’t the end of this kind of research; it’s a stepping stone on the way to making informed changes.’