(Above: Satellite image of Hurricane Alicia)
Robert Halvorson, winner of our 2016 Gold Medal, has worked on tall building projects all over the world. Here he describes how and why he came to run structural tests on a skyscraper - during a category three hurricane.
When designing a tall building the challenging part is providing the lateral stiffness and strength in the structure necessary to resist lateral loads – the forces put on the building by high winds and seismic events.
When we were working on the Allied Bank Plaza (now known as Wells Fargo Plaza) building in Houston, Texas, this was particularly relevant, as the US Gulf Coast area is prone to high hurricane winds. Our team worked closely with the University of Western Ontario to study the building design in a wind tunnel, but we were acutely aware that there are a lot of assumptions that go into such testing – so we decided to conduct our own tests on site to back up the wind tunnel findings.
So it was that a colleague and I hired accelerometers and a fancy tape recorder to place on top of the building and take measurements. All we needed was a storm, and we got one soon enough – Hurricane Alicia. I borrowed my wife’s car and headed downtown as the storm rolled in. The entrances to the tower carpark were sandbagged to preclude flooding, so we had to park a distance away and walk through the storm to the building, where the building engineer took us up in an elevator.
The storm was really picking up, and already we could feel the sway of the building in the elevator – the building engineer’s eyes grew wider and wider as we ascended, especially when we reached the 70th floor, where we could really feel the whole structure moving. The building engineer left us as quickly as he could and headed downstairs to safety.
Still, I found it fascinating to be up there during such a powerful storm, which saw the building reach a peak acceleration of 44 milli-g, well beyond the normal storm criteria of about 20 milli-g – in other words it was being subjected to loads twice as bad as you would expect from the worst event in an average decade. It meant we really got to test the building under the most trying conditions, and feel the way it was reacting.
It was quite an experience. I could actually see the floor in front of me moving up and down, and sense the building twisting, as the building deflected – it was like being on a cross channel ferry during a storm.
After the storm we headed back down, pleased with the experience – but unfortunately found that my wife’s car had been badly damaged by flying debris. I had to drive the wrecked vehicle home, and naturally she was less than pleased with my test results!
Shortly afterwards we compared our findings with those of the University wind tunnel, and were greatly impressed to see that the wind tunnel results were very accurate: this was one of the first, if not the first, times the technology had been tested and confirmed in a real building during such high wind speeds, so it was a really useful experiment and a great example of how truly exciting and hands-on structural engineering can be!
Structural engineering is a great career choice for anyone who gets a kick out of this kind of experience: to be inside a structure you’ve designed, learning first-hand how your work performs under the most challenging conditions – there really is no feeling like it.