When we consider glass, we often think of its fragility – so the idea of walking across glass bridges is, for many of us, just a little scary. After all, a bridge in China’s Jilin province recently shattered due to high winds, leaving a tourist uncomfortably stranded 100 metres above the ground.
Regardless, glass is widely used in construction because of its unique properties. In reality, the type of glass used in structures is very thick and sturdy.
“Glass is a very strong material. The kinds of glass used in structural applications is very carefully manufactured,” says Dr Colin Caprani, a senior lecturer in civil engineering at Monash University.
“Tempered glass and laminated glass panels are used. A little bit like windscreens but bigger, and kind of thicker and layered.”
Despite being strong, glass does break – just look at the Jilin Bridge – but what sets it apart from other bridges is the way it breaks.
“There’s really two ways a material behaves,” says Caprani. “If you think of a plastic fork or a plastic spoon and you bend it, it doesn’t shatter, it kind of bends a lot. That’s called a ductile failure.
“It means that the material has been separated, but it has deformed so grossly so largely that its very obvious. It has been overstressed, and the other type of failure is what we’re familiar with.”
But glass is a brittle material, which means it doesn’t bend and malform in the same way – instead, everything might look fine until it completely shatters. This can be dangerous because there is no warning of collapse.
“That’s why people will probably find it surprising that glass is used in bridges,” says Caprani. “And there are bridges that are wholly glass [with] no steel in them at all.”
The glass used in these bridges is the same type of structural glass used in buildings, staircases, balconies and balustrades, and it can be incredibly thick, depending on its purpose. Designing the glass is still a relatively niche area of expertise.
“It’s still quite a specialist activity in structural engineering, but it’s increasingly used by architects,” says Caprani.
The brittleness of glass means that a glass bridge has to be treated slightly differently to steel bridges that can bend under weather conditions.
“With structural glass, we have to be careful with [glass panel] connections, and how it’s connected to a steel member or a concrete member that’s near it,” says Caprani.
“We try and avoid what are called stress concentrations, so we’ll have connections that are padded to allow for some movement. We don’t want to have a rigid connection for the glass because it can precipitate shattering, so the connections with structural glass need to be held [in a way] that allows a little bit of movement at the connection point.”
So even though the glass itself is rigid, the entire bridge still has movement and flexibility built into it, because it is built of separate pieces, often padded with something like neoprene.
“Our structural engineering spidey senses would be tingling far more when it comes to structural glass because it’s so sensitive to these very localised kinds of pinching, which is what a stress concentration is,” explains Caprani.
“We really are super careful to try and avoid those stress concentrations in structural glass.
“They [might have an] anticipated movement of five or ten millimetres [at these points], which cumulatively over a long bridge would allow the bridge to sway quite a lot, you know, hundreds of millimetres in the lateral direction”.
But, as is the case with all bridges, things can happen that make the materials break if it gets pushed beyond this point – although it isn’t very common.
“Sometimes it goes wrong, the Millennium Bridge in London is a good example of when vibration began to take place, it was so large that it was very apparent what was happening,” says Caprani.
The Millennium Bridge’s problem was apparent on its opening day, 10 June 2000. Its movements were caused by a phenomenon called synchronous lateral excitation. It was described in a 2005 article in Nature: “Soon after the crowd streamed on to London’s Millennium Bridge on the day it opened, the bridge started to sway from side to side: many pedestrians fell spontaneously into step with the bridge’s vibrations, inadvertently amplifying them.”
In the case of the Jilin Bridge, its integrity was put to the test because of extremely high winds.
“If you’ve ever tried to carry a big sheet of wood or something in the wind, you know how strong wind is,” says Caprani.
“When wind comes along [it can] start deforming a bridge. The bridge will deform to take the load offered by the wind and, you know, that’s normal.
“If the way the glass was connected to the steel structure didn’t allow for enough movement and flexibility between the glass and the structure that was deforming – that’s when you get to this place where the glass is being put under a stress concentration that it wasn’t designed for, and then can shatter.
“So, my take on what occurred [at Jilin] was just that: in high winds, the bridge was deforming more than had been allowed for in the connection between the glass and the steel bridge.”
So why even build bridges out of glass?
“Being pretty – that’s going to be the main [reason] by far,” says Caprani, with a laugh. “The extra caution that comes along with using glass will be overshadowed by its other benefits.”
Perhaps surprisingly, glass bridges are relatively low maintenance, although they are annoying to keep clean.
“Property durability is probably one of the main benefits,” says Caprani “[Glass is an] extremely durable material, it’s not going to corrode, it’s not going to discolour and so on. You just have to get out there and clean it.
“Another great benefit is that glass is incredibly strong as well, believe it or not – as long as it’s not brought to that point where it can it can shatter.
“And, you know, for the aesthetic, you’re paying the price of having this very careful and cautious detailing around where the glass connects to the rest of the structure.
“[You] can enjoy a barbecue on a pergola completely made of glass.”
Now that’s some classy glass.