Now that we are facing the reality of tall buildings we will have to employ special engineering techniques to stop them wobbling and to bring services up to the top, writes Emma Cullinan.

In the rising pressure to build taller in Ireland the debate has surrounded the new look that our cities will take on. Now that some tall buildings have been given the go-ahead we are facing this reality. While high rise is a new concept for us, it is also a new challenge for designers and engineers in this country.

With the globalisation of expertise - with many architects and engineers having worked abroad and companies such as Arup and Buro Happold spanning the globe - such knowledge is readily available in Ireland, it just hasn't been put to use much here.

Also, while we are looking at building tall, the towers that we are seeking planning permission for - at an average of 30 storeys - would be dwarfed in cities such as New York, where the Empire State has 102 storeys, and Kuala Lumpur, whose Petronas towers rise to 88 storeys. Liberty Hall, with 16 floors, seems a midget by comparison.

But 20 storeys seems to be a critical height, above which the provision of services and structural stability up-top takes on new dimensions.

Building taller isn't just a matter of piling on more floors, instead there is a whole school of engineering that enables us to reach for the sky.

For a start, such buildings rise to where the wind is strong enough to push them so hard that they sway about unless they are "dampened". Wind also tends to rush down tall buildings - because it's sometimes easier than going through or around them - creating lots of blustery excitement at their bases, unless the gales are designed out.

Services, such as water and lifts, need to climb higher and, ergo, faster to ensure that the needs of the people at the top of the building are served efficiently. While that may just mean the provision of a drink of water, it could also encompass the means to put out a fire or provide an escape route. Something brought sharply to mind after 9/11.

Engineer Chris Bakkala, who works in engineering firm Buro Happold's Dublin office, has grappled with such issues, both in the US and in Ireland. The firm he worked with in New York did the engineering on the Petronas towers in Malaysia designed by Pelli Clarke Pelli Architects.

When living in New York, he saw how citizens can embrace tall buildings. "Some Irish people may be afraid of tall buildings but we haven't yet had a chance to experience it in the public realm. Where I come from people take ownership of buildings such as the Chrysler. It's right, then, that planners should put the onus on developers to make those buildings right for all of us."

He also saw how tall buildings transformed New York's fortunes. "In the early 1970s New York was a depressed city. Then the World Trade Center was built and developed a vertical community of traders. Those towers created an even broader community and New York became a major international financial centre."

Buildings that taper at the top, such as the Chrysler, Empire State, Petronas, Bank of China, Hong Kong and the fondly named Gherkin in London, may be aesthetically pleasing but such shapes are also practical. In earlier days many tall buildings were rectangular stacks and, as Bakkala says, you could often ask the question: "Why stop there?" The tapered or wedge shaped buildings look more friendly, although developers may worry about the diminishing floor space at the top.

They are also far more wind-friendly. Wind takes the path of least resistance when faced with an object. Some will go around it and some will head straight down the building. If the building is tapered at the top the wind won't hit it so hard. This reduces what is scarily called the "overturning moment": the OTM becomes greater when you throw more "sail area" higher up, so tapered buildings, by reducing high sail, have greatly reduced overturning moments.

Just as airplanes are designed to work with the wind, buildings can be too. Many of the lessons learned about tall structures have been gleaned through trial and error. The effects of wind gushing down the side of tall buildings was felt keenly at the Flatiron building in New York (1902, by Daniel Burham). It was here that men used to gather on gusty days in order to catch the site of women's skirts being blown skywards. The expression "22 Skidoo" was thought to have originated here when police officers would say: "22 Skidoo guys, cross the street," to shoo them off the scene. "From 20 floors upwards, engineers talk about the pedestrian-level effect," says Bakkala.

Ways of stopping wind heading at great speeds towards the ground include putting a canopy at the base of the building to send the gust circling back upwards when it hits it.

It's also possible to slow down wind by playing with the texture of the building or adding on projecting items, such as metal bars. "These trip the wind on its own feet," says Bakkala. "If we stick something out there by way of roughness we can create little turbulent eddies on the side of the building, so as wind tries to race down the surface it gets bumped, and bumped, and bumped."

While heavy gusts of wind may be inconvenient as you pass or enter and exit a tall building, that's nothing to the fear of being in a building that sways at its apex. This can be an issue at 20 or 30 storeys, depending on a building's slenderness and its height to width ratio.

Structural engineers of tall buildings have access to a series of graphs which show how much a building will sway and, as a result, determine "occupant comfort curve accelerations". And things used to be a lot more rocky in the past, before experts learned how to dampen down the movement.

Bakkala, who is a member of an international group of experts who share information on tall buildings, says: "If you're on the top-most floor of a very tall building and a gust of wind hits it, you'll sort of feel good. You think, 'I can feel that wind but you know what? I'm safe.' You do feel the impact but it's gone quickly. But what can be a problem is resonance. This can make you feel uncomfortable because it lasts for longer and can make a person feel unsafe, even if the building is sound," says Bakkala.

Resonance - or resonant frequency - is when a structure begins to move in a natural rhythm. It's the reason why armies break step when they cross bridges, otherwise the regular footsteps can make a bridge swing more and more violently.

Engineers, then, also have to break - or dampen - the movement.

While those working in an office at the top of a tall building may not notice the natural sway if they're walking around, someone who lives high up in an apartment will be more aware of it when they lie down, relaxing with a book in the evening, and won't appreciate the feeling of being on a boat.

Ways of keeping the building from swaying beyond its "occupant comfort" are to brace it externally: both the Bank of China in Hong Kong, and the "Gherkin" have stability trusses on their perimeters.

Another method is to use large I-beams in elevator shaft walls to counter lateral loads and stabilise floor plates, says Bakkala.

Another way is to combine water storage with "dampening" (no pun intended). "Columns of water at the tops of buildings can slosh at a frequency that coincides with a resonant frequency to damp it out of a response: a bit like dancing with a partner who steps on your toes.

"This is a wonderful way of using a very simple material to make the occupant feel safe and comfortable while getting rid of harmonic resonance," says Bakkala.

Another thing learned, by mistakes, by the builders of towers was how to stop floors plates tilting. "It was found that concrete tends to shrink," says Bakkala, "and this becomes crucial when you build higher than 22 storeys. When you think of concrete you have to see it as a squishy fluid: like liquid jelly. Concrete shrinks - a process known as viscoelastic shortening - which you would never really know about in a 10-12 storey building, but when you go above 20 storeys it matters.

"In tall buildings it used to be popular to have concrete central elevator shafts with the rest of the building in steel. But what they found, in America, after a couple of false starts, is that the concrete core begins to creep while the steel perimeter doesn't. On the lower floors you wouldn't notice a thing but on the upper floors, if you put an orange on your desk, it wouldn't stay there."

Building towers also has implications for getting people and services up to the top quickly. Lifts have to climb faster, especially in office buildings where conventional elevators, stopping at every floor, will mean that some workers may take an age to get to their office.

To save on floor space, the solution (used in the Petronas towers and other super tall buildings, like the World Trade Center) is to have a number of lifts going from the ground floor, and then introducing "sky lobbies" higher up the building, where just one or two lifts will take cloud-based workers up higher. A bank of six lifts rising all the way up through the building would take up valuable floor space. Another option is to have one lift that visits all floors and an express lift that stops at just a few.

Services, such as water can also break their journey, for different reasons. If you had one water pipe running up the building then water would gush out of taps on the lower floors due to the high pressure.

Putting a break in the pipe means that users will only get say 20 floors worth of pressure, rather than 30 or 40.

While no one likes to think of fire escapes, the Twin Towers made them a poignant issue. In the Petronas towers, the sky bridge that spans between the two, will allow people to escape from one building across into the other. Bakkala also thinks that we need to challenge the tradition of not using lifts in a fire.

"Using an elevator to evacuate makes sense and we need to design for this," he says. "For instance, by providing back-up if there's an electrical failure or having lifts that will return to the ground floor and have a sign saying that they're not working when they break."

Bakkala fondly recalls stories he was told about the way that skyscrapers were built in America. "The workers would toss hot rivets across to each other and stuff them into holes 60 floors up above the streets of New York. Can you imagine the damage if one fell? It's not done like that anymore!"

A lot has been learned since then about how to build tall but it's an area of expertise that Ireland is going to have to catch up on.

"What an opportunity we have to learn from a body of international experience and develop great design solutions and a great urban identity," says Bakkala.