Buildings

Roma Agarwal wrote this awesome book on constructions (buildings, bridges etc) titled Built. Like many Westerners, she switched fields during college, from physics to structural engineering. The reason for her pivot? She stumbled upon engineers making the stuff that would be used by physicists!

“One was designing a metal holder for a glass lens – a simple task, you might think, except that the whole apparatus had to be cooled to minus 70 Celsius. Metal contracts more than glass, and unless the holder was cleverly and carefully designed, the cooling metal would crush the lens.”

It was an eye-opening moment for her:

“Suddenly, it became very clear to me: I wanted to use physics and maths to solve practical problems.”

And boy, did she become a passionate engineer, as the book shows.

“A major part of the engineer’s job is figuring out how structures can withstand the manifold forces that push, pull, shake, twist, squash, bend, rend, split, snap or tear them apart.”

The solution is easy… in theory:

“It is vital for an engineer to understand where the forces are flowing, what kind of force it is, and then to make sure that the structure transmitting the force is strong enough for the job.”

Gravity, she says, is the easiest force for an engineer to deal with. It acts in a single direction after all, and thus easy to plug into a maths equation even before computers. On the other hand:

“Other equally destructive forces are not so easily reduced to equations.”

But first, concrete and steel were game changers, they were “so much stronger than timber” allowing us to “build taller towers and longer bridges”. And with taller buildings, wind became a key factor to contend with. Unlike gravity, wind is “random, fluctuating, unpredictable”. No wonder computer simulations are so critical today in the construction of skyscrapers.

There are two solutions for the wind problem: (1) At the center of a building, build a spine or skeleton like structure that holds it together; or (2) Build an exoskeleton, an outer shell, around the building that takes the beating and transmits the wind force to the base safely. Regardless of the solution, a building will still sway at high winds. Yes, buildings do sway! Strangely though:

“The swaying in itself is not a problem, what’s important is how fast the building sways and for how long.”

The limits on both aspects (sway duration and sway speed) are based on the threshold at which human inhabitants feel uneasy. Tall buildings use a massive pendulum called the “tuned mass damper” to absorb the energy of swaying, thereby slowing the speed of the sway and causing the sway to die out fast. You’d think something so functional would be hidden, and for the most part it is. Except in Taiwan’s Taipei 101 tower, where the pendulum is a tourist attraction!

Buildings also need to factor in for earthquakes, specifically to make sure the natural frequency of the building does not match that of the earthquake.

Even before terrorism, engineers had learnt the hard way the need to design buildings to “resist explosions”. Why? They’d seen a gas explosion in one apartment leading to “disproportionate collapse” of the entire building. She cites the Bombay Stock Exchange that did not collapse even after terrorist bombs were deliberately targeted at a load bearing column. Another learning came too late, after the exit staircases in the 9/11 towers collapsed, leading to many getting trapped on the higher floors. Since then, exit staircases are protected with concrete etc to ensure they last long enough for people to evacuate.

Given her knowledge and enthusiasm, I was not at all surprised that she ended the topic of buildings with this line:

“Never challenge a structural engineer to a game of Jenga: we know which blocks to remove - how to take chunks out of a structure so that it doesn’t crash.”