Why Do Skyscrapers Sway?

If you've ever been near the top of a skyscraper and thought you felt the building move, you probably did.

Before you start looking for the stairs, that's usually a good thing.

Skyscrapers are designed to move.

It sounds completely backward. We spend our careers making structures strong, stable, plumb, and secure. The last thing most people want to hear while standing 70 stories above the ground is, "Don't worry. The building is supposed to be moving."

But when you're building hundreds or even thousands of feet into the air, a structure that can flex slightly can actually perform better than one that is completely rigid.

Welcome to our new series, Why Is It Built Like That?

We're starting at the top.

The Higher You Go, the Windier It Gets

At ground level, buildings, trees, terrain, and other obstacles interrupt the wind.

Go hundreds of feet into the air and things change.

Wind pushes against the enormous surface area of a skyscraper. The taller the building, the more significant those forces become.

And wind doesn't simply push steadily from one direction.

It changes speed.

It changes direction.

It creates pressure around corners and across different surfaces of the building.

A skyscraper has to deal with those forces continuously throughout its life.

Trying to make the entire structure absolutely rigid isn't necessarily the answer.

Instead, engineers design tall buildings to safely flex.

Think About a Tree

A large tree provides a pretty good example.

When strong wind hits a healthy tree, the trunk and branches move.

They bend.

They sway.

Then they return toward their original position.

If everything were completely rigid, strong wind could create enormous stresses at particular points.

Skyscrapers operate on a much more engineered version of the same basic principle.

The structure can move slightly while remaining within carefully calculated limits.

That movement helps the building handle forces created by wind and, depending on location and design, seismic activity.

How Much Do They Actually Move?

This depends entirely on the building.

Height, shape, structural system, wind conditions, surrounding buildings, and engineering all affect how much movement is possible.

At street level, you probably won't notice anything.

Near the top of a very tall building, movement can sometimes be noticeable, particularly during high winds.

Here's the interesting part.

A building can be structurally safe and still move enough to make the people inside uncomfortable.

Humans are pretty good motion detectors.

We notice movement.

Our inner ears definitely notice movement.

So engineers aren't only designing tall buildings to remain structurally sound. They also have to consider how the building feels to the people occupying it.

Nobody wants to spend the afternoon on the 80th floor feeling like they're working from the deck of a boat.

Engineers Have Ways to Calm Things Down

One of the most fascinating solutions is something called a tuned mass damper.

Picture an enormous weight installed high inside a building.

As wind causes the building to move one direction, the damper moves in a way that helps counteract that motion.

Some are enormous.

The famous damper inside Taipei 101, for example, is a huge suspended steel sphere weighing hundreds of tons.

And yes, people can actually see it.

Other buildings use different damping systems, structural cores, bracing, aerodynamic shapes, or combinations of engineering strategies to control movement.

Which brings us to another interesting feature of modern skyscrapers.

Those Weird Shapes Aren't Always About Looks

Look at newer skyscrapers around the world and you'll notice something.

They're rarely simple giant rectangles anymore.

Some taper.

Some twist.

Some have setbacks.

Others have openings or unusual edges.

Architecture certainly plays a role, but sometimes those shapes also help manage wind.

Wind flowing around a tall structure can create repeating swirling forces called vortices. Those forces can contribute to building movement.

Changing the shape of the building can disrupt how those forces develop.

So that strange-looking skyscraper may not simply be an architect trying to get attention.

There may be some serious engineering hiding behind the shape.

Now Imagine Working on the Outside

Here's where things get especially interesting from an access standpoint.

The building moves.

The wind is stronger.

You're hundreds of feet above the ground.

And somebody still has to work on it.

Glass has to be installed.

Exterior systems need maintenance.

Façades require inspection.

Sealants fail.

Materials need repair.

Buildings don't stop needing maintenance just because they're tall.

Access systems used on high-rise structures have to account for conditions that simply don't exist on a three-story building.

Wind becomes a major factor.

Anchorage becomes critical.

Communication matters.

Weather conditions can determine whether work happens at all.

And the farther workers get from the ground, the less forgiving poor planning becomes.

Movement Doesn't Mean Failure

This is probably the biggest misconception.

People naturally associate movement with instability.

On many jobsites, movement absolutely can indicate a problem.

But engineered movement in a skyscraper is different.

The building isn't randomly shifting around.

Engineers calculate how the structure will respond to expected forces and design systems capable of handling them.

The goal isn't necessarily to prevent all movement.

The goal is to control it.

That's a very different thing.

Construction Is Full of Things That Look Wrong

That's exactly why we're starting this series.

Construction is full of things that don't make sense until you understand the reason behind them.

Buildings move.

Bridges expand.

Concrete gets intentional gaps.

Steel bends.

Foundations sometimes float.

Structures that look simple can hide incredible engineering.

And sometimes the thing that looks like a flaw is exactly what keeps everything working.

At Southwest Scaffolding, we spend a lot of time thinking about how workers safely reach the structures our industry builds and maintains.

But every once in a while, it's worth looking at the structure itself and asking a very simple question:

Why is it built like that?

In the case of skyscrapers, the answer might make you slightly uncomfortable the next time you're standing on the top floor.

Yes.

It's moving.

And that's exactly what the engineers expected.