Envelope #47: Bangkok skyscraper collapse

Good morning! Andy from Back of the Envelope here.

By now, you've probably heard about the M7.7 earthquake that struck Myanmar last week. My heart goes out to the tens of thousands affected 😥.

Just like the email I wrote back in 2023 about the Turkey earthquake, this one is tough to write. So many lives lost… and many loved ones left behind.

But, just like I said back then: we're structural engineers.

We talk about failures. Earthquakes. Collapses.

Not to sensationalize, but to learn.

And to make things a little safer next time.

Most of the failures near the epicenter make sense: older buildings, soft stories, weak construction.

But one failure... stood out.

It happened 685 miles away — in Bangkok.

A modern, under-construction high-rise: the State Audit Office tower.

Let’s dive in.
(Estimated read time: ~4 minutes)

Quick disclaimer: The official investigation is still ongoing. Facts are still trickling out, and there’s a lot of speculation floating around. I’ll do my best to separate the signal from the noise.

My heart dropped when I saw this video:

X/Twitter video link

and this:

Reddit video

…the speed at which the building came down is terrifying.

And the fact that it’s so far from the epicenter? That’s what really shook me.

So I started digging.

What we know so far

Confirmed facts:

• 33 stories, 449 feet tall

• Budget: ~$63M USD

• Still under construction, but topped out in April 2024

Likely facts
(based on a Chinese technical article that was later taken down — but archived. Link below):

• Structural system: prestressed flat slab (no beams) with exterior concrete columns and core walls

• 500 KSC (≈7000 psi) concrete for vertical elements

• 350 KSC (≈5000 psi) concrete for slabs

Reasonable inferences:

• The building likely followed local Thai code, which references recent ACI standards

• Bangkok is generally considered a low seismic risk zone (no nearby active faults)

• But the city sits on deep soft soil, which can amplify long-period seismic waves

• This soil amplification effect was supposedly added to the seismic design maps in recent years (link to the research at the end)

And now... speculation

According to a LinkedIn post (link below), the recorded acceleration response spectrum in a nearby station shows a peak at around 1.2 and 1.8 seconds (see yellow and blue lines below, which represent acceleration in the east-west and north-south direction).

That’s a bit different from a typical response spectrum where short periods dominate.

Let’s do a quick back of the envelope calc:

At that period, the spectrum suggests the building may have experienced ~2 m/s² acceleration.

Yet, if designed per code (assuming that’s what the brown line represents), the target design acceleration might've been closer to 0.6 m/s².

That’s more than 3x the design force!

So what does that mean?

It suggests the building may have experienced seismic demands well beyond what it was designed to handle.

And if the core walls weren’t designed for high ductility, and if the lateral displacements were large, the gravity system (columns and slab joints) may have failed due to deformation incompatibility.

Once that happens, collapse can become a cascade…

Of course, this is just a hypothesis. The truth may look very different as more data comes out.

But I’d like to point out one thing:

In the aftermath of disasters like this, it's easy to reach for blame:
design flaws, construction errors, corruption, cost-cutting.

And you’ll see that all over the web.

But sometimes, even when everyone follows the rules, nature changes the rules.

The Bangkok collapse reminds us that codes aren’t guarantees. They’re approximations built on what we know.

And events like this? They expand what we know.

That’s how we get better.

Alrighty that’s all for now. Thanks for reading!

Yours truly,

Andy Lin
Back of the Envelope/Structural Engineer HQ

PS:

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Click here to learn more: https://go.sehq.co/durafuse

PPS:

Links mentioned above:

• Chinese technical article (use Google Translate to convert it to English): Archived link

• Seismic hazards in Thailand - a compilation and updated probabilistic analysis: Read here

• LinkedIn post on response spectra: View post

• Google Maps link to the tower: Link