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Structural Engineer Answers City Questions From Twitter

Structural engineer Dr. Nehemiah Mabry answers the internet's burning questions about city building. How are underwater tunnels made? What city has the best urban designer? How do bridges not collapse? How do floating cities work? How do sink holes happen? Nehemiah answers all these questions and much more! Director: Lisandro Perez-Rey Director of Photography: Constantine Economides Editor: Ron Douglas Expert: Dr. Nehemiah Mabry Line Producer: Joseph Buscemi Associate Producer: Brandon White Production Manager: Eric Martinez Production Coordinator: Fernando Davila Casting Producer: Nicole Ford Camera Operator: Chris Alfonso Audio: Brett Van Deusen Production Assistant: Ryan Coppola Post Production Supervisor: Alexa Deutsch Post Production Coordinator: Ian Bryant Supervising Editor: Doug Larsen Assistant Editor: Ben Harowitz

Released on 03/07/2023


I'm Dr. Nehemiah Mabry.

I'm a civil structural engineer.

Today I'll be answering your questions from Twitter.

This is City Support.

[upbeat music]

All right, @jbricericejb asks,

How do you safely demolish a 28 story building?

So the goal when you're demolishing a building

is to get it to implode, or basically collapse

directly on its own footprint.

The structural drawings are studied.

Low bearing supports are identified,

and detonators are placed there

so that they can sequentially,

or at a certain time, all be executed.

And then the remaining portion of the building

has nowhere to fall but directly down.

And then the weight of the building itself

then causes it to continue to collapse upon itself.

@ErinOfBoston asks, How are underwater tunnels made?

I.e. Sumner Tunnel?

First, you have to make sure

that you can get down under the water to begin the work.

What we do is place makeshift dams

down into the water to seal out certain parts of the water.

And then the water inside of that dam

is literally pumped out.

So now you have a section of the water that workers

or machinery can get down.

And we have what we call a tunnel borer machine

that can then tunnel through in the direction

that we want the tunnel,

and also keep the pressure or the water out.

And that being done, allows there to be

some sort of prefabricated tunnel.

And by prefabricated I mean, it was created

outside of the water to then be placed into the boring hole.

Sometimes this is done on both sides

of the tunnel at the same time.

And then when it is done, the makeshift dam is removed.

The water then continues.

And the tunnel at that point

is connected to the road that leads to it.

@archi_tradition asks What city has the best urban design?

Amsterdam in the Netherlands is known for its canals,

which create a pretty nice grid.

I would say in general, Singapore has been recognized

as a country that has pretty good urban design.

Newer cities have the advantage of looking at

how cities were designed in the past

and identifying areas that were perhaps not as optimal.

You wanna make sure that there's

a significant amount of housing,

and that there's more room to develop as necessary.

You wanna make sure that there is affordable housing.

And then you wanna make sure that there's efficient routes

from where there are primary residential neighborhoods

to the areas where people do a lot of their working,

such that you don't have to have everyone driving,

individually creating more carbon emissions.

You wanna see green spaces.

Essentially harmony between the ways that people live,

they work, and they play.

@meowjennayy asks, Isn't it crazy how someone

designed roads and highways to get places?

Like how did they know where to go?

In some respects, we have our indigenous ancestors to thank,

particularly here in America.

Oftentimes these roads were footpaths.

People were traveling from one place to another

and they were just basically trying to go

in the optimum directions to get from one place to another,

to another natural resource.

Roads over time kind of naturally took the place

of what were just heavily trodden paths.

@SydneyRaye497 asks Honestly,

how the heck did engineers reverse the flow

of the Chicago River in 1900?

Naturally, the Chicago River had a flow

north into Lake Michigan.

As the city began to grow,

a lot of pollution from the city would go

into Lake Michigan, and they ran into a problem

of actually consuming their own pollution

when they got their water from Lake Michigan.

What they basically did was locate a high point

not too far west of Chicago,

and they dug a canal that redirected the water.

And because it was on the other side of the high point

the gravity naturally caused it to flow downstream

as opposed to upstream.

And so they basically just rerouted a section of it.

@engineers_feed asks, What is the most

mind-blowing engineering marvel you know about?

One of my favorite bridges is the Millau Viaduct.

It's actually the tallest bridge in the world.

It's about a mile and a half long.

And the height of the towers

are actually taller than the Eiffel Tower.

It's a cable-stay bridge, one of my favorites.

Cable-stay bridges transfer the loads

or the strength that the cables experience

to support the deck of the bridge.

And they transfer it back to each individual tower.

It really looks sleek because it has to be symmetrical.

And the fact that it's a tall bridge

means that you can get nice clouds and fog.

That makes for some pretty stunning photography.

@Miles_B, Question for New York City.

If you had to do it all over again,

would you build elevated trains instead of subways?

They're basically pros and cons

for building underground versus aboveground.

Where if you're aboveground you have the convenience

of being able to work on it quite easily.

One of the disadvantages is that

you're more exposed to the weather.

An advantage of having subways underground is

they don't have to worry about the constant snow load

or the winds that say a road would have to deal with.

At this point, I think it's probably better underground.

You really don't want the additional congestion

of having to repair around all of the people

that are already in New York City.

@nikoentombed asks,

How the frick are skyscrapers even made?

This is a pretty basic skyscraper design.

It gets narrower as you get up higher.

But a lot of times these structures are designed

by either reinforced concrete,

or very, very large steel I-beams.

This is a very small cutout model of one

where you have a web, a narrow web.

And then you have a wide flange at the top or bottom.

I-beams can form horizontal supports.

Or very thick ones can be used as columns for a skyscraper.

And the more you want your particular I-beam to carry,

the thicker you'll make these different components.

@oberley_ asks, To be honest,

if you aren't using number nine steel rebar bars

in concrete beams, what kind of civil engineer are you?

So what she is referring to is the steel bars

that we use inside of concrete

to basically reinforce the strength.

Steel is very strong

in what we like to call tensile strength.

It can resist being pulled.

Whereas concrete not so much strong in that type of force.

It's much stronger in compressive strength,

being pressed on.

So in certain parts of our concrete

we would place steel bars of different sizes

to be able to reinforce the strength

in that area of the structure.

And the number of the rebar represents

the diameter of that particular steel bar.

Now number nines,

it represents one square inch of cross section,

so it makes the math a lot easier to calculate.

Every time we add one number nine rebar,

we've just added one square inch

of steel reinforcement for that area.

@leahleisen, How are rope bridges built?

So the first step is just to see how can you get

that first rope from one side to the other.

It probably was either walked over,

or hiked over at first.

Or perhaps thrown or shot over using an arrow

in some of the earlier cases.

And once that first rope reaches the other side

now you have a system to get other things across

using a system of pulleys and levers.

These rope bridges are very basic cases

of suspension bridges.

@bon_bon222, How do bridges not collapse?

We design bridges to be able to last

for 50 to 70 years on average.

So in the case of a suspension bridge,

what you have is a long wire cable

connecting from one support to the other.

They have vertical cables that are going down

from that cable that are in tension,

or being pulled down by the weight

of that particular bridge deck.

When this is happening,

that is actually transferring the load

back to the top cable.

And then the top cable is responsible for transferring

that load back to the tower.

That then supports, then distributes the load

across these columns,

and transfer it back to its foundation.

We run a lot of tests on the type of material

to make sure that it can withstand

all of the temperature cycles, all the wet, the dry cycles,

the fatigue from the constant use of traffic.

We do this to a point that we have a high reliability,

or a high level of confidence it's not gonna collapse

for a significant amount of time.

@inbury asks, The civil engineers in this city

really have zero clue on traffic flow.

How can 90% of lights be red as you get to them?

Traffic studies are based on data.

They look at the number of cars that are actually

passing through a particular intersection,

or heading in one direction at different times of the day.

They measure this sometimes by using sensors in the ground.

Sometimes there are cameras.

Or they can be old-fashioned

and actually have a person go out there

and literally count the cars.

Whenever this changes, they're able to optimize the phases

or the changing of the lights at different times of the day.

They want to prioritize

where there is a heavier flow of traffic.

Roads that have a high volume of traffic,

there's a higher speed, they're given more time

for their red, and even yellow lights.

Because they don't wanna jam up traffic in that direction.

So that being said, perhaps you're on a road

that doesn't have high priority,

or there needs to be a refresh

in the traffic studies of that area.

@torontokyo, Did the London Bridge actually fall down?

Yes, multiple times.

There was a bridge around the 11th century

that was reportedly torn down by a Viking invasion.

However, after that was rebuilt,

the nursery rhyme you're referring to

could likely be speaking of the fact

that during the longest tenured bridge of about 622 years,

the London Bridge actually went through a lot of fire.

It experienced a lot of wind or tornadoes.

It experienced a lot of rain.

And so over time, it became a joke that not only

the London Bridge falling down, but one poem

written about the same time states,

The London Bridge is Broken Down,

speaking to the constant need of repair that it was in.

Next question.

What do you think?

How will our future cities look like?

A, B, or C?

Any highly advanced city is gonna need to make sure

that there is an integration of green spaces,

not only for our oxygen,

but just for our enjoyment and wellbeing.

The more that we get rid of our natural green spaces

the less we are able to have that oxygen

naturally generated in our environment.

I see that they're all high vertical structures.

And that really speaks to the fact that we're gonna need to

get more and more comfortable with building up.

'Cause building out isn't gonna always be an option

@ASCETweets asks, How will floating cities work?

What can civil engineers do now to help us get there?

First of all, American Society of Civil Engineers,

I would think that you would know the answer already.

Shout out to y'all.

Or you could look at things such as artificial islands,

or oil rigs, or even cruise ships to get an idea

of how these cities could be worked.

Oftentimes, they are built with anchors

connected to the ocean bed, or the sea floor,

to make sure it doesn't float away.

Other times they can have very long columns

of significant weights distributed underneath the platform

so that it's able to be steady.

And then weight can be built,

or structures can then be built on top of it.

@BabylonOnReplay asks,

I know it doesn't rain frequently in California,

but why the frick does the drainage suck so bad

in so many areas?

Like did the city engineers not consider rain at all?

I'm pretty sure they considered rain,

but they consider a certain amount of rainfall.

And that was based on the historical data

at the time that they designed the city.

As the environment begins to change,

and things begin to switch up,

sometimes the amount of rainfall that was once expected

to be very very rare becomes more frequent.

And so oftentimes you do have puddling

that cause engineers have to go back and redesign.

We begin to now have new information that allows us

to design more for the present than for the past.

And so you could be experiencing something

that just is in need of a revised design.

@katherinegould asks, Question for engineer types.

If you walk under a bridge and you see crumbling concrete

and exposed rebar, is that bad?

Asking for the city of Cleveland.

Well, it's not necessarily bad, to be honest.

The concrete that is at the bottom

is actually used as cover to cover the steel rebar.

We've only designed it so that the steel

is actually resisting most of the strength.

The problem however, is that that steel is now exposed.

And so as the steel begins to corrode and rust,

then it actually does get bad.

And so if you see exposed steel,

in some cases it just needs to be cleaned and recovered.

In other cases it could be pretty bad.

@chhrissyy, How are sinkholes created?

My sister, you cut a hole in the counter

and put a sink in it. [laughs]

Sinkholes are basically just areas of non-compacted soil.

Sometimes there was water there that has been drained out

and so now you have this big old gap

underneath the surface that whenever it experiences

enough weight at the top, it could sink.

@litindustrial asks, How would you design a new city

in the desert with an unlimited budget?

This brings to mind a city that's already been proposed

in Saudi Arabia known as The Line as its nickname, Neom.

Pretty much very narrow city that stretches several miles.

There will literally be two walls

that are able to support several overhangs,

or cantilevers as we call it, that can support green spaces,

residential housing, transportation.

When you're just looking at a short footprint,

and you're building up as opposed to out,

you can increase the optimum travel

from one end to the other

by making public transportation just a straight line.

Being in the desert, when you're thinking structurally,

sand is actually pretty strong.

It has a high bearing capacity.

That being said, you have to bring in additional cement

and other materials to make sure

that this sand can consolidate

and not be as shifty as we know sand to be.

I think it's pretty challenging,

but I'm not one to rule out what human ingenuity can do.

@alexis_brittney asks,

What does building a bridge even have to do with math?

How we design a bridge.

We have to start by saying,

whenever a force is applied over an area

we divide the area that's gonna experience that force

by the value of that force to get what we call the stress.

So there's math right there.

We measure the strength of the concrete

in terms of pounds per square inch.

How many pounds per square inch of area

can this take perpendicularity?

We call it compression strength.

The cylinders are taken into the lab

and put into a very high strength compression testing device

until it breaks, until it fails.

And then we can say to ourselves,

this concrete has a failure strength

of 5,000 pounds per square inch.

We then come back to our structure, our bridge,

and we determine if that concrete

is strong enough to hold up the weight

that we're expecting this bridge to experience.

In other words, it's a lot of math.

@RossMeenagh asks, How is everybody fine with the fact

that skyscrapers sway back and forth in the wind?

Skyscrapers have to worry mostly about wind.

So as wind is hitting one side of the building,

it's going to naturally want to sway a little bit.

If wind is able to rush by it extremely fast,

and it's feeling a lot of positive pressure on this side,

it creates what we call a negative vortex,

which can cause it to vibrate pretty heavily.

Throughout a very tall building or a skyscraper

are installed dampers,

which basically counteract the swaying of the building.

A certain floor may have a very large weight,

such that as the building begins to sway

say this direction, the damper will then move

in the opposite direction to kind of counteract

the amount that is being swayed.

But they are hidden from your typical user of the building.

They have hydraulic actuators that push

from one side to the other, usually matching the frequency

that the building is naturally swaying in the wind.

@cadetkelly_ asks, Concrete versus cement?

What's the difference?

Cement is actually a powder

that creates a paste when mixed with water.

And concrete has not only that cement,

but also rocks to make it one big recipe.

So cement is actually an ingredient inside of concrete.

@StClemon asks, These monstrous elevated freeways,

whoever thought those were a good idea?

Well, the elevation of a freeway has to take

into consideration the vertical space,

or the vertical clearance,

of whatever is traveling underneath it.

Whenever you have multiple intersections,

and several freeways passing other freeways,

you have to make sure

that the minimal point is still high enough

for whatever traffic is traveling underneath it.

But you can't just step it up whenever you need to.

And so oftentimes the vertical curve

needs to be gradually created over time.

The alternative would be to go under it.

It wouldn't be as easy to maintain.

And so this calls for certain areas

to look unnecessarily higher than they need to be.

But it's ultimately to try to accommodate

a very smooth experience for those who are traveling on it.

Those are all the questions I have time for today.

I had a lot of fun.

Thank you so much.

I hope you enjoy and learn something.

Thanks for watching City Support.

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