- California gets earthquakes,
which means that the building codes here
have some pretty strict requirements.
There's a lot you can do with computer simulation,
but at some point,
new building techniques have to be tested in the real world.
The massive 10-storey building behind me
has been constructed to test and certify a new design,
a "post-tensioned rocking wall".
Basically, there's a massive, heavy wood and steel spine
that runs all the way up from the floor to the roof,
bracing and dampening the whole thing,
which, along with new fireproofing,
might allow wooden buildings to be far taller than ever before.
Does it work? Well, to really test it,
you need an earthquake, so we're about to get one.
The building is sitting on the Shake Table here
at the University of California San Diego.
This whole site is locked down because that table is ready to go.
There's about to be a test.
But I was here a few months back
while that building was still under construction,
and one of the folks in charge showed me around
while everything was powered down.
We started in the equipment building,
where there's colossal accumulators and pumps
that pressurize hydraulic fluid to 5,000 psi,
which is more than a hundred times the pressure of your average car tire.
This is really big engineering.
So how much can the shake table move?
- If you're going along the long axis, the x direction,
we can go plus/minus 35 inches.
Going in the lateral, the y-axis,
we can go plus/minus 15 inches.
And in the z, up and down, it's plus/minus five.
We have the largest payload capacity in the world.
We can move 2,000 tons.
So about five million pounds of mass.
This 10-story building is about 700,000 pounds total.
So that's maybe 10%, 15% of our capacity.
- Wow. - Yeah.
- They're going to shake a 10-storey building back and forth,
and it's not even a big deal for them.
Also, it's not just up, down, and sideways.
The shake table also does roll,
pitch,
and yaw
When it was originally built, about 20 years ago,
it did only go back and forth in one direction.
They can still do that.
They can shake in precise mathematical sine waves
if they want to for a particular experiment.
But these days, they can also replay actual records
of real-life earthquakes.
Although, that turns out to be far more complicated than I thought.
- Before we build anything on it,
we need to do some tuning for the control system.
So bare table, when there's nothing on the table,
we do some tuning.
Then, when you put the specimen on,
those dynamics, the specimen dynamics actually affect
the control loop. - Right.
- So we need to do some further tuning
and actually filter out some of those frequencies
in order to successfully move the shake table.
- Oh, because if the building that you've made
has a resonant frequency...
- It interferes with the control system.
- Right. - So we need to filter out
those resonant frequencies. - Oh.
So you're not just playing back an earthquake.
You're trying to get as close to that as possible
while dealing with whatever the building wants to do as you shake it?
- Right, if you're doing multiple shakes.
So the first few tests,
the building hasn't been altered in any way.
- Then the building might shift or break
or you might change- - You might change something.
- Right.
- And that might affect the controls.
- Next up on the tour was the connector
between the power building and the shake table,
where again, there is stuff that feels like over-engineering
until you realize just how much mass this thing has to move.
- Everything here is color coded,
so the red is high pressure and blue is return.
These large pipes here, they're 12-inch diameter,
they're like over an inch thick.
Inside this pipe is 3,000 psi pressure.
- I feel like every curve in that pipe
was something that had to be really carefully engineered,
'cause every time 3,000 psi fluid has to change direction,
you're going to get water-hammer effects.
- Oh, absolutely.
- The hydraulic shock, right, that's the term,
it will add vibrations to the table itself?
- Exactly, it'll cause a lot of headache,
not to mention the damage that it can do
if one of these connections breaks.
- Yeah. - Then you're getting,
you know, 3,000 psi oil shooting everywhere.
- And that's why the whole area is locked off right now.
Everything is charged up.
Warning lights are flashing, a test is about to happen.
It's very unlikely there'll be anything wrong.
This is not the first test.
They started small a few weeks ago. They're working up.
The safety margin is massive.
But right now, inside and under those buildings
is not technically a safe place for humans,
just in case.
So from here, 5,000 psi in the accumulators,
3,000 psi running through these pipes
at who knows what speed.
- Right.
And they come over here and make a U-turn!
- Agh! It does a U-turn!
- Yeah, this is why we have a damper system here
is to make sure... - Good grief.
- ...there's no vibrations here.
So this was very highly engineered
to make sure everything's okay. - Yeah, it was!
Don't ask me exactly why there's a U-turn in the pipes.
It's to do with retrofitting new improvements
around existing concrete structures.
But I do know that no sane engineer
would put a U-turn in that pipe
unless they absolutely had to.
- Now we're actually under the shake table.
So this is the shake table platen.
The 10-storey building is currently sitting on it.
This is what we move.
Everything you see on this side is mirrored.
So right now we're on the south side.
We have six vertical actuators.
So there's three of them on this side,
three of them on the north side.
The vertical actuators do the up and down movement
and then the roll and the pitch movements.
- Okay, oh yeah. - Right, so you can-
- 'Cause you can actuate each one a different amount
and that will do- - Right, you can lift one
and drop the other. - Yeah.
- And they're not actually attached to the shake table.
So this thing is a glide plate.
The vertical actuator actually has a 32-inch diameter circular surface.
And that creates like a vacuum chamber between the glide plate and the surface,
very low friction. That vacuum gets filled with oil.
So when we push the actuator up,
that table gets lifted up with it,
but it can still freely slide around.
What we definitely don't want
is for that seal to ever open.
Right?
- Okay, yeah, absolutely. - You don't want the actuator to be unseated
from that glide plate. - We're laughing,
but what would be catastrophic.
Okay. - That would be catastrophic, right.
-So last stop on the tour,
we've seen how it moves up and down.
Next is the side to side.
- Right over our heads are our horizontal actuators.
So there are two- - I hadn't looked up.
- There's two horizontal actuators- - Wow.
- In a V shape on either side,
and they're the ones
that allow the x and y and that yaw motion.
- These are...? - These are what we call hold-down struts.
They're passive actuators, if you will.
They're basically a piston with nitrogen filled on one side.
- Right. - Constantly applying a downward force.
So we have three of them. There's a couple of reasons for it.
One is, remember when I said we don't want those vertical actuators to ever unseat?
- Right. - So you're always pulling down.
- Okay. - You're never going to unseat those.
And the other one would be for the overturning moment
of the shake table.
We have a 10-storey building on it.
If it wants to tip over, it's going to want to lift and-
- So it's always- - This stops that.
- Getting pulled down for safety?
- Always a restoring force, yeah.
- I want to just make that clear.
They're constantly pulling down on the table
because that building on top
can have so much inertia
that it might just want to keep tipping over.
So it's got to fail safe.
The amount of safety systems here,
if anything trips, presumably everything just shuts down
as quickly as it can. - Everything shuts down,
but it's a controlled shutdown.
You can't leave it off-axis and you can't just be like,
"Okay, return to zero immediately,"
because you- - Right.
- You might actually do more damage
on that reversal. - Yeah.
- It has to be a controlled, slow ramp down
to bring it back to its zero position.
- So here we go,
the area around the building has been cleared.
The scientists in the control room are going through the startup sequence.
This is just one test in a hundred,
another routine earthquake!
And I don't mean to undercut the drama here,
but the point is that the building will survive.
This is not testing to destruction. They're not even going to 100% power.
This is just another day for the Shake Table.
- "Commencing in five, four, three, two, one..."
- I know it's fine, but I do want to turn and run.
I absolutely want to turn and run.
Wow.
- "Next test will commence in 10 minutes."
- All right.