- So let's begin the tale of the story
of the George Washington Bridge
from the social perspectives.
So from the social perspective,
Ammann needed the support of
politicians and the community.
He needed to sell his vision.
And his vision had to be dramatic,
it had to get the attention of the community
and of the politicians,
just the general public.
By 1922, Ammann had formed a close
but informal alliance with
the governor of New Jersey,
who at the time was George Silzer.
And Silzer was very interested in improving
the transportation, automobile transportation,
in New Jersey.
He felt that that would really
help strengthen the economy of New Jersey.
So he was very supportive of Ammann's idea
of an automobile bridge in northern Manhattan
where you see it exists today.
In December of 1923,
Ammann wrote a 22 page report,
which he gave to Silzer,
about the bridge.
And this report wasn't just about the
engineering aspects of the bridge,
but it also included traffic, financing,
and just the impact on the regional development.
So he gave this report to Silzer
who in turn gave the report to
the Port Authority of New York.
When Lindenthal found out about this report
and what Ammann was doing,
he was really upset, I guess,
to put it mildly,
and he called Ammann names like
unethical and dishonorable.
Of course, that wasn't Ammann's intention.
Ammann tried to convince Lindenthal of that,
but I can understand both points of view
and how Lindenthal was seeing this
from his side.
So Ammann becomes, essentially,
a politician for about three years,
to move forward to sell people
the idea of this bridge.
And finally, on July 1st 1925,
Ammann was hired by the Port Authority
to design what we know now to be
the George Washington Bridge.
So when this bridge was built,
we can imagine the transformation of communities,
now we have New Jersey connected
to New York City with automobiles,
there's no longer that problem
of the ferries being overworked
and overcrowded with automobiles
trying to cross the Hudson River on the ferries.
Which is what Ammann was seeing,
which was again what was inspiring him
to say this needs to be an automobile bridge.
And the next question, the next social question,
because the social is tightly tied to the economy,
is how was he able to double, essentially,
the longest spanning bridge in the world,
and still keep it economical?
And to answer that question,
I'm going to come to it when
we talk about the scientific aspects
of the George Washington Bridge.
But first, let me talk a little bit
about the symbolic aspects.
So from the symbolic aspects
we have both a symbol of the past
and a symbol of the future
in Ammann's vision and Ammann's
design for this bridge.
The symbol of the past were the heavy stone towers,
which you don't see in the bridge today.
But Ammann's design, his proposal was for
heavy stone towers.
So the steel skeleton that you see today
the towers representing the George Washington Bridge
that was the skeleton that was intended
to support the stone facade, the granite facade,
that was intended to go on those towers.
But again, just like the Bayonne,
this was during the Depression
and to save money, that stone facade
was left out.
But Ammann's vision was very similar
to what we know to be
the Brooklyn Bridge.
So we compare a sketch made by Cass Gilbert
of the design of the George Washington Bridge,
as was intended by Ammann,
compare it to an image of the Brooklyn Bridge,
you'll see some similarities,
you see the stone arch abutments approaching the bridge,
and you see the heavy stone towers.
The difference is that for the George Washington Bridge
the foundation is not on a caisson
like in the Brooklyn Bridge.
The Brooklyn Bridge towers needed to be heavy stone
to sink those caissons,
in this case, caissons were not used
so the heavy stone towers
were not needed.
The symbol of the future is the very thin deck.
So if you look at an image of
the George Washington Bridge
under construction and when it was finally completed,
we see that the deck of this George Washington Bridge
was very, very thin.
At the time of completion, it only had one deck.
So the top deck has eight lanes of traffic,
today it has two decks,
a lower deck has added six more lanes of traffic.
So the original design was designed for
two decks of traffic,
but the second deck was intended
to be put on there much later and
it was put on there about 30 years after
the completion in 1931.
So this very thin deck was a symbol of the future.
Symbol of future suspension bridges
that were to be built in the 1930s.
And it was this aesthetic that would
lead some bridges into some trouble,
as I'll talk about in a moment.
So now let's look at the scientific aspects
of this bridge, and the forces.
In particular, let's look at the forces
on the tower.
So we have on the tower, the cables.
And on top of that tower,
we have the cables in the backspan
that are pulling, the cables remember
are in tension in this bridge.
So there's this force, F,
that's pulling down via the cable on the tower.
And we can take that force F and
split it into a horizontal and a vertical component.
A horizontal component, H,
and a vertical component, V.
And if you would draw a box
around those arrows, those arrows
should be drawn so that
they start and end at a corner of the box.
And that will give you an indication of the magnitude
of the force.
So the lengths of those arrows
are a relative indication of the size of each force,
the relative size of the force F
H, and V.
The horizontal force H is equal to
QL squared over eight D.
Where Q is the load, it's a load
in terms of force per length
so kips per foot, for example.
L is the span, remember the span
is the distance between tower to tower.
So it is the distance between two towers.
And D is what we call the sag.
So if you take that equation,
H equals QL squared over eight D,
we can say that QL over 8 represents the load,
and L over D represents, essentially,
the form of the bridge.
So again, with L, the distance between the towers.
D the sag, or the distance between
the top of the tower and the bottom of that cable.
We can play with those two dimensions
and arrive at different forms for bridges.
So if the bridge had shorter towers,
or if the bridge had taller towers
they would have a different value, D.
So the question I have for you is,
which design would result in the smallest force
in the cable.
Would it be the design
with a shorter tower?
Or a design with a taller tower?
Or if you have no idea what I'm talking about
you can answer, C,
no idea, I'm still confused.
The answer is that the taller tower
has the smaller cable force.
So let's see why.
So if we look again, we draw that box.
We know that the vertical force V,
doesn't change depending on that distance
that sag D.
So V stays constant.
But that force F will change.
You can imagine that the angle of that
force F is going to change if the tower
is tall or the tower is short.
So in which case, if you have that angle F
change and it becomes longer H
also becomes larger.
Or you could just go back to the equation
H equals QL squared over eight D
and we know that if D is smaller, H is larger.
So larger towers or taller towers
means smaller cable forces,
smaller cabler forces means smaller cables.
So why not design the towers of suspension bridges
as tall as possible.
Because that would reduce material in the cable
but the problem is that that adds material to the tower.
So it's really a trade off between the cost
of the tower and the costs of the cables.
Continuing on the scientific aspect of the study
let's answer that question of how
Ammann was able to design the George Washington Bridge
in a way to keep it economical.
And the way he did it was looking
at the traffic loads.
How much traffic load will he design
this bridge for?
So he was designing essentially
for a low probability of full truck loading.
A low probability that every single lane
would be back-to-back full of trucks.
So at the time the equations
or the methods that engineers used for
designing bridges was such that
traffic load, or the live load as it's called,
these are the loads of the vehicles crossing the bridge
is equal to the maximum conceivable load,
so a bridge full, back-to-back of trucks,
times one half, so about a half
of a fully loaded bridge.
That's what was designed at the time.
Ammann used some equations using the length
of the bridge, the span length and the number of lanes,
using these variables he came up with a new proposal
which essentially reduced the
probability of full truck loading.
So he proposed, instead of one half,
a number for a bridge of this span,
the number would be on the order of 1/6.
So he reduced the load, or the live load,
for something of this span,
from something of the order of
23 kips per foot or 23000 pounds per foot
to on the order of 8000 pounds per foot.
And this difference in how much load
he was designing for was the difference
of seven million dollars in savings
for the George Washington Bridge.
So the success of the George Washington Bridge
led to a trend in bridge design
that would become dangerous.
This trend of thin decks,
the symbol of the future.
So next, let's look at this trend of bridge design,
and what it meant for the bridges
designed in the 1930s and on.