I'm in Sydney
Australia and I'm here at the harbor and
I'm really excited to see the bridge
Sydney Harper
bridge I'm particularly excited because
I used to live in Newcastle pontine in
the northeast of England where they had
the Prototype of this bridge a smaller
one where they were trying out the
design the same shaped design
but not as wide because the river TI in
Newcastle is not as wide as the harbor
is
here and I was talking to Brady about
the bridge and he wanted to know why on
Earth would you build a bridge out of
steel which is easily corroded by salt
from the sea and it rusts with oxygen in
the air when it's wet why wouldn't you
build it from concrete or something
that's less liable to corrode and the
answer is that steel is so much
stronger now you can't just use pure
iron because pure iron is rather soft
and with a structure as big as this it
would bend under its own weight but
steel which is a mixture of iron with a
small amount of carbon and sometimes
some other metals as well is very very
strong steel swords are much better than
iron
swords don't break when you hit your
enemy which can be quite embarrassing if
you're in the middle of a fight and so
steel is a really good material to build
something like this
bridge and this bridge I looked up has
more than 58,000 tons of steel in it at
the same time you've got to join the
steel together in modern Bridges they
use a technique called welding where
they heat the steel to a high
temperature and essentially melt the
edges together so it becomes one solid
piece of metal here they used a slightly
older technique called
riveting you make a hole through the two
pieces you want to join and put a steel
or iron Rod through it and then you bang
the two ends of the rod so they form
heads like the head on a screw and and
that holds the things together and it
holds it very
rigidly there's an expression in English
being riveted which means really fixed
on something like I hope you're riveted
on my videos now which just to stress
how strongly a rivet will hold things
together there's still the question of
rusting because the steel when it gets
wet will react with oxygen in the air so
you have to paint it and if you have a
really big bridge you paint it all the
time you have a gang of workers who
start painting at one end and by the
time they've got to the other end it's
time to start again and I guess that
that's what they do here as well there
are several bridges in the UK where
they're constantly painted painting puts
a layer over the surface of the iron and
essentially stops the oxygen getting to
it
the alternative strategy is to put
pieces of some metal that corrodes more
easily like zinc and then the zinc
corrodes
preferentially this is what happens
where you have galvanized iron the sort
of iron that you put on sheds and
buildings in the country where you have
iron covered by a thin layer of zinc and
the zinc corrodes away but it corrode
sufficiently slowly that the roof exists
as long as you need it any metal when
you heat it up expands in case of a rod
it gets
longer and obviously the longer the rod
the more it'll
expand this is a big problem in railway
lines if you don't allow for expansion
when the weather's hot the railway lines
bend and buckle and the trains fall off
and with a bridge it's even worse so
what you have to do normally is that you
fix one end of the bridge and the other
end of the bridge is on rollers so when
the bridge gets hot and expands it
physically moves on the rollers
backwards and forwards and the precise
way that the rollers are implemented
differs from each bridge I don't know
whether this bridge actually uses
rollers or it has some other sort of
expansion
joint and it's an interesting point
because building a bridge is an
engineering problem and Engineering is
different from science because often
there several different solutions to the
same problem in
engineering you just have to look at all
sorts of different sorts of cars which
all are solving the problem how do you
move from one place to another in
science there's usually only one answer
why do two hydrogen atoms bond together
you've either got the right answer or
not I was pretty impressed when I first
saw the
bridge it's always a bit difficult when
you've seen something in pictures so
many times to know what it'll be like I
was surprised that there were so many
buildings and things around it because I
suppose in photos people usually try and
show it
isolated