oh hi guys squeaks and i were just playing with some blocks you know it may not seem like it but the cities and towns that we live in are all built using the same rules as our little pretend village over here it's just that the real ones are bigger and one of our viewers six-year-old hannah from the uk wanted to know how some of the things that we build stay standing so she sent us a really great question why are bridges so strong excellent question sometimes when a road or railroad track needs to go across
something big like a river or a deep valley experts called engineers design and build bridges to do that job and bridges can be really busy take for example what's said to be the world's busiest bridge the george washington bridge in new york city look at all those cars and trucks it has to be pretty sturdy to carry so many people in cars for a bridge to carry that much weight it has to be built a special material like iron and steel but it takes more than tough materials to make a strong bridge so let's look
at how bridges work one very simple kind of bridge is called a beam bridge when we say simple we really do mean simple a beam bridge can be just a log that you use to walk across a stream or put a long strip of cardboard between two short blocks that's a beam bridge too all bridges can hold a certain amount of weight but what happens if we put too much weight on a beam bridge let's find out it collapses so a bridge that carries trucks and cars which are very heavy would have to be stronger
than a bridge that carries bikes or people on foot which are lighter so how do we make stronger bridges well over time people have learned that certain shapes can be used to make stronger bridges take a look at this railroad bridge it has to be strong because it carries trains what shape do you see that's right triangles and that's not by accident the fact is triangles are really strong shapes for building if you put force on one side of a triangle it bends but if you put force on its point it keeps its shape that's
because the two sides of the triangle are pushed down by the force and the bottom gets stretched out to both sides each side feels the force but none of them bends and this makes the triangle a really sturdy and stable shape this is why you'll see lots of triangles and bridges both above the part that you actually travel on called the deck and below it the long string of triangles that you see in a bridge is called a truss trusses help a bridge spread out the weight that it has to carry but not all bridges
are made of trusses if a bridge has to cross a really wide body of water it might be too difficult or expensive to build a truss bridge so engineers design another kind of bridge called a suspension bridge the golden gate bridge in california is a great example of a suspension bridge suspension bridges work by using a force called tension tension is just pulling something tight suspension bridges are made of a deck that's hung or suspended from thick cables that stretch from one side of the bridge to the other these cables are supported by tall towers
and then are held down tightly or anchored on both ends suspension bridges are strong because the force on the bridge gets spread out the weight of the cars or trains or horses whatever is traveling across it pulls on the cables creating tension those cables then pull down on the towers and also pull on the anchors on either end of the bridge to hold up the deck i could go on and on about all kinds of clever bridges but basically bridges are strong not only because they're made of strong materials but also because of the smart
designs dreamed up and planned by engineers so thanks for asking hannah and thank you for hanging out with us at scishow kids see you next time
