Physics Week Three

Week Three of our co-op physics class focused on bridges - specifically, suspension bridges.  The class was divided into two teams, and each team was given a kit to construct a model suspension bridge. 

Naturally, the kids managed to turn it into something of a race, even though that wasn't exactly the intent.  Here is the "winning" team with their bridge:

When the models were completed, we talked about how the suspension bridge works.  What is being "suspended," and why?  In very simple terms, the bridge deck is suspended from the cables and supported by the towers and anchorages.  By using suspension cables, the load can be distributed more effectively, making the bridge more durable than one where the load is carried only by supports underneath the bridge deck.

Gravity pulls down on the bridge deck and everything on it.  This downward pull is balanced by the upward pull of the suspender cables, which in turn tug downward on the main cables.  The main cables run over the towers and are anchored in huge solid anchorages on shore.  The downward pull of the main cables generates a horizontal force, which would shear the towers if they had to resist it alone.  That's why the cables run over and past the towers to the anchorages.  The bridge deck is often reinforced by trusses underneath to prevent oscillation caused by the wind.

The basic forces at work are:

• compression - a force that crushes something together
• tension - a force that pulls something apart
• shear - forces in opposite directions, but offset, not pushing at the same spot.  When two forces are pushing past each other, what's between them tends to be cut or broken.

For fun, we also looked at some pictures of suspension bridges, from the famous Golden Gate Bridge

(image from Wikipedia)

to the footbridge in Capilano, British Columbia (which I chose to show them, because I've been there).

I also found some fascinating pictures of bridges that would truly scare the living daylights out of me, given the fact that I have a fear of bridges anyway.  These come from the website Travelet, on a page titled "Most Dangerous Bridges in the World" - check out the website for all the pictures, and ask yourself if you'd be brave enough to cross some of those bridges.

This one isn't too bad, I guess - Carrick-a-Rede Rope Bridge in Ireland:

But less likely to inspire my confidence were these...

An Inca rope bridge

Better yet, this engineering masterpiece over Borit Lake in Pakistan.  I love how they left the broken "unsafe" bridge there right beside the brand-spanking-new-and-improved model.  Hmmm...

Still, I think I'd prefer the relative safety of the warped slats of wood and randomly strung handrail fence to the Glen Nevis Wire Rope Bridge in Scotland...

Are those people in the picture smiling???  How far is the drop???

I thought it was very interesting to find out that the very simple principles of that wire rope bridge, and the Inca bridges that had been long in use by the time the conquistadors arrived in South America, are the same physics behind the modern engineering of the incredible Akashi Kaikyo Bridge in Japan (with a total length of almost 13,000 feet, and the longest suspension span in the world at 6532 feet)

(image from Wikipedia)

Although personally, I'm still scared of bridges. 

The information for this lesson came from the instruction booklet with the bridge kits, and from the website How Stuff Works.

This is the third post in a series - Physics Week One (gravity and friction), Physics Week Two (structural engineering)

Does knowing how suspension bridges work make you feel better about their safety? Let me know in the comments!

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This post was added to the Throwback Thursday Blog-Style link-up hosted by Tots and Me... Growing Up Together! on February 18, 2016.

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