Most cable stayed bridges have 2 towers like this one. The cables that support the deck run from the deck to the tower.This 7th grade team built this model at OMSI.. The lines (string) attach to the wood cross beams that are screwed to the underside of the deck. Then they are connected to the towers. There’s another set of lines going from the other side of the tower back down to the section of the deck- the side deck. Here’s a team of 7th grade girls with their cable stayed model. Looks like there are 6 lines from each tower supporting the deck and the 50 pound live load. If that was all there is, the bridge would collapse- the towers would tip in towards each other. What’s preventing this? Notice the single line from the top of each tower that is tied to a C-clamp that is clamped to the table top. That single line takes the place of the multiple lines found on most real bridges. -Like the Tillicum Bridge below.
Take a look at the part of the deck between the 2 towers. The weight of the deck and the people and buses- all this pulls on the cables, which pull on the towers. To balance this pull- and negate the pull on the towers, there’s an identical set of cables pulling on the towers on the opposite side.
In recent years, engineers have come up with a new design—a single tower bridge.
The Erasmus Bridge in the Netherlands, has only 2 thick cables (back stays) to balance the series of cables holding up the deck.
Santiago Calatrava, a famous Spanish engineer, went a step farther and eliminated the back stays altogether.
The Alamillo bridge in Spain has the tower leaning backwards. Now the weight of the deck (and the cars and people on the deck)-plus the weight of all the concrete in the tower is enough to balance the pull of all the other cables. Jake used this idea to build this model. The weight of the wood tower wasn’t nearly enough to counter act the pull of the 6 lines holding up the deck- so we used a few clamps to hold down the tower.
Which brings us to our cable stayed model.
The tower doesn’t weigh enough to balance the weight of the deck, so we hung a “skip” from the top of the tower and put a 4.7 pound brick in. This is enough ctwt. to balance a load ( 1 brick) placed half way along the deck.
Mateo explains it below.
So if we want to put a heavier load on the deck, and we don’t have any more bricks to increase the counter-weight- what would happen if we increase the distance from the pivot point to the ctwt.?
Increase that distance and we increase the Moment. Mateo added 2 wood extensions- bolted to the back of the tower and suspended from the tower, and hung the “skip” . So now we have the same ct.wt. (1 brick), but it’s farther away from the fulcrum, so we’ve increased the Moment.
