An impractical bridge, but would it work?

[Dylman3003]

What I'm asking is this: I'm researching bridge designs for a balsa wood bridge, but I'm wondering why that the ellipse of a curve of a bridge support system is never completed?

I understand that bridge that had an ellipse bisected by a roadway would be impractical in reality, but would it be more efficient than a half ellipse in my case where I'm building a balsa wood bridge
(Disclaimer: This is not homework)

 

[BobG]

An arch or sphere or what have you is strongest when the pressure is pushing inward. The structure is being compacted into itself.

If the pressure is outward, an arch or sphere isn't as strong.

Gravity only pushes straight down. That's pushing inward on the top half of an ellipse and outward on the bottom half.

 

[Dylman3003]

But if the ellipse is completed, shouldn't the force downward be redistributed as a compression force against the top when (the load is centered) the bottom of the ellipse pulls downwards? In effect, shouldn't this new sideways force on the top directly compress against that of the top trying to pull downwards?...

i may have to use a lap joint than a perfect butt joint for this statement though

 

[Studiot]

First of all, welcome to Physics Forums.

but I'm wondering why that the ellipse of a curve of a bridge support system is never completed?

Who said they are never completed?

Take a look at the picture of the Quebec bridge in post#13 in this thread.

You will see two fully completed "ellipses" supporting a centre drop-in "half" ellipse span.

Self weight is the controlling factor in most bridges since the weights (densities) of real materials needed to withstand traffic loads and environmental degradation far exceeds that of balsa. Self weight is usually (by far) the largest load imposed on a bridge.

Given the above you can immediately see why saving half the material is often attractive.

What you are describing would work - a ring structure will span between a couple of supports.

What bridge builders are doing by cutting it in half is to replace the forces that would be present (in a full ring) at the cut with forces supplied by the supports. This replaces the weight of half the ring with supports forces.

Of course to supply these (quite enormous) support forces can only be done at suitable sites.

Does this help?