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?

 

[PJC01]

As a scientist, my response to your question would be that while an ellipse-shaped support system for a bridge may seem more efficient in theory, it may not necessarily be practical or feasible in reality. The design of a bridge involves a complex balance of structural integrity, materials used, and the forces acting upon it, and an ellipse-shaped support system may not be able to withstand these forces effectively.

Additionally, constructing an ellipse-shaped support system would require precise calculations and measurements, which may not be possible or practical when building a balsa wood bridge. Balsa wood is a lightweight and delicate material, and any slight miscalculation or error in construction could compromise the overall stability and strength of the bridge.

Furthermore, the design of a bridge is also influenced by factors such as cost, time, and resources. Building a half ellipse support system may be more efficient in terms of these factors compared to a full ellipse, making it a more practical option.

Ultimately, the design of a bridge should be based on careful consideration of all these factors, rather than simply aiming for the most efficient shape. I would recommend exploring different bridge designs and consulting with experts in the field to determine the most practical and effective option for your balsa wood bridge project.