Bridges Are Us

[robnisch]

Engineers say a cable-stayed bridge could be in Port of Baltimore’s future

Given the contours of modern ships and the Port of Baltimore, engineers see a cable-stayed bridge in the city’s future.

www.baltimoresun.com

There must be some structural engineers on these forums.
I have a little old knowledge of the subject.
It would seem that the trend is cable suspension. Like the new bridge going from Detroit to Windsor.
Thoughts?

 

[bilby]

The choice appears to be dictated more by fashion than by any structural questions. All the major bridge types seem to be structurally sound and in a similar ball-park price-wise, though if you want a high clearance and a long span (as you must over a major shipping lane), the only real options are suspension, cable-stayed, or cantilever. The later being most expensive, but having a lower profile - so they are useful near airports, where you need to have a high water clearance for ships, but also a low maximum height.

The fashion in the 1980s and '90s was for suspension bridges; Today it seems to have swung towards cable-stay designs.

Then there's the soon-to-open Neville Bonner bridge across the Brisbane river, which was designed by a committee who, unable to agree on what kind of bridge to build, built one that was a bit of everything.

 

[steve_bank]

My guess would be cost is the issue.

If a suspension bridge uses less steel it is cheaper.

It may be cheaper to build.

Try searching on 'suspension bridge costs versus other designs.'

There should be sites that go through the structural design issues and tradeoffs.

 

[lpetrich]

Cable Stayed vs. Suspension Bridges - The Gerald Desmond Bridge

A few key advantages of cable-stayed design include the following:

• Using balanced-cantilever construction whereby bridge segments are added symmetrically to both sides of the tower to balance the load, the cables act as both temporary (during construction) and permanent supports for the road deck.
• The symmetrical building of the road deck balances the load, eliminating the need for large ground anchorages.
• Cable-stayed bridges are far less costly for road-deck lengths of 500 to 3,000 feet (the Bridge Project has a 2,000-foot main span); and they can be built in far less time.
• They require much less steel cable and use more precast concrete sections, which accelerates construction.

and
What is key differences between cable stayed and cable suspension bridge? - The Constructor

Cable stayed bridges have a competitive span range of 150m to 1000m, with a deck span: depth ratio of 1:40 to 1:200

Suspension bridges, however, have a competitive span range of 800m to 2000m, with a deck span: deck ratio of 1:100 to 1:200

•  Suspension bridge
•  Cable-stayed bridge
•  Cable-stayed suspension bridge - a hybrid design

From Wikipedia on cable-stayed bridges:

Cable-stayed bridges were being designed and constructed by the late 16th century,[1] and the form found wide use in the late 19th century. Early examples, including the Brooklyn Bridge, often combined features from both the cable-stayed and suspension designs. Cable-stayed designs fell from favor in the early 20th century as larger gaps were bridged using pure suspension designs, and shorter ones using various systems built of reinforced concrete. It returned to prominence in the later 20th century when the combination of new materials, larger construction machinery, and the need to replace older bridges all lowered the relative price of these designs.[2]

 

[steve_bank]

If you have not seen it, Tacoma's Galloping Gertie bridge failure.

Mechanical resonance.

Google Search

www.google.com

 

[bilby]

The main reason for the (in)famous failure of the Tacoma Narrows bridge was the rectangular cross-section of the bridge deck.

Modern suspension bridges have an inverted aerofoil cross-section, so that the net vertical component of any wind load is downwards, pushing the deck into a condition in which all of the suspension cabling attatched to it is in tension.

Resonance played a role in amplifying the problem, but had the deck had a mosern shape, there wouldn't have been a vertical oscillation large enough to matter, regardless of resonance effects.

For a bridge that really did suffer from serious resonance issues, you need to look at the Millennium Bridge, a foot bridge across the Thames, which when built, had such a strong horizontal plane resonance at the frequency of human footsteps as to be unusable without the addition of expensive (albeit fairly simple) damping measures.

 

[steve_bank]

Some engineering trivia, vibration and resonance.

Back in the 80s I ran a vibration test on a camera mounted in a gimbal for mounting on an aircraft. The gimbal was hogged out of a solid aluminum black and you could not deform it with a sledge hammer. The table moved maybe 1/8 of an inch. When the table frequency hit the resonance f the gimbal it deformed like plastic.


Resonance modes of a flat plate. Animation 1 is called the 'oil can mode'.

Vibration of plates - Wikipedia

en.wikipedia.org

Interesting video if you have not seen this demo before

Google Search

www.google.com

There are two modes seen in the bridge video. One is a torsional or twisting mode, and a sine wave can be seen.

The other node is a transverse mode, a vertical sine wave can be seen across the bridge.

Elect cal or mechanical resonance means big deviation and swings with small energy inputs.

Quantum resonanceis the same general principle. A rectangular infinite potential well is a quantum resonator with modes. When the wavelength of photons bouncing back and forth in the well match the length of the box a resonate standing wave forms. A gas laser tube.