Suspension bridge ideas

While I'm an amateur bridge designer, I can't seem to shake my conviction that the professional designers have somehow got themselves stuck in a traditional rut - and are probably wasting millions of taxpayers dollars in the process.

More catenary cables

More catenary cables

This idea keeps the catenary cable near to the body of the bridge.

That saves a considerable length of vertical suspension cable.

Some of the weight of the body of the bridge is supported by the cable that reaches the tower near its base. This weight no longer has to be supported by the upper part of the tower - which can consequently be made lighter and more cheaply.

The base of the tower is already likely to be very strong - and is probably already very well positioned to sustain this force.

The upper catenary cable no longer has to bear the entire weight of the bridge - since the regions near the towers have their weight borne directly by other cables.

Keeping the catenary cable close to the bridge's horizontal member results in greater stabilising forces against lateral perturbations.

This design reduces the extent to which the bridge acts like an aeolean harp - and minimises wind resistance.

The shorter cables have higher notes assocaited with them - and higher frequencies are more easily damped and are less likely to result in vibrations.

The lower catenary cable need only carry a small fraction of the weight of the main catenary cable - and can thus be made of lighter and cheaper stuff.

The cost of the additional catenary cables is likely to be greatly outweighed by the cost of all the vertical cables that are no longer needed, the cheaper towers, and thinner catenary cables that can be used.

The lower catenary cables should extend right across the entire length of the bridge. There is likely to be a section in the middle of the bridge where they run for some distance horizontal, level with the roadway. That is fine.

Application of the strategy described here can be expected to reduce both the weight and cost of large suspension bridges - allowing them to more easily attain larger spans.

Even more catenary cables

Even more catenary cables

Some existing bridges have partial implementations of this idea:

Brookline bridge

Tenkenji bridge

Tobiuo bridge

Meiko nishi ohashi bridge

Slanting struts

Slanting struts

Apologies for the primitive nature of this diagram.

Here the idea is to reduce the intensity of the stress point the cables have endure at the top of the tower.

The overall height of the bridge is reduced slightly in the process.

Each tower has to bear about one third of the force from the cables that the original single tower had to bear - so the cost is not three times as large as using a single tower, but is broadly comparable.

Additionally, all the towers can be made slightly shorter than the original one, resulting in some savings.

As a consequence of this ideas, the cables are stabilised by the towers slightly nearer to the bridge's central point.

The three towers can profitably be connected by cross cables - again assisting with stability considerations of the entire structure.

I observe that slanting struts were included in one of the old proposals for a Gibraltar bridge:

Gibraltar bridge proposal

Here the struts extend a substantial distance horizontally - in what appears to be an attempt to reduce the tensile-only span.

The slanting struts in this design also act as anchors for the vertical support cables.

I should add that there are plenty of problems associated with the idea of slanting struts.

It may sometimes prove beneficial overall - but I think it would need an exhaustive cost-benefit analysis to see if it's worth it.

I do think the idea is worthy of further investigation, though.

For those interested in the history of the ideas described here, some relics of an earlier page have been preserved here.