Tensile cable stayed bridges

Large compression structures are expensive and heavy, and place a limit on the span of these types of bridges.

This page documents a more tensile design. The pattern is based on that used by cable stayed bridges, but it manages without a roadway that must sustain large compressive forces.

Here is a model of such a bridge:

Bridge model

It resembles a cable-stayed bridge - but the roadway is light and slender - and is incapable of supporting any compression forces, as it would in the classical cable-stayed pattern.

Suspension/cable-stayed hybrid

I think a hybrid approach between suspension and cable- stayed bridges is likely to be the best way to deal with this issue.

First, to provide some visual context, here are the two types of bridge in question:

Suspension bridge

Cable-stayed bridge

The first observation is that if you pull on either end of the cable-stayed bridge hard enough then you can take its roadway out of compression - at the expense of reintroducing ground anchors at either end.

Ground anchors - and their cost and associated environmental disturbance - are often cited as one of the disadvantages of suspension bridges.

However, if sustaining horizontal compression forces is what is required, the earth is by far the cheapest, strongest and best strut imaginable - and shifting the compressive forces away from the ground and into a man-made strut running parallel to it is unlikely to be a very intelligent move.

However pulling on the ends of the roadway is not a very satisfactory solution. If the roadway is made of the same material all along its length, then as the applied force takes the compression out of the roadway nearest the piers, it is simultaneously applying a huge stretching force at the point furthest from the piers.

What it would be nice to be able to do is to apply tensile forces of different magnitudes at different places along the bridge's length - so the areas near the bridge piers have the greatest forces applied to them. This can be done by using a number of cables, running parallel to the roadway:

Cable-stayed bridge - normal

Cable-stayed bridge - hybrid

If all the cables run across - carrying the same strain all along their length, then the opposite problem is introduced - albeit much more mildly - the roadway becomes under compression near its mid point.

There are two simple solutions:

• Anchor off one cable in six or so;
• Have different sections of the long cables carry different strains - so the horizontal forces balance.

In the following model, the upward curve in the roadway has been deliberately magnified, so its shape is visible.

The cables anchor very simply to the roadway - and do not run across the full length of the bridge.

Ideally, the thickness and tensile strength of the road should vary along its length, according to the tensile force it needs to resist - but it is not critical if this doesn't happen.

Bridge model - left hand side

Bridge model - right hand side

Properties

The large ground anchors at either end of the bridge are back - the resulting bridge is rather like a suspension bridge in that respect.

The result scales upwards to large spans like a suspension bridge does. The main problem that prevents cable-stayed bridges from covering large spans is eliminated by using this approach.

Relevance to suspension domes

However, in other cases the need to handle the large compression forces from bridge's cables can easily become a substantial point of difficulty.

The struts and trusswork required to sustain compression forces are fundamentally heavy - and adding them is likely to increase cost, weight - and the stresses and strains on all other parts of the structure.

Usage

Possibly. However, so far I have not encountered the design in the literature - and the evidence I have gathered so far about existing implementations is not very strong.

One of the main observable features of such a design is that the roadway forms a catenary curve - similar to that commonly found in many suspension bridge designs.

While most cable-stayed bridges seem to have pretty flat roadways, some do bulge upwards in the middle - e.g.:

Seine River, Normandy

Seine River, Normandy

Unfortunately, there are other possible reasons for the road of such a bridge bulging upwards in the middle besides it being in tension.

Such an upward bulge in the roadway:

• could form a compression arch - which may be seen as being desirable;
• increases shipping clearance;
• reduces the lengths of the cable stays;
• increases the angles the cables make to the roadway;
• improves drainage: avoiding water pooling on the roadway;
• can improve aesthetics sometimes.

The idea of reducing the lengths of the cables (and increasing the angles the cables make to the roadway) would suggest that the resulting bridge roadways form two straight sections, with something close to a fold in the middle.

That does appear to be the pattern used in a number of bridges - e.g.:

Queen Elizabeth II bridge diagram

Queen Elizabeth II bridge

The profile of a hybrid cable-stayed/suspension bridge is rather different - it tends to be flattest in the middle, with the curvature concentrated near the ends:

Bridge model - roadway flattest in the middle

This suggests that such bridges are not really using the principle described on this page - and instead have a roadway that bulges upwards in the middle for different reasons.

Another possible approach to detecting such bridges involves examination of the construction method - via photographs.

A classical cable-stayed bridge would be constructed from the piers outwards. I have collected some photographs of this process - see the links at the bottom of this page.

Construction of a hybrid cable-stayed/suspension bridge would be most likely to involve throwing a lightweight framework right across the full span, and then fleshing the structure out.

So far, I have not found any evidence of this latter technique being used.

Near the bridge piers

It can't possibly make sense to allow tensile forces to run all the way across the length of the bridge, just to remove a few minor compression forces at either end of the bridge - yet that is what happens when cables are applied near the piers.

It seems reasonable to allow portions of the roadway near the bridge piers to bear some compression loads - by using a simple trussed structure there - in order to prevent the resulting horizontal forces being carried right across the bridge.

Hybrid bridge FAQ

History

Links