Bridges & Catenaries

Suspension Bridges

Suspension systems are ideal where long spans are required, as in highway and pedestrian bridges, supporting conveyors, pipe lines and overhead passageways in industrial plants, and overhead crossovers above railroads.

When appearance, durability, utility and ease of construction are considered, suspension bridges are often the most economical to build. For example, flood damage to exposed piers is eliminated and difficult or dangerous pier foundations can be avoided with a suspension-cable construction. Often the entire problem area is spanned; the foundations can be located at economical installation points where they are least likely to be damaged. Great clearance is obtained since the supporting structure is above the floor and has no intermediate supports.

Stiffening trusses may be incorporated into the design of foot bridges and similar bridges, where they also may serve as hand railings. These trusses add relatively little to the cost of the structure, and they ensure a bridge free from disturbing floor movement.

Structural strand and wire rope is used for the main cables, suspenders and wind cables of highway, pedestrian and pipeline suspension bridges. Structural strand is manufactured through 5 1/2” diameter and wire rope up to 7” diameter.

Pre-stretching greatly reduces the constructional stretch of the structural strand or wire rope and improves the overall elastic stability. While in the pre-stretcher, overall lengths and intermediate tower and suspender points can be measured to close tolerances under prescribed tensions.

Tied Arch Bridges

In a tied arch bridge, the bridge deck is suspended by structural strand or wire rope hangers hung from a steel or concrete arch. Tied arch bridges normally cross short to medium spans. Structural strand has been used in tied arch bridges having span lengths of more than 1,000 feet.

Cable Stayed Bridges

The cable stayed bridge is a relatively new type of bridge, in which structural cables radiate diagonally from one or more towers or pylons to a connection point on the bridge girder. This bridge form allows a very efficient use of material, which results in a lighter structure and less massive foundation.

Cable stayed bridges have been built with a main span as long as 2,300 feet between the towers. Frequently, the limiting constraint on span lengths is the permissible height of the pylon.

Galvanized helical structural strand has been specified for cable stays as have several other cable configurations. Various types of socket attachment and corrosion protection systems have been used with varying degrees of success. Zinc-poured attachment of sockets is recommended. Corrosion protection systems are too varied and rapidly evolving to recommend a particular system.

Vertical Lift Bridges

In a vertical lift bridge, the movable span is balanced by counterweights located in the towers at each end of the span.

Each corner of the span is connected to the counterweights by sets of large wire ropes which operate over parallel-grooved sheaves at the top of the towers. Using powered winch drums, smaller wire ropes raise and lower the movable span.

The lengths of the counterweight ropes in each of the four corners must be matched closely to ensure equalization of tension. Uniform stretch also is an important factor. In vertical lift bridges where counterweight clearances are limited, ropes should have minimal constructional stretch. Counterweight ropes can be pre-stretched to reduce constructional stretch, and measured under tension to ensure closer control of rope lengths. Normally, operating ropes do not require pre-stretching since minor length adjustments can be made at the drums.

Catenaries

The most elementary structural suspension system is a catenary, which is similar to that of a suspension bridge. This system usually requires end towers and abutments to resist the tension in the catenary and a stiffening structure to eliminate the flutter in the roof system.

Preloaded Catenaries

This system consists of a central tension unit connected to an exterior compression ring by radial cables. Preloaded catenaries are ideal where a clear span, free from central supports, is required. To eliminate flutter, a relatively heavy load of precast or poured-in-place concrete may be placed on top of the cables.

Tents

This system consists of parallel assemblies or radial assemblies extending from one support point to various abutments, with the roofing material spanning between the assemblies. This system, in addition to its requirement of vertical posts within the covered space, makes no effort to solve the flutter problem. Essentially, the cables are sloping catenaries governed by the laws of statics.

Grids

To avoid flutter without adding heavy weight, grids of interlacing cables are sometimes used to dampen the catenary assemblies. In some cases, these surfaces contain reverse curves (convex) created by cables having opposite curvatures; usually, these convex cables have an initial tension and mirror the concave catenary cables.

When flutter problem has been solved by placing a mass on top of the cables, such as precast concrete planks, this additional mass adds to the superimposed weight. Damped cables, on the other hand, do not require additional weight to avoid flutter.

A properly damped, suspension system, consisting of cables designed to resist all superimposed static loads, may be covered with a light roofing material.

A number of such suspension roofs and systems have been built, and they have demonstrated a complete absence of flutter and a high degree of rigidity. Though much lighter in weight, their rigidity is comparable to, or higher than, conventional structural elements of steel trusses or girders.