Overhead Transmission Lines ABB

Structural design of steel latticed towers

Overhead Transmission Lines ABB

Structural design of steel latticed towers

•Specific criteria and design approaches recommended for steel transmission towers•Self supporting and guyed structures•Suspension and tension towers•Tower outline definition•Geometrical and Mechanical checks

Overhead Transmission Lines ABB

•Specific criteria and design approaches recommended for steel transmission towers

It is recommended that the line designer provides at least the following informations:

•Applied loads, including safety factor, at attachment point of insulator/earthwire

•Wind load on support

•Load combinations

•Ultimate limit state for each load combination

•Deformation of structure

Overhead Transmission Lines ABB

•Self supporting and guyed structures

Towers could be designed as self-supporting or as guyed towers. The difference between the two type of configuration is that the stability of self-supporting tower is guaranteed by its foots with relative foundations, while the guyed tower use some pretensioned wires connected between tower and relative foundations.

Overhead Transmission Lines ABB

Self supporting structures

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Guyed structures

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•Suspension and tension towers

Three basic tower definitions are recommended:

1. Suspension the conductor phases pass through and are suspended from the insulator support point at the tower

2. Strain the conductor attachment points are made by attaching the conductor to a dead end clamp, a compression or bolted fitting and connecting the clamp through the insulator string directly to the tower. In order to connect electrically the adjacent span a jumper is looped through or around the tower body

3. Dead – end conductor attachment point is the same of strain tower, but this type of tower has a permanent different tension on each side of tower

Overhead Transmission Lines ABB

•Suspension tower

Overhead Transmission Lines ABB

•Strain / Dead end

Overhead Transmission Lines ABB

•Tower outline definition

The geometric configuration of latticed transmission tower is based on the overhead ground wire shield coverage, number of circuits, conductor phase arrangement selected to satisfy the electrical clearances and right-of-way requirements.

The development of tower configuration starts with the upper portion. This section of the tower is designed for the selected vertical and horizontal phase spacing and electrical clearance around each conductor.The configuration should be as compact as possible around each conductor.

The lower portion of the tower is designed next and depend to useful height, clearance to ground and extension or reduction of height request by tower spotting.

The wider the tower base gives smaller loads on foundation but increase the length and the weight of bracing members. Therefore an economical balance must be reached between the tower base width and the size of the bracing members.

Overhead Transmission Lines ABB

Geometrical configuration of structure

The geometrical configuration of the head of tower is characterized by:

•Type of transmission line ( voltage, circuits, conductors)

•Electrical clearance

The height of tower depend on:

•Sag of conductor

•Electrical clearance to ground

Loads on foundation are affected by:

•Width of base of tower

Overhead Transmission Lines ABB

•Geometrical and Mechanical check

The arrangement of tower members should keep the tower geometry simple by using as few members as possible.

A tower is described by a design drawing which shows overall dimensions, joints, and member locations.

For purpose of analysis a tower can be represented by a model composed of members interconnected at joint. Members are normally classified as primary and secondary member. Primary members form the triangulated system that carries the loads from their application points down to the tower foundations. Secondary members are used to provide intermediate bracing points to the primary members and thus reduce unsupported length. Secondary members can be easily identified on a drawings as members inside a triangle formed by primary members.

Overhead Transmission Lines ABB