ABB Structural Design of Steel Latticed Towers

5/22/2018 ABB Structural Design of Steel Latticed Towers

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Overhead Transmission Lines ABB

Structural design of steel latticed

towers


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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


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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 combinationDeformation of structure


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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.


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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


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Suspension tower


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Strain / Dead end


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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.


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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


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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 applicationpoints 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.


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Documents

ABB Structural Design of Steel Latticed Towers