AVS Australia Techsheet

SERVICEABILITY INDEX - DEFINITION

Serviceability index = (factored strength over factored load)

Overhead line structures serve in the public domain and as such, the safety (and serviceability) requirements of these building, or structures, are defined in relevant regulatory design codes and by established industry-wide practice.

Interpretation of the national building codes as they apply to overhead are set down in simplest form in the standard:

AS/NZS 4676: Structural design requirements for utility service poles. SA/SNZ.

and more comprehensively in the guide:

ESAA C(b) 1 : Guidelines for design and maintenance of overhead distribution and transmission lines. ESAA/SA.

The structural design and performance assessment methods prescribed in these documents are based on limit state concepts.

Where a structure (or part of it) may fail to fulfill basic functions required of it, it is said to have reached a limit state. Two limit states are identified, namely the:

Ultimate strength limit state: where the criteria specified relate to the strength and stability of all or part of the building; and the

Serviceability limit state: where the criteria specified deal with deformation and deflection limit which effect the appearance or function of the building.

A number of limit state models are defined in the AS/NZS 4676 standard and the HB C(b)1 guide, with which a line structure shall be analysed for compliance.

These models are represented by the general equation:

this is the Limit state model equation: (factored strength greater than factored load)

where

is the nominal strength of the component,

is the strength factor which takes into account variability of material, workmanship, etc.,

is the wind load pertinent to each loading condition,

is the line reliability factor applied to weather loads,

are the applied loads pertinent to each loading condition (E.g. Live, Dead, Snow, Ice, Seismic, Conductor tension, etc.), and I

are the load factors which take into account variability of load, importance of the structure, safety implications, etc.

This equation provides a definition of the modeled 'serviceability index' SI, where:

this is Serviceability index: (factored strength over factored load)

Conforming structures have SI’s greater than 1.0.

With this usage of the word ‘serviceability’ in context, a high index (SI >> 1.0) indicates a highly serviceable structure with excess strength; a low index (SI < 1.0) indicates risk of failure and a need for reinforcement or replacement.

Distribution companies' have an obligation under the Australian standards to undertake the following policy:

In compliance with its statutory obligations as the operator of overhead Iine networks, a distribution company requires that all new line construction (including the replacement of existing line structures) at least meet the design requirements set down in the latest releases of:

AS/NZS 4676 for utility service poles, and

HB C(b)1 for distribution and transmission lines

and that proof of this is documented with reference to the so-called serviceability index of factored strength over factor load that corresponds to the design requirements that model the most severe loading condition, and to the modeling equation itself .

Furthermore, a distribution company should only recognise an assessment of the safety (and serviceability) of an existing line structure as valid, if the same design requirements are applied in the analysis.

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