Conclusiones y Recomendaciones

ENATS 43-40 was therefore developed as an example to show what would be the consequences of adopting the ENATR 111 approach to lines that were in nearly all

respects similar to lines that were being erected at the time. Larger conductored lines had previously been erected to one of the heavy designs, the latest at that time being ENATS 43-20. Small conductored lines were erected to one of the light designs with BS 1320 being the subject of the Baldock recommendations and little changed in its various editions up to and including ENATS 43-10.

ENATS 43-40 was accordingly pitched to show that low cost lines, like the light designs, could still continue to be erected in those areas where they had served well in the past. Indeed, it was shown to be reasonable to erect lines with larger conductors to similar designs in these areas. It was also pitched to show the cost of attempting to provide security equivalent to that previously provided for heavy lines in the more exposed areas if small conductors were to continue to be used in these areas.

The layout of ENATS 43-40 reflected the probabilistic base of the design by providing a benchmark approach.

4.2.6.2 The full specification

The specification used structures very similar to those in use in current standards at the time but incorporated minor modifications to improve structural efficiency. Additionally, some supports have been specifically modified to address the principle of failure containment.

To use the specification in practice the engineer uses maps to decide the weather parameters that are likely to represent the site. This reflects the height above sea level of the site, correcting from local experience for any relevant topography that is likely to have an adverse effect. Knowing the conductor that is required, the engineer then converts these extreme weather conditions into equivalent conductor loads using the tables. A check is made on the maximum spans that may be used both in terms of conductor strength and propensity to clash during storm conditions. Propensity to clash is also a function of conductor spacing and so an awareness of the supports likely to be required is needed, although this tends to be a slightly iterative process so nothing needs to be firm and fast at this stage.

The engineer is then aware of the:

• worst design wind and ice expected at the site incorporating a probabilistic assessment

• equivalent ultimate conductor loadings which should be catered for incorporating the load factors and reliability factors that are characteristic of this specification • maximum spans, which should be used for the chosen conductor, conductor

spacings and clashing performance.

Based on the ultimate conductor loadings and knowledge of the support arrange- ments that provide the required conductor clearances, the engineer can now use the tables provided to determine the support requirements in terms of required strength. A further iteration is required to determine the failure containment principle. In essence, there is a requirement to provide failure containment measures wherever the designed line is in an area where, in extreme conditions, an unacceptable risk of conductor failure or clashing exists.

Traditional and probabilistic design standards 53 Where relatively strong conductors are used with wide conductor spacing, this requirement is considered to be inherently met. Where specific failure containment measures are required the requirement is considered to be met provided specific structure types are used with a specified frequency throughout the line. Again, the concept is that the line shall withstand without failure all likely extreme conditions and shall offer a level of resistance to cascade failure. This occurs when the failure of one component (usually a broken conductor) causes a pole breakage, which then overloads the next pole etc., and a series of poles collapse in a domino fashion until typically a section pole is reached which is strong enough to withstand further collapse. 4.2.6.3 Variants of the specification

It was envisaged at the time that ENATS 43-40 was prepared that other specifica- tions using the ENATR 111 methodology would be prepared as mentioned earlier. In particular, it was envisaged that designs with both higher and lower resistance to the likely extreme weather conditions identified for each location would be needed to meet the requirements of each company.

There are situations where it may be desirable to re-build existing lines to ENATS 43-40 standards, but where applying it strictly is impractical. In such cases ENATR 111 provides an umbrella of protection for the engineer who chooses to develop his own ENATS 43-40 variants.

As has been explained earlier in this chapter, deterministic design is based on well-defined specific loads and strengths, and sets margins of safety between cap- ability and stress. Probabilistic design accepts that all real components have a strength distribution rather than a specific single value. This is particularly significant for nat- ural components such as wood poles. ENATS 43-40 is based on a combination or semi-probabilistic design that uses a specific deterministic design strength subject to a probabilistic range of load conditions. ENATS 43-40 was borne out of a tech- nical specification ENATR 111 that was produced from historical knowledge and meteorological data. The ENATR 111 methodology was demonstrated in the devel- opment of the only national specification designed at that time in anticipation of the commencement of the 1988 regulations. This specification, ENATS 43-40, was approved before the regulations took effect.

It is actually notable that ENATS 43-40 was intended only to demonstrate the practicality of using the ENATR 111 methodology. Thereafter it was intended that alternative specifications to meet the particular needs of individual companies with both higher and lower needs in terms of reliability and, conversely, cost would be drawn up.

4.2.6.4 Design loads

Wind and ice loads

In ENATR 111 the weather load combines the factors of ice and wind pressure. The component load takes into account the gust factor (normally taken as 1.5× wind speed) in assessing the overall load. The conductor loads can be split into horizontal and vertical components.

The vertical component is the mechanical result of the conductor weight and the ice weight and is known as the maximum conductor weight (MCW). The horizontal load is made up of the wind pressure acting on the ice accretion envelope. This is known as the maximum conductor pressure (MCP).

The combination of these forces is the resultant, i.e. the maximum conductor resultant (MCR), that may occur at any particular point along the component load line. In the case of angle poles the effect of conductor tension can be significant. The maximum conductor tension (MCT) is defined as the maximum conductor tension at 0◦C at the MCR loading.

Further considerations of downpull etc. are also required but will not be considered here. Briefly, because in flat country the conductors will sag between the poles, they will be applying a downward force or downpull on the poles (trying to force them deeper into the ground – a problem in peat bogs). Normally this downpull angle is assumed to be 1:10. However, if the line crosses a valley and there is a pole in the dip, the conductors may go up from the pole, applying an upward force to the pole (especially in cold weather as the conductors contract). This is known as uplift and the conductors could be trying to pull the poles out of the ground on a badly designed line.

Section 4.2.8 looks at the deterministic snow/ice loads that are part of BS EN 50341/BS EN 50423 that are now adopted for new lines in the UK. This is a simpler version than in ENATS 43-40 Issue 1 and is similar to that used in ENATS 43-20.

Load-factored design

In preparing ENATR 111 the industry had supported a load-factored design approach for wood pole lines in preference to a full probabilistic design base similar to that applied in the then new steel tower design base to reflect the wider coefficients of variability associated with the materials and loading cases in wood pole design.

The load-factored approach used a more or less rigorous probabilistic analysis of applied loads and of anticipated conductor deflections together with a more or less traditional approach to the ultimate strengths of components and a geometric, partial-factored approach to set reliability levels.

Weather zones

It was appreciated that the weather load would not be the same for all areas. Met- eorological data was used to generate wind ordinates (1 to 6) and wet snow ordinates (A to E) for different regions or zones of the UK (Table 4.2). As can be seen, the wind ordinate increases from 190 N/mm2_{wind pressure in steps of 190 N/mm}2_{and the} snow ordinate from 5 mm of radial ice in 5 mm steps. In practice, the wind ordinate, 6, only occurs for land at >400 m in northern and western Scotland and rarely in places where overhead lines are built. In these areas there is in fact no land at this height and so effectively the wind ordinate ranges from 1 to 5. Most of the UK where wood pole lines are constructed is covered by a 2B or 2C weather zone (380 N/mm2wind pressure and 10–15 mm of radial ice load).

Traditional and probabilistic design standards 55

Table 4.2 Wind/ice load areas

Wind ordinate 1 2 3 4 5 6 Wind pressure (N/mm2) 190 380 570 760 950 1140 Ice ordinate A B C D E

Radial ice thickness (mm) 5 10 15 20 25

Software design

The original design tables etc. published in ENATS 43-40 Issue 1 could be replic- ated and amended for differing conductor arrangements using proprietary software in MS-DOS and SuperCalc formats as issued with that specification. Details of these packages can be obtained from the Energy Networks Association, although the SuperCalc program used for the development of the spreadsheet design folders is now not readily available.