GB2423999A - Tower with shock absorbing means - Google Patents

Abstract

A pylon or other tower includes shock absorbing means 5, 7, which absorb or dissipate forces, such as wind generated uplift or compression forces, travelling along the length of a structural member of said tower. The absorbing means preferably comprise one or more plates 5 which join together two structural members and may comprise part of a structural member. The plate may be sized the same as a conventional splice plate. The absorbing means 5 may be located on a leg 2 or alternatively, or in addition, absorbing means 7 may be located in the bracing 6. The absorbing means includes fixings means which align with existing tower fixing means. Preferably the material of the absorbing means has a Young's modulus less than 205kN/mmé and includes a coating for protection against bi-metallic corrosion/oxidation, it is preferably a material with an Ultimate Tensile Strength equivalent to high tensile steel.

Description

IMPROVED TOWER CONSTRUCTION

This invention relates to the field of the construction of tower structures. The invention is particularly applicable to electrical transmission line towers which are used to support electricity transmission lines, but may also find application in any tower structures which are required to withstand significant lateral forces, for example telecommunications masts, cranes, oil rigs etc. Transmission line towers support the electrical conductors which form the basis of any electricity transmission and distribution network.

In the United Kingdom alone there are in the region of 50000 steel lattice overhead transmission line towers, 23000 of these forming the National Grid Transco (NGT) 275/400kv supergrid', with the remainder forming the lower (132kv) distribution network. Generally, the NGT supergrid' is in the region of 40 years old, with the local distribution network being considerably older, in the region of 70 years.

Transmission line towers typically have four orthogonally spaced legs, each rigidly set into a foundation structure. Whilst the foundation design can vary from structure to structure, almost all towers connect to a foundation "stub" steelwork via a butt-joint connection which is held in place with bolted "splice plates".

Towers need to be able to withstand lateral forces caused by adverse weather conditions, without failing. The key issue in avoiding tower failure is the tower's resistance to uplift or compression caused by wind etc imposing lateral forces on the tower. In other words, as lateral * II. *I* * *..

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* . I.. * * I S * I I * Si I * S * 0I* 0 * S wind force is incident on the tower, the leg first impacted by the wind is subjected to a lifting force ("uplift") as the leg tries to lift out of its foundation. Given the rigid structure of the tower, a lifting force on one leg results in a corresponding downward force on the diametrically opposite leg which significantly increases the compressive forces on this leg.

During a storm event, the incident direction of wind forces may be rapidly changing, further increasing stresses on the tower structure as legs are alternately subjected to uplift and compression forces.

In addition to the everyday forces imparted on the towers through environmental loading (wind, ice and other weather conditions etc), another event which might cause failure of the tower is the failure of the transmission line (the "conductor") which is supported by the tower.

Conductors are supported between towers under tension and therefore, if a conductor breaks, the towers at either end thereof will be subjected to a high shock loading of relatively short duration as the shock of the conductor breaking passes through the legs of the tower. The effect of such a "broken wire" condition is illustrated in Figures lB and 1C.

Under such conditions, the tower as a whole may be subjected to forces that cause it to fail. Under most Design Codes/standards, a tower "failure" is defined as any one leg of the tower displacing by more than 10mm during a single event (e.g. a storm or broken wire condition) EU code EN50341 sets regulations governing the strength * .1* *I* * *I.

a. S * S S S * S. S S S S 55 * * uS S S * S * * S S S S S S * IS. * a a a requirements for transmission line towers. This standard is perceived as very onerous in comparison with strength requirements to date in the United Kingdom. As a result, there is a desire to improve tower foundations in order to minimise the risk of failure, as defined by the EU code.

For example, new constructions of tower foundations may be bigger and stronger (and hence more expensive) than traditional tower foundations. Existing tower foundations may be strengthened in situ by the casting of concrete slabs around the base of the tower and by the installation of load-bearing piles. This method is particularly expensive, disruptive and time-consuming to implement, perhaps taking a month to reinforce a single tower.

The UK mobile telecommunication networks consist of approximately 35QQQ steel lattice structures having varying foundation types but having similar tower - foundation connections to transmission line towers (as described in more detail below) . Typically these structures are in the region of 20 years old and similar issues concerning the risk of failure apply, meaning that the present invention is equally applicable to such structures.

Therefore, there is a need for an improved tower construction that seeks to overcome the above-described problems.

According to a first aspect of the invention there is provided a transmission line tower including shock absorbing means which means, in use, absorbs or dissipates forces travelling along a longitudinal axis of * SI. . ..

S I * I I * * II S a * p * * * Ph * I S * * . - 5 0 a p * a *5S S p i * a structural member of said tower. Such forces may be uplift forces or compression forces resulting from lateral forces incident on the tower. Instead of transmitting the forces from the structural members of the tower to the tower foundation, the shock absorbing means, which is preferably located intermediate the structural member and the tower foundation, absorbs or dissipates the forces so as to reduce the risk of tower failure.

Preferably, the shock absorbing means comprises one or more plates which serve to join two structural members of the transmission line tower. In a preferred form, said plate is of a suitable size and shape to enable it to directly replace a conventional splice plate joining two structural members of a transmission line tower which meet at a butt-joint junction.

Advantageously, said shock absorbing means is situated in a leg of said tower.

Alternatively, or in addition, said shock absorbing means is situated in the bracing of said tower.

Preferably, said shock absorbing means includes fixing means which, in use, align with fixing means present on said tower.

Preferably, said shock absorbing means comprises part of a structural member of the tower. The shock absorbing means need not necessarily be a plate bridging a junction of two structural members of the tower. Alternatively, the shock absorbing means may replace part of a single structural member so that the shock absorbing material is located intermediate the two parts of the structural * lie It* 4 *11 II ? * * I * * I C IS * .. I. C I * * * S S * S I C 5 S.. * S S member.

In a preferred embodiment, said shock absorbing means comprises a material whose Young's modulus is less than 205kN/mm2.

Preferably, said shock absorbing means includes a coating for protection against bi-metaljjc corrosion and/or oxidisation. Towers are generally painted or coated in order to prolong their life and it is expected that the shock absorbing means may be painted or coated with the same material as the rest of the tower.

Preferably, said shock absorbing means comprises a material of Ultimate Tensile Strength substantially equivalent to that of high tensile steel.

According to a second aspect of the invention, there is provided a construction including shock absorbing means as described in any of the preceding paragraphs. It will be understood that the shock absorbing means described above may be used in any suitable constructions where undesirable lateral forces may be encountered, for example telecommunications masts, cranes, oil rigs etc. Preferred embodiments of the present invention will now be more particularly described by way of example only, with reference to the accompanying drawings, in which:

Figure 1 (PRIOR ART) is a schematic side view of a

transmission line tower leg;

Figure 1A (PRIOR ART) is a schematic top view of a

transmission line tower, showing incident wind forces; I,. * * 5. I I I S * I I S *I S I I S $ S I S I I I S asa S S S

Figures lB and 10 (PRIOR ART) show forces on a

transmission line tower resulting from one or two broken wires respectively; Figure 2 is a schematic side view of a transmission line tower leg embodying the first aspect of the present invention; Figures 3A, 3B and 30 are side, plan and front elevations respectively of a splice plate; Figure 4 is a perspective view, partly in cross-section, showing how the splice plates are used to fix together a tower leg and a stub leg; Figure 5 is a cross-sectional view, through the tower leg, showing how the splice plate is fixed thereto; Figures 6A, 6B and 60 are side, plan and front elevations respectively of a smaller version of the splice plate, for use on tower bracing; and Figure 7 shows the transmission line tower leg of Figure 2, subjected to lateral forces.

Some relevant terminology, used throughout this

description, is explained below:

"Tower leg" refers to the main structural steelwork supporting the tower. Generally there are four tower legs per structure, one in each corner, that run from ground level to the tower peak.

The "stub" or "stub leg" is the structural steelwork that is cast into the foundation of the tower. The tower legs * I.. *.* . I..

a. * . . . S * I. S S S S SI * S *SS * S S S * . S * I I * I S S.. S * I * are attached to the stub legs. In use, the "stub" transfers loading from the tower structure to the foundation.

"Structural members" are the component load bearing parts of the tower construction, for example the tower legs, the stub legs and bracing such as that illustrated in Figure 1 (reference numeral 6) . Generally the main load bearing/structural members (including tower legs and stub legs) are fabricated from high tensile steel which is capable of withstanding considerably more force/load than mild steel.

"Splice plates" are bolted across the butt-joint junction between each tower leg and stub leg.

A schematic side view of a typical transmission line tower leg is shown in Figure 1. The leg comprises a main leg part 1 (generally referred to as a "tower leg") which is attached to the main tower structure and a stub leg part 2 (generally referred to as a "stub leg") which is firmly fixed into a reinforced concrete foundation 3.

The tower leg and stub leg are rigidly attached to one another by means of one or more high tensile steel splice plates 4 having fixings (e.g. bolts) therethrough. Each leg is also provided with high tensile steel bracing 6.

The two-part leg arrangement is needed in order to facilitate construction of the tower. As shown in Figure 1A, a typical tower has four orthogonally placed stub legs 2A, 2B, 2C and 2D. When a new tower is to be constructed, the stub legs are installed first. Each stub leg site is excavated, the stub leg placed therein and aligned inwardly with the correct angle or "hip * *.. *** * *..

S. I * . . a * a. * * I * 10 * . St. S S S * * . . S * I S * S S.. I * S * slope". Each stub leg is then surrounded by a cast concrete foundation 3 in order to rigidly fix it in position.

When all four stub legs have been installed, the tower structure can be constructed upwardly from the stub legs.

Each stub leg 2 is fixed to the main leg part 1 by means of one or more splice plates 4.

Figure 1A illustrates schematically the effect of wind force on the tower. An incident gust of wind, indicated by arrow W in Figure 1A first reaches the tower at leg A. The lateral force of the wind causes leg A to be subjected to a lifting force ("uplift") as the leg A tries to lift out of its foundation 3A. Given the rigid structure of the tower, a lifting force on leg A results in a corresponding downward force on the diametrically opposite leg C. The present invention seeks to alleviate the effect of these forces by providing shock absorbing means and/or load damping means in one or more of the tower's legs.

Figure 2 shows a tower leg in which the conventional splice plates are replaced with shock absorbing/load damping splice plates 5. Each shock absorbing/load damping splice plate 5 is preferably sized and shaped such that it can be retrofitted to an existing tower in place of a conventional splice plate.

Whereas the skilled person would be inclined to reinforce a conventional tower leg (in particular the splice plate area) to provide a lower risk of tower failure, the present invention utilises the discovery that choice of a suitable shock absorbing material for a splice plate can * *** **S * *..

I. I * S * * I. S * * * ** - I * *q S S * S * * I * S S I S * St. * S S provide sufficient damping properties to mitigate the effects of uplift, without compromising the structural integrity of the tower construction.

It is envisaged that several alternative materials may be suitable for use in the shock absorbing splice plate.

Broadly speaking, the material should preferably have the following properties: a. a lower Young's modulus than high tensile steel (which is circa 205kN/nim2) b. a similar elastic limit and yield point to high tensile steel with a linear slope to the elastic limit c. a similar UTS (Ultimate Tensile Strength) to high tensile steel.

d. the material should be ductile, i.e. not brittle.

e. the material should be stable under compressive load.

f. preferably, the material could be coated to protect against bimetallic corrosion and/or oxidisation.

Figures 3A-3C show detail of the shock-absorbing splice plate 5. The splice plate 5 is of a thickness, length and width selected according to the requirements of the tower to which it is to be attached. The splice plate 5 is provided with bolt holes 8, whose diameter and pitch preferably match those of the conventional splice plate that is to be replaced. The number and arrangement of bolt holes can be selected according to the particular tower concerned, but selection to match the plate that is going to be replaced means that the splice plate 5 can * I.. *** . *** a. * . * S * I. S S S S *S * S *. * e * * * . S S S S * * S S.. 5 S S * easily be retrofitted to existing towers, in situ, at greatly reduced cost compared with other methods of reducing the risk of tower failure.

Referring to Figures 4 and 5, the tower leg 1 and stub leg 2 both are of generally L-shaped cross-section and meet at a butt-joint junction region 9.

As shown best in Figure 5, each of the two parts of the L-shaped section is provided with an outer splice plate (5A, 5A' respectively) and an inner splice plate (5B, 5B' respectively) . In order to fix the tower leg and stub leg together, the outer and inner splice plates are bolted together, using bolts 10 which are fitted through the bolt holes already present in the region of the junction 9.

It may be possible to fix the tower leg and stub leg together using splice plates on only one of the two parts of the L-shaped section (for example using only splice plates 5A and 5B) . Alternatively, it may be possible to fix the tower leg and stub leg together using splice plates on only one side of the L-shaped section (for example using only outer splice plates 5A and 5A', or possibly even only splice plate 5A) . However, the preferred arrangement is illustrated in Figure 5.

It is envisaged that additional reinforcement of the splice plate, particularly in the region of the bolt holes, may be provided to resist any tendency of the bolt holes in the tower leg and stub leg to elongate when subjected to the relevant forces. Such reinforcement should not interfere substantially with the shock- absorbing properties of the splice plate.

* Is. I.. S **I I. I * S S * I. S I I * I* * I III S S S S * . S S S * S S * *** I S S Alternatively, or in addition to the splice plates 5, the transmission line tower may be provided with shock absorbing means 7 in the bracing 6 (shown in Figure 7) Such shock absorbing means 7 may take the form of shock absorbing plates 7, smaller than those in the tower leg and sized according to their location in the bracing.

It 1S envisaged that shock absorbing means could be provided at several suitable locations throughout the transmission line tower. Whereas the above described shock absorbing means is located at a junction of two structural members of the tower, it is possible that, alternatively, the shock absorbing means could completely replace a conventional component of the tower and therefore not need to be situated at a junction.

Referring now to Figure 7, the arrows F indicate typical forces which could occur in the tower leg 1, stub leg 2 and bracing 6 and which would be mitigated or dissipated by the shock absorbing means (splice plates 5 and 7) Although the splice plates of the present invention have been described above in relation to transmission line towers, it is envisaged that they may be used in any suitable constructions where undesirable lateral forces may be encountered, for example telecorrmunicatjons masts, cranes, oil rigs etc.

Claims (12)

* *** S.. S *5 S S S S S * S. S S S S *S * . *5S 5 S S * * . . S S * * * * *I. S * S * CLAIMS

1. A transmission line tower including shock absorbing means which means, in use, absorbs or dissipates forces travelling along a longitudinal axis of a structural member of said tower.

2. The transmission line tower of claim 1 wherein said shock absorbing means comprises one or more plates which serve to join two structural members of the transmission line tower.

3. The transmission line tower of claim 2 wherein said plate is of a suitable size and shape to enable it to directly replace a conventional splice plate joining two structural members of a transmission line tower.

4. The transmission line tower of any of the preceding claims wherein said shock absorbing means is situated in a leg of said tower.

5. The transmission line tower of any of the preceding claims wherein said shock absorbing means is Situated in the bracing of said tower.

6. The transmission line tower of any of the preceding claims wherein said shock absorbing means includes fixing means which, in use, align with fixing means present on said tower.

7. The transmission line tower of any of the preceding claims wherein said shock absorbing means comprises part of a Structural member of the tower.

8. The transmission line tower of any of the preceding * I.. *** . I..

*s. S S S S * S. I S I I IS * I III S I S S * S S S S S S * S S.l S S S S claims wherein said shock absorbing means comprises a material whose Young's modulus is less than 2O5kN/mrn2.

9. The transmission line tower of any of the preceding claims wherein said shock absorbing means includes a coating for protection against bimetallic corrosion and/or oxidjsation

10. The transmission line tower of any of the preceding claims wherein said shock absorbing means comprises a material of Ultimate Tensile Strength substantially equivalent to that of high tensile steel.

11. A construction including shock absorbing means as described in any of claims 2-10.

12. A transmission line tower substantially as described herein with reference to any appropriate combination of Figures 2-7.