Posted: May 16th, 2013 by Peter Botsoe
Comments (6)

First prototype Tension pylon (with mock-up tubular diamond) erected

Flanged foundation connection

Picture 1 of 14

The exciting news continues. We erected our prototype tension T-pylon in Denmark at the DS SM ( site in Rodekro on the 11th of April alongside the suspension pylon erected 9th January. Furthermore, we added a set of earrings to the suspension pylon to understand the visual impact ahead of mechanical testing. As with the erection of the suspension pylon, the construction activity was not straight forward because we had two days of high wind greater than 30mph precluding any lifting operations with the crane.

First impressions are the two prototype members of the T-pylon family look very much in keeping with one another – this is obviously an important consideration because they must look similar and also support the wires at equivalent points to minimise their visual impact when installed in a line.

The two pylons are slightly different in that the internally flanged tension pylon monopole is around 0.3m wider at the bottom and the top (2.3m and 1.4m respectively) compared to the slipjointed suspension pylon, but this is only just discernible to the eye and, even then, only close up. Another difference lies in the manner which the wires are held, the suspension pylon suspends the wires whereas the tension effectively pulls the wires round corners and must remain standing if multiple suspension pylons fail. Due to these high loads, a steel tubular diamond is used to cater for the situations where wires could fail e.g. after mechanical faults or extreme weather conditions. Due to time constraints, we erected a mock-up tubular diamond to understand how to construct it, handle it and visualise how we would work with it when installing wires on a line of pylons or undertaking any future maintenance activities.

The pair both stand 35m high, roughly a third lower than a traditional standard height lattice pylon, and are both made of uncoated steel so that they appear similar to the Angel of the North which is fabricated from a weathering steel which requires next to no maintenance because it is designed to build a strong, protective, rusty outer layer.

Game of two halves

Following our experiences erecting the suspension pylon in January, we learned that pre-assembly of the cold rolled steel crossarms and interconnecting cast iron heart and horn ( could significantly optimise the site construction activity. This technique meant that we could erect the tension T-pylon using a two stage approach.

With this in mind the mock-up tubular diamond and monopole sections were taken to the construction site in advance of the heavy lift operations and constructed on trestles on the ground. This meant that we found it easier to align the two key connection points for the tubular diamond (to the horn (1 off M45) and monopole (1 off M12)) and were not affected by the weather which would have been the case had we tried to make the connections ~30m in the air.

Inside job

On day 1 we lifted the lower half (on this design, 18m and ~35tonnes) of the monopole into place with a large crane and bolted it using 96 off M48 hot dip galvanized bolts. The highest loads on the structure are due to wind loading on the monopole body, two sets of nuts and bolts are therefore required to ensure the bottom is held fast. One set is located external to the monopole whilst the other is accessed via a purpose-built hatch which is flush mounted with the monopole surface. Inside the monopole, a ladder (with rest platforms) ascends the monopole to a working platform ~18m above ground where a further set of 44 off M48 bolts is used to connect the top half.

Once the bottom was installed and secured, wind speeds picked up meaning that we could not complete the pylon. Two days later with a 5am start, on the day of our departure, we felt confident that the pylon would be erected. When we arrived at site, the top half of the pylon ~13m mast and 31m wide crossarm with mock-up tubular diamond weighing in at ~40 tonnes was then lifted, guided into place and bolted down. (The bolts and flanges on the prototype add a further ~4.3tonnes to the total weight.)

As with the suspension pylon, we used two cranes on the day, one for the heavy lift and the other to support the lower end of the mast sections as they were raised. By ensuring the section were a metre above ground level, the assembly was then uprighted. This technique reduced the risk of the section being dragged along the ground and being damaged.

The conclusion we came away with was that although a day should be enough to erect a suspension pylon, as we showed in January, each tension pylon is likely to take a little longer – probably around a day and a half.

So, the big news is we now have a tension pylon next to our prototype suspension pylon and we’ve completed the climatic load tests (full ice and wind) on the suspension pylon without any issues. We’ve also mechanically tested the diamonds that connect to the suspension pylons to ensure that they are sufficiently robust to withstand the loads.

We also have a pair of diamonds from Lapp ( /Mosdorfer ( and Pfisterer Sefag ( that have undergone preliminary electrical testing in Sweden – more on that in the next post.


  1. Robert Jones /

    What would the terminal towers look like, how would they interface with a Sub Station or Cable Sealing End Compound ?

    • Peter Botsoe /

      Hi there Robert,

      Thanks for your post. The terminal pylons as you note are located at either end of a line…(which could consist of a number of pylons – suspension, tension and probably flying angle) prior to its connection to equipment. This pylon takes the full line tension and hence has to be quite robust.

      In designing the terminal pylon, our (Bystrup and us) basic principle was to maintain the T-shape. In then end, two designs were developed, the Flat T – a single T-shaped monopole & the Double Diamond – two monopoles supporting two steel diamonds. The Double Diamond is a slight departure from the T-shape but takes from the tension pylon design which has 2 tubular diamonds connected to the crossarm and mast of the T. (Note: Different designs of Double Diamond can also act as a tension pylon for line deviations of up to 90 degrees.) The aim with both designs was to develop a shorter structure than the standard T at 35m to blend into the substation & sealing end compound surroundings.

      The Flat T at just under 23m connects all three wires to the slightly uplifted crossarms. The two earth wires are supported above the main crossarm at the appropriate distance to provide shielding from lightning strikes.

      The Double Diamond at just under 28m holds the wires in the same V-configuration as on the suspension & tension pylons. The earthwire is installed in the same location i.e. at the top of the steel tubular diamond.

      Each pylon has its pros and cons…(Flat T is shorter but wider(crossarm & mast) than the Double Diamond and has a single monopole/monopile foundation) but as noted in a number of posts, the line design considers a number of impacts from construction, environmental through to landscape/visual amenity and sustainability. In addition, the output of public consultation will further inform the choice of pylon.

      The design of the Flat T and Double Diamond continue to evolve so unfortunately I can’t post any images now. We’ll post some photomontages of the T-pylon family in the coming weeks contextualised against a landscape.

      Thanks for taking time to comment.

  2. David R Smith /

    Whereas I support the idea that the fabricated pylon that we all lover hate, needs a new looking at, I regret that NO-ONE seems to have gone back to basics.
    1/ There is no functional requirement for the conductors to be at different heights: they can then all be at the same minimum height.
    2/ In first order approximation, cable sag, and sway is proportional to the square of the separation of the pylons.
    3/ Windage forces on pylons increase with height, with the square of wind-speed, and wind speed increases, in the first approximation with height.
    So the bending moment on a tower increases with the FOURTH power of height.

    A carefully considered design is here offered, which aims to comply with the above design requirements:

  3. David R Smith /

    An early design is shown here:
    but though originally for 6 power conductors and 3 earths, to keep the geometry correct, the height approached 60% of the present 400kV pylons, at which height, the simple ‘lamp-post’ technology would be stressed. Hence the design is now downgraded to 3 power conductors, and two earths.

  4. David R Smith /

    This is a numbers thing:
    There are two Pythagorean triangles concerned here:
    3: 4: 5, and 9: 40: 41.
    The poles are 41 units long, and when they form a pair of shear-legs, with the feet separated by 18 units, the height of the isosceles triangle formed is 40 units.
    The height of this triangle forms the hypotenuse of the 3: 4: 5 triangle, so that as the two pairs of legs lean outwards from their common feet, the height of the top cross wire above the feet is 32 units, and the separation of the tops of the shear-legs is 24 units.
    If earth conductors are run along the leg tops, and the power conductors are spaced according to voltage, then, though the 3: 4: 5 triangle is an imperfect approximation to half an equilateral, it is near enough, so that the natural distance between the outer power conductor, and the earth conductor, is close to that between it and the pole itself. As we move away from 60 degrees, this approximation is degraded.
    This is why only three power conductors can be borne by two pairs of legs.
    There is however not outstanding reason why two sets of legs may not be placed side by side, so that the inner pairs of legs join to form a four pole pyramid, and there, the tie down cable is redundant.
    The legs then stand on a 18 unit x 48 unit rectangle, with tie down points a further 24 units out on the centreline, under the outer leg tops.
    The simple ratios derived by using these two pythagorean triangles enables quick and easy general design of the installation.


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