The exciting news continues. We erected our prototype tension T-pylon in Denmark at the DS SM (www.ds-sm.dk) 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 (www.hegerguss.com) 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.
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 (www.lappinsulators.com) /Mosdorfer (www.mosdorferccl.com) and Pfisterer Sefag (www.sefag.ch) that have undergone preliminary electrical testing in Sweden – more on that in the next post.