e-mosty September 2017: Mersey Gateway Bridge. Arenales Bridge. Mersey Gateway Bridge. Arenales Bridge. BLWT. - Page 10

3.3.2 Pylons The north and south pylons feature a hammerhead which supports the deck on bearings whereas the central pylon is monolithic with the bridge deck. The lower parts of the pylons were completed using standard forms with a plywood face and poured in four lifts. The hammerheads required shutters (formwork with the frames that served as a platform) to exact measurements, with the use of CNC (computer numerically controlled) machines to ensure accuracy for putting the formwork together. Once the central hammerhead was complete, the formwork was removed and the platform was reused for construction of the pier tables. About 12 sections of the pier table were built offsite and transported onto site, lifted onto the hammerhead by crane and then had concrete poured into them. Together the assembly made up the pier table onto which the form travellers were installed. A significant challenge for the designers of the pier table formwork was the requirement to transfer the high vertical loads into embedded anchors within the lower pylon. As the lower pylon geometry dictated the maximum possible number of anchors, a sequenced construction in layers was adopted to avoid the accumulation of vertical forces, ensuring that the loads can be safely supported, making the process more manageable. The upper pylons – above road deck level – were constructed by pouring concrete into the surrounding formwork, using 5m lifts. All four formwork platforms were raised those 5m by way of two hydraulic rams sitting at about the second level within the formwork. When the concrete had cured sufficiently – at least 25N/mm² - the vertical guide rails were first moved upwards 5m and then secured by huge doorknob-like anchors screwed into pylon. The formwork was unbolted from the side of the pylon and hydraulic rams, one pushing and one pulling, slide the formwork up 5m on the vertical guide rails. It was again bolted onto the pylon for security. One of the challenges common to all bridge construction projects is high wind speeds. The advantage of the hydraulic system for the Mersey Gateway was that it could withstand higher wind speeds than standard cranes, meaning windy conditions caused fewer disruptions. Additionally, as crane use was restricted on-site, using this system meant that other areas of the construction project could fully make use of crane- time. To complete an upper pylon, it took around 21 lifts. Once the pylon was completely poured, only then was a crane erected to take the formwork off its anchors and lower the sections back to ground level where it could be fully dismantled either on site or taken off- site. Figures 8 + 9: South pylon 3/2017