Huge roofs on Top o' th' Cow

They are believed to be the biggest freely supported concrete domed roofs in the UK. Stewart Tennant, director of GHA Livigunn, describes one of the most technically demanding projects of his career

Two service reservoirs designed by GHA Livigunn are believed to have the largest freely supported concrete domed roofs in the country.

Working in association with United Utilities (UU), KMI+ and MWH, the Cheshire-based structural design engineers developed the 4.15Ml reservoirs at Top o' th' Cow, an exposed rural location outside Bolton, Lancashire.

A planning requirement stipulated that the new service reservoirs should have domed roofs in order to sympathise with the appearance of the original tanks, which had become local landmarks. This meant that GHA Livigunn also had to incorporate a freely supported domed roof design.

The capacity of each reservoir was also specified to match the effective capacity of the two existing reservoirs at Top o' th' Cow. One of the tanks had already been removed from service and demolished, which meant that the remaining steel tank could not be withdrawn from service until at least one reservoir had been commissioned.

The new service reservoirs are 33m in diameter and 6m high, with 300mm-thick walls. The walls are reinforced with special structural mesh S786 (H10s at 100) and post-tensioned to achieve the client's requirements of a 0.1mm crack-limiting criteria "critical aesthetic appearance".

The walls are designed to resist the thrust from the domed roof and internal hydrostatic pressure in ring tension. A wall thickness of 300mm was adopted as this was considered to be the minimum thickness that could be poured in a single lift using a 170mm slump C35 concrete.

The base slab of each service tank sits on an embankment formed using imported fill from a nearby construction project. This ensures gravity flow of all tank contents into the water distribution network.

The post-tensioning design involved close liaison with specialist sub-contractor, Balvac. Although the designers explored a number of lightweight options for the roof domes, reinforced concrete was adopted as it met the required design life of 60 years UU's security requirements. Each roof was designed as a flat dome, with a rise of 3.3m and contractor SGB was responsible for falsework and formwork to ensure the roofs would not become loaded during stressing operations.

The design solution was achieved through careful analysis of the site and the use of considerable 3-D visualisation and finite element modelling to create an elegant structure in a landmark location. The design was carried out using STRAND 7 advanced Finite Element Analysis, with verification by traditional analysis methods.

This demonstrated to all stakeholders how a valve house 50% smaller than originally proposed could be built without compromising access or maintenance requirements.

The net result of this design alternative was the creation of sufficient space for the two new service reservoirs to be built simultaneously on what was an extremely compact site and also avoided the need to remobilise specialist contractors.

The Top o' th' Cow contract was a UU AMP 4 project tendered on a design and build basis, with GHA Livigunn and KMI+ being awarded the project.

GHA Livigunn's director, Stewart Tennant said: "Without a doubt, the design of the Top o' th' Cow service reservoirs was one of the most technically demanding structural engineering projects of my career - and one of the most challenging design projects faced by GHA Livigunn."

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