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Willmott offers geometric solution to Pontypridd puzzle

The contractor is giving voice to a council-backed redevelopment of the South Wales Valley town, in the shape of £40m multi-use scheme.

Project: Taff Vale redevelopment, Pontypridd
Client: Rhondda Cynon Taf County Borough Council
Contract value: £40m
Main contractor: Willmott Dixon
Contract type: JCT Design & Build
Start date: March 2018
Completion date: Spring 2020

The small town of Pontypridd in south Wales has a lot to shout about.

It was here that the Welsh national anthem was written in the 19th century and, more recently, Ponty (as it is also known) has achieved fame for being the birthplace of Tom Jones.

Unfortunately for the town’s local population, the successful redevelopment of Cardiff just 19 km south along the Taff Valley has seen the local town centre stagnate somewhat. Now the Rhondda Cynon Taf County Borough Council is aiming to kickstart Ponty’s own resurgence.

The £40m Taff Vale redevelopment will see Willmott Dixon build two new office blocks and an architecturally striking flagship building containing a library, community facilities and council offices, a leisure and fitness centre as well as retail units and a café.

“It is a very important project for our client Rhondda Cynon Taf County Borough Council and the town of Pontypridd,” says Willmott Dixon senior operations manager Dave John.

“Cardiff’s successful regeneration has drawn business and employment opportunities away from the area. This project will provide badly needed high-quality office space for local businesses as well as new library and community focus for the town.”

Three main buildings

Recreating some of the success the Welsh capital has enjoyed following its redevelopment won’t be easy. For a start, there are some technical challenges to overcome.

On a site perched on the west bank of the River Taff as it runs beneath Pontypridd’s Old Bridge (once the longest single-span stone bridge in the world), the scheme consists of three main buildings: these are buildings A and B, which are two five-storey steel-framed office blocks with a reinforced concrete basement level, and the flagship library and gymnasium building.

All the buildings on the site share the same reinforced concrete basement and podium design.

Willmott Dixon Taff Vale 8

Willmott Dixon Taff Vale 8

Each building has jump-formed concrete stair and lift cores

A 375 mm-thick slab at podium level with concrete columns on a 6.5-7.5m grid is based on a forest of 680 CFA piles, 650 mm in diameter and stretching 25 m deep into the earth.

Piling contractor Westpile used a 130-tonne rig to install the piles, which required a night-time road closure and close monitoring of the main bridge into the town. Each building has jump-formed concrete stair and lift cores. This will anchor the steel frame of the buildings as they creep skyward and each will feature composite metal decking floor slabs.

“The two office blocks are very similar in design and construction,” Mr John says. “One already has a tenant lined up but the tenants for the second are being sought. The flagship building is more complicated.”

Statement roof design

‘Complicated’ may well be an understatement. Just the materials used for its construction have probably made the lives of the structural engineers detailing the design difficult.

The long list of materials used reads as follows: steel, cast in-situ reinforced concrete, brick, glass, prefabricated timber cladding units, and individually sized and shaped zinc cladding panels – there is barely a construction technique that isn’t due to be tried on the scheme.

But it is the shape of the library building that is causing the biggest issues. Its architectural statement design sees its roof and cladding curve in two directions, ensuring its installation (and design) is a challenge for the project team.

“We have experience in delivering these sorts of awkwardly shaped structures. We worked on the Royal Welsh College of Music and Drama in Cardiff, which is similarly curved and complicated,” Mr John explains.

Willmott Dixon Taff Vale 5

Willmott Dixon Taff Vale 5

The library’s roof and cladding curve in two directions

The curved roof is of semi-monocoque design. A curved steel frame springs from the reinforced concrete core. This frame then supports prefabricated timber cassettes that are bolted back to the steel.

Thin timber planks are then used to form the curves of the façade before individual zinc shingle panels are fixed using a “secret” clamp to form the outer face. There are 6,345 zinc shingles in total, each around the size of a ceiling panel.

To the naked eye, there appear to be places where the steel appears so out of shape it looks as though local fabricator Morgans of Usk has made a terrible mistake, but it is nothing more than an optical illusion.

Danish bricks bring elegance and challenges

Although the technical challenges that the curves of the library building offer may focus the minds of the Willmott Dixon site team, the two office buildings are not without their own challenges.

Initially the team had looked at using brick-slip precast concrete panel systems to clad both buildings, but eventually decided to go down the traditional bricklaying route.

“We’re confident that our supply chain will be able to support the decision. We looked at precast but went against it – we just don’t think it’s the right solution for this scheme,” Mr John says.

But although the team is looking at traditional skills to complete the work, it has shifted from the traditional 215 x 102.5 x 65 mm UK brick size to a longer, wider, and shallower 228 x 108 x 54 mm Danish standard.

To combat any potential issues with supply due to Brexit, the project team has already purchased the bricks. But even a diversion as seemingly simple as the dimension of the bricks has meant a few headaches for the site team.

“The longer, thinner bricks will look fantastic but a step away from the norm like that does make things more difficult than you would expect,” Mr Owen says.

“Setting out is different. That slight change affects where you place expansion joints, how you work around windows and piers. Now you have half bricks where you wouldn’t normally expect them. Tolerances are tighter,” he explains. 

Overhanging façade

South Yorkshire-based specialist Constructional Timber used a Finnish product, Laminated Veneer Lumber (LVL), to manufacture the cassettes.

LVL is produced using 3 mm-thick veneers stripped from softwood. These veneers are glued together to form a large section of engineered timber from which structural sections, shaped according to the design, can be cut.

“In places the curved façade splays out creating an overhang of around 2 m from the base of the building,” explains Willmott Dixon construction manager Jarred Owen.

Willmott Dixon Taff Vale 11

Willmott Dixon Taff Vale 11

The site runs beneath Pontypridd’s Old Bridge

“There are areas where the design is such that the windows are almost tilting back on themselves. Getting the drainage details right in those areas has been important.”

Glazed curtain walling completes the exterior of the flagship building at its ground floor level. Again, this is being installed at a splayed angle which sees it falling back on itself. Only robust engineered timber beams around the base of the zinc cladding and the top of the curtain walling prevent this.

“There are areas where the design is such that the windows are almost tilting back on themselves. Getting the drainage details right in those areas has been important”

Jarred Owen, Willmott Dixon

Public realm works include heavy landscaping with tree-pits cast into the podium slab, ready to accommodate the roots of the trees and shrubs planned. There are future plans for a footbridge across the River Taff to link the development with the Ynysangharad War Memorial Park on the opposite bank.

The Willmott Dixon team has plenty of time to get to grips with the construction challenges at Pontypridd.

The team is not due to complete the project until late spring in 2020, just in time to see the green grass of Wales comes into its own.

BREEAM drives excellence

Under the terms of the contract, the project team is obliged to ensure the building hit the Excellent standard under the BREEAM sustainability assessment method.

Although not a final requirement, Willmott Dixon has used BIM throughout the project to help measure and map its progress and that of the project team toward its final BREEAM target.

There are some photovoltaic panels on the rooves of buildings A & B, rainwater harvesting tanks in some of the plant room areas, and as well as the use of prefabricated timber cassettes, there are some modular construction methods in the build too.

But according to Mr John, it is thanks to the strong sustainable design and the effort the team is putting in to make sure the scheme helps deliver training and job opportunities for the local community that will really make a difference.

“We have 2280 weeks of targeted recruitment and training to deliver on this project,” he says.

“We have twinned with the University of South Wales to offer students experience on the project and a Community Liaison Officer to help us offer mentoring and experience for the unemployed and those not in training or education.

“The target sounds daunting initially, but there are plenty of areas where we and our supply chain can offer real help and long-term employment to the local community” Mr John adds.

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