|Date:||25th November 2008|
Gloucester Quays shopping mall installation raises the roof
In Gloucester English Architectural Glazing are installing an innovative roof on the city's latest retail development. By Paul Thompson
Project:- Gloucester Quays shopping mall
Main contractor:- Bovis Lend Lease
Atrium roof package:- £7.5 million
Lead subcontractor:- English Architectural Glazing
Roof steelwork:- Adey Steel Glazing/Cladding LMC
Gantry work:- Atrium Gantries
The city of Gloucester has always revelled in its reputation as the workhorse to its more thoroughbred neighbour Cheltenham.
Even in the days when its dockside warehouses were packed to their oak rafters with goods brought from around the world up the river Severn for distribution to the Midlands and beyond, Gloucester has struggled to hang on to the wealth it generated.
Most rich merchants preferred to spend their money in the neighbouring spa town and seemingly that is still the case.
The relative wealth of the two towns is quite similar, but much of the cash that the citizens of Gloucester earn is spent in the trendy boutiques and designer shops of Georgian Cheltenham – a tidal wave of money that Gloucester council is keen to divert back into the city of origin.
The Gloucester Quays shopping centre is being built by main contractor Bovis Lend Lease but one of its quirkiest features is the gull-wing glazed roof being installed by specialist contractor English Architectural Glazing (EAG).
Spanning the mall of the shopping centre, the atrium roof features 40 mm-thick glass panels weighing up to 360 kg supported on a latticework of steel beams that form the main trusses.
In all, the weight of the steel spanning the main atrium and the two smaller east and west malls totals almost 250 tonnes, with a further 160 tonnes of glazing units put on top of that.
A flying start
The gull-wing design over the main atrium features a total span nearing 50 m with the two 'wings' each spanning almost 20 m.
The curved radii of the wings spring from the 10 m-wide central breast and are pinned to support the trussed wings. These are propped on 203 mm square universal columns fixed onto a steel base plate with bolts grouted into the reinforced concrete deck of the car park roof which surrounds the glazed atrium.
The 60 m long 'breast' or spine of the roof is supported on three supporting 'trees' which sprout from the reinforced concrete floor of the mezzanine, three levels below the roof.
They are bolted and grouted into the floor and the loadings from the roof pass through these trees, through the mezzanine floor and into the reinforced concrete columns which support the mezzanine level.
During installation steelwork contractor Adey Steel, used military trestle supports to prop the trusses as they were installed. With tough tolerances of plus or minus 2 mm across the entire roof, the fabrication and installation teams had to be right on top of their game.
The larger 15 m-long sections of steel trusses were fabricated in Adey's workshop and transported to site where they were lowered into place using three tower cranes. The smaller 7.5 m-long intermediate sections were fabricated on the third floor concrete slab by the on-site fabrication team. Using the trestle systems enabled Adey to place the spine with the required level of accuracy, according to Adey project manager Tom Hamilton.
"There were six of the hydraulic trestle systems. They allowed us to get the girders where we wanted them. We fixed the core girders first then dropped them down onto the trestles, then the spine girders," he says.
The advantage of the trestle propping system is that it enables the site team to check the structure for line and level before fully fixing, enabling any minor adjustments to be made.
The bolted connections are then fixed and the hydraulic trestles are dropped in sequence to help ensure the structure is loaded incrementally, avoiding any hogging across the roof.
The framework proved to be exactly within the specified tolerances of plus or minus 2 mm, allowing them to get on with the task of placing and fixing the glazed units.
Manufactured by German producer Rickert, the units feature a sandwich of glass and air with an overall thickness of 40 mm.
A 10 mm clear, toughened heat soaked glass outer pane featuring a patterned inner face and ground edges is separated from a 12.8 mm annealed clear laminated inner pane of glass by a 16 mm air-filled cavity.
The innermost face of the unit, face 3, features a low emissivity coating to help improve the thermal performance of the whole structure.
"The call-off period for the glass is 6-8 weeks and then they are delivered to Telford for bonding to the aluminium frames. That takes about two weeks so it can be as much as 10 weeks after the glass has been called off that the units arrive on site. It's not a problem because we know we have planned that period in," says EAG site manager Rudi Pfeiffer.
The largest of the units are more than 3.5 m wide by 1.5 m long and weigh 360 kg but when the sucker attachment that enables the crane to pick them up is included this total tops 480 kg.
This high mass forms the basis of the placement as the 70 steel clips that help fix the panel into the frame are little more than locators – it is the mass that keeps the units in place.
Once lowered into position a 20 mm build-up of mastic is squirted into place in the 20 mm wide gap between the frames, followed by a 30 mm layer of caulking.
This is left to go off for four or five days before the final 20 mm layer of mastic is applied to complete the joint.
The contract has been heavily weather-dependent and the team has been battling to stay on track with the installation.
Thankfully there have been other areas where EAG has been able to divert attention on the many down days suffered thanks to torrential rain and high winds.
"We initially planned on the glazing taking three to four weeks. In the end it has taken about four months but if you look at the number of days when we have been able to get up on the roof and start placing the units they add up to around the original four-week estimate," says Ian Billson, another of EAG's project managers on the scheme.
The project should see the team through the turn of the year and into 2009. Unfortunately for the citizens of Gloucester, they will miss the New Year sales.
Reprinted with permission from Construction News.