Steel frame success on London commercial development
21 May 2008 By Contract Journal team
The new eight-storey office development at 60 Gresham Street in the City of London has presented the construction team with more than just the usual challenges associated with a confined inner-city project.
The site is rectangular and bounded by roads on three elevations. And none of these streets, including Gresham Street, are straight. So to maximise the available space, the building's shape, which is dictated by these thoroughfares, is irregular.
The site was occupied by a collection of buildings, some dating back to the mid-19th century. Once these structures had been demolished three retained features - a police station's facade and its internal wooden staircase, and a pub entrance - remained, which had to be incorporated into the design.



When complete, the building will provide about 5,500m2 of Grade A-specification office and retail accommodation in the heart of the City, a stone's throw from St Paul's Cathedral.
"Many of the challenges are typical with central London projects," says Chris Jarman, project manager for Costain's project engineer SKM Anthony Hunt. "All three of the project's street facades are slightly skewed because of road alignment and as we reach the fifth floor the structure has to step-in at every floor so it doesn't interfere with adjacent buildings' views."
Then there were the retained features that had to be incorporated into the new structure. The new steel frame goes up alongside the old staircase and over it, with no connections.
"The staircase couldn't take the loads, so it remains structurally independent," says Jarman.
The retained police station facade is along an alley to the rear of the new building and will form one wall of the new building's rear. This portion of the new building goes up to fourth-floor level and abuts an existing building on Old Jewry, one of the three roads that bound the site. The retained pub entrance, on the other hand, is connected to the steel frame, although it retains its own structural integrity.
Erecting the new steelwork around these retained features, coupled with the site's irregular shape, means the structure has been constructed with a variety of grid patterns and spans, with the longest approximately 15m.
"There are no straight lines in this project, except the lift shafts," says Jose Aira-Rodriguez, Bourne Steel's project manager, which is undertaking all steel fabrication and erection on behalf of Costain. "The facades are all skewed and to incorporate the retained features we have a real mixture of elements."
The design had to maximise all available space - including the floor-to-ceiling heights - and keep all the services in the supporting floor beams. Cellular beams are ideal for this kind of project as they are fabricated and supplied to site with service holes already in their depth, making floor construction quicker and more economical.
Fabsec cellular beams are used throughout the project and were stipulated by structural engineer SKM Anthony Hunt because of the services alignment and the number of beams and columns that then have to sit on top of these sections.
"The beams are all bespoke for this project and many have elongated 750mm service holes to accept the unique service bundles," says Jarman. "They are also supporting other steelwork in some places, and in these areas the beams have no holes so they keep their structural integrity."
About 200t of Fabsec beams have been supplied, all of which are 550mm deep and supplied in lengths up to 15m, with the shortest member being 3m long.
"By using these beams we've maximised the available space by keeping services in the floor depths and retaining the required floor-to-ceiling heights," adds Jarman.
Steelwork starts at ground floor level and erection has followed a regimented programme, whereby two floors are constructed at a time and then fully decked while the next two levels are being undertaken.
"As well as incorporating the retained features into the steelwork we are also marrying into an existing building along the south elevation up to the fifth floor," adds Rodriguez.
When the build reaches its fifth floor, the structure's steps kick-in and floor space decreases from about 800m2 on the fourth floor to 380m2. The subsequent floors then decrease gradually until the top (eighth) floor, which is 143m2.
Because of the building's irregular shape each floor has incorporated at least two grid patterns, one off-set to the other by about 45 degrees. "This was necessary to take into account the structure's shape," explains Rodriguez.
60 Gresham Street is scheduled for completion by this summer.
Fact file:
Project: 60 Gresham StMain client: Gresham DevelopmentArchitect: Frederick Gibberd PartnershipStructural engineer: SKM Anthony HuntMain contractor: CostainSteelwork contractor: Bourne SteelSteel tonnage: 500t

source:
http://www.contractjournal.com/Articles/2008/05/21/59290/steel-frame-success-on-london-commercial-development.html