Article in NSC February 2022
Steel frame completes masterplan
Structural steelwork has ticked all of the boxes for the successful completion of the final distribution centre at ICON Harlow.
Construction activity in the distribution sector shows little sign of waning, as shopping habits change, more and more out-of-town distribution warehouses are needed to supply online retail outlets. Sometimes referred to as sheds, distribution centres vary in size, from small buildings covering a few hundred square metres to enormous warehouses with a floor space equivalent to multiple football pitches. One thing that does not change is the fact they are invariably built with structural steelwork, as the material dominates the framing systems used in this sector with a market share exceeding 95%.
Clients like the speed of construction that can be achieved using steel, while distribution centres usually require large internal column-free spaces, which are best formed with a steel portal frame design. “From cradle-to-grave, steel offers re-use and recycling opportunities without degradation of its performance properties, unlike any other main framing material, making steel the only real contender that ticks all the required boxes of time, cost and sustainability for industrial buildings,” says Saunders Architects Associate Director Kerry White.
Mr White is specifically referring to ICON Harlow, an 18-hectare site occupying a brownfield plot previously used for a GlaxoSmithKline facility. ICON Harlow will, on completion, offer over 65,000m2 of distribution space in five warehouse buildings, each with dedicated access, servicing and car parking. The north-west Essex town of Harlow is in an area viewed as an ideal location for distribution centres. London is less than 30 minutes away, and ICON is just over four miles from the M11 motorway and some of the major routes to the Midlands and the North of England.
Known as Building E, this distribution centre is the fifth structure on the site and concludes the overall masterplan. Building E is the first distribution centre on the site that main contractor ISG and steelwork contractor Billington Structures have built, but Saunders Architects and structural engineer Fairhurst have been involved in the overall project from its original site acquisition in 2016, through planning and delivery stages. As well as the construction of the distribution centres and a multi-storey van park over the last two years, the work has also ensured the scheme fits into the surrounding area with a landscape strategy, that has retained and enhanced the existing landscaping around the perimeter of the site and introduced new green areas to support a wide range of species that support the local ecology.
Measuring 145m-long × 65m-wide and 12.7m-high, Building E is not only the last structure to be built at ICON Harlow, it is also the largest and will offer 10,390m2 of floor space. The main warehouse part of the structure has two 32.5m-wide spans, formed with two pitched portal frames. The perimeter columns are set at 8m bays, while internally, the one line of valley columns is arranged in a hit-and-miss configuration, creating more valuable floor space.
The spans are formed with a series of rafters that have a splice at midpoint. The two sections were lifted into place individually, using two mobile cranes, and all of the bolted connections were completed while the steelwork was being held aloft. Stability for the building is achieved from the portalised frames and cross bracings, which are located in each corner of the structure so as not to interfere with the all-important dock levellers, of which there are ten.
“Steel was the most efficient means of achieving the required clear spans and to meet construction programme,” adds Mr White. “The speed of erection was important as the building needs to be watertight as quickly as possible to allow the internal trades to join the critical path at the earliest opportunity. With a steel structure and an optimised purlin design we could hit the sweet spot in efficient frame design and also maximise the distance between outer columns thus reducing the amount steelwork and foundations.”
Having an experienced sub-contractor on board early on in the project is essential to take advantage of the benefits of building with steel. A quickly proposed steel frame design, together with timely approval of the fabrication drawings from the wider design team, meant that the design process was completed on time and handed back to the specialist fabrication and installations teams. Highlighting steelwork’s speed of programme, Billington Structures were contracted to complete the steel erection work in six weeks and, despite some inclement weather, the job was still finished early.
Collaboration was key to the speed and success of the steelwork erection programme, says ISG Project Manager Kevin Clement. “The weather was very poor and as the ground is predominantly clay, persistent rain meant water was very slow at draining away. Consequently, we had to coordinate crane positions with Billington and use sacrificial crane mats and install stone over some of the footprint to enable a quick and safe completion of the steel erection.”
A lot of coordination was also implemented between trades, because although the steelwork programme was prioritised, it was completed while the groundworks (see below) were also being undertaken on other parts of the site.
Within the main warehouse area, Building E features a high-level internal plant deck, supported by columns. Having internal plant decks is an estate-wide feature as they are said to maintain the building aesthetics, while offering safe access for inspection and maintenance, which will not be affected by external weather conditions.
Working their way out of the site towards the plot’s access road, the final part of the building to be erected was the western elevation that includes a three-storey office block offering approximately 99m2 of floor space. The offices are formed with a traditional beam and column design, creating clear column-free spaces. The steelwork supports metal decking and concrete topping for a composite flooring solution.
Building E has achieved a BREEAM ‘Very Good’ rating and it incorporates a number of sustainability features including future-proofing for PV roof panels, electric vehicle charging points with a capacity to allow for more points in the future, energy efficient water equipment and fittings, while an increased building fabric performance with very low air leakage means that the building services have a lower energy consumption during operation. The speculatively-built Building E was completed in December 2021.
Building E and the adjacent Amazon distribution centre represent phase two of the ICON Harlow masterplan. Prior to the latter building being erected, an extensive earthworks programme was undertaken over the entire phase two plot.
Fairhurst Partner Hugh O’Neill says: “The phase two land had significant level changes due to the former uses. Following the demolition works and in order to tie these two distribution centre plots into the existing infrastructure, a substantial amount of earth had to be moved across the site to form the two development plateaus. The finished levels were designed to minimise the amount of excess spoil to be removed from site. In some areas, the plot was backfilled to a depth of 2.5m and a ground improvement scheme, utilising cement stabilisation techniques, was adopted to allow the buildings to be supported on traditional pad foundations with the warehouse slabs being ground bearing.”
Allowing a seven-day curing period for the pad foundations, the steelwork erection for Building E closely followed on behind the groundworks.
|Steelwork Contractor||Billington Structures|