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Paul Hitchins of Halfen looks at a regeneration development in Lewisham that has benefited from the new generation of HIT balcony connectors to help achieve impressive sustainability credentials.
Lewisham has undergone significant regeneration in recent years and as part of the ongoing improvements Barratt London was selected by Lewisham Council as their partner in conjunction with London & Quadrant Housing Trust to develop proposals for the site at Loampit Vale.
Following discussions with local residents and key statutory bodies such as the Greater London Authority and CABE, planning permission was granted in September 2009, and construction started on site in April 2010, with the site due to be completed by June 2015.
The proposals for Renaissance embrace 21st Century urban living and will deliver an exciting mixed-use development including a new leisure centre which will achieve BREEAM ‘Excellent’ standard. In addition, 788 new homes are under construction of which 149 are affordable, achieving Code 4 of the Code for Sustainable Homes. In addition public spaces and open piazzas coupled with an energy centre, green roofs and access to mainline rail services highlight the development’s sustainable credentials.
As part of that commitment one of the challenges facing the design team of Assael Architecture and Adams Kara Taylor included the need to meet stringent levels of thermal performance for the many balconies whilst maintaining elegance of design and ease of construction. Structural thermal bridges such as at balconies may lead to moisture problems resulting from decreased temperatures on internal surfaces as well as loss of heat. A thermally insulated balcony connector would reduce the transmission heat losses in the area of the connections and prevent condensation and mould growth. In addition, the design team’s requirement for F120 fire rating meant that a traditional thermal break would require extra fire protection.
Halfen worked with the design team and Barratt London to provide solutions to these and other issues using the new generation of Halfen HIT insulated connectors incorporating innovative CSB technology.
The CSB technology combines the shear and compression components of the thermal break into one thus eliminating costly stainless steel shear rails and yet providing 55% more shear capacity. This has a dramatic effect as far as transportation and installation is concerned and provided a key benefit to the construction team at Loampit Vale when it came to the curved cantilevered balconies that are the highlight of the frontage. At these locations, where the internal reinforced concrete slab drops from 250mm down to the balcony slab at 200mm, Halfen were able to adjust the bar heights given the relatively low numbers of bars required, so that the faceted units could overlap providing the required radius whilst maintaining the continuous thermal break.
The concrete installation team from Masterson Holdings were able to rapidly install the insulated connector units in conjunction with the required reinforcement from above without having to cut through transportation bars and with fewer projecting bars to cause potential clashes. With produced lengths of 25,50 and 100cm cutting to size on site could be avoided with the unit locations pre planned. Barratt project engineer Hardik Bhatt commented that the installation was quick and simple providing a major saving in time.
Once installed, the HIT insulated connectors are providing the design team with unparalleled levels of thermal performance. With the removal of shear bars that run from the balcony to the internal structure the new CSB units provide a 37% lower linear thermal transmission coefficient than other thermal breaks with the provision for the engineer of a detailed verification of the additional value for thermal bridges.
In addition the requirement for F120 fire rating is accommodated as standard in the new HIT design as part of the robust body construction. This means that the design team has been able to avoid costly additional fire protection in areas where the thermal break is applied with confidence knowing that the load bearing elements of the units are fully insulated.
Halfen have also been able to address another of the issues associated with balconies and access balconies which is the transfer of vibration into the main structure of the building and distribution into adjacent rooms as airborne sound. In laboratory experiments the HIT thermal separation significantly reduces the impact sound pressure level in comparison to a continuous floor slab by up to 12dB(1)
The design team were able to rest assured that the Halfen HIT CSB insulated connectors were compliant with the most stringent standards. The requirement described in BRE IP1/06 which is cited in Building Regulations Approved Documents Part L1 and L2 and Section 6 in Scotland is that the temperature factor used to indicate condensation risk must be greater than or equal to 0.75 for dwellings and residential buildings. With a range of 0.868 – 0.942(2) this is easily met by the HIT CSB range.
The fire protecting properties were verified by the Institute for Materials Research and Testing MFPA Leipzig. The classification of the entire structure in fire resistance class REI 120 according to DIN EN 13501-02 or in class F120 according to DIN 4102 is specified in the National Technical Approval Z-15.7-293.
Halfen HIT design programs provide verifiable structural calculations using the geometry of the balcony and the constraints regarding concrete cover and fire protection. This calculates the correct HIT unit that has been type tested and officially approved by the German Institute of Construction Engineering. All approvals for individual cases within the framework of the National Technical Approval Z-15.7-238 or Z-15.7-293 are also provided. This is part of the Halfen integrated safety concept and is complemented by NHBC acceptance in the UK.
(1) Standardised tapping machine test according to DIN EN ISO 10140
(2) The values were determined according to DIN EN ISO 10211 by the Institute for Materials Research and Testing at the Bauhaus University MFPA Weimar.
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