The latest BBA Certification awarded to the Schöck Isokorb is the culmination of an unprecedented standard of assessment by the BBA. It has resulted in a much more demanding new generation of Agrément Certificates for thermal break products and the emergence of a new ‘Leader Certificate’. This new ‘Leader Certificate’ incorporates significantly more detailed content on structural stability and performance of the balcony connector.
The rigorous assessment by the BBA involved tests for structural stability, fatigue stresses and fire. As a result, the enhanced Certificate includes a full description and specification of each component. Specification of threaded steel bars to connect the system to steel balcony; limitation for deflection of the concrete slab and steel; the requirement for fatigue stresses due to different temperatures and for natural frequency of the external concrete slab and steel balconies; thermal bridging at the junction between the balcony and the wall. In addition, it covers behaviour in relation to fire.
The BBA’s Project Manager, Seyed Tajallifar, emphasises that: “All future Agrément Certificates of this sort will follow this format. The items covered in the Schöck Certificate will be included in any other balcony connector Certificate when they are due for Reissue.”
Chris Willett, MD at Schöck comments: “We have always welcomed our collaboration with the BBA, which dates back over several years. This new generation BBA Agrément Certificate takes a huge step in providing the highest level of assurance to the construction industry and we are certain that it will lead to further specifications of our product in the future”.
For a free copy of the Schöck Thermal Bridging Guide and / or the Thermal Bridging Solutions brochure – contact the company on 01865 290 890 or visit www.schoeck.co.uk
On the north side of the Thames in central London, the three riverside embankments, Chelsea, Victoria and Albert, are the result of extensive civil engineering works that reclaimed marshy land, narrowed the width of the river and provided a large-scale new area of development during the late 19th century. Today almost 150 years later, much of the area is once again being transformed as the result of a scheme designed to replace many of the outdated buildings which have characterised these areas for so long. There will be high specification property construction, promenades and parks; and at Millbank, one of the major residential riverfront developments is Riverwalk.
Riverwalk features two organically shaped buildings of seven and seventeen storeys, connected by a central podium and incorporating 116 high specification one, two, three and four bedroom apartments, plus penthouses. The design focus is on light, space, service and exceptional views across the Thames. Aesthetically the buildings are enhanced by horizontal bands of limestone on the curving facades between the glazing and ceramic panels, with the stonework wrapping around the lower parts of the balconies to create a continuous organic shape.
With such a high specification development, early consideration was given to the avoidance of the thermal bridging at the critical balcony connections. Thermal bridges would result in higher heat transfer through the assembly and colder surface temperatures on the warm side of the assembly.
Some of the consequences of this being higher energy use for heating and cooling, non-compliance with UK Building Regulations, potential building structure corrosion and the risk of mould growth and associated health concerns from respiratory problems. To help minimise any risk of thermal bridging, the structural thermal break module specified throughout the Riverwalk development is the latest generation Schöck Isokorb for concrete-to-concrete applications, the type KXT.
This latest example of Schöck engineering means even better performance, which is critical in meeting the inceasingly stringent EU guidelines and the imminent need for nearly zero-energy building standards. The Schöck Isokorb type KXT pushes the technical boundaries in meeting these demands.
In addition, through close collaboration with its clients, Schöck has also now optimised the load capacity of the Isokorb range. As a result, the load capacities most frequently in demand have been identified and appropriate refinements applied to the product portfolio. The range now provides planners with not only a structurally optimised product solution for the construction of cantilevered components, but cost advantages as well.
Verifiable performance values
The product offers such a high level of insulation, that in Germany the Passivhaus Institute in Darmstadt has awarded the product with the low ‘thermal bridge construction‘ certificate and confirmed its suitability even for Passivhaus construction. The product has also just been awarded the very latest BBA certification. All units meet full compliance with the relevant UK building regulations, which require that the temperature factor used to indicate condensation risk (fRSI) must be greater than or equal to 0.75 for residential buildings. The range also provides Local Authority Building Control Registration and there is compliance too with the UK government Standard Assessment Procedure (SAP 2012) concerning CO2 emissions from buildings and respectively heat losses through non-repeating thermal bridges.
Here, the lambda values of the Schöck Isokorb enable energy loss in various connective situations to be reduced by as much as 84% to 91%.
For a free copy of the Schöck Thermal Bridging Guide and / or the Thermal Bridging Solutions brochure – contact the company on 01865 290 890 or visit www.schoeck.co.uk
Innovative product developments, unrivalled performance certification and an exciting range of software support packages, all combine to strengthen Schöck’s continued position as the number one supplier of verifiable performance thermal break technology.
This year, Schöck pushes the boundaries further with its new generation of Schöck concrete-to-concrete Isokorb, offering unrivalled thermal performance for both standard and Passivhaus construction. Also on display is the Isokorb type AXT – offering a more thermally efficient and more durable alternative to wrapped parapets and minimum savings of 10% on construction costs.
The rest of the Isokorb family, for concrete-to-steel and steel-to-steel connectivity are on display too – all with the reassurance of the new BBA certification, verifying Schöck to the most up-to-date industry requirements. Among the latest software packages there is the new Calculation Software for the Isokorb type KS and the Thermal Bridge Calculator. A brand new customer focussed website is being launched and you can get up-to-date information on the latest BIM developments.
The Schöck team is of course on hand to discuss any of the new developments and to offer ‘live’ consultancy advice on any project specific requirements you may have.
Lightweight support structures, such as free-cantilevered steel balconies, can be prone to vibration when people move about on them more heavily than usual. As designs become ever more lightweight and competitive in cost terms, particularly with very large cantilevers, the vibration behaviour of a structure takes on even more importance.
So when considering the development and structural design of steel balconies, there are a whole host of influencing variables that should be taken into account. In selecting the method of connection to the building slab, the challenge lies not only in choosing a component that ensures effectiveness as a thermal break; but one that offers a safe structural design solution as well. An additional factor is that it also needs to be compliant with the necessary serviceability requirements.
Acceptability of vibrations
The Building Regulations Part L defines the limit values for thermal bridges; and Eurocode 3 specifies the required verifications, such as for vibration, in serviceability limit state. Such verification is the responsibility of the structural engineer, whose function it is to calculate the natural frequency of the construction depending on its utilisation. That being said, when considering the design of the balcony construction and thermal insulating element, it is prudent to ensure that the natural frequency is greater than the limit frequency specified by the structural engineer. Generally, vibration of floors is considered to be a serviceability issue, primarily related to discomfort. As the perception of discomfort varies from one individual to another, no precise limit can be imposed that will guarantee satisfaction for everyone during the lifetime of the balcony. Assessment of acceptable vibration is therefore not straightforward. However, a logical approach is to design structures so that their natural frequency is sufficiently beyond potential excitation frequencies.
Depending on the type and utilisation of the structure, published data indicates limit frequencies of between 4 Hz and 7.5 Hz. Experience has shown that adopting a limit frequency of 7.5 Hz for steel balconies not only eliminates the possibility of undesirable vibration, it also enables the design of cost efficient structures.
If planned properly, the Isokorb type KS for concrete-to-steel thermal connectivity allows almost any balcony geometry to be designed without constraints. To assess how prone to vibration separated balconies using the Isokorb type KS might be, Schöck has employed the latest Natural Frequency Calculator software package. This is a free service which uses geometric and material variables and enables project teams to identify the optimum solution for steel cantilever balcony connections early in the design process. In most cases it will be found that the natural frequencies of normal balcony constructions are still above the recommended limit frequencies when utilising the Isokorb type KS.
The Schöck Isokorb type KS incorporates 80mm of insulation and is 180 mm wide and between 180 and 280mm high, to allow flexible adjustment for differing slab thicknesses. It can be prefabricated, reducing assembly time on site and can bear extremely heavy loads. This combination of features makes the unit ideal in meeting the various thermal and structural demands involved in the design of modern balconies. Which in the case of cantilever steel balconies, with a thermally broken connection to a concrete slab, normally sees the elements exposed to both vertical and horizontal bending moments and shear forces. The KS has a shear-bending interaction which, based on project specific loadings, allows for a much more flexible design and results in greater tolerance when designing steel balconies.
Recommendations for planning and designing steel balconies
The following factors influence the natural frequency and therefore the vibration behaviour of free cantilever steel balconies. Particular attention should be paid to these factors when planning and designing thermally broken steel balconies:
·Balcony geometry, especially the cantilever length and spacing between the connections
·Rigidity and proper execution of the stub bracket, especially on stepped thresholds
·Sufficient transverse rigidity of the balcony construction
·Incorporation of the stiffness of the chosen thermal insulating element into the planning process
Demandingly large balconies at Rathbone Market
A good example of unusually large cantilever steel balconies being incorporated is on the Rathbone Market scheme, part of the Canning Town and Custom House regeneration programme. A three-phase development delivering a new library, around 652 new homes and a new market square at the heart of the scheme. Here the balconies are an unusually demanding 2.3 metres in depth.
The striking façade of this building in Berlin is the concept of Sergej Dott, a locally-based pop art painter who chose medicinal herbs to represent the building’s use as a major medical centre. The Treptow Medical Centre, named after the district in which it is located, is a 19th century listed building and recent refurbishment work has seen the interior transformed into twelve surgeries and a pharmacy. The final flourish of the restoration activity is the addition of the pop art herbs, each one metre in diameter, up to twelve metres high and 600kgs in weight, all grouped around three sides of the building.
To incorporate them into the structural design, additional steel beams had to be installed inside the building at varying floor levels and Lorenz Linnhoff, whose engineering firm carried out the structural planning for the building comments: “The optimum solution for fastening the giant flowers to the building envelope was the Isokorb KST, which made it possible to transfer the load of the flower installation via the steel beams to the load-bearing building”.
The KST has been developed to create thermal breaks in steel structures, where the modules are used to transfer tensile forces and pressure/shear forces. With this application the product is used only for the latter purpose, as on this occasion it has to perform a structural function rather than ensure thermal partition. Each flower is connected by two KST modules at the top of the stalk and their modular capability meant that off-site pre-fabrication was possible, helping to reduce on-site costs. Two head plates were welded to the stalks off site and another two head plates attached to the main steel structure with the KST modules bolted to them. Then flower stalk was then lifted into position and the two plates simply bolted to the exterior connections of the KST units. The community arts and culture scene in Treptow wil benefit too, as each of the herbs is to be sponsored, with the proceeds donated to various art institutions.
The Kidwells Estate, a 1960s-built initiative just outside Maidenhead town centre, in Berkshire, is undergoing major regeneration which involves the construction of seven new blocks. These vary from three to six storeys of structural concrete frame and are part timber clad. Long cantilever walkways that service multiple flats on the complex form an unusual feature of the new development and the demands of the walkways presented a technical challenge for the structural thermal breaks used in their construction.
Now known as Parklands, the former Kidwells Estate, is a landmark £27.1 million project, scheduled for completion later this year. It is a scheme for One Housing and in replacing seven 1960s-built apartment blocks, the redevelopment will provide 204 new mixed tenure high quality homes, consisiting of 75 homes for rent to existing and new tenants, nine shared ownership homes and 120 homes for private sale. On completion the development will provide three and four bedroom family homes; as well as one and two bedroom apartments. One Housing manages over 15,000 homes across 27 London boroughs and surrounding counties; and in the last three years has delivered more than 1,500 new affordable homes, with plans to build a further 3,600 by 2019 to help meet the housing shortage.
The unusual construction challenge involving the walkways at Parklands, is brought about because the design consideration, both aestheticaly and for the benefit of residents and visitors, requires as much natural light as possible to be available in the areas around the cantilever walkways.
To meet this requirement, extensive light wells have been introduced at regular intervals along the length of the galleries. These wells are effectively a series of elongated spaces, which separate the walkways from the building for large sections. As a result, there are far fewer structural connection points between the walkways and the building than would normally be anticipated, so the connectivity opportunities are very limited. The walkways themselves being cantilever concrete construction into internal reinforced concrete frame.
Schöck is the supplier of the structural thermal breaks and because of the restricted connectivity point opportunities, the products had to resist a much greater shear load than would be the case with a more conventional design. In addition to this, the support condition of the external columns and edge beams also had to be taken into account and there was a limitation too on slab thickness for the dowel connections.
The solution required a relatively unusual combination of products and the issues were resolved by Schöck designing in its type HPC Isokorb. It is a product utilised only if horizontal, tension and compression forces are present in the design and additionally it is necessary to integrate the type QP90+QP90 Isokorb as well, for linear or point connection to support high shear loads. Schöck type SLD Q50 heavy duty stainless steel dowels were also incorporated to enable the transfer of vertical shear forces and allow two directional lateral movements at the expansion joints. The result was a completely secure thermally insulated load-bearing connection at all of the restricted, but key structural points.
The Isokorb structural thermal break range enables connections to be made between concrete-to-concrete, concrete-to-steel and steel-to-steel – and the many different unit types available, combined with their ability to enable the transmission of shear, bending moment, tension and compression forces, also means that the options available effectively run into hundreds when the different combinations are taken into account.
All units provide BBA Certification and LABC Registration and meet full compliance with the relevant UK building regulations.
The requirement described in BRE IP1/06 – a document cited in Building Regulations Approved Documents Part L1 and L2 and Section 6 in Scotland – that the temperature factor used to indicate condensation risk (fRSI) must be greater than, or equal to, 0.75 for residential buildings, is easily met by incorporating the Schöck Isokorb.
A comprehensive 236 page ‘Technical Guide’ is available, which displays in detail the complete range of thermal break applications for all construction types – concrete-to-concrete, concrete-to-steel, steel-to-steel and concrete-to-timber.
A new free ‘Thermal Bridging Guide’ is also available on request
A PFI Extra Care complex at Abbey Hulton village, outside Stoke-on-Trent, is another UK project benefitting from a faster build time using innovative off-site manufacturing. It combines Isokorb structural thermal break units from Schöck with the Hollowcore system.
Holdcroft Fields at Abbey Hulton, provides 175 self-contained one and two bedroomed apartments with extensive communal facilities for people over 55.
Designed to meet BREEAM Excellent standards, there are two linked three-storey blocks around two central courtyards and many of the apartments feature spacious balconies. So the prevention of thermal bridging is a critical consideration.
Quite apart from heat loss, condensation can lead to structural integrity problems and may even encourage mould growth, which has serious medical implications. One of the most effective countermeasures on the market is the Schöck Isokorb structural thermal break.
It dramatically reduces thermal energy loss in connective areas and enables inner surface area temperatures to remain well in excess of those likely to cause condensation.
To meet the steel cantilever requirements on the project, the Isokorb type KS14, for concrete-to-steel connectivity is being installed in conjunction with the Hollowcore floor system. The method involves the precasters breaking out selected cores and cast reinforcement bars in their works.
The thermal breaks are then fixed to a template, to match the broken out Hollowcore,. complete modules are supplied to site, dropped into position and the broken out Hollowcore filled with insitu concrete.
The Hollowcore has voids extending its full length, with a huge weight saving over floor slabs of equal thickness or strength, and efficiencies in transportation and material costs. With slab sizes on the project typically 1.2m wide by 7.5m long, they are also faster to install and provide an immediate working platform for following trades.
The Isokorb type KS14 has verifiable performance values, providing BBA Certification and LABC Registration, as well as comfortably exceeding the requirements of BRE IP1/06 and Part L of the Building Regulations.
The temperature factor used to indicate condensation risk (fRSI), must be greater than, or equal to, 0.75 for residential buildings and this is comfortably met by incorporating the Isokorb.
Contact Schöck on 01865 290 890 or email: email@example.com