Freyssinet promotes post-tensioned concrete technology in the construction of schools and colleges
Freyssinet introduced post-tensioned concrete building floor slabs to the UK in the early 1980’s. Since that time they have championed post-tensioning technology and contributed to its acceptance as a standard system of construction.
Offering a full service package, their in-house design team can support clients from conception through detailed design and site installation utilising experienced site managers and operatives. Freyssinet sets high and specified performance criteria positioned above the usual industry standards.
Last year Freyssinet successfully completed a number of post-tensioned slab contracts for the construction of schools and colleges throughout the UK. These included:
• The Learning Centre, Ebbw Vale Campus, Wales
• Bourneville College, Birmingham
• The Big Build, Rutland
• Orminston Academy, Birmingham
• Sir Robert Woodard Academy, Lancing
Freyssinet have recently been awarded further post-tensioned floor slab contracts for Wade Deacon High School in Widnes and the Learning Resource Centre at Stafford College. Freyssinet first got involved with the Wade Deacon project over a year before it was finally let, working with VINCI Construction to give cost advice on a concrete alternative to the then conforming steel framed structure with precast concrete flooring units. The post-tensioned alternative saved significantly on the build cost and the flat soffits assisted the architectural planning and services layout.
The structure consists of a spine block running the full length of the 106m long historic façade. Springing from the spine are three teaching wings, each comprising an atrium bounded by classrooms on either side. The three storey structure has 9,750m2 of suspended PT slabs, all being 230mm deep, each floor will be concreted in six pours.
The post-tensioning system selected by Freyssinet’s in-house design team is the 4B13 bonded system. At level 1, two 1000x350mm deep PT beams have been used to span up to 13.7m to allow a pair of columns to be omitted thus creating a feature access way. And on the roof level the same sized PT beams cantilever 3.3m thus avoiding having to run columns down through teaching space below.
Post-tensioned concrete is an ideal choice for academic building construction. It reduces the depth of the structural members meaning the floor to floor height is reduced. Post-tensioned concrete also has excellent deflection control due to the inbuilt parabolic profiles of the tendons, which directly counter deflection effects.
There is plenty of experience within Freyssinet of building post-tensioned concrete academic buildings. The majority of these have been designed and detailed by our in-house team of qualified and experienced engineers and technicians.
Post-tensioning provides a great many advantages, including:
• Increasing spans and load carrying capacity;
• Reducing slab thickness;
• Eliminating beam downstands;
• Reducing self weight, quantities of materials and loads on foundations;
• Considerably reducing and simplifying passive reinforcement;
• Reducing the number of expansion joints and columns;
• Better punching shear characteristics;
• Outstanding behaviour in seismic zones;
• Increased cracking resistance;
• Increased waterproofing due to concrete compression;
• Reduced excavation in basements;
• Deflection control;
• Removal of the formwork within 3 days;
• Environmental benefits, less concrete, less steel, fewer lorry journeys.
The environmental credentials of post-tensioned construction are one of the major selling points when the client’s team are trying to determine the construction material. Typically, post-tensioned slabs save about 25% of the slab depth and hence 25% of the concrete volume, when compared to the Reinforced Concrete (RC) equivalent. The steel content (reinforcement and post-tensioned combined) is often reduced by 65%.
In addition to academic buildings, post-tensioning also suits a wide variety of projects, such as:
• Offices
• Retail developments
• Underground car parks and multi-storey car parks
• Hospitals
• Business centres
• Industrial buildings
• Sports buildings
• Residential towers
Freyssinet Limited work with VINCI Construction UK to win contract
Streatham Hub is a multi-million pound regeneration project by Lambeth Council in partnership with Tesco. The Hub site covers the old ice rink and leisure centre between Streatham High Road and Streatham Station.
The development includes the construction of a replacement sports centre and ice rink, the Tesco store, a bus interchange close to the station, a public square and new homes. The Hub is expected to create around 600 new jobs for Streatham when it opens in 2013.
The project involves an undercroft car park, two storeys of retail store and a transfer structure above that which supports the residential blocks. To one side is the bus interchange, energy centre and service yard all above more undercroft parking. To the other side is a link block and the leisure centre, including the new ice rink and swimming pools.
The total post-tensioned slab area on the project is expected to be 21,250m2.
During the tender stage Freyssinet supported VINCI with value engineering options which assisted in winning VINCI the contract. The majority of these options are now being taken forward into construction as follows:
• The Tescos Store was a 600mm deep cobiax (voided) slab spanning 16×7.5m car park grid. This has been replaced by 2400x550mm deep PT beams and 250mm PT slabs, thus saving 50mm of dig across the 75x75m store footprint.
• The Bus Hub and Service Yard support HA loading and comprised 1600x900mm deep RC beams with 450mm RC slabs. The adopted PT solution has 425mm fl at slabs with 3m2x550mm OA depth drop heads at columns.
• The Link Block contains a 400mm deep PT transfer slab with 1200sq x 550 OA deep column caps at level 1. This carries 5 levels of metsec residential block above. Previously, in RC, this slab was a 600mm deep solid slab.
• The ground level of the Link Block has been post-tensioned too (previously 600mm deep RC cobiax, now 350mm PT flat slab). By post-tensioning isolated beams, previous head room clashes with vehicle ramps below have been solved.
• The Ice Rink slab was designed as PT by Halcrow Yolles from the outset because of the extremely fine deflection control required. It typically consists of 1000x1400mm deep beams and 300mm deep slabs.
Freyssinet Limited was appointed as the specialist PT contractor for this contract in August 2011 and design works have commenced. Construction work will commence in April 2012 and the PT slabs are due to be completed by January 2013.
Streatham Hub, along with Hoe Street and Wade Deacon School are examples of contracts where Freyssinet is supporting VINCI Construction bid teams at tender stage. This synergy between companies is giving them an edge through proposals to use post-tensioning and innovative engineering.
Freyssinet Join Alliance to Revolutionise Offshore Wind
VINCI Construction UK Ltd., and sister VINCI company – Freyssinet International & Cie, have created a joint venture to revolutionise the delivery of offshore wind farms. Developed with support from the Carbon Trust by GBF®, a consortium of Gifford, BMT Nigel Gee, and a specialist arm of Freyssinet – the alliance’s low cost, low risk and low impact approach includes a radical solution for deploying foundations without the need for costly specialist vessels or heavy lifting at sea. The team’s approach also includes a manufacturing process for assembling entire wind-turbine structures onshore before deploying them at sea.
The team’s product comprises a Gravity Based Foundation to be slip formed on land. The tower and turbine will be installed in the construction yard in a continuation of the base production line – economies of scale and a slick methodology will ensure the complete assemblies are produced economically. The completed units will be deployed and installed on the seabed with the assistance of a Transportation & Installation Barge (TIB). The TIB is a low cost, low maintenance dumb barge, built specifically for this process.
Based on UK Government commitments, a six-fold increase in output from wind will be required by 2020. The VINCI Construction UK – Freyssinet JV will construct and deploy the GBF® concept which is set to change the economics of offshore wind farms suitable for gravity bases The JV believes that its methods will enable deployment of turbines in rougher seas and deeper waters than conventional alternatives. A further key advantage is that a completely different supply chain is called upon compared to the better known monopile or jacket solutions.
VINCI Construction UK and Freyssinet International will jointly develop the innovative solution. Close partnerships and collaboration will be critical to the success of the project, and the JV will be working with developers, turbine manufacturers and others, including other companies from the wider VINCI Group to ensure the right arrangements and skills are in place.
There are also considerable environmental benefits to the proposed offshore scheme. The foundations themselves can be made from recycled materials, and the environmental impact throughout the lifecycle of the structures will be minimal. The foundations are designed such as that they can be completely removed at the end of their useful life and hence will have minimal long term impact on the environment once they’ve been decommissioned. While providing local employment opportunities, the projects will also, of course, help the UK to meet its renewable energy targets and deliver sustainable energy in the years ahead.