Large steel framed building, with very distinct architectially visable steelwork
The Leadenhall Tower will be 224m high, 50-storey office building with a distinctive wedge shape in the city of London, offering 56,000m² of office accommodation. Informed by the need to keep particular views of St Pauls Cathedral clear the architects, Rogers Stirk & Harbour, have created the tapering shape. It forms part of a 'cluster' of towers in that part of London alongside the Heron Tower, Swiss Re and the Pinnacle.
At ground level there is a 28m high public space underneath the building footprint, extending the adjacent public square. The tower features a distinctive external steel braced 'megaframe' stability structure - a first for a building of this height - and features extensive architectural steelwork detailing throughout.
On the north side of the building all 20 passenger lifts travel visibly at speeds of up to 8 m/s within glass shafts suspended from the steel framework. The joints in this megaframe are very important for the architecture, the structural performance and the construction of the building. Arup is providing the full structural engineering and services design for this building. It is normal practice in the UK for the detail design of the joints to be carried out by the steelwork contractor. Because of the importance of the joints, their unusual nature and the risk that they might present to tenderers, British Land asked Arup to detail the design of the major joints in the building.
MegaNodes Design Process
The steelwork and concrete have been modelled in Tekla BIM software. Local meganode models could therefore be extracted into Rhino via STP file format. In Rhino the plate surfaces are cleaned up and used to create the geometry in Nastran via the IGES file format. The Nastran model uses a very fine mesh of first order tetrahedral elements, with each model typically containing in excess of one million elements. Tetrahedral elements were used because they allowed for a dramatic reduction in the time required to mesh each joint, and although not traditionally as accurate as hexahedral elements, sensitivity studies were undertaken to establish the best compromise between accuracy and model setup and run time.
The resulting analysis design data was then modelled in the 3D Tekla model and a set of design fabrication sequence drawings were established for each joint type. This allowed the practicality of the various welds to be investigated and identified those that would need to be one sided and/or in some cases would only be sealing welds. The sequence was informed by the access requirements to perform the welding and importantly the access for testing of the welds. A key element of the fabrication drawings was to demonstrate to potential fabricators that despite the complex geometry the large majority of the welding was with single pass double sided fillet welds.
The Tekla Model
Currently comprises 9000 steel members modelled, four storeys of concrete basement construction and 125 piles. Throughout the modelling process technicians worked within the multiuser environment and all 2D drawings remained live within the Tekla model, meaning they are always all up to date with the latest information. Internal processes are streamlined as the model is used as a visual communication vehicle between Engineer and Technician to improve the checking process.
The structural Model and MEP CADDuct Models was appended for coordination and clash checking
The Tekla model was provided as part of the tender process meaning the main contractors can bid the work knowing exact quantities and work involved in the mega-nodes saving them a huge amount of time and money calculating how the building will be constructed and fabricated. Arup's have been working on internal processes that changes their deliverable to a 3D building information model on all major projects which they believe is essential for a complete BIM process to work well.