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Civic Vierendeel Truss Design David Brown of the SCI considers the design of Vierendeel trusses A recent theme in NSC has been the design of trusses – which also feature in the Vision Tameside project. At the ends of the 100 m wings, each façade is extended as a cantilever from the main structure. Wind loading on the façade extensions causes out-of-plane (horizontal) bending of the cantilevers, so the vertical orientation of the trusses is a little deceptive. Vertically, the trusses carry the façade loading. In the vertical direction, the trusses are Vierendeel trusses. Named after the inventor – Arthur Vierendeel – these trusses have no diagonal members. The classification as a truss is rather deceptive, as they are really continuous frames with moment resisting connections between the vertical members and the chords. The bending moments in the truss members are at a maximum near the supports, so in a simply supported Vierendeel truss it would not be unusual to have larger members towards the supports, possibly diminishing in size towards the centre of the span. With a cantilever Vierendeel truss, the bending moments tend to be larger at the central support. Compared to Pratt or Warren trusses, a Vierendeel truss will probably have more expensive connections (as they are moment resisting), but has the advantage of no diagonal members – so storey-height Vierendeel trusses are sometimes used as transfer trusses within buildings, as access through the truss is maintained. Vierendeel trusses can be analysed “by hand” – a pin is assumed at the mid-point of the members and an approximate analysis completed. The method is described and an example presented in older editions of the Steel Designers Manual (SDM). Most designers will simply use software to readily determine forces, moments and deflections, which is the method encouraged by more recent editions of the SDM. With all trusses, the design of the joints is key, with the objective to remove the need for local reinforcement, which would be expensive. This is particularly so with Vierendeel trusses, where the bending moments must be transferred across the joints. Joint resistances can be checked using the guidance in BS EN 1993-1-8, with free software available from Tata Steel. It is highly likely that an iterative process of checking joint resistances, revising member sizes and re-analysis will be needed to reach an optimum solution, both structurally and economically. This best practice iterative approach was followed for the Vierendeel trusses in the Vision Tameside project – although like so much elegant engineering, the trusses will be concealed by the cladding in the completed structure. 24 NSC June 17 as horizontal diaphragm elements to transfer the laterally applied wind and stability forces to discrete vertical braced bays. These braced bays then transfer the applied lateral forces to the building’s piled foundations through an arrangement of continuous steel diagonal bracing members located within the main column and floor beam frames. What is already in the ground from the demolished TAC building also played a role in the choice of a steel-framed solution for Phase Two. The demolition specification for the original Council office building required the pile elements of the building’s foundations to be left in the ground. This requirement meant that the construction of the foundations to the new building needed to be set out around these existing piles, while also providing the necessary support to the new building superstructure and its proposed column positions. Consequently, a composite steel frame building was adopted to minimise the weight of the superstructure and to enable the new foundations to be offset around the existing piles in an economical solution. “A composite steel frame building offers a lighter frame solution than a comparable reinforced concrete frame, offering economies in the foundation design, while the use of composite floor slabs within the steel frame avoids the use of traditional formwork associated with RC frames, offering a faster form of construction,” says TPS Associate Director Andrew Forshaw. New piles were installed to a depth of 20m and, to keep the construction programme on schedule, the installation of pile caps and then the steel erection were started before the piling had been completed. This meant, that at one point, all of these trades were on-site at the same time, following each other in a sequential manner. Steelwork was erected by Elland Steel Structures in six phases, working around the project clockwise and then finishing with the central element. The composite floors above the store area are subject to higher loads than the individual building wings to suit the buildup of finishes forming the podium deck to the elevated public realm area and its associated occupancy loads. To accommodate this design feature slightly larger steel sections were used for the store. Vision Tameside Phase Two is scheduled for completion in early 2018. 23 Elland Steel’s erection programme nears completion


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