Project Name: Pembroke College Footbridge

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1 Project Name: Pembroke College Footbridge Categories: Structural Heritage Award Pedestrian Bridges Commercial or Retail Education or Healthcare Community or Residential Arts or Entertainment Sports or Leisure Small Practices Small Projects Highway or Railway Bridge Infrastructure or Transportation Structural Designer: Price & Myers Brief Description ( Characters): Sculptural Corten steel footbridge with singular 9m long glass balustrades linking the new and existing college buildings. Location: Oxford, England Date of completion: February 2013 Cost: 155,000 excluding concrete podium abutment Client: Pembroke College Principal Contractor: Kingerlee Architect: BGS Architects Temporary Works Designers: Kingerlee Consulting Structural Engineers: Price & Myers Project Contact: Gemma Ahkin, Publicity Price & Myers 30 Newman St London W1T 1LT Project Representative: (person that will be called on stage to collect award if won) Robert Nilsson, Engineer (IStructE membership No.) Price & Myers 30 Newman St London W1T 1LT

2 Vision and excellence As part of their new masterplan development, Pembroke College desired a pedestrian bridge between the historic college buildings and the new, linking past and present. Working on the design of the first bridge in Oxford city centre in just under a century was an honour and a challenge. The last built bridge was the Bridge of Sighs over New College Lane in The vision was to provide a light skyway that was robustly anchored in the Fellow s Garden of the existing college and then gracefully projected over the listed Ancient Monument stone wall and Brewer Street below to gently touch the more delicate, modern construction of the new build. Creativity and innovation Initially, the sole robust portion of the bridge was to be the piled concrete abutment and podium in the Fellow s garden. The bridge itself was to be its own entity, only relying on the podium for vertical support. Multiple scheme options arose from brainstorming sessions with BGS Architects, but the key that helped shape the bridge was the road below. Despite Brewer Street being a generally quiet side street with one-way traffic, the bridge would still have been required to withstand highway-level vehicular impact loadings had it impeded on the 5.7m clearance zone above road level. Designing out these implications was paramount in reducing costs, simplifying the overall design, and producing a practical structure. With access levels already fixed either side, the maximum structural depth to clear the 5.7m was 135mm over the 10.3m span; an unlikely feat simply supported. The weight of the concrete podium was thus embraced, allowing for a fully fixed connection to a haunch in the bridge deck. The haunch would then taper as it reached across the road, reducing it to a 135mm structural depth. The deck gradually evolved into a highly sculptural form that is deceptively simple: a hollow monocoque fabrication of welded Corten steel plates to form a box section morphing from triangular into rectangular in cross-section with internal rib strengthening exposed along the soffit to express the natural load paths of the structure. To keep with the simple and pure nature of the deck and to enhance its presence, a single sheet glass panel was chosen for the balustrade, supported laterally by a stainless steel handrail beam following the same structural principles as the deck. Modelling and analysis The theoretical structural behaviour of the bridge deck and handrail beam is relatively simple and can be idealised as a clamped cantilever. A complicated analysis is thus not required. However, the bridge is the marriage of 3 separate and distinct structural components that are geometrically linked and mutually dependent on each other. The singular glass balustrade became the crux of the design. The primary function of the glass pane is to infill between the handrail and deck. It does not contribute towards the stiffness of the deck nor does it support the handrail beam vertically or laterally. However, its out-of-plane stresses could be greatly amplified by deflection of the handrail and deflection and vibration of the deck. A comprehensive finite element analysis was carried out on the singular glass panel checking localised stresses around the oversized holes where the handrail beam joins along its span. A combined deck and handrail finite element model was used to check frequency, damping, and footfall. The bridge deck was sufficiently stiff over the span, but to isolate the glass and not cause unwanted excitation, the glass was supported vertically on individual bearing pads at outer quarter points along its span to tell the glass where to bear. During the design process, there was much contention about using a glass balustrade over a city street. The primary concern being that if both 10mm sheets within the laminate shattered, the panel when loaded laterally would deflect considerably and potentially pop out of the lower deck slot. A load test was thus commissioned by the College and a mock-up panel with support conditions mimicking the proposed design was constructed. Both sheets of the laminate glass panel with SentryGlas interlayer were intentionally shattered, loaded, left to sit for 2 weeks to

3 monitor creep, and then finally hit with an additional 50kg ball drop impact loading from height. The test was a success and approval for the footbridge was granted shortly afterwards by the Oxford Highways Department. Geometrically, the bridge was fully modelled in the parametric software Digital Project. As all components of the bridge are interlinked, a small change to one component could have had an exaggerated effect on another. The parametric modelling ensured flexibility in the design and was a very useful tool in fine-tuning the form of the bridge with the architect and client. All curvatures forming the geometry of the deck derived from developable surfaces to facilitate fabrication. All fabrication drawings for the bridge deck were interpreted and created directly from the Price & Myers 3D model. Sustainability All chosen materials react intrinsically with their external environment. The exposed Corten surfaces will produce an aesthetic rusty protective patina that will weather naturally and nicely within its climate. Aside from the upper surface of the deck where water is allowed to collect below the oak treads, no other surfaces require treatment or protective coatings for the full life span of the bridge. The untreated oak cladding to the handrail structure is envisaged to weather naturally into a silvery-grey shade to match the finish of its supporting stainless steel. Value The project was funded by the College with the significant aid of private and public donors. The simple yet meaningful form of the bridge became the symbol of the masterplan project and community fundraising campaign, dubbed Bridging the Centuries. Conclusion The bridge has been well received by the College and it has become a highlight within their newly completed grounds, frequently enjoyed by not only its students and staff, but also the public. The streamlined shape of the bridge merges art with precision engineering. Pedestrians walking across the bridge have found that the uninhibited plane of the glass balustrades unexpectedly but pleasantly abstract and blend together the surrounding buildings into a mirrored collage of the old and new. Images Quintin Lake Quintin Lake

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5 Kingerlee Robert Nilsson Drawings

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