Greenside Place Link Bridge, Edinburgh

• tubular steel lattice footbridge
• nonlinear buckling analysis
• natural frequency check

Greenside Place Link Bridge is an unusual S-shaped, helical steel lattice structure that spans 50m across Leith Street in Edinburgh, Scotland.  Buro Happold Consulting Engineers used LUSAS Bridge to carry out a static analysis, a nonlinear buckling analysis, and a natural frequency check of this innovative structure for its client Coal Pension Properties (c/o La Salle Investment Management)

Overview Greenside Place Link Bridge replaces a previous link bridge across Leith Street and connects the St. James Centre to the Nottingham Place Terminal of the Greenside Place underground car park. It is a free standing structure of curved steel tubes of 139.7 x 10 Circular Hollow Sections (CHS) spiralling around six longitudinal 193.7 x 16 CHS located in the top and bottom of the bridge cross-section. The 5m wide x 4m high helix is supported by splayed legs of 457 x 25 CHS springing from concrete plinths. Aluminium planking is used for the decking. Glass panels frame the sides of the walkway and also provide a roof to the structure for pedestrian protection.

Modelling A CAD model for the structure already existed and so the geometry for the bridge was imported using a DXF file. To assist with the assignment of attributes such as geometric and material properties in LUSAS, named groups were used. This very useful facility allows common parts of a model to be viewed or selected in isolation and provides greater control over subsequent processing of results. Thick beam elements were assigned to the line features representing the main steel members. Joint elements were used to model the pinned connections between the supports and the helical tube. Fully fixed supports were used at connections with the concrete base. Long-term loads including differential settlement, and short-term loads from pedestrians, wind, and temperature were assessed. Accidental loads from vehicle impact with the concrete piers were also considered.

A static analysis of the proposed structure clearly showed that the bottom members of the helix immediately above the inclined supports were overstressed and some form of modification or stiffening would be required for the final structure. Iva Trifkovich, structural engineer at Buro Happold explains: "The helical structure, which provides shear transfer between the top and bottom chords, was globally very stiff - the only problem we had was locally around the supports. We considered different strengthening strategies, including a steel plated stiffening option, but this didn’t solve the problem." In the end 193.7 x 20 CHS diaphragm rings were used at each support location. Subsequent analysis showed that the insertion of these rings lowered the stresses in the helix local to the supports to an acceptable level. Iva Trifkovich said: "Because of the complex nature of the structure and its S-shaped form, it would not have been possible for us to be sure of the exact stresses and forces in the helical members without the use of LUSAS."

A nonlinear buckling analysis was carried out to determine the critical (lambda) factor for when the structure would start to behave nonlinearly. An initial imperfection was applied to the deformed buckling shape and the structure was incrementally loaded. It was shown that the structure remains linear within the range of the design loading.

Whilst dynamic response to pedestrian loading can be a problem for some footbridges, a straightforward natural frequency analysis showed that the first horizontal mode shape of 2.55Hz and the first vertical mode shape of 6.32 Hz were both above the critical frequency range for pedestrian comfort according to the UK BD37/01 design code. Gain frequencies were outside of the critical walking range too. Iva Trifkovich explains: "We compared gain frequencies against pacing and jumping frequencies for both vertical and horizontal movement of the bridge and got satisfactory results, showing that pedestrians will not feel any discomfort when crossing the bridge."

"Because of the complex nature of the structure and its S-shaped form, it would not have been possible for us to be sure of the exact stresses and forces in the helical members without the use of LUSAS."

Buro Happold wish to acknowledge the contribution made by the following organisations on this project:

Client: Coal Pension Properties (c/o La Salle Investment Management)
Contractor: Raynesway
Steel Fabricator: Westbury
Architect: Broadway Malyan
Quantity Surveyor: CBA

Share this article