Engineering analysis and design software
Bridge design and engineering

Case Study

The Helix Bridge

  • Glass and steel retractable footbridge
  • Global modelling of stresses in glass, adhesive and cantilevered walkway
  • Localised modelling of key individual components

Helix Bridge, Paddington Basin

The Helix Bridge at the Paddington Basin development adjacent to Paddington Station is a retractable, composite glass and steel pedestrian bridge. It has a helical frame which rotates during deployment to give the appearance of it corkscrewing across the canal. Davy Markham used LUSAS to ensure that the proposed form of construction would not excessively stress the glass or adhesive bonds whilst operating under self-weight.

Redevelopment scheme

The Paddington Basin development is one of the largest urban regeneration projects in Europe. As part of a masterplan to restore links within the local community, a network of new footpaths; a towpath along the canal to Little Venice; and several pedestrian bridges, such as this Helix Bridge, by Marcus Taylor, and the Rolling Bridge, by Thomas Heatherwick were designed and installed on behalf of Paddington Development Corporation.

Construction

The glass and stainless steel helix structure measures 7.2m long x 3.5m in diameter and surrounds a 2m wide cantilevered, carbon steel walkway which can be retracted to allow the passage of water traffic. The composite tube is fabricated from a tubular helix or ‘corkscrew’ formed from 140mm diameter Circular Hollow Sections to which are bonded a number of curved, trapezoidal sheets of toughened laminated glass. Welded along the full length of the helix are six 80mm square-section transoms or cross members, which provide mounting points for the laminated glass panels. The 15mm thick glass is bonded to the transoms with structural-grade glazing adhesive and the joins covered by sealing strips. Analysis with LUSAS was required to ensure that the proposed form of construction would not excessively stress the glass or adhesive bonds whilst operating under self-weight.

LUSAS model of structure

Modelling and analysis

A model of the whole structure allowed global stresses and displacements in the glass and steel members of the assembly to be obtained and assessed. 3D beam elements modelled the tubular helix section and the transoms. 3D shell elements modelled the glass panels, the sealant running around the "circumferential" joints, and the adhesive used to fix the glass to the transoms. Elements representing the adhesive and sealant were easily isolated to enable contour plots of stresses and strains in these critical regions to be produced. A broken glass scenario, as well as a wide range of operating loads and conditions was also considered.

Stress in sealant under self weight load case
In-service stresses in cantilevered walkway Von-Mises equivalent stresses in glass under self weight

Localised models investigated stresses at selected intersections of the Circular Hollow Section helical members and Square Hollow section transom members. CHS wall thicknesses were evaluated and an optimum thickness obtained. Regions of individual transom members that required reinforcing were also identified.

Stress in reinforced Circular Hollow Section Stress in reinforced Square Hollow Section

"Finite element analysis with LUSAS was essential on this project and proved that the differential movement between the glass and the steel helix was within the strain limits of the silicon adhesive."

Phil Snowsil, Senior Design Engineer, Davy Markham

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