Structures in the Low Countries: the footbridge Lichtenlijn in Knokke

This is the first post in a series on Structures in the Low Countries where I am spending time this Fall doing research and meeting interesting people.

On a beautiful Fall afternoon, I stumbled across the Lichtenlijn footbridge in Knokke Heist, a resort town at the Belgian coast.

With its undulating shape, the bridge evokes the image of waves and sails. (credit Sigrid Adriaenssens)

The bridge reminds us of sails, the wind and waves.  It connects the dike and a nature reserve and enhances the location of two historic lighthouses that guide ships to safe shores (hence the bridge’s name Lichtenlijn which means Line of Light in Dutch).

The slenderness and the expressive overall geometry evokes the image of a hammock tensioned above the local flora. (credit Sigrid Adriaenssens)

The bridge reminded me of what the French philosopher Saint Exupery  wrote in Terre des Hommes (1939):

“It seems that perfection is attained not when there is nothing more to add, but when there is nothing more to remove.”

The Knokke-Heist footbridge ( 2008), was designed by Ney and Partners unlike traditional structures. The overall geometry is based on hanging cloth model principles (like the shape of a hammock). The bridge has a curved plan and is supported at two intermediate mast supports and at the abutments.  The plan curvature responds to the site conditions and the suspension at the masts provides interesting viewpoints for the pedestrians and cyclists on the bridge.

The bridge is suspended at 2 intermediate mast supports and supported at 2 abutments. (credit Sigrid Adriaenssens)

The overall geometry was further refined to comply with the CNC manufacturing constraint of single-curvature steel sheet bending, and was then numerically optimized to maximize the overall stiffness of the bridge.

Viewpoints on the bridge

The latter task presents a typical topology optimization problem that consists of distributing a given amount of material in a design domain subject to load and support conditions, such that the bridge stiffness is maximized.

Topology optimisation result in openings in the bridge which act as view concentrators.

The figure below shows the optimal thickness distribution in the bridge surface for different values of the thickness ρmean, which is a measure of the total material volume constraint.  By combining topology optimization with form finding and CNC manufacturing constraints, a 3D typology was found that might not have been conceivable in a purely analytical or intuitive fashion.  At the Form Finding Lab, we investigated this bridge and its topology optimization, you can find out more about it  here.  Soon we will visit the form found cupola over the Dutch Maritime Museum in Amsterdam. Stay tuned for more interesting structures in the Low Countries!

Optimal material distribution in the thickness of the shell

Location: Elisabethlaan, Knokke Heist, Belgium

Author:  Sigrid Adriaenssens

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