Case Study

Wooden Tower, Helsinki Zoo

  • Wooden lattice structure
  • Ultimate and serviceability limit state checks for applied loadings

The wooden observation tower at Helsinki Zoo in Finland was the winning design in a competition held for architectural students of Helsinki University of Technology. Situated on a small island just outside the city, and built on one of its highest hills, it provides visitors to the zoo with panoramic views over the sea and surrounding area. LUSAS Civil & Structural was used in the design and analysis to prove the safety and structural behaviour under imposed loading.

Overview

Similar in structural form to the Downland Gridshell at the Weald and Downland Open Air Museum in Singleton, UK, but far smaller in scale, the Helsinki University tower is a 10m high, two storey, irregular-shaped wooden lattice structure built from 72, twelve metre long, glue-laminated battens. The battens were factory bent to 7 different preformed shapes ready for assembly but to achieve the final shape they had to be bent and twisted further during the construction process. The wooden members are connected with double-sided nail plates and 10 mm through-bolts to form a space truss. In all, there are more than 600 joints in the tower. Two floor levels are supported by edge beams at the perimeter and connected to the truss by deck bolts to give the necessary horizontal stiffness and support to the lattice members. A steel framed opening in the shell provides access to wooden stairs. The whole structure rests on a hidden reinforced concrete foundation via specially designed steel joints.

   

Modelling and analysis

A 3D CAD model of the architect’s complex irregular geometry was imported into LUSAS using the DXF interface. Supports, loading, and section properties were defined to enable a full structural analysis using 3D beam elements to be carried out. Lauri Salokangas of Helsinki University said: "We found the ability of LUSAS to import CAD files extremely useful. It saved us a great deal of time in re-creating the model in LUSAS".

In the Ultimate Limit State the maximum compressive normal force obtained due to most unfavourable load combination of self weight, live load and wind load, was 17.8kN. The corresponding stress in the most stressed member was about 5 N/mm2, well below the compression strength for wooden laminates. No cause for alarm was found with regard to sideways buckling in the most stressed lower struts.

In the serviceability analysis LUSAS was used to determine the stiffness and frequencies of the structure. In spite of individual slender members the structure turned out to be very stiff in the vertical direction. Serviceability Limit State loadings caused only a few millimetres deflection on the floor levels. This was also experimentally verified in a load-test on the finished tower, where an applied load of 21kN on the first floor caused only a 5mm deflection.

The eigenvalue analysis showed that the fundamental frequency in the weaker horizontal direction was about 5Hz. This corresponded reasonably well with the measured one of 4.5Hz on site.

"We found the ability of LUSAS to import CAD files extremely useful. It saved us a great deal of time in re-creating the model in LUSAS."

Lauri Salokangas, Helsinki University

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