Case Study

Analysis of a glass reinforced plastic arch for the U2 stage set

• Investigation into wind loading effects
• Tensioning of arch for lifting process

Top architectural and engineering consultants Atelier One used LUSAS Civil & Structural analysis from FEA Ltd to help in the design of a self supporting GRP arch - part of a 30m high structure forming part of the stage set for the rock band U2's current world tour.

For ease of transportation between concerts the 7.8m high by 12m wide upper arch has to be constructed from 40 separate GRP panels. Each 25mm thick panel is connected vertically to its neighbour by 8 coffin latches for ease of assembly. Steel base frames fit inside the lower 4 panels and a central steel lifting frame fits into the top of the arch. Four tensioned cables connect the top of the lifting frame to the base frames and help to strengthen the arch during the lifting process. After assembly horizontally on the ground, the arch is lifted into position and fixed to the GRP-clad steel arch portal legs via the base frames.

LUSAS was used to investigate stresses and forces in the GRP panels and coffin latches caused by the lifting process and, more critically, from a specified design wind loading of 130kmh (80mph) where suction effects tended to pull the joints between panels apart.

The finite element model was created in LUSAS from DXF data supplied by Atelier One's in-house CAD system. LUSAS QSL8 semiloof shell elements modelled the GRP panels and JL46 joint elements were used to model the coffin latches. The circular and rectangular hollow steel sections of the lifting and base frames were modelled with BSL4 beam elements. Self weight and wind loadcases were applied and a linear static analysis obtained stresses in the GRP panels and forces in the coffin latches for comparison with test results.

Independent tensile testing of the coffin latches resulted in a breaking load of 2 tonnes being achieved. By analysing the upper arch with LUSAS Civil & Structural the force in the latch joints under the worst-case loading condition was found to be in the order of 1.5 tonnes, meeting the design requirements.

LUSAS was also used to study the effects of increasing the tension in the 4 inner lifting cables that connect the top of the lifting frame to the base frames. The aim of this was to attempt to further reduce the overall forces in the outermost latches by pulling the component panels together. By applying an initial stress loading to the elements representing the cables and repeating the analyses it was shown that due to the geometry of the arch and the location of the cables that there was no benefit in increasing the tension force beyond that initially specified.

With a tight design schedule to meet, dedicated engineers and the use of LUSAS Civil & Structural software ensured that the contract was completed in time for pre-tour stage assembly and rehearsals.