A high-tech, earthquake resistant steel and glass roof structure for the Crowne Plaza Hotel in Caserta

Detailed information

The anti-seismic roof structure

The roofing plan is a square and it is based on those previously built four buildings. Its corners are cut because of the staircases. Large dimensions of 60 x 60 m and a height of 6 m made it necessary to have a tubular arch structure made of steel. The arches are placed at a distance of 4.5 m and another steel sub-structure supporting the rhomboidal glass is included between the arches. As the hotel was built in a seismic zone where the top plan seismic shifts are equal to 300 mm, according to regulations it was necessary to build a roof that could be rigidly connected to one of the four buildings through a pre-compressed concrete system and that could run free along the other three buildings.

The anchors are composed of a horizontal frame on three sides that is made of a reticular steel structure. The aim was to balance the drifts of the arches and transfer the weight on to the top of the columns of the existing buildings. Moreover, 300 mm gaps were created in order to leave the buildings free to swing in any direction during an earthquake. The sliding supports that make it possible to face the shifts were implemented for this project through an upper plate bolted to the reticular steel structure, a neoprene-steel support, a PTFE disk, an anti-dust lining, an inoxidizable steel disk, and the support. The steel structure is connected to the pre-compressed reinforced concrete beam through the pre-compressed cables of the steel structure.

Technical features

The roofing was built with transparent and rhomboidal elements: stratified glass with outer toughened plate of 6 mm thickness and an inner plate of 4 mm thickness. A PVB plate (Polyvinyl butyral) combines the glass. The glass plates were subjected to breaking tests: in case of distributed load, breaking took place a bit beyond 4 kN/M2 and its effect was only on the toughened plate. Moreover, it was shown that the PVB is capable of maintaining the load of the plate after breaking for 36 hours without collapsing. Glass leans on the main arches and on the secondary structure. Despite a structural silicon connection between the glass plates, there is a high-density polyethylene support lining, which is moulded in order to collect the water that is likely to seep into the glass.

A special study was carried out together with the EMPA of Dubendorf concerning the resistance of the roofing to hail: an estimated diameter of the hailstone hitting the roofing at high speed was determined, and then an empirical test was carried out by launching a 40 mm diameter polyamide sphere. This made it possible to establish the breaking features (the procedure was standardised for the polycarbonate roofing). The test demonstrated that breaking takes place at higher speeds and especially if the impact is at central part or at a part 10-12 mm from the free edge the roofing doesn’t break.

Incremental launching

Incremental launching was necessary because of the short time to construct the roofing. A working platform was built and the final part of the roofing was constructed on the area where the pre-compressed concrete plate was located. Then the cross arches of the first group were built with longitudinal connections. After the launching of the first group of arches, a second group, connected to the rear part of the first one, was assembled and the group was launched in the same way until it joined the existing part. Glass was assembled before launching in order to save time and this procedure ensured relatively safer conditions for the workers. Arches were pre-assembled on the ground.

Due to this, during launching, the structure had a one-way and cross behaviour instead of a two-way behaviour. This is the reason why pre-stressed chains and stabilising elements were added during launching and were removed at the end of the launch.

Anti-fire analysis

At the end of the construction, the effect of a possible fire spreading from a hotel room, where the flames coming out of the windows can cause an increase in the temperature of the dome structure, was examined. The pattern of the fire and the temperatures produced by it were created and the resulting structural analysis showed that the arch edge was stressed beyond the elastic limit.

The control strategy was to insert a material with lesser rigidity corresponding to the resistance features of the material at the estimated temperature into the arch and to consider the adjoining elements as hinges on the two ends. The formation of plastic hinges was therefore accepted.