Hotel Motto Vienna: a rooftop extension with Cofrastra® composite flooring system

Detailed information

A complex building project at lofty heights with a striking glass roof extension

A good example of these roof extensions is the former Hotel Kummer, a Gründerzeit era property steeped in history situated on the corner of Mariahilfer Strasse (71a) and Amerlinstrasse. With a 50 million Euro investment from Wertinvest in partnership with Schadekgasse 20, the hotel has been gutted, refurbished, and extended with a new two-storey, barrel-shaped roof structure added. This new structure links all the different parts of the building, coordinating with the magnificent, original facade and connecting with the new, modern cityscape.

The building has remained a hotel but has been given a new name: Hotel Motto. It will operate with 91 rooms, a restaurant, a bar, and meeting areas. A branch of a well-known clothing brand has taken a lease on the ground floor and an organic bakery with an outdoor café will move in next door. The striking roof structure, designed by architect Arkan Zeytinoglu and set back slightly from the facade, will house a restaurant and roof terrace that will be open to the public.

Cofrastra®: a composite flooring system with numerous advantages

In terms of structural engineering, the project has been a considerable challenge. During the course of the work, a wide variety of ceiling constructions were uncovered within the fabric of the historic building, including in-situ concrete ceilings, wooden beam ceilings, and brick ceilings. In planning for the additional roof levels, the civil engineers of Thomas Lorenz ZT GmbH decided to use steel, or more precisely a steel composite ceiling construction because of the many advantages it offers.

Working in partnership with the manufacturer, the engineers chose Cofrastra® 70 composite slabs from ArcelorMittal Construction Austria. This composite slab profile, made from galvanised sheet steel with additional embossed cams, serves as both the formwork for the in-situ concrete and the lower reinforcement layer to absorb the bending moments. The stackable trapezoidal-shaped panels were ideal for the city centre site because they can be easily transported and stored efficiently without taking up unnecessary space. The ribs also reduce the dead weight by up to 15% for the same static height, crucial for the deflection of the slab and resulting in a slim construction with lower loads on the building stock below. Fire protection (REI 90) is achieved by simple bar reinforcement in the ribs (Ø12mm) without additional time-consuming measures and is in line with EN1994-1-2.

Assembly of the 1100 metre-square composite slab could largely be carried out by hand without the use of a crane, saving valuable crane capacity and construction time. The special dovetail geometry allows for a reversible installation of building services and suspended ceilings so that the new composite ceiling, as well as the original ceiling constructions, remain hidden from view.

Key details for Cofrastra® composite floors

Loads: additional dead loads - 2.7kN/m2; live loads - 5kN/m2

Sheet thickness: variable, between tN=0.75mm and tN=0.88mm

Spans: 1.60m single span system – 5.20m double span system with two supports during construction

Floor height: 15cm and 22cm

Concrete requirements: 200l/m2 – 125l/m2

Floor weight: < 500kg/m2 - ca. 320kg/m2