It is required from now designed hotel, public and sometimes residential buildings to be multifunctional. This requirement creates the need for different structural systems on different floors. For example, a typical functional solution is the location of large open space in the lower floors, forced by the location of conference rooms, restaurants, exhibition halls or reception-representational areas of the hotel. However, in the upper floors, their living function, and thus a dense system of internal walls, does not require using costly slabs with large spans.

The easiest and cheapest way to get large spans, and thus spaces of the lower floors, free of support, would be to use reinforced or prestressed concrete beams carrying the upper floors. With the spans of such beams measuring over a dozen meters and loaded by several floors, the required height of the beam made of prestressed concrete far exceeds the acceptable sizes limited by the permissible height of the slab. Sometimes, a good way to reduce the height of beams is step by step introduction of prestressing (Post-tensioned transfer slab over the tunnel at the Warsaw Old Town). On the other hand, it is a time and cost consuming process.

A common solution is to use prestressed slabs with large spans on all floors as a pre-tensioned (usually made of hollow concrete slabs) or post-tensioned concrete versions. Therefore, each slab carries a load of its floor independently. Such a solution, though simple and effective, increases the costs of building execution and is not favoured by investors.

While searching for an effective solution to the presented problem in a designed 7-storey hotel building, a non-standard solution for the structure of the building inspired by a bridge box cross-section had been proposed. The space of the first floor includes a restaurant room while the upper floors are designed for hotel purposes. The original design plan assumed the use in the first floor two rows of columns carrying higher floors. It was attempted to increase its attractiveness by eliminating internal columns due to the representative character of the space of the first floor. The limited structural height of the slab to 0.70m and the maximum distance of external supports measuring 14.5m did not allow for making the beams capable of transferring the load of four floors of the hotel, even when using prestressed concrete. The post-tensioned load-bearing system, consisting of two slabs connected every 4.2m by reinforced concrete walls (webs) was designed. In this way, the I-section with a height of full floor was made. Vertical reinforced concrete walls of the first floor acting as a web of I-beams are further interrupted by a hole resulting from the location of the corridor in the building. Its location in the middle of the span of the beam (outside the cumulative shear forces) is not an obstacle. At the slab level of +8.10m, constituting a tension zone of the formed beam, the post-tensioning in the form of four 7L15.5 cables, located in each of the four beams under the walls, was used. The spacing of prestressed supporting elements of the building is 4.20m. Three masonry structures storeys of the third, fourth and fifth floor with reinforcements in the form of reinforced concrete small columns were designed on the created post-tensioned concrete structure. The span of created load-bearing concrete structure is from 13.0 to 14.45m. The C35/45 concrete (according to Eurocode 2) was to be used to complete the slabs with prestressed elements.

One of the problems to be solved was to locate air-conditioning ducts with large cross-sections. Due to the fact that the length of the beams covers the entire width of the building segment, and their height uses the full available height of the slab, it was impossible to avoid passing of the ducts through the beams. It was decided to limit the height of the duct up to 250mm, leaving 250mm of the lower zone of the beams for cable location. The ducts took substantial width because of the reduced height of cross-section. Finally, 2 ducts with a width of 600mm were accepted.

The holes with dimensions of 600×250mm were to be located near the wall due to the presence of the reinforced wall above, transferring the shear forces. The holes were spaced only about 600mm from the edge of the wall beyond the zone of greatest stress caused by prestressing. Additionally, the beam width was increased from 500 to 600mm in the section measuring 600mm from the edge of the wall and on the width of the column. It was decided to safely anchor the ordinary reinforcement of the beam and the column. This procedure is also very important from the point of view of the effort of the element caused by prestressing at the supports. It causes a reduction of the stress at the supports, caused by the eccentric application of force.

The abandonment of the approach commonly used in the design of reinforced concrete buildings and the use of reinforced concrete structure within the one floor (two slabs and connecting reinforced concrete walls) as cooperating load-bearing structures allowed to achieve the span of the slab measuring 14.5m without changing the structural system of higher floors.

Such a solution allows to design a large, free of support spaces, and selected floors in multi-storey buildings without using costly slabs with large spans in higher floors.