River House - Bratislava, Slovak Republic
The River House is the main building of the multifunctional River Park complex on the left bank of the Danube River. The complex consists of four blocks which include 203 luxury residences with a magnificent view of the river, above-standard offices oriented towards the castle hill and a five star deluxe hotel. The four elevated blocks have three common underground storeys, 264 x 53 m, in which parking spaces and technical facilities are situated.
Substructure and basement
The foundations of the River House comprise 900 and 1.200 mm diameter bored piles embedded to the stone base (idem for the other three blocks). In the basement a "white tank" waterproof concrete system is applied which has to resist hydrostatic pressure from underground water (average height 4-5 m and extreme height 9.6 m). In areas where the counterweight of the buildings is not enough, the basement slab of the "white tank" is anchored into the stone base by prestressing bars.
The River House has an irregular trapezoidal shape and floors with curved rims. The overall plan dimensions are 104 x 24 mm with one dilatation block. The 3rd - 8th storeys have a conventional bearing structure with flat slabs spanning between columns and cores. As the whole building stands on two cores and a pair of piers, a transfer structure was needed on 1st and 2nd storey. With regard to large spans and cantilever overhangs combined with load from eight storeys, a prestressed concrete structure came across as the most efficient and reasonable solution. A system consisting of two longitudinal and twelve transverse deep beams was proposed to carry the columns of upper storeys. Transverse (secondary) deep beams with the height of one storey (3.100 mm) and thickness of 600 - 750 mm are supported by prestressed longitudinal (primary) deep beams with the height of two storeys (7.700 mm) and a thickness of 750 - 1.000 mm. The largest span of the longitudinal beams is 31.3 m long and the largest cantilever overhang is 15.9 m long. Both of them are prestessed by eight cables consisting of 15 unbonded strands in HDPE ducts. Compressive forces generated by prestressing have affect only on the cantilevers due to high stiffness of the cores and piers. In spans between supports only vertical actions were employed. The application of prestressing has resulted in lower consumption of reinforcing steel and reduction of the thickness of deep beams. In addition its advantageous effect on crack widths and stiffness of the structure is no less significant. The flat slabs of the typical storeys have a complicated shape with several levels, various thicknesses and spans. The largest bay has the span of 8.80 x 7.35 m, and the largest cantilever overhang is 3.45 m long. For loads on the transfer structure a maximal reduction of the slab thickness was needed, but on the other hand the façade and heavy acoustic partitions posed strict claims on the deflection of the slabs. For these reasons the thickness of the slabs varies from 250 mm to 400 mm.
Building of the transfer structure
The most difficult part of structure was the cantilever above the Danube River for which a special steel supporting structure had to be applied (made-to-measure for this construction). The cables were stressed in two phases to avoid overloading of the structure by vertical actions from prestressing when the counterweight from the upper storeys had not been enough yet.
Monitoring of the deformations
The façade, mainly in the office part, consists of large glass tables with very tight gaps between them. There are rigid and heavy acoustic partitions in the residential part. These constructions are very sensitive to deflections of the load bearing structure. For these reasons long-time monitoring of deflections had been proposed in the most critical parts of the structure. The last measurements show very good accordance with the results of analysis.
Use of Scia Engineer
For the structural analysis of this structure IDA Nexis (ESA-Prima Win) was used. The challenges: optimisation of prestressing and dimensions of the structure, detailed seismic analysis, structural analysis, calculation of deflections, etc. The program allowed us to change the structure often during the design process, promptly and easily. Nowadays we are using Scia Engineer and with this program the design process of the presented structure would certainly be still more effective.