Application of Higher Strength Concretes in Tubular Structures

Abstract

Most buildings constructed in the Netherlands are constructed with steel and/or reinforced concrete as the main load-bearing material. The use of concrete in high-rise buildings has the advantage of being able to build rather stiff and fire proofing structures thanks to the large applied amount of concrete. Structural stiffness is important in high-rise structures since the maximum lateral displacement at the top has to be limited to meet the required comfort level in a building. An often-used type of structure is the framed tube structure. Several studies have pointed out that the tubular structure is able to achieve great heights. It highly improves the building’s stiffness, resulting in less lateral displacements. In the last decades, higher buildings are being built in seeking the ultimate height limit of a structure. While buildings were being built higher and higher, the quest for higher quality materials continued as well. Until now, many new types of concrete have been developed, which have better properties than its predecessors. This thesis applies two new types of concrete in a framed tube structure, namely High Strength Concrete (HSC) and Ultra High Strength Concrete (UHSC). It appears that higher strength concretes can be applied in the structure. Its behaviour changes since the higher strength concretes have a different modulus of elasticity. The structures are proven to give better performance: the lateral displacements reduce and higher structures can be built while still fulfilling the requirement to maximum deflection. Obviously, the higher strength concretes come with a higher price. Consequently, the structure becomes more expensive. However, thanks to the better material properties, the building can be built higher. To acquire knowledge in whether the higher building is feasible, the costs per floor are calculated. A higher building contains more commercially available area and the analysis shows that the costs per unit floor area decrease by 14% if HSC is applied. Despite the fact that the building with an UHSC structure contains more commercially available area, it is not beneficial due to the high price of the UHSC mixture: the costs increase up to 25%. The Very High Strength Concrete (VHSC) mixture is not studied in this thesis, but is expected to provide costs and structural performance that lay in between the HSC and UHSC performance. Several changes to the structure are recommended to optimise the structure’s costs-performance ratio. The most important recommendation is to utilise the material properties of the higher strength concretes as much as possible. This is achieved by applying a hybrid structure: combining higher strength concrete with ordinary concrete in one structure. HSC is applied in only the lower eight floors, while in the remaining 22 floors of the structure OC is applied. The thesis shows that the application of higher strength concretes in framed-tubular structures is possible. It provides better performing structures and, in some cases, a reduction in costs. While the HSC models proved to provide a good performance to costs ratio, the UHSC models currently do not.