ABSTRACT
Reinforced concrete thin-walled cores represent typical shear-resistant structural arrangements for high-rise buildings. The cores can be used as single elements or other shear resistant structures can be disposed in order to collaborate with them in counteracting external lateral loads. Regarding the structural behaviour the cores can be considered as thin walled elements with open section exhibiting pure torsional rigidity coupled with warping rigidity. When only one core embodies the shear resistant structural complex as it happens for buildings with suspended floors or for buildings with a central core and perimetral columns, the need of increasing the overall structural rigidity can become a basic prerequisite of design specifications. Among the operational techniques feasible for increasing the lateral rigidity of particular interest are the ones not requiring to increase the dimensions of the core, allowing to maintain unchanged the usable internal surface of the building. At this subject the insertion of outrigger systems, exhibiting adequate flexural rigidity allows to markedly increase the flexural and the torsional rigidity of the cores, so that this procedure can be regarded as a profitable solution. The stiffening effect of the outriggers is quite small but not negligible for buildings with suspended floors while it becomes significant for buildings with perimetral columns which are forced to collaborate with the cores by the outrigger system. The contribution given by the outriggers in restraining lateral displacements is different depending on the flexural or torsional behaviour of the cores is dealt with. In the first case the interaction forces between the cores and the perimetral columns can be reduced to an axial load and two bending moments, while in the second the interaction forces also produce a bimoment. The flexural interaction between cores and outriggers has been widely investigated for buildings with perimetral columns, /!/, Ill, /3/, and has been recently extended by the authors to the case of suspended buildings, /4/. Regarding the torsional interaction a detailed analysis of the stiffening effects produced by the outriggers has not been exhaustively developed. In particular investigations devoted to define the basic parameters governing the coupled structural
Tall buildings stiffened by outriggered thin walled cores 229
that its value for a prescribed load P, allows to evaluate the statical efficiency of the outrigger. When m outriggers are acting at the abscissa z, we can immediately generalize Eqs. (1) applying the principle of superposition. According to this assumptions we write
Eqs. (4) represent the most general expressions of the actions applied to a thin walled core by a distribution of outriggers acting at the abscissa z~ and loaded by longitudinal forces P;. The related state of stress and deformation can be determined by solving the differential equations governing the statical behaviour of the core. In particular we can refer to unit values of the actions expressed by Eqs. (1). In this way we obtain the Green functions of the problem, which, according to the principle of superposition, allow to easily calculate the state of stress and deformation of the core subjected to a general distribution of interaction forces.
