Tall Building Structures With Multi-Outrigger Systems

The control of top drift and base moment in the core of a tall building structure under lateral loads has become two main concerns in structural design of tall buildings. The outrigger-braced system is regarded as one of the most effective ways for increasing structural stiffness and has been widely used in tall building structures. (Iyengar, 1972). Considered the design of one-outrigger structures and suggested that the optimum location of an outrigger should be close to the mid-height of the building (0455 of the total height from the top). (Colaco, 1978. Iyengar, 1978. Goldberg, 1975) found that the optimum locations for two outriggers are 0-312 and 0-685 of the total height from the top. analysed the structural behaviour of outrigger structures under uniformly distributed horizontal loads. In their study, the optimum locations of outriggers, top drift and base moment reduction efficiencies were expressed by a non-dimensional characteristic parameter which is a function of the flexural rigidity ratios of core-to-column system and core-to-outrigger system. (Saul, et. al., 1976). Took a one- outrigger structure as an example to investigate the effects of several structural parameters (the stiffness ratios of core-columns and core-outrigger, the variation of structural properties along the building height) on the optimum location of an outrigger and the reduction of top drift. (Tall Buildings and Sustainability, 2002). Examined three different computational models for the analysis of multi-outrigger structures and showed how to minimize the extreme non-uniform distributions of structural inner forces by selecting several major structural parameters. the optimum locations for a two-outrigger system. (Stafford and Coull, 1991). Investigated the effect of lintel beams in the outer tube on the structural behaviour of outrigger structures. (Taranath, 1988). Presented some problems to which attention should be paid when using outrigger-braced structures, which include irregularities of structural inflexibility and internal strengths, feeble stories impelled by outrigger bracing, structural plans and choice of stiffness for outriggers and seismic design for such structures. (Bao, 1992). Proposed the design criteria for reinforced concrete tall building structures with outriggers. Zhu proposed a methodology for verifying the key vibration period of outrigger-braced structures.

The building can have one or a few belt truss; the more trusses utilized, the better the coordination of bracing won't meddle with the building's capacity. The structural guideline of utilizing belt trusses at the top and mid-height of a building appears to be temperate in requisitions up to more or less 60 stories. The anxiety outline in Figure shows the relative productivity of pivoting the belt trusses to the border columns instead of settling them unbendingly. In the event that the trusses were to be consistently joined with the columns, the whole framework might go about as an unit, hence using just a minor rate existing apart from everything else opposing limit of the core, whose walls are moderately near the neutral axis of the building.

This is indicated by the continuous distribution of stresses shown for the rigid frame in Figure. On the other hand, belted musses that are cantilevered from the core and hinged to the perimeter columns better develop the moment resisting capacity of the core while still engaging the exterior columns as in the rigid system. In fact, since the hinged shear connections induce no bending moments into the columns, the axial capacity of the columns is increased relative to that for the case of fixed shear connections. The response of a core frame building with belt trusses to lateral loading is shown in Figure. This Figure schematically shows the reduction of moment in the shear-core for a one-outrigger system and a two-outrigger system compared to that for a no-outrigger system. The point when the frame is pivoted to the core of the structure, the core carries on as a cantilever and its top is allowed to turn. The frame itself barely opposes any revolution. Provided that the frame is fixed to the core by a belt truss, however, any pivot at the top of the framework is limited, since the edge columns secure the belt truss. There is then no bowing minute in the columns. The fractional fixity gave at the top of the framework by the belt truss is reflected in the minute outline in Figure. The framework no more

This study has presented a general review of structural systems for tall buildings. The design issues for preliminary design and optimization have been briefly summarized, and a rational methodology of design was shown. This enables optimization of initial structural systems for drift and stresses, based on gravity and lateral loads. Some insight into the design of many types of tall building structural systems and their subsystems was provided based on past experience in tall building design. The design issues are efficiency of systems, stiffness, member depths, balance between sizes of beam and column, bracings, as well as spacing of columns, and girders, and areas and inertias of members. Drift and accelerations should be kept within limits. Good preliminary design and optimization leads to better fabrication and erection costs, and better construction. The cost of systems depends on their structure weight. This depends on efficient initial design. Efficient structural design also leads to a better foundation design, even in difficult soil conditions. The structural steel weight is shown to be an important parameter for the architects, construction engineers and for fabrication and assembly.

REFERENCES:

Bao, S.H. (1992). “Flexure-torsion coupled vibrations for tall building frame-shear wall-thin walled member cooperating systems,” China Civil Engineering Journal, 25(1), pp. 12-20. Colaco, J.P. (1978). “Elastic Analysis”, Tall Building Monographs, Tall Concrete Buildings, Vol. CB., Chapter 9. Goldberg, J,E. (1975). “Approximate Methods of Analysis of Tall Buildings”, Proceedings of US National Conference on Wind Engineering, Fort Collins, CO. Iyengar, H.S. (1972). “Preliminary Design and Optimization of Tall Buildings”, Proceedings,

Iyengar, H.S., (1978). “Elastic Analysis and Design”, Tall Building Monographs, Tall Steel Buildings, Vol. SB., Rob J Smith, M. R. W. (2007). "The damped outrigger concept for tall buildings." The structural design of tall and special buildings 16: pp. 501-517. Saul, W.E., Jayachandran, P., and Peyrot, A.H. (1976). “Response of N-Degree Tall Buildings to Stochastic Wind”, Journal of the Structural Division, Proc. ASCE, May. Stafford Smith B and Coull A. (1991). Tall Building Structures: Analysis and Design. Wiley: New York. Stafford Smith, B., and Coull A. (1991). Tall Building Structures: Analysis and Design, John Wiley and Sons. Tall Buildings and Sustainability (March 2002). Economic Development Office, Corporation of London. Taranath BS. (1998). Steel, Concrete, and Composite Design of Tall Buildings (2nd edn). McGraw-Hill: New York. Taranath, B. S. (1988). Structural Analysis and Design of Tall Buildings, McGraw-Hill Company.