City Research Online

Abstract

In recent years, global urbanisation trends created pressing demands for residential and office space in all major cities of developed and developing countries. These demands are increasingly addressed by height-wise urban development. This is underpinned by the advent of new high strength materials and stiff lightweight structural components which enable designing and construction of tall buildings with large height-to-width aspect ratios. Indeed, slender tall buildings with rectangular floor plans achieve economical land utilisation in congested urban environments and facilitate efficient inner space organisation. However, such structures are prone to excessive wind-borne oscillations in the crosswind direction due to vortex shedding (VS) effects generated around their corner edges. These oscillations may generate floor accelerations beyond code-specific occupant comfort thresholds under moderate frequently occurring wind actions leading to serviceability failure. In this regard, the design of most slender tall buildings with rectangular plan view is commonly governed by serviceability occupant comfort criteria. At the same time, the latest sustainability requirements for new-built structures urge for minimising material usage as the portion of global carbon emissions due to building material manufacturing is rising. In this regard, this thesis addresses occupant comfort and material usage requirements in wind-excited tall buildings by equipping structures with innovative passive inerter-based dynamic vibration absorber (DVA) motion control configurations in conjunction with novel DVA-equipped tall building design approaches for weight minimisation.

To this aim, the thesis contributes an optimal tuned mass damper inerter (TMDI) design approach in which TMDI stiffness and damping properties are numerically determined via a computationally efficient scheme to minimise floor accelerations in wind-excited buildings for given inertial TMDI properties (i.e., inertance and secondary mass) and inerter element connectivity. Optimally designed TMDIs for a wide range of inertial properties and various inerter connectivities are obtained for a benchmark slender 74-storey building subjected to experimentally calibrated spatially-correlated crosswind force field accounting for VS effects. Design charts on the TMDI inertial (mass-inertance) plane are furnished demonstrating that fixed structural performance level in terms of occupant comfort can be more efficiently achieved through lightweight TMDIs if compared with classical tuned mass dampers (TMDs) as long as sufficient inertance is provided. Further, TMDI sensitivity to host structure properties and to reference wind velocity is shown to decrease by increasing inertance or by spanning more floors in connecting the secondary mass with the host structure by the inerter.

Moreover, attention is focused on examining the efficacy of the TMDI motion control potential for different dominant mode shape of buildings. This is facilitated by putting forward a novel analytical two-degree-of-freedom (2DOF) dynamical model representing TMDI-equipped slender buildings treated as continuous tapered cantilever beams with varying geometrical properties and, thus, mode shapes. It is found that reduced free-end displacement and TMDI stroke are achieved for structures in which the ratio of flexural rigidity over mass decreases faster with height, resulting in vibration modal shapes with higher convexity. The latter is quantified through the average modal curvature shown to be well-correlated with TMDI motion control improvement. It is concluded that appropriate building shaping extends the applicability of the TMDI to structures in design situations where connecting the inerter away from the free-end is practically and economically challenging.

Inspired by the above findings, a local structural modification, top-storey softening, is proposed in conjunction with optimally tuned top-floor TMDI for improved occupant comfort performance in typical core-frame slender buildings. Comprehensive numerical data pertaining to a parametric investigation for a 34-storey steel-concrete composite core-frame structure demonstrate that the proposed top-storey softening reduces attached TMDI mass/weight requirements and inerter force for fixed floor acceleration performance and inertance. It further reduces TMDI stroke and achieves increased robustness to TMDI stiffness and damping properties as well as to the assumed inherent structural damping. It is concluded that by leveraging inertance and top-storey lateral flexibility, the proposed solution can efficiently control VS-induced floor acceleration with small additional gravitational (added weight) and horizontal (inerter and damping) forces.

Lastly, an innovative framework for the optimal design of wind-excited DVA-equipped tall buildings subject to serviceability comfort criteria is proposed, which enables minimising material usage for occupant comfort-governed building structures by exploiting the motion control capability of inerter-based DVAs. The framework relies on a novel optimal structural member sizing Lagrangian formulation for minimum-weight structural design, in conjunction with optimal DVA tuning for occupant comfort under crosswind excitation. The applicability and usefulness of the framework is exemplified by application to a routine occupant-comfort-sensitive 15-storey steel moment resisting frame (MRF) building equipped with a ground floor tuned inerter damper (TID). The inclusion of the TID together with the herein proposed design framework achieve up to 67% steel tonnage savings in meeting the ISO 6897 occupant comfort criteria. Pareto optimal solutions further demonstrate that the self-weight of lateral wind-load resisting structural systems can be traded to TID inertance, potentially leading to significant material usage reductions.

Overall, numerical data furnished in this thesis demonstrate that the herein contributed optimal design formulations and algorithms as well as the innovative inerter-based DVAs are quite promising in achieving new types of sustainable and resilient slender tall buildings to wind excitation which can address current and future demands for residential and office space in modern city centres.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TA Engineering (General). Civil engineering (General)
Departments:	Doctoral Theses School of Science & Technology > School of Science & Technology Doctoral Theses School of Science & Technology > Engineering

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