Design of an outrigger structure for tall timber buildings

Abstract

New innovative timber materials, such as Cross Laminated Timber, brought up new challenges for architects and engineers for timber construction in recent years. All over Europe many multi-storey housing projects were developed in CLT, which set new boundaries on structural limitations. Due to diaphragm action in CLT panels larger stiffness in structures can be attained resulting in better opportunities for more challenging structures and taller buildings. Today the tallest timber residential building in the world is located in London and has ten floors, all floors scattered with CLT shear walls to resist lateral wind loading. Taller buildings usually have multiple functions such as a commercialand residential function, which require flexibility in plan design. Multi-storey buildings erected in concrete and/or steel are frequently designed with a central core and attached frame structure to implement flexibility in design of rooms and open spaces. CLT has similar structural properties as concrete shear walls; hence a natural replacement for sustainable construction. However, a frame structure reduces the lateral stiffness in comparison to a structure with shear walls all over the building's footprint. Considering building designs in concrete and steel, the application of storey height horizontal trusses, so-called outriggers, connected to a central core improves the stiffness of a structure significantly. However, tall timber buildings have certain properties which are disadvantageous for horizontal sway and dynamic behaviour of the structure, which are the low mass density and relatively low stiffness in comparison to concrete and steel buildings. Serviceability is commonly a decisive aspect in structural timber design, thus in particular for tall timber buildings. Within this research the objective is to investigate the structural behaviour of a timber outrigger structure for tall buildings through a fictive twenty-storey building design, where serviceability is accentuated. Eurocodes are adopted for design of structural elements as well as connections in the Ultimate Limit State. Design in the Serviceability Limit State has been underpinned by Dutch building regulations set in NEN 6702. A design process is followed starting with architectural sketches of a fictive twenty-storey building and preliminary structural analysis of multiple stability systems. The outrigger structure has been analysed on dynamic properties, such as the fundamental frequency, and dynamic response due to fluctuating wind loading obtained from a wind spectrum in EC1. Structural elements have been verified on strength in accordance with EC5, where structural forces are obtained from Finite Element analysis in the computer program STAAD Pro. Structural connections are designed according to EC5 likewise, for which aspects like prefabrication, assembly processes, fire safety and noise control are incorporated. Slip of fasteners in connections are analysed by various STAAD Pro models, resulting in a sense of the effects on the structural behaviour regarding to horizontal sway and horizontal accelerations. In an early stage of the design process it became apparent that serviceability governs the structural design in buildings elements. The proposed outrigger structure with a CLT core and glulam frame structure including outriggers is capable to resist both lateral wind actions and vertical loading. Although the fundamental frequency was computed on rather low values, i.e. 0.58 Hz., the dynamic response to the time varying wind loading was well below permissible values in accordance with NEN 6702. Taking into account slip behaviour in connections both static deformations and horizontal accelerations increased to critical values. It appeared that connections should be considered in an early design phase, since they are as relevant for the stability of the structure as the system itself. Anyhow, with incorporation of thorough design of connections between timber members it can be concluded that timber has high potential for tall buildings up to twenty storeys and perhaps even more.

Date of Award	31 Aug 2012
Original language	English
Supervisor	A.J.M. Jorissen (Supervisor 1) & S.P.G. (Faas) Moonen (Supervisor 2)