Wind Loads on Tall Building Structures

Video of an aeroelastic model

Introduction

Recent trends towards tall, slender, flexible and light-weight buildings have resulted in a large number of buildings being susceptible to wind induced motion and human perception of building motion has become a critical consideration in modern building design.

The Australian and New Zealand Wind Loading Standard, AS/NZS 1170.2:2011 stipulates that structures having a height to breadth ratio of 5 or more should be designed using dynamic analysis. The dynamic wind loads can be accurately estimated by means of a wind tunnel study. This can result in a rationalisation of the structure and has the potential to effect considerable savings in the cost of the structure.

The Australian and New Zealand Wind Loading Standard, AS/NZS 1170.2:2011 also stipulates that the wind loads of structures taller than 200m or with a natural frequency less than 0.2Hz must be analysed using the wind tunnel modelling technique.

Methodology

Either an aeroelastic test rig or a high-frequency base balance rig is used in order to measure wind-induced base moments for the design of the structural frame. This study can also determine translational vibrations and assess them against various occupant comfort criteria. The interference effects of nearby buildings can also be identified and remedies proposed to alleviate problems.

The high- frequency base balance technique is applicable to the large majority of tall buildings and has the advantage of allowing the structural engineer to easily update the results in the case of a change in the building’s stiffness, mass or natural

frequency. The technique allows one to update the results fairly quickly, without compromising the accuracy of the results. With this setup, Windtech Consultants uses the modal analysis technique. This technique is particularly useful when analysing the wind tunnel results for buildings that have complex 3D coupled mode-shapes. Windtech has also developed a technique to account for the effect of close natural frequencies between the modes of vibration.

Windtech have also devised a technique for cases where there is a rigid link between separate tower buildings to accurately determine the effect of the rigid linkage on the load transfer between the two towers and to accurately determine the appropriate load cases.

The assessment of occupant comfort based on the measured levels of tip accelerations (x, y, ? and combined) and comparisons with respect to different occupant comfort criteria. Accelerations are normally presented for three different levels of damping.

Windtech’s wind tunnel data for structural loads on tall buildings has been verified against results from two other leading wind tunnel laboratories.