Building Structures- Case Studies & Concepts
343419 - Assessment of Vortex-Induced Vibrations of Flexible Buildings Using Wind Tunnel Experiments on Section Models
Friday, April 20
11:00 AM - 12:30 PM
Location: Ballroom B
Flexible buildings with low frequency and light damping properties are susceptible to moderate amplitude vortex-induced (locked-in) vibrations at low wind speeds. The lock-in response, while less destructive than flutter, can lead to user discomfort, material fatigue, and non-structural failure. When designing this type of structures, engineers are often faced with two important questions:
• What wind speeds will cause vortex-induced vibration on the structure?
• What vortex-induced response amplitude will occur on the structure?
The experimental results of a rectangular section model (with aspect ratio of 4) and a circular cylinder model are being used to develop an analytical procedure to answer the above questions in this work. Experiments were performed on two degrees of freedom spring-mounted rigid section models in smooth and nominally two-dimensional flow conditions in an open-return, low-speed, low-turbulence wind tunnel at the University of Illinois, Urbana-Champaign. Reynold’s number ranged between 0 and 2500. Two lock-in regions for both vertical motion and torsional motion were observed on the rectangular section. Normalized vertical amplitude (Amplitude/D) in first lock-in region was 0.006 and second lock-in region 0.045. D is a characteristic dimension perpendicular to flow. Rotational amplitude for first lock-in region was 0.002 radians and second lock-in region 0.019 radians. Single lock-in region was observed on the circular section and maximum normalized vertical amplitude of 0.2 was measured.
It will be shown that by matching the reduced wind speeds (U/(fnD)) of a model section with that of the full-scale structure at different modes of vibrations, lock-in wind speeds for full-scale building can be reliably determined from similar wind tunnel tests. U is wind speeds and fn natural frequency of section model. Although numerous analytical models and methods have been developed to date, reliably estimating amplitude of vortex-induced vibration has proven to be difficult. A simple method will be presented that utilizes combined mass and damping ratio (also known as Scruton number) properties to approximately estimate the amplitude of vortex-induced vibration of full-scale buildings.
This article presents an efficient and simple analytical procedure that can be used to assess if a building is susceptible to vortex-induced vibration and to determine if more rigorous procedures are warranted. This topic is of interest to engineers and researchers in US and aboard who are working in the field of building aerodynamics and aeroelasticity.