Wind engineering

W i n d e n g i n e e r i n g - a n a d v a n c i n g s c i e n c e N Cook* and P Blackmore**

Wind engineering is a relatively young field, but one with a long ancestry, having been formed from diverging lines of mathematics, statistics, meteorology, physics and structural engineering. The field owes much to the explosive expansion of fluid dynamics at the beginning of this century, in the rush to perfect and exploit the aeroplane. The foundations of wind engineering can be traced back to the late seventeenth century, although the exploitation of wind effects on buildings can be traced much further back to Egypt in 2000BC where the design of the town of Kahan showed a working knowledge of 'architectural aerodynamics'.

In the early years of wind engineering, improvements in structural design and the understanding of wind effects on structures were reactive in response to wind failures. Amongst the most significant of these failures were: the Brighton Chain Pier (1836) - collapse by oscillatory motion; Tay Bridge (1879) - collapsed in a severe storm; Tacoma Narrows Bridge (1940) - collapse by oscillatory motion; and the Ferrybridge Cooling Towers (1965) - collapse of three cooling towers in the downwind row caused by wake buffeting from the upwind towers. By the early 1960s theory and experimental practice were sufficiently developed to allow a new appeciation of the problem, in particular the significance of turbulent statistical aerodynamics. The first international conference on wind effects on build- ings and structures was held in 1963, where the foundations of modern industrial aerodynamics were laid.

Wind engineering has advanced considerably in the last few decades, but failures still occur. Total collapses are rare, but each year an average of over 250,000 buildings in the UK are damaged by the wind at an estimated cost in excess of £50million. This does not include the cost of loss of service and other consequential losses which can often exceed the original repair costs. However, during single severe storms, such as those of October 1987 and January 1990, the number of buildings damaged by wind action can be greater than one million. In some of the counties worst hit by the 1990 storm over 71% of council-owned houses were damaged.

The key to prevention of damage is design by modern wind engineering principles. The problem is twofold; predicting the wind climate, and understanding the aerodynamic behaviour of structures. The Building Research Establishment in the UK has particular expertise in the statistical methods used for predicting extreme design wind speeds from meteorological information, and the BRE methodology is now generally adopted as the norm in wind engineering. However, the

*Geotechnics and Structures Group, BRE. **Wind Loading Section, Building Research Establishment, Garston, Watford, WD2 7JR, UK. Tel: (0923)894040 Fax: (0923) 664099

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/= gl Wind loads on the proposed new roof of a refurbished stand at Ibrox Park - home of Glasgow Rangers FC - were examined in the BRE boundary layer wind tunnel to aid the structural design of the roof

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The new boundary layer wind tunnel at BRE, wh ich is the m o s t a d v a n c e d building aerodynamics wind tunnel in the world

,~ Crown Copyright 1991 - Building Research Establishment

114 CONSTRUCTION & BUILDING MATERIALS Vol. 5 No. 3 SEPTEMBER 1991

Wind engineering of wind on buildings in environments ranging from open country to densely-built city centres, at scale factors between 1:1000 and 1:100. This tunnel, which complements the two other boundary layer tunnels, was designed to incorporate a number of novel and innovative features:

The control computer which runs and stores the data in the testing programs in the BRE environmental wind tunnel

principal strength of BRE is in modelling the behaviour of the wind and its interaction with structures. The principal tools used for this work are its boundary layer wind tunnels (Figures 1 and 2) designed to reproduce, at model scale, the earths' naturally turbulent winds, and BRERWULF used to simulate local dynamic loads on cladding systems.

Boundary layer wind tunnels evolved from aeronautical wind tunnels as the importance of accurately simulating the turbulent atmospheric boundary layer became clear. These tunnels generate velocity and turbulence characteristics that accurately represent those of the natural wind by using devices such as grids, walls and roughness elements on the floor of the tunnel. The boundary layer wind tunnel is now an established tool in the field of wind engineering and is used for a wide range of measurements including:

• fluctuating surface pressures for the design of cladding and structural elements,

• overall forces and moments for the design and stability of the overall structure,

• loading and response spectra for predicting dynamic response and occupant comfort,

• wind speeds and turbulence around buildings for the comfort and safety of pedestrians.

The newly-commissioned BRE boundary layer wind tunnel (Figure 2), the most advanced of its kind in the world, has a long length of roughness elements to grow the boundary layer naturally. It can simulate the effects

• Aerofoil-slatted walls and roof in the working section keep the flow around the model more accurate and allow larger (higher blockage) models to be tested. This is the first tunnel in the world to utilise this device.

• A novel method of changing short lengths of the flow roughness elements permits infinite adjustment in height between zero and 200 mm, so allowing simulations to be modelled precisely and rapidly changed.

• A 14 m long simulation development section to give better simulations.

• A sophisticated computer controlled system (Figure 3) allows remote control of wind speed, turntable angle and anemometer probe position.

• The computer-controlled system allows control of data acquisition and analysis, allowing overnight operation and greatly increasing the throughput and range of tests.

But why, it may be asked, is it necessary to go to the trouble of wind tunnel tests? The simple answer is that it is not necessary for any but the most complex of structures that are outside the scope of wind loading codes and standards. In these cases the designer often has no other recourse short of full-scale testing. However, even more conventional shaped structures can still benefit from wind tunnel testing because the design loads obtained from a wind tunnel test are tailored to the structure in its environment, so are far more precise than the crude and conservative estimates obtained from a code.

Another advantage of wind tunnel testing is that it can identify high-load areas on the structure which, if the testing is carried out at an early stage in the design process, allows these features to be optimised or eliminated from the final design.

Simply knowing the magnitude and distribution of wind pressures over a surface is sometimes not enough for the design of some multi-layer cladding or roofing systems. In these cases it is often desirable to have an understanding of the load paths through the structure, both pneumatic and mechanical, and how the load is shared by each element of the system. That is why BRE developed BRERWULF an acronym for BRE Realtime Wind Uniform Load Follower, an apparatus which can reproduce fluctuating wind loads as instantaneous uniformly-distributed pressures over plane sections of buildings.

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