Theme paper: Low-rise structures--Wind loading and flow field

Theme paper: Low-rise structures--Wind loading and flow field

Journal of Wind Engineering and Industrial Aerodynamics, 29 (1988) 197-201 197 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Nether...

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Journal of Wind Engineering and Industrial Aerodynamics, 29 (1988) 197-201

197

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

THEME PAPER: LOW-RISE STRUCTURES--WIND LOADINGAND FLOW FIELD

KISHOR C. MEHTA

Department of Civil Engineering, Texas Tech University, Lubbock, Texas 79409 (USA)

ABSTRACT This session containing sixteen papers deals with research concerning wind loads on and v e n t i l a t i o n in low-rise buildings. The two areas relate to d i f f e r ent types of wind; wind loads are of concern during strong winds while the v e n t i l a t i o n of buildings is of interest during periods of no winds or low prevailing winds. A majority of the papers deal with experiments in wind tunnels though a few papers describe f i e l d observations and computational methods. The theme paper surveys the papers in the session and challenges the researchers to be cognizant of the r e a l i t i e s of wind in nature. INTRODUCTION A vast majority of constructed buildings and structures.

f a c i l i t i e s are low-rise (less than 20m)

Even though i n d i v i d u a l l y these buildings and struc-

tures do not j u s t i f y intense engineering attention, as an aggregate they represent a vast amount of professional work.

I t is appropriate to devote s u f f i c i e n t

time at the conference to review, update, and exchange information gathered by researchers on low-rise buildings and structures. A variety of papers are assembled in this session.

The subject matter

ranges from scale distortion in a wind tunnel to ventilation flow rate in near zero wind.

Even with this wide range of subject matter a common theme of the

papers is that the results presented are applicable to wind related problems of classes of buildings and structures, rather than an individual building or a structure. Wind related problems on low-rise buildings and structures can be divided into two categories.

One category is resistance to damage in strong winds and

primarily deals with wind loads.

The other category

is

serviceability of

buildings during prevailing wind and is concerned with the operation of v e n t i l a tion and exhaust systems in buildings. with in the papers.

Both categories of problems are dealt

A majority of the studies are through wind tunnel experi-

ments, though wind load related problems are studied to a limited extent through f i e l d observations and computational techniques. papers

of this session

in relation

to categories

The theme paper surveys the of problems and challenges

198

f u t u r e researchers to be cognizant o f the r e a l i t i e s

of the nature of wind in the

field.

SESSION PAPERS There are a total of sixteen papers in the session; of these, twelve are related to wind loads and four deal with ventilation and exhaust systems in buildings.

As can be expected, a majority of the papers present results of

studies conducted in wind tunnels.

Two papers discuss results of f u l l - s c a l e

work (one of them presents measurements conducted in strong winds while the second one interprets damage documentation e f f o r t s ) .

Another paper compares

results of the f u l l - s c a l e measurements and wind tunnel experiment of the same building. Three papers relate to assessing wind loads through computational techniques. Some of the highlights of conclusions from these papers are as follows: 1.

Wind tunnel technology has made s i g n i f i c a n t strides in measuring loads on various building shapes, though experiments of low-rise buildings in a wind tunnel can run into d i f f i c u l t y because of possible distortion of scale.

2,

Turbulence is a very important parameter in wind tunnel experiments.

3.

Measurements of building response in the f i e l d and interpretation of building damages in a windstorm are d i f f i c u l t to accomplish; however highly valuable information is obtained from these exercises to improve the prediction of wind loads and associated building responses.

4.

Wind tunnel results approach the ones obtained from measurements in the f i e l d i f wind characteristics are correctly simulated in the wind tunnel experiments.

5.

Serviceability of ventilation and exhaust systems in buildings during prevailing winds is an important problem of wind engineering that needs additional basic studies.

FUTURE CHALLENGES As the content of the papers in this session and at this conference i n d i cates, wind tunnel technology is a very important part of wind engineering. Correct simulation of wind that flows over and around buildings is c r i t i c a l i f reliable results from wind tunnels are desired.

The boundary layer wind tunnels

simulate closely wind speed and turbulence characteristics of stationary winds. The winds in extratropical cyclones and frontal passages are r e l a t i v e l y steady and close to being stationary. this type of wind.

Thus, wind tunnel results are f a i r l y reliable in

199 Unfortunately, most of the damages to buildings and structures occur in windstorms such as hurricanes (typhoons or cyclones), thunderstorms, or tornadoes.

The winds in these storms are often d i f f e r e n t from the stationary winds

in extratropical cyclones. Figure I.

A wind speed recording in a thunderstorm is shown in

As i l l u s t r a t e d in the figure,

the mean component of wind speed

increases and decreases f a i r l y rapidly with the movement of the thunderstorm. In addition, the wind speed fluctuates s i g n i f i c a n t l y about the mean component. Turbulence levels in hurricane winds are known to be f a i r l y high.

Turbulence

characteristics in thunderstorms and hurricanes need to be understood to simulate these characteristics in wind tunnels. An added characteristic of wind that is not i l l u s t r a t e d in Figure 1 is the fluctuations

in wind direction.

Wind recordings

have shown that

standard

deviation of wind direction fluctuations in a 12-minute duration can be as high as 8 degrees when wind was blowing essentially from the same direction.

This

observation suggests that wind direction fluctuates s i g n i f i c a n t l y in nature. These wind direction fluctuations can have s i g n i f i c a n t influence on peak pressures on a building surface. To assess pressures on a building surface, a f i e l d experiment site was established on the campus of Texas Tech University. sions 9.1 m x 13.7 m x 4 m.

The building is of dimen-

Along with the building, a 49 m high meteorological

tower was constructed; these f a c i l i t i e s are shown in Figure 2. instrumented with wind speed and wind direction instruments Pressure taps with d i f f e r e n t i a l building surfaces. strong winds.

pressure transducers

are

The tower is

at four levels. installed on the

A microcomputer records the wind and pressure data during

These data w i l l

provide wind characteristics and associated

pressure data of the real world. Prediction of the performance of the ventilation and exhaust system in a building depends on wind speed, wind direction, atmospheric s t a b i l i t y , and wind flow around the building.

The wind direction fluctuations and atmospheric

s t a b i l i t y are important parameters that are not well established.

The f a c i l i t y

shown in Figure 2 has the potential of obtaining f i e l d data for the ventilation and exhaust system of the building. The challenges in the future are to duplicate f i e l d data in wind tunnels for low-rise buildings.

In addition, upgrading of wind tunnel technology to

include fluctuations in wind direction and to simulate thunderstorm type turbulent wind w i l l

require new approaches. Improvement in wind characteristics

simulation w i l l provide better prediction of wind loads in windstorms, assist in mitigation of damage, and permit prediction of v e n t i l a t i o n and exhaust system performance in buildings.

:ig.

1.

Wind speed r e c o r d i n g

E

in a thunderstorm.

f

JUNE 16, 1986 LUBBOCK N W S (I KNOT = I. 15 MILES)

C

201

Fig. 2.

Field experiment site on the campus of Texas Tech University.

CONCLUSIONS Significant advances have been made in the prediction of wind loads through the use of atmospheric boundary layer wind tunnels.

Reliable field data for

wind loads on low-rise buildings and for ventilation systems are desirable to improve wind tunnel technology.

Incorporation of wind direction fluctuations

and simulation of thunderstorm type turbulent winds in wind tunnel experiments w i l l provide better wind load predictions.