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Meaninterferenceeffectsamongtallbuildings

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ImpactFactor:1.84·DOI:10.1016/j.engstruct.2004.03.007

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Engineering Structures 26 (2004) 1173–1183

www.elsevier.com/locate/engstruct

Mean interference effects among tall buildings

Z.N. Xie a,b, M. Gu a,�

a State Key Laboratory for Disaster Reduction in Civil Engineering, Tongji University, Siping Road 1239, Shanghai 200092,

People’s Republic of Chinab Department of Civil Engineering, Shantou University, Shantou 515063, People’s Republic of China

Received 28 June 2003; received in revised form 1 March 2004; accepted 8 March 2004

Abstract

The mean interference effects between two and among three tall buildings are studied by a series of wind tunnel tests. Both theshielding and channeling effects are discussed to understand the complexity of the multiple-building effects. The results show thatthe upstream interfering buildings cause certain shielding effect by decreasing the mean wind load on the downstream principalbuilding. For buildings of the same height, the shielding effect increases and, therefore, the interference factor (IF) decreases, withthe increase of the breadth of the interfering buildings. However, due to the channeling effects, two adjacent interfering buildingscan significantly enhance the mean wind load on the principal building. In addition, the variation of the shielding effect is foundto be significant when the heights of interfering buildings range from 50% to 125% of the height of the principal building. How-ever, higher interfering buildings may cause stronger channeling effects.# 2004 Elsevier Ltd. All rights reserved.

Keywords: Tall buildings; Mean wind loads; Wind tunnel test; Interference effects; Channeling effects

1. Introduction

Generally, the mean interference effects of tall build-

ings present ‘‘shielding effects’’ where the presence of

existing nearby buildings (hereafter referred to as inter-

fering buildings) tends to decrease the mean wind load

on the principal building. For a pair of buildings of

equal size in tandem arrangement, Sakamoto and

Haniu [1] found that the drag force of the downstream

building reduced to zero when the upstream building

was three times the building breadth away (center-to-

center spacing) from the downstream building and

the mean drag force could be negative when the

spacing was less than this critical distance. The shield-

ing effect decrease with the increase of the spacing

between the two buildings. However, Taniike [2] found

that the shielding effects could be still noticeable when

the upstream building was located at a place 16 times

of the building breadth away from the downstream

building. In his paper, he indicated a mean interference

factor of 0.8, or, a shielding of 20% of mean wind

loads on the principal building. However, due to the

channeling effect, the interference factors can be greater

than 1.0 in some special arrangements of the buildings.

In other words, the adjacent buildings can also amplify

the mean wind loads acting on the principal building.Some recent studies aimed at providing the general

recommendations on the wind-induced interference

effects. On the basis of the existing wind tunnel test

results, English and Fricke [3] employed a well-trained

neural network to predicate the interference effects

between pairs of buildings located in proximity in a

variety of geometric configurations and boundary-layerwind flows. Khanduri et al. [4] also tried to give the

general guidelines of wind-induced interference effects

between two buildings. Kwok [5] made a review on this

topic. In his paper, he summarized along-wind and

across-wind and torsional interference factors between

two buildings, and analyzed mechanism of the inter-ference. However, due to the huge amount of experi-

mental workload and the complexity of the interrelated

parameters, most previous investigations have mainly

focused on the interference effects between two build-

ings, that is, one interfering building and one principal

building. Only a few studies on the interference effects

1174 Z.N. Xie, M. Gu / Engineering Structures 26 (2004) 1173–1183

among three buildings have been reported whichshowed that the interference effects among three build-ings could be more significant than those between twobuildings [6].

This paper focuses mainly on the behaviors of themean interference effects among three buildings. Themean interference effects in the present study are repre-sented as the interference factor (IF) defined as:

IF ¼

Mean base moment of a buildingwith interfering buildings present

Mean base moment of an isolated buildingð1Þ

Furthermore, only the interference effects of themean along-wind base moment are considered in thisstudy since the mean across-wind base moment can beneglected for an isolated building with square section.In fact, there are two main kinds of effects involved inthe wind-induced interference effects on tall buildings,namely, the mean interference effects and the dynamicinterference effects. Previous studies have shown thatthe dynamic interference effects are more significantand more severe than the mean effects. This paperfocuses on the mean interference effects. The studies ofthe dynamic interference effects among three buildingswill be discussed in another paper.

2. Experiment setup and data processing

Wind tunnel tests were conducted in the STDX-1Boundary Wind Tunnel of the Department of CivilEngineering at Shantou University. The main test sec-tion of STDX-1 for the building model is 20 m long,3 m wide and 2 m high. The test section has an adjust-able roof that provides a negligible pressure gradient inthe downstream direction. The maximum wind speedof the wind tunnel can reach to 45 m/s. According tothe Chinese Load Code (GB50009-2001 [7]), theexposure categories B and D (corresponding to expo-nents of the power law of mean speed profile of 0.16and 0.30, respectively) are simulated at a length scaleof 1/400 by setting spires, barriers, and rough elementsin the test area. The simulated mean wind profiles(V=Vg) and turbulence intensity distributions e (%) forthe two exposure categories are shown in Fig. 1, whereVg is the mean wind speed at the gradient wind height.The gradient heights for the exposure categories B andD are 350 and 450 m, respectively; and accordinglythose of the simulated wind fields in the wind tunnelare 0.875 and 1.125 m, respectively. In order to make acomparison and investigate the mechanism of the inter-ference effects, some configurations were tested in uni-form flow in which the turbulence intensity e is lessthan 1%.

The measurements in this study are carried out bymeans of a Nitta’s universal force–moment sensormodel no. UFS-4515A100 and the attached signal con-

ditioner and amplifier. The technical specifications ofthe sensor used are shown in Table 1.

The lowest natural frequency of the model-balancesystems can reach up to 112 Hz, which is much higherthan the concerned frequency range of the aerody-namic forces acting on the building models. The con-ditioned and amplified analog signal is transmitted to aScanivalve’s Zoc/EIM-16 module and eventually digi-tized by Scanivalve’s sampling platform.

A 600 mm tall and 100 mm wide, square modelmade from foamed plastics as a core and light woodplate (1 mm thickness) as ‘‘clothes’’ is used as the prin-cipal building. The model has the same length scalewith that of wind simulation, i.e., 1/400, representing areal building of a height of 240 m. Two groups ofupstream building models are used as the interferingbuildings. The first group of interfering buildings hasthe same height h as the principal building, where h(=600 mm) is the height of the principal buildingmodel, and square cross-sections with differentbreadths of 0.5b, 0.75b, 1b, 1.5b, and 2.0b, where b

Fig. 1. Distributions of wind profile and longitudinal turbulence

intensity.

Table 1

Specifications of Nitta’s UFS-4515A100 sensor

Component F

ull scale range A ccuracy

Fx, Fy 4

40 N

Linearity: 0.2% F.S.

Fz 8

80 N

Hysteresis: 0.2% F.S.

Mx, My, Mz 5

1 Nm

Z.N. Xie, M. Gu / Engineering Structures 26 (2004) 1173–1183 1175

(=100 mm) is the breadth of the principal buildingmodel. The second group of interfering buildings hasthe same cross-section as the principal building but dif-ferent heights of 0.5h, 0.75h, 1h, 1.25h and 1.5h. Allbuilding models are orientated with one face normal tothe wind direction and the spacing between them variesas the test parameters in the along-wind direction (x)and the across-wind direction (y) in a grid systemshown in Figs. 2 and 3, respectively. More than 7400cases of building arrangements were tested in thepresent study.

In order to quickly process the huge amount of testdata, a system of Windows-based software platformthat integrates the radial basis function-based artificialneural network (ANN), statistical analysis and data-base management is developed. All the interference fac-tors from the tests can easily be stored in the databasewith the software. The software system can also be

used to analyze the interference characteristics andmodel the interference effects by the ANN-basedmethod. With the help of this software, the interferencefactors at various positions can be calculated andvisualized quickly and accurately by modeling the testdata.

3. Experimental results and discussions

3.1. Results for two identical square buildingsand comparison with previous studies

English [8] synthesized several existing wind tunneltest results that were obtained for different simulatedterrains and concluded a regression equation to predictthe mean along-wind interference factor of the down-stream building for twin buildings arranged in tandem.The formula is given in the polynomial form as:

IF ¼ �0:05 þ 0:65xþ 0:29x2 � 0:24x3 ð2Þ

where x ¼ log½Sðhþ bÞ=hb�, S is the clear spacingbetween the two buildings, b is the breadth of thebuildings, and h is the height of the buildings.

To check the reliability of the results of the presenttests, the interference factors of two tandem-arrangedbuildings under the uniform flow condition, exposurecategories B and D, are compared with Eq. (2), asshown in Fig. 4. The comparison shows a good agree-ment between the results measured in exposure cate-gory D and the regression results from Eq. (2).However, differences are found in exposure category B,and especially in uniform flow. It can be seen that thedeviation of the interference factors decrease with theincrease of building spacing in the different categoriesof terrains. The maximum difference is found in thespacing of 3b to 6b. The smoother the upstream ter-

Fig. 2. View of the principal building and the interfering buildings.

Fig. 3. x–y coordinate grid for locating the interfering buildings,

principal building is fixed at (0, 0).

Fig. 4. Interference factors for two tandem-arranged buildings, x

denotes the center-to-center spacing between the interfering building

and the principal building.

1176 Z.N. Xie, M. Gu / Engineering Structures 26 (2004) 1173–1183

rain, the more significant the shielding effects of theupstream building. In addition, interference factors ofzero are seen in the figure at about 2b, 3b and 4b forthe three types of terrains, respectively. The position ofthe zero value in exposure category B with a ¼ 0:16 isat a center-to-center spacing of 3b, that is almost thesame as that of the result observed by Sakamoto andHaniu [1] under the similar terrain condition of openterrain.

For the results of other building arrangements,Khanduri et al. [9] gave the IFs caused by an upstreambuilding at the region of [2b–8b, 0–4b] in open terrainby means of synthesizing the results given by Taniikeand Inaoka [10] and Saunders and Melbourne [6].Huang [11] also conducted some similar experiments inthe same terrain of exposure category B. Table 2 liststhe above-mentioned results, together with the corre-sponding results that are predicted by the well-trainedneural network with the test data from the presentstudy. The results show the good consistency and theefficiency of the ANN-based method presented in thispaper. Of course, differences still can be found in thetable due to the use of different terrain categories andbuilding aspect ratios in different studies.

Fig. 5 presents the IF contours for the two equal sizebuildings in uniform flow and exposure categories Band D. Clearly, the shielding effects vary with thedegree of roughness of the upstream terrain. One cansee that each contour has a negative region corre-sponding to the negative IF, which means that theprincipal building is subjected to a reverse wind dragforce. The negative IF region increases with thesmoothness of the upstream terrain.

3.2. Results for configurations of three equal sizebuildings

3.2.1. Tandem arrangementCompared with the mean interference effects of two

tandem-arranged buildings, the mean interferenceeffects of the three tandem-arranged buildings (anarrangement of three buildings placed on behind theother in the along-wind direction, i.e. yA ¼ 0 and yB ¼0 for the two interfering buildings, see Fig. 3) are moresignificant than those of other kinds of arrangements.Fig. 6 gives the distribution of the interference factorsfor three tandem-arranged buildings in exposure cate-

gory B. It can be seen that the minimum interferencefactor is close to zero at smaller building spacing,which is slightly different from that of two-buildingconfigurations where IF equals to �0.2. In general, theshielding effects are dominated by the nearer upstreaminterfering building, especially at smaller spacing. Simi-lar trends can be found for this configuration inexposure category D.

3.2.2. Side-by-side arrangementTable 3 gives the interference factors of the three

buildings in side-by-side configuration, where yA andyB denote respectively the across-wind center-to-centerspacing, i.e. the across-wind coordinates defined inFig. 3, of the two interfering buildings and the princi-pal building. It can be seen from the table that whenthe two interfering buildings are located at the sameside of the principal building, they still can produceshielding effects on the principal building, resulting inan IF of 0.94. However, adverse effects of IF > 1 canbe found for most of the arrangements and themaximum IF is found to be 1.10 when the two inter-fering buildings are located at yA ¼ �3:2b andyB ¼ 3:2b, respectively. This indicates that the twosymmetrically located interfering buildings can increase10% or even more wind load on the middle principalbuilding, that is to say, the channeling effect in thiscase is more significant than that of the two buildingsin side-by-side arrangement.

3.2.3. Staggered arrangementFor the interference effects of three buildings, four

variables (i.e. two x-coordinates and two y-coordinatesof the two interfering buildings) are included in each ofthe configurations and the results are very difficult tobe expressed with simple contours. In this study, a sub-stitute scheme is used to analyze the multi-variable testresults by fixing one interfering building (model A) at acertain position and varying the spacing between theother interfering building (model B) and the principalbuilding. An example is shown in Fig. 7 where thebuilding model A is fixed at (6.1b, �2.4b).

In order to compare the interference factors of thethree-building configuration with those of the two-building configuration, model A is considered as anadditional interfering building to the two-building con-figuration where the interference effects have been

Table 2

Comparison of the IFs of twin-building configuration

Interfering building at (x, y) (5

b, 1.5b) (5b, 2.5b) ( 5b, 4b) (8b, 0) (8 b, 1.5b) (5b, 2.5b) ( 8b, 4b)

Khanduri et al. [9] 0

.78 0.90 1 .0 0.57 0 .74 0.86 0 .99

Huang [11] 0

.74 0.98 1 .03 0.63 0 .71 0.93 1 .00

Present study 0

.73 0.96 1 .03a 0.57 0 .64 0.88 1 .02a

a Denotes extrapolation, see Fig. 3.

Z.N. Xie, M. Gu / Engineering Structures 26 (2004) 1173–1183 1177

shown in Fig. 5b. It can be found from the comparison

results in Figs. 7 and 5b that the introduction of model

A increases the most significant shielding region of

IF 0:4 in the two-building configuration, with the

maximum longitudinal spacing changing from 5b to

about 6b. The slight shielding region of 0:8 < IF 1 is

also broadened with the introduction of model A.

These results indicate that the shielding effects are

enhanced in the three-building configuration.However, the above-mentioned distribution in Fig. 7

is just a local description of the interference effects of

three-building configurations and cannot give the com-

plete information of the interference effects for the con-

figuration. Statistics analysis for a thorough description

of the interference effects is therefore needed and the

results are shown in Fig. 8, where p represents the per-

centage of the positions of the corresponding inter-

ference factor over the whole test positions of the

configurations. From this figure, one can see that p is

35% when IF is about equal to 1.0 for the two-building

configuration, but only about 13% for the three-build-

ing configuration. In general, for different levels of

IF 0:9, the value of p of three-building configuration

is greater than that of two-building configuration.

These results once again indicate that the shielding

effects of three-building configuration are more signifi-

cant than two-building configuration. However, due to

the channeling effect, the IF is found to be about 1.1

for 2% of the complete set of interfering building

arrangements, as shown in Fig. 8. That means that

there may be static amplification due to the existence

of two nearby interfering buildings.

configuration of two equal size buildings in different upstream terrains. (a) Uniform flow;

Fig. 5. IF contours for the (b) exposure category B; (c)

exposure category D.

1178 Z.N. Xie, M. Gu / Engineering Structures 26 (2004) 1173–1183

3.2.4. Channeling effectThe channeling effect was mentioned in ASCE 7-98

[12] and other literature [13], but it has not been dis-cussed in any detail in the previous studies. The reasonmay partly be that this kind of effect is insignificant,compared with the above-mentioned shielding effects.However, the maximum IF is 1.04 in the present test inthe two-building configuration when the interferingbuilding is located side-by-side at (0, �3.2b). For theconfiguration of three equal size buildings, themaximum IF can increase up to 1.10 when the twointerfering buildings are located at (0, 3.2b).

Fig. 9 presents the IF distributions from the presenttest for the two interfering buildings at y ¼ 3:2b inexposure categories B and D. The most significantinterfering positions in the two categories of terrain arefound to be the same. From Fig. 9, one can see thattwo side-by-side upstream interfering buildings producealmost no static amplification effect on the principalbuilding. Only when one of the two interfering build-

ings is located side-by-side with the principal buildingand the other one is arranged upstream, the inter-ference factor can be 1.04, the same as that of thetwo-building configuration; and the region whereIF � 1:04 in exposure category B is much larger thanthat in category D. It can be concluded that the chan-neling effect could be more significant in the smootherterrain.

3.3. Effects of breadth ratio

To investigate the effects of the breadth ratio (here-after referred to as Br) of across-section of the inter-fering buildings to the principal building on theinterference effects, five types of interfering buildingmodels with different breadths are tested. These inter-fering models have the same height as the principalbuilding model but with different breadths. Thebreadth ratios adopted in the test are 0.5, 0.75, 1.0, 1.5and 2.0. The results are discussed in the following.

3.3.1. Two-building configurationGenerally, larger Br of interfering building produces

stronger shielding effects. In most of the interferingpositions, the interference factor decreases with theincrease of Br. However, due to the channeling effectdiscussed in the previous sections, the interfering build-ing with side-by-side arrangement can produce adversestatic amplification effects on the principal building.These adverse effects can also increase with the increaseof Br. Fig. 10 presents the variation of this adverseinterference effects with respect to different breadths of

nterference factors of the three equal size and tandem-arranged buildings in exposure cat

Fig. 6. I egory B.

Table 3

Mean interference effects for side-by-side arrangement in exposure

category B

yA y

B IF

�3.2b �

1.6b 0.94

�3.2b 1

.6b 1.04

�3.2b 2

.4b 1.09

�3.2b 3

.2b 1.10

�2.4b 1

.6b 1.04

�2.4b 2

.4b 1.06

�1.6b 1

.6b 1.04

Z.N. Xie, M. Gu / Engineering Structures 26 (2004) 1173–1183 1179

the interfering buildings located at (0, �3.2b), i.e. inside-by-side arrangement with the principal building. Itmight be anticipated that a parabolic relationship existsbetween IF and Br. A maximum value of IF ¼ 1:16 isrecorded when Br ¼ 2, as shown in Fig. 10. This indi-cates that the interfering building with Br ¼ 2 canincrease 16% mean wind load on the principal buildingwhen the center-to-center spacing of the two buildingsis 3.2b.

3.3.2. Three-building configurationMore interfering buildings generally produce more

significant shielding effects, and the shielding effectsincrease with the increase of Br. The statistical proper-ties for the interferences effects of five types of inter-fering buildings are shown in Fig. 11. From this figure,one can see that the most notable shielding regionof IF 0:4 increases quickly with the increase of Br

while the regions of 0:5 IF 0:9 remain unchangedrelatively.

The increase of the building size could also enhancethe adverse static amplification on the principal build-ing when the two interfering buildings are located atsome critical locations. For the five types of breadthsof interfering buildings, the critical position for both ofthe two interfering buildings are found to be about(0, 3.2b) in the present test grid region shown inFig. 3. The corresponding maximum interference fac-tors for different Br in exposure category B are listed inTable 4. The table shows that the maximum IF increa-ses with Br, and a maximum value of 1.195 is found forthe interfering buildings of Br ¼ 2. This indicate thatthe two symmetrically located larger sized interferingbuildings of Br ¼ 2 can increase 20% wind load on themiddle principal building.

3.4. Effects of height ratio

To investigate the effects of the height ratio (here-after referred to as Hr) of the interfering buildings tothe principal building on the interference effects, fivetypes of interfering building models with differentheights were tested. These interfering building modelshave the same breadth of that of the principal buildingmodel but different heights. The height ratios for thetest are 0.5, 0.75, 1.0, 1.25, and 1.5.

3.4.1. Two-building configurationFig. 12 shows the effects of Hr on the interference

effects for the two-building configuration. The resultsshow that the interfering building with Hr ¼ 0:5 pro-duced insignificant shielding effects; on the other hand,heights greater than 1.25 produce similar interferenceeffects. This means that the shielding effects of the

interference factor vs. the relative positions of interfering building model B for interfering

Fig. 7. Variations of the building model A fixed at

(6.1b, �2.4b) in exposure category B.

Fig. 8. Comparison of the distribution of the interference effects

between the configurations of two and three equal size buildings in

exposure category B.

1180 Z.N. Xie, M. Gu / Engineering Structures 26 (2004) 1173–1183

interfering building with Hr 0:5 could be neglected,while the interference effects are almost the same whenHr � 1:25. So the mean interference effects may only besensitive to the height ratio in the range0:5 Hr 1:25. However, for Hr � 1:25, the channel-ing effects become significant than those of the casewith Hr ¼ 1.

3.4.2. Three-building configurationMore interfering buildings generally enhance the

shielding effects. Fig. 13 presents the statistical distribu-

tions of interference effects for the cases of the two

interfering buildings with different height ratios in

exposure category B. The results show that two lower

interfering buildings of Hr ¼ 0:5 produce insignificant

interference effects, with most of the interference fac-

tors being within the range [0.9, 1.0] in exposure cate-

gory B. Also, the interference effect of these two lower

Fig. 9. IFs distributions of three-building configuration for two

interfering buildings A and B fixed at yA ¼ 3:2b and yB ¼ �3:2b. (a)

Exposure category B; (b) exposure category D.

Fig. 10. Interference factors vs. breadth ratios of interfering build-

ings located at (0, �3.2b) in exposure category B.

Fig. 11. Comparison of the distribution of the interference effects of

different breadth ratio configurations (three-building configurations,

exposure category B).

Table 4

Maximum IF for different interfering building size due to channeling

effects (three-building configurations, exposure category B)

Br M

aximum IF

0.5 1

.03

0.75 1

.05

1.0 1

.10

1.5 1

.15

2.0 1

.195

Z.N. Xie, M. Gu / Engineering Structures 26 (2004) 1173–1183 1181

interfering buildings becomes even less significant inthe higher turbulence of exposure category D. Theresults indicate that the effects of interfering buildingwith a height less than 0.5h can be neglected. Onlywhen the heights of the interfering buildings are greateror equal to 0.75h does shielding effects become notable.

In contrast to the significant change in interferencefactors for interfering buildings with Hr ¼ 0:5; 0:75and 1:0, factors for interfering buildings with Hr ¼1:0; 1:25 and 1:5 vary only marginally. However, asindicated in Fig. 13, a slight difference for IF ¼ 0:7between the configuration of Hr ¼ 1 and that of Hr >1 in exposure category B is found. It shows that theshielding effects of the two interfering buildings withHr ¼ 1 are greater than those of the two interferingbuildings with Hr > 1. Fig. 14 presents the variationsof the IF with respect to the height ratios of the inter-fering buildings when the two interfering buildings arelocated at (6.1b, �1.6b) and (6.1b, 1.6b), respectively.From this figure, one can see that the interferencefactors decrease rapidly with the increase of Hr in therange from 0.5 to 1.0, but for interfering buildings ofHr � 1, the interference factors increase marginally

with the increase of Hr. It can be also found fromFig. 14 that the shielding effects in smoother terrain ofexposure category B are more significant than those incategory D.

Based on the above results, it can be summarizedthat the sensitive height of interfering buildings for themean interference effects are in the range from 0.5h to1.25h, while the interference effects remain almost thesame for higher interfering buildings. However, higherinterfering buildings cause stronger channeling effect,and the static amplification may increase with theincrease of the height of the interfering buildings.A maximum value of 1.13 of IF is recorded when thetwo interfering buildings with Hr ¼ 1:5 are located at(0, 1.6b) in terrain category D.

3.5. Simplification of the results for three-buildingin arbitrary configurations

Since four variables, i.e. two x-coordinates and twoy-coordinates of the two interfering buildings, areinvolved in the analysis of the interference effects ofthree-building configurations, the interference factorscannot be simply expressed in a single contour as in thetwo-building cases. The problem is how to deduce arelatively simple and yet precise enough representationmethod for practical applications from the complexdata from the wind tunnel tests. A reduced interferencefactor (RIF) contour for three-building configuration isthus proposed by synthesizing the effects over thewhole test domain.

Let PA(x,y) and PB(x,y) be the location coordinatesof two interfering buildings; the interference factor canthen be expressed as

IF ¼ f ðPA;PBÞ; PA;PB 2 X ð3Þ

Fig. 12. Comparison of the distribution of the interference effects of

different height ratio configurations (two-building configuration,

exposure category B).

Fig. 13. Comparison of the distribution of the interference effects of

different height ratio configurations (three-building configurations,

exposure category B).

Fig. 14. Interference effects of different height ratios while two

upstream interfering buildings are located at (6.1b, 1.6b).

1182 Z.N. Xie, M. Gu / Engineering Structures 26 (2004) 1173–1183

where X denotes the whole position domain of the

interfering buildings in the test. Simplifying the above

four-variable problem to a lower two-variable one, the

so-called RIF can be expressed as

RIF ¼ gðPAÞ ¼ maxPB2X

f ðPA;PBÞ ð4Þ

From Eq. (4), the reduced interference factor, RIF,

can easily be expressed by a simple contour in the simi-

lar way of the two-building configuration. From

Eq. (4), it can also be seen that a RIF value is always

greater than or equal to the corresponding IF. In order

to make the estimated interference factor in terms of

Eq. (4) close to the real one, i.e., IF, the interference

factor for practical purpose may be determined by the

following equation

IF

¼ min gðPAÞ;gðPBÞð Þ ð5Þ

Of course, the calculated result from Eq. (5) will also

be greater than or equal to that of Eq. (3), i.e. IF

� IF.

However, from the practical point of view, interference

factors defined by Eq. (5) are conservative.Based on the above definitions, Fig. 15 shows the

distributions of the RIF for the configuration of three

identical buildings in exposure categories B and D,

respectively. Distributions of only half of the region are

drawn in the figure due to the symmetry of the RIF.

Fig. 15 also shows the shielding effects in exposurecategory B are stronger than those in category D.Meanwhile, stronger channeling effects can also be seenin exposure category B, that is to say, the maximumstatic amplification in exposure category B is more ser-ious than that in category D.

An example is shown here to explain briefly how toapply Eqs. (4) and (5) in practical use. For two inter-fering buildings A and B located at PAð4b;� bÞ and PB

ð9b;� 2bÞ in exposure category B, one can obtain twoRIFs of gðPAÞ ¼ 0:81 and gðPBÞ ¼ 0:87 by interpolat-ing from the distribution of the RIFs shown in Fig. 15a.Then according to Eq. (5), the interference factor for

this configuration is IF

¼ minð0:81;0:87Þ ¼ 0:81.

4. Concluding remarks

The mean interference effects between two andamong three buildings with different configurationshave been studied by a series of detailed wind tunneltests. A good agreement between the current study andthe existing results in two-building configurations isfound, which ensures the reliability of the results andconclusions proposed in the present study. For inter-ference effects of three-building configurations, theinterference factor is simplified to an easier expressedRIF to simplify the experiment results. The mainresults are summarized as follows.

g. 15. The RIFs of three equal size building configuration. (a) Exposure category B; (b) exposure category D

Fi .

Z.N. Xie, M. Gu / Engineering Structures 26 (2004) 1173–1183 1183

1. Generally, the effects of the upstream building(s)show shielding effects and the corresponding meaninterference factors are less than 1.0. But the staticamplifications, due to the channeling effects, couldalso lead to an increase of 10% of the mean windload on the principal building when the two equalsize interfering buildings are located at (0, 3.2b),or, in other words, the three buildings are arrangedside-by-side. The observed maximum increase of themean wind load can be 20% depending on the sec-tion size and spacing of the buildings.

2. The interference effects are sensitive to the breadthof the interfering buildings. Larger upstream build-ings could produce more shielding effects on theprincipal building and, meanwhile, side-by-sideinterfering buildings with larger size can producemore serious channeling effect on the principalbuilding.

3. The height of the interfering buildings could alsoaffect the wind load on the principal building. Theresults show that interference from lower interferingbuildings with Hr 0:5 is negligible while the sensi-tive height ratio of interfering buildings is in therange from 0.5 to 1.25. For higher buildings, theshielding effect is constant. However, higher inter-fering building may cause stronger channeling effectsand the static amplifications will increase with theincrease of the heights of the interfering buildings.

Due to the complex of the problem, the above dis-cussions and conclusions on the effects of the geometryof interfering buildings are still in the qualitative level.More tests and efforts are therefore needed to improvethe understanding in this area.

Acknowledgements

This research is jointly supported by the NationalScience Foundation (50321003), the Foundation forUniversity Key Teachers by the Ministry of Education,

and the Science Foundation of Guangdong Province

(010455). They are gratefully acknowledged.

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