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KSCE Journal of Civil Engineering (2012) 16(1):119-131

DOI 10.1007/s12205-012-1163-y

119

www.springer.com/12205

Structural Engineering

Wind-Induced Mean Interference Effects Between Two Closed Spaced Buildings

Jignesh Arvindbhai Amin* and Ashokkumar Ahuja**

Received March 5, 2010/Revised March 1, 2011/Accepted May 12, 2011

Abstract

The mean interference effects between two rectangular buildings located in close proximity in a geometrical configuration of Land T plan shape are studied through wind tunnel test on 1:300 scale rigid models. The mean surface pressure distributions on allthe walls of two buildings located in close proximity as well as in an isolated position are measured over an extended range of winddirections. The mean responses of pair of buildings namely, block-1 and block-2 subjected to interference effects are evaluated usingexperimentally obtained wind loads and, subsequently compared it, with the responses of a similar building in an isolated position.Effectiveness of upstream building location and wind orientation in changing the mean wind pressure distributions and responses ofupstream and downstream building are also assessed. At wind incidence angle of 0o, presence of upstream block-1 reduces the meanalong-wind displacement of block-2 of L and T shape arrangements up to 25% and 71% respectively as compared to that ofcorresponding block in an isolated position. However, the presence of upstream block-1 increases the maximum mean torque onblock-2 of L and T shape arrangement up to 28%, and up to 88% respectively as compared to maximum mean torque developedon a similar block in an isolated position.

Keywords: wind pressure, building arrangements, interference effects, wind orientation

1. Introduction

There are many situations, where analytical methods and avail-

able database can not be used to estimate wind loads and their

associated structural responses of high-rise buildings. For example,

the shape of the building is uncommon, the pair of buildings

located in close proximity or buildings clustered together in

groups, as office buildings grouped together in city centre. The

question of interference effects from other adjacent buildings of

similar height on the structural loading and response of tall

building arises in later cases. Due to mutual interference, the

wind loads on the subject building may be considerably different

from those measured on an isolated building. The interference

effects between two or more bodies in close proximity are quite

important in the fields of building aerodynamics (Kelnhofer,

1971; Khanduri et al., 1998) and vehicle aerodynamics (Kato

and Nishikawa, 2002; Zhang et al., 2010). The main parameters

affecting interaction between adjacent buildings are the type of

upstream terrain, shape and size of the buildings, the incidence

wind directions and last but not least, the building arrangement

and spacing. Neighbouring buildings may either decrease or

increase the flow-induced forces on a principal building. The

torsional response of buildings having 25 to 40 storeys is also

significantly affected by the presence of near by structures.

However, most wind engineering codes and standards offer little

guidance to the designer for evaluating the wind forces on a pair

of rectangular buildings located in close proximity.

There have been fairly good amount of interference studies

between smaller group of two or three buildings with specific

arrangement of buildings. Harris (1934) found that torque on the

Empire State Building in New York would be doubled, if two

building blocks were built across the two streets adjacent to

building. However, resurrection of studies on the interference

effects occurred in the early seventies. This sudden interest could

perhaps be traced back to the collapse of three out of the eight

natural draft-cooling towers at Ferrybridge, England in 1965,

which was attributed to the interference effects. Kelnhofer (1971),

Melbourne and Sharp (1976), Saunders and Melbourne (1979)

and Ahuja et al. (1991) have measured the effects of changing

the relative height of upstream building on wind loads on a

downstream building. They observed that mean along wind

loads on downstream buildings were reduced by increase in the

height of the upstream building due to shielding, but dynamic

loads increased.

Sakamoto and Haniu (1988) found that the drag force on the

downstream building reduce to zero, when the upstream building

was located three times building breadth away and the mean drag

force could be negative, when the spacing was less than this

critical distance. Common sense suggests that the interference

effects between two buildings should decreases by increasing the

*Associate Professor, Dept. of Civil Engineering, Sardar Vallabhbhai Patel Institute of Technology, Vasad 388 306, India (Corresponding Author, E-mail:

[email protected])

**Professor, Dept. of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee 247 667, India (E-mail: [email protected])

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Jignesh Arvindbhai Amin and Ashokkumar Ahuja

120 KSCE Journal of Civil Engineering

separation distance, such that beyond certain spacing, buildings

behave as isolated under the action of wind. However, Taniike

(1992) found that the shielding effects could be noticeable, when

the upstream building was located at a distance of 16 times

building breadth from the downstream building. He indicated a

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

wind loads on the principal building. English (1993) suggested a

third order regression polynomial to predict the mean along wind

shielding factors for a pair of rectangular prisms in tandem.

Kwok (1995) summarized along-wind, acrosswind and torsional

interference factors between buildings and analyzed mechanism

of the interference. English and Frick (1999)and Khanduri et al.

(1997, 1998) summarizes research developments in the area of

wind-induced interference effects and suggests a neutral network

approach for the assessment of interference effects and design

wind loads on a buildings located in a variety of geometrical

configurations. Khanduri et al. (2000) studied and tried to give

general guidelines for wind induced interference effects between

two buildings.Tang and Kwok (2004) investigated the interference

excitation mechanisms on translational and torsional responses

of an identical pair of tall buildings. Xie and Gu (2004)studied

the mean interference effects between two and among three tall

building using a wind tunnel tests.

However, due to huge amount of experimental workload and

the complexity of the interrelated parameters, most previous

investigations have mainly focused on the interference effects for

specific arrangements of buildings and wind orientations. This

paper focuses mainly on the wind pressure distributions and

mean interference effects between pair of rectangular buildings

located in close proximity in a geometrical configuration of L

and T plan shape. In particular, the present study represent the

level of interferences as reflected by the distribution of mean

wind pressure coefficients, mean forces/displacements and torque

developed on the buildings due to unevenly distributed forces on

the building walls. Pressure measurements are restricted to open

country type flow, as the interference effects due to adjacent

buildings are most pronounced in flow with low turbulence

levels (Khanduri et al., 1998). In facts, there are two main kinds

of effects involved in wind induced interference effects on tall

buildings namely, the mean interference effects and the dynamic

interference effects. Previous studies have shown that the mean

interference effects are also significant in certain cases (Khanduri

et al., 1998; Xie and Gu, 2004).

2. Experimental Pogramme

2.1 Feature of Experimental Flow

The experiments were carried out in a closed circuit wind

tunnel under the boundary layer wind flow at the Department of

Civil Engineering, Indian Institute of Technology Roorkee, India.

The wind tunnel has a working test section of approximately 8.2

m in length with a cross-section of 1.2 m (width)0.85 m

(height). The experimental flow was simulated similar to

exposure category-II of Indian wind load code IS: 875 (part-3) at

a length scale of 1:300 by placing the grid of horizontal bars at

upstream edge of the tunnel and roughness devices. Terrain

category-II represents an open terrain with well scattered ob-

structions having height generally between 1.5 m to 10 m and

having exponents of the power-law (n) of mean speed profile of

0.143. The wind velocity in the wind tunnel at the top level of

models has been maintained as 15 m/s. The gradient height for

exposure category-II is 300 m; and accordingly that of the

simulated wind fields in the wind tunnel is 1 m. Models are

placed at a distance of 6.1 m from the upstream edge of the test

section. A reference pitot tube is located at a distance of 5.0 m

from the grid and 300 mm above the floor of wind tunnel to

measure the free stream velocity during experiments. The

simulated mean wind velocity profiles and turbulence intensity

distributions are plotted in Figs. 1(a) and 1(b) respectively.

2.2 Details of Models

The models used for the experiments were made of transparent

perspex sheet of 6 mm thick at a same geometrical model scale

with that of wind simulation, i.e. 1:300. Height of all the models

was kept 300 mm for comparison purposes. The L and T

shape model arrangements are made of two models, namely

block-1 (50 mm75 mm) and block-2 (125 mm50 mm). In

case of L shape arrangement, block-1 (L1-1) was placed at the

edge of block-2 (L1-2), whereas in case of T shape arrange-

ment, block-1 (T1-1) was placed at the center of block-2 (T1-2)

with 1 mm of spacing between two building models. The plan

and isometric views of arrangement of building models are

shown in Fig. 2. Both the models were instrumented with more

than 140 numbers of pressure taps at six different height levels of

25, 75, 125, 175, 225 and 275 mm from bottom to obtain a good

distribution of pressures on all the walls of building models.

These pressure taps have been placed as near as possible to the

edges of the faces to attempt to capture the high pressure

variation at the edge of the faces.

3. Pressure Distributions and Discussion

Mean, maximum and minimum pressure coefficients on all the

Fig. 1. Wind Flow Characteristics; (a) Velocity Profile at the Test

Section, (b) Turbulence Intensity with Height at Test Section

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Vol. 16, No. 1 / January 2012 121

walls of rectangular building models arranged in L and T

shape configurations were evaluated from the fluctuating wind

pressure records at all pressure points over an extended range of

wind incidence angles of 0o to 90o at an interval of 15o.

3.1 L-shape Building Models Arrangement

The general characteristics and observed pressure distribution

on different walls of the building models arranged in L-shape

configuration are summarized as follows.

Figure 3(a) shows the mean surface pressure coefficient con-

tours of building models arranged in L plan shape configuration

at wind incidence angle of 0o. At wind incidence angle of 0o,

front wall-A of model L1-1 is subjected to positive pressures and

the values of mean pressure coefficients varies between 0.30 and

0.93. However, wind pressure coefficients distribution does not

remain symmetrical about the vertical centerline as in the case of

rectangular/square bluff body. It increases from left edge to right

edge, towards the re-entrant corner of wall-A. Inner walls-B and

C are also subjected to pressures of almost uniform intensity.

Although wall-B of model L1-1 is parallel to wall-F1, which is a

side face, it is subjected to pressures and not suction due to the

blockage of wind flow by wall-C of block L1-2, which causes

stagnation of flow in that re-entrant corner. It is the peculiar

characteristics of models arranged in L-shape. Side walls-D and

F are subjected to negative pressure, which increases slightly

from windward to leeward edge due to separation of wind flow.

The leeward wall-E of block L1-2 is subjected to suction but the

variation of suction along the height as well as along the width is

almost negligible being under the wake region.

As the angle of wind incidence increases, the intensity of posi-

tive pressures on wall-A is reduces. However, higher positive

pressure still exists near the re-entrant corner. Fig. 3(b) shows the

mean wind pressure coefficient contours at wind incidence angle

of 30o. At skew wind incidence angles, the parts of inner walls-B

and C towards the re-entrant corner are subjected to higher wind

pressures as compared to pressures on corresponding faces at

wind incidence angle of 0o. Wind incidence angle in the range of

30o to 60o, governs the cladding design of walls-B and C depend-

ing up on the dimensions of inner walls forming the re-entrant

corner and spacing between two models. Wall-D is subjected to

peak negative pressure coefficient of -1.25 at wind incidence

angle of 30o.

When the wind blows at an angle of 45o, larger region of

stagnant air is formed in the re-entrant corner of L shape

models arrangement. The size of this region is highly dependent

on the spacing between two models, slenderness of the models

and dimension of inner walls forming the re-entrant corner.

Taller the model, smaller the spacing between two models and

uniform dimension of the inner walls-B and C implies a larger

stagnation zone in the re-entrance, as the flow tends to contour

the sides rather than flow into the cavity. Beyond the wind

incidence angle of 45o, wall-A of model L1-1 is subjected to

suction and peak negative pressure coefficient of -1.75 is noticed

at wind incidence angle of 60o. Wall-E and F of block L1-2 is

subjected to negative pressures up to wind incidence angle 90o

Fig. 2. Plan and Isometric Views of Building Models;(a) L-shape

Arrangement, (b) T-shape Arrangement

Fig. 3. Mean Pressure Coefficient Distribution on Models L1-1 and

L1-2: (a) Wind Incidence Angle-0o, (b) Wind Incidence

Angle-30o

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and the values of mean wind pressure coefficients are observed

in the range of -0.4 and -0.75. Contours for other wind incidence

angles are not included here due to paucity of space.

3.2 T-shape Building Models Arrangement

The general characteristics and observed wind pressure distri-

bution on different walls of the models arranged in T shape

configuration are summarized as follows.

Figure 4(a) shows the mean wind pressure coefficient contours

on the models arranged in T plan shape configuration at wind

incidence angle of 0o. At wind incidence angle of 0o, the flow is

symmetrical about central vertical axis and the stagnation point

is observed approximately at 0.8 h, where h is the height of the

model. Wall-A is subjected to mean pressure coefficients in the

range of 0.32 to 0.92. The sidewalls-B and H of model T1-1 of

T shape arrangement are subjected to negative pressures due to

separation of flow from upstream corner, whereas sidewall-B of

block L1-1 of L shape arrangement is subjected to positive

pressures. The exterior edge of wall-C and G of model T1-2 are

subjected to positive wind pressures due to direct incidence of

wind flow on that smaller area, whereas the portion towards the

re-entrant corner of wall-C and G are partially submerged by the

shear layers emanating from the upstream edge/block, thus

subjected to suction. The variation of pressures and magnitude of

pressure coefficients on wall-B of upstream model is signifi-

cantly affected by its location with respect to downstream model

and size of wall-C. Side walls-D and F and leeward wall-E of

model T1-2 are subjected to negative pressures. The pressure co-

efficients on sidewall-D are decreases from windward to leeward

side. The negative pressures on leeward wall-E are uniformly

distributed, being under the wake region. Suction on the leeward

and sidewalls of building models are significantly affected by the

ratio of width of walls-B and wall-C, which is forming the re-

entrant corner.

At wind incidence angle of 15o, inner walls-B and C are subjected

to higher pressures as compared to pressures on corresponding

walls at wind incidence angle of 0o. Fig. 4(b) shows the mean

wind pressure coefficient contours at wind incidence angle of

30o. As the wind incidence angle increases, the mean pressure

coefficients on walls-B and C are increases up to 45o. Wall-D and

A of models T1-2 and T1-1 respectively are subjected to peak

suction at wind incidence angle of 30o and 60o respectively. Wall-

D of model T1-2 is subjected to peak localized negative pressure

coefficients of -1.39 at wind incidence angle of 30o. Beyond the

wind incidence angle of 30o, Walls-G and H are not directly ex-

posed to the flow, being rather under the wake region influence,

and these walls are subjected to mean pressure coefficients in the

range of -0.4 to -0.61. At wind incidence angle of 90o, inner

walls-B and C are subjected to positive pressures similar to L

shape models arrangement. However, the highest positive pres-

sures on front wall-D are no longer in the middle of the walls as

in the case of square/rectangular building models but are moved

to position near the re-entrant corner.

4. Prototype Buildings

The prototypes selected for the study are hypothetical rein-

forced concrete moment-resisting framed tall buildings. The pro-

totypes are 28 storey buildings with a total height of 90 m. In

case of L and T plan shape arrangements, two buildings/

blocks, namely block-1 and block-2 are separated by the separ-

Fig. 4. Mean Pressure Coefficient Distribution on Models T1-1 and

T1-2; (a) Wind Incidence Angle-0o, (b) Wind Incidence

Angle-30oFig. 5. Plans of Prototype Buildings; (a) L-shape Arrangement, (b)

T-shape Arrangement

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Vol. 16, No. 1 / January 2012 123

ation joint as shown in Fig. 5. The grades of the concrete and

steel reinforcement used in prototype buildings are M25 and

Fe415 respectively. The descriptions of the buildings and frame

elements are shown in Table 1.

4.1 Calculation of Wind Loads on Prototype Building

The wind loads on each node of the prototype buildings are

calculated from the experimentally obtained non-dimensionalize

mean wind pressure coefficients at different pressure points on

the respective models, as follow:

(1)

Where,

Ae: effective frontal area (strip) considered for the building

at heighty,

Cp,mean: mean wind pressure coefficient at heighty,

Fy,proto: static wind load on the building node at heighty cor-

responding to strip areaAe,

V: designwind velocity at the roof height of building;

: density of air.

The building is considered to be located Terrain category-II. It

represents an open terrain with well-scattered obstructions having

height generally between 1.5 m to 10 m. The design wind vel-

ocity at the roof height of building is considered as 50 m/s for a

50-year return period.

5. Response Evaluation of Prototype Buildings

Block-1 and block-2 of L and T shape arrangements and in

an isolated condition were analyzed at wind incidence angles of

0o, 15o, 30o, 45o, 60o, 75o and 90o using the experimentally

obtained mean wind loads on the corresponding models. The

location and dimensions of block-1 significantly affects the wind

loads on block-2. The modification of wind loads on a building

due to adjacent buildings is known as interference effects and it

can be quite significant. Generally, the arrangement of buildings

and direction of wind determine the extent of interactions. In this

study, efforts are made to quantify the effects of shielding and

interference between a pair of rectangular buildings located in

close proximity with geometrical configurations of L and T

plan shape. To estimate the severity of mean interference effects,

wind loadings/displacements on a building in the presence of an

adjacent building is compared with the wind loadings on a

similar building in an isolated. Results are expressed in terms of

a dimensionless ratio known as interference factor (IF), defined

as:

(2)

5.1 Effects on Block-2

Interferences effects as reflected by the mean displacements,

torque, mean torque as a normalized eccentricity, base moments

and base shear developed on block-2 due to the presence of

block-1 at different wind incidence angles are summarized as

follows.

5.1.1 Displacement Along the X-axis

Figure 6 shows the mean displacements at the top of section x-

x of block-2 of both L and T shape arrangements and in an

isolated position along the X-axis at different wind incidence

angles. The part of broader front wall of block-2 of both L and

T shape arrangements behind block-1 is not subjected to any

wind forces due to the shielding effects of block-1 at all wind

incidence angles. Generally, mean alongwind forces on a down-

stream building are reduced due to shielding by the upstream

building. It is noticed that block L1-2 of L shape arrangement is

subjected to significantly higher mean displacement along the X-

Fy pro to, Cp mea n, Ae1

2--- V

2=

IF

response of block-i with interfering building (block-j) present for specific angle

response of block-i in isolated position at corresponding wind incidence angle-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------=

Table 1. Description of the Buildings and Frame Elements

S. No. Particulars Details/Values

1 Ground storey height 3.6 m

2 Remaining other storey heights 3.2 m

3 Size of beams 300 mm600 mm

4 Size of columns (from ground storey to tenth storey) 850 mm850 mm

5 Size of columns (from tenth storey to twentieth storey) 750 mm

750 mm6 Size of columns (from twentieth storey to twenty eight storey) 600 mm600 mm

7 Thickness of floor slab 150 mm

Fig. 6. Displacements at the Top of Block-2 Along the X-axis

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axis as compared to corresponding block T1-2 of T shape

arrangement at all wind incidence angles. This is mainly due to

wall-C of block L1-2 of L shape arrangement is subjected to

positive pressures at all wind incidence angles, and therefore

wind forces on it, are acting towards the wall surfaces. Whereas

in case of T shape buildings arrangement one re-entrant corner

towards the upwind is subjected to positive pressures and another

re-entrant corner towards downwind is subjected to negative

pressures at all skew wind incidence angles. Hence, wind forces

on front broader wall of block T1-2 on either side of block T1-1

(i.e. wall C & G) are acting along the opposite directions. At wind

incidence angles of 0o, outer part of walls-C and G of model T1-

2 are subjected to positive pressures due to direct incidence of

flow on that small area, whereas the inner portion towards the re-

entrant corner is partially submerged by the shear layers emanat-

ing from the upstream edge/block, thus subjected to suction. So

net wind forces acting along the X-axis on block T1-2 are signi-

ficantly reduces as compared to block L1-2 at all wind incidence

angles. Beyond wind incidence angle of 30o, block L1-2 is sub-

jected to higher displacement along the X-axis as compared to

the displacement of a similar block in an isolated position. At

wind incidence angle of 90o, block-2 of both L and T shape

arrangements is subjected significant mean displacement along

the global X-axis, which is perpendicular to wind direction,

whereas mean displacement of an isolated building along the X-

axis is almost negligible.

Table 2 shows the mean displacements along the X-axis of

block-2 and the interference factors (IFx)at different wind inci-

dence angles, which are calculated according to Eq. (2). At wind

incidence angle of 0o, the presence of upstream block-1 reduces

the mean alongwind displacements of block-2 of both L and

T shape arrangements up to 25% and up to 71% respectively,

as compared to the mean alongwind displacement of a similar

block in an isolated position. It is observed that the displace-

ments of block-2 are sensitive to the depth, location of the inter-

fering block-1 and wind orientations.

5.1.1.1 Effects of Location of Block-1

Block L1-2 is subjected to higher displacement along the X-

axis as compared to block T1-2 at all wind incidence angles. The

location of block-1 on upstream side at the edge of block-2

produces less interference on block-2 as compared to its location

at the center of block-2. In general, the effect of upstream

interfering block-1 is to provide some shielding regarding the

total forces on the subject block-2 at wind incidence of 0o to 30o.

At wind incidence angle of 30o, wind forces acting along the

global X-axis on block L1-2 and a similar block in an isolated

position are almost same, whereas block T1-2 is still subjected to

comparatively lesser wind forces along the X-axis as compared

to a similar block in an isolated position. Hence block L1-2 and

block T1-2 has IFx of 1.01 and 0.5 respectively, at wind

incidence angle of 30o. At wind incidence angle of 30o to 60o,

block L1-2 has IFx in the range of 1.01 to 1.81, whereas block

T1-2 has IFx of 0.5 to 1.06.

At wind incidence angle of 60o to 90o, block-2 of both L and

T shape arrangements is subjected higher wind loads along the

X-axis as compared to the corresponding block in an isolated

position, due to the blockage of wind flow from block-1.

5.1.2 Displacement Along the Z-axis

Figure 7 shows the mean displacements at the top of section z-

z of block-2 of both L and T shape arrangements and in an

isolated position along the Z-axis at different wind incidence

angles. The mean displacements of block-2 of both L and T

shape arrangements along the Z-axis are slightly affected due to

the presence of block-1. The displacements of block-2 along the

Z-axis and interference factors (IFz) for block-2 of both types of

arrangements at different wind incidence angles are shown in

Table 3.

5.1.3 Torsion

Figure8(a) shows the torsion developed at the base level of

block-2 of both L and T shape arrangements and a similar

building in an isolated position. Block-2 of both L and T

Table 2. Displacements (mm) at the Top of Block-2 Along the X-axis and IFx

Angle 0o 15o 30o 45o 60o 75o 90o

block-(L1-2) 136.16 152.45 165.01 164.20 155.98 151.45 132.45

block-(T1-2) 52.88 68.83 81.82 90.53 91.44 88.08 94.00

isolated 181.06 173.47 162.59 132.94 86.12 29.03 0.00

IFx (L1-2) 0.75 0.88 1.01 1.24 1.81 5.22 -

IFx (T1-2) 0.29 0.40 0.50 0.68 1.06 3.03 -

Fig. 7. Displacements at the Top of Block-2 Along the Z-axis

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Vol. 16, No. 1 / January 2012 125

shape arrangements is subjected to noticeable higher mean

torsion as compared to the corresponding block in an isolated

position at most wind orientations. The mean torque on down-

stream block-2 of both types of arrangement is significantly

increases, because the front broad wall of block-2 is partially

shielded by block-1 on upwind side. At wind incidence angle of

15o to 90o, wall-C of block L1-2 of L shape arrangement is

subjected to positive pressures, hence wind forces on that wall

are acting towards the wall surface. Whereas in case of T shape

arrangement, one re-entrant corner towards the upwind is sub-

jected to positive pressures and another re-entrant corner is

subjected to negative pressures, hence wind forces on front

broader wall of block T1-2 on either side of block T1-1 (i.e. wall

C and G) are acting along the opposite directions. Therefore

block T1-2 of T shape arrangement is subjected to significantly

higher torsion as compared to block L1-2 of L shape arrange-

ment and a similar block in an isolated position at wind inci-

dence angle of 15o to 90o.

The maximum mean torsional response of block-2 of both L

and T shape arrangements increases by up to 28% and up to

88% respectively as compared to the maximum mean torsional

response of a similar block in an isolated position. Table 4 shows

torsion developed on block-2 of both L and T shape arrange-

ments and the interference factors (IFT ) for torsion on block-2.

The mean torsional moment My about vertical axis is also

expressed as a normalized eccentricity:

(3)

Where d= maximum building width/depth, Vb = , Vbxand Vbz is the base shear along the global X and Z-axes respec-

tively. Fig. 8(b) shows the torque at the base of buildings as

normalized eccentricity. Torsion arises from a number of causes:

building shape, wind direction, interference effects, and dynamic

response. It is easily observed that the torque on a high aspect-

ratio building would increases, if wind on the broad face is

partially shielded by an upwind building. Block-2 of L shape

arrangement has a maximum eccentricity of wind forces is 9%,

whereas block-2 of T-shape arrangement has a maximum eccen-

tricity 28%. However, similar block in an isolated position has a

maximum eccentricity of 5% to 7% except at wind angle of 75 o.

The increase in eccentricity of block L1-2 and T1-2 occurs

despite a reduction in the total shear flow. The effect of inter-

e

d---

My Vb

d---------------=

Vbx2 Vbz

2+

Table 3. Displacements (mm) at the Top of Block-2 Along the Z-axis and IFZ

Angle 0o 15o 30o 45o 60o 75o 90o

block-L1-2 5.70 4.52 8.26 20.11 38.58 45.37 50.64

block-T1-2 0.57 9.85 8.68 22.63 40.46 44.16 48.49

isolated 0.98 4.53 13.24 33.49 43.42 46.62 48.74

IFz (L1-2) - 1.00 0.62 0.60 0.89 0.97 1.04

IFz (L1-2) - 2.18 0.66 0.68 0.93 0.95 0.99

Fig. 8. Variation of Torsion on Block-2 with Wind Incidence Angle;

(a) Torsion, (b) Torsion as a Normalized Eccentricity

Table 4. Torsion (MN-m) and IFT of Block-2

Angle 0o 15o 30o 45o 60o 75o 90o

block-(L1-2) 15.93 15.93 21.83 22.38 16.49 16.57 11.40

block-(T1-2) 0.31 31.20 33.01 31.44 28.10 27.68 24.81

isolated - 7.80 16.5 15.78 4.09 -17.54 -

IF (L1-2) - 2.04 1.32 1.42 4.03 0.95 -

IF (T1-2) - 4.00 2.00 1.99 6.87 1.58 -

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fering block-1 is to provide some shielding regarding the total

force on the subject building, but the torque is increases. Most of

the building codes and standards have lagged behind the recogni-

tion of this important load type (Boggs, 2000). The Indian stand-

ard (IS-875, (part-3), 1987) does not specify the torsional load.

ASCE 7-98 (ASCE 1998) specifies an unbalanced load equival-

ent to an eccentricity of e/d = 3.625% at 87.5% of maximum shear.

ASCE 7-02 (ASCE 2002) had increased the torsional loading to

an eccentricity of 15% at 75% of the maximum wind shear, and

Eurocode 1 (Eurocode1, 1995) specifies an eccentricity of 10%.

But 10% is a typical value, and twice of it or even more is not

uncommon. For most cases, these codal provisions have been

adequate because the lack of required torsion is compensated by

conservative force specifications but not for all the cases.

5.1.3.1 Effects of Location of Block-1 and Wind Incidence

Angles

Block T1-2 is subjected to significantly higher torque as com-

pared to block L1-2 at skew wind incidence angles. Generally,

the location of block-1 on the upstream side at the center of

downstream block-2 produces higher torque on block-2 as

compared to its location at the edge of block-2.

As the wind incidence angle increases, the interference factor

(IFT) for torsion on block-2 of both types of arrangements re-

duces up to 45o. At wind incidence angle of 60o, block L1-2 and

block T1-2 is subjected to noticeable higher torsional interfer-

ence factor (IFT) and beyond that, it reduces from wind incidence

angle of 60o to 75o.

5.1.4 Base Moment (MxandMz)

Thebase momentMxabout global X-axis on block-2 of both

L and T shape arrangements are shown in Fig. 9(a). It is

observed that the base momentMxon block-2 is almost slightly

affected due to the presence of block-1 at all wind incidence

angles.

Figure 9(b) shows the base momentMzabout the global Z-axis

developed on block-2 of both types of arrangements at different

wind incidence angles. Beyond wind incidence angle of 30o,

block L1-2 is subjected to significantly higher base momentMzas compared to the corresponding block in an isolated position

due to the blockage of wind flow by block L1-1, which causes

higher wind forces on wall-C of block L1-2 along the global X-

axis. Up to wind incidence angle of 60o, block T1-2 is subjected

to substantially lower base momentMz as compared block L1-2

and a similar block in an isolated position. Block L1-2 is subject-

ed to substantially higher base moment Mz as compared block

T1-2 at all wind incidence angles. Table 5 shows the base

momentsMzon block-2 and interference factors (IFMz) for block-

2 of both types of arrangement. As expected, the IFMz for base

moments are almost similar to that of IFx for displacements of

block-2 along the X-axis.

5.1.5 Base Shear (VbxandVbz)

Figure 10(a) shows the base shearVbx along the global X-axis

developed on block-2 of both L and T shape arrangements.

Beyond wind incidence angle of 30o, block L1-2 of L shape

arrangement is subjected to significantly higher base shearVbx as

compared to a similar block in an isolated position, due to the

blockage of wind flow in the re-entrant corner, which causes the

higher wind loads along the global X-axis on wall-C of block

Fig. 9. Variation of Base Moment on Block-2 with Wind Incidence

Angles; (a) Base Moment-Mx, (b)Base Moment-Mz

Table 5. Base Moments Mz(MN-m) and IFMz of Block-2

Angle 0o 15o 30o 45o 60o 75o 90o

block-(L1-2) -287.16 -320.22 -348.17 -347.40 -329.99 -318.32 -283.09

block-(T1-2) -111.14 -144.50 -171.17 -190.40 -192.27 -186.04 -198.47

isolated -380.94 -365.70 -343.27 -281.03 -181.88 -54.74 -

IF(L1-2) 0.75 0.88 1.01 1.24 1.81 5.82 -

IF(T1-2) 0.29 0.40 0.50 0.68 1.06 3.40 -

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Vol. 16, No. 1 / January 2012 127

L1-2 as compared to the loads on the corresponding wall of an

isolated block and block T1-2. Up to wind incidence angle of

60o, block T1-2 of T shape arrangement is subjected to sub-

stantially lower base shearVbx as compared to block L1-2 of L

shape arrangement and similar block in an isolated position. The

base shearVbzalong the Z-axis on block-2 of both L and T

shape arrangements are slightly affected due to the presence of

block-1, as shown in Fig. 10(b).

5.2 Effects on Block-1

Interferences effects as reflected by the mean displacements,

torque, mean torque as a normalized eccentricity, base moments

and base shear developed on block-1 due to the presence of

block-2 at different wind incidence angles are summarized as

follows.

5.2.1 Displacement Along the X-axis

At 0o wind incidence angle, front wall-A of block L1-1 is

subjected to unsymmetrical wind forces, due to the blockage of

wind flow by block-2 on downstream side, whereas the front

wall of block T1-1 is subjected almost symmetrical wind forces.

The wind loads are not acting on the rear side of block-1 of both

L and T-shape arrangements due to the shielding effect of

block-2. Therefore block-1 of both L and T shape arrange-

ments is subjected to significantly lower displacement along the

X-axis as compared to a similar block in an isolated position, up

to wind incidence angle of 45o. Fig. 11 shows the mean displace-

ments at the top of section x-x of block-1 of both types of

arrangements along the global X-axis at different wind incidence

angles. At wind incidence angle of 0o to 33o, block T1-1 is sub-

jected to slightly higher displacement along the X-axis as com-

pared to block L1-1, whereas beyond 33o, block L1-1 is subject-

ed to slightly higher displacement as compared to block T1-1.

Interference Factors (IFx) for displacements along the X-axis of

block-1 are shown in Table 6.

5.2.2 Displacement Along the Z-axis

A downstream interfering building has very little effects on the

loads and responses of an upstream building for most locations.

However, for locations of close proximity, a downstream build-

ing can significantly alter the wake characteristics of the up-

stream building thus resulting in higher loads on the side faces.

Fig. 12 shows the mean displacements at the top of section z-z of

block-1 at different wind incidence angles. At wind incidence

angle of 0o, block L1-1 is subjected to significantly higher mean

Fig. 10. Variation of Base Shear on Block-2 with Wind Incidence

Angle; (a) Base Shear Vbx, (b)Base Shear Vbz

Fig. 11. Displacements at the Top of Block-1 Along the X-axis

Table 6. Displacements (mm) at the Top of Block-1 Along the X-axis and IFx

Angle 0o 15o 30o 45o 60o 75o 90o

block-(L1-1) 52.03 43.25 28.59 19.41 56.50 47.75 44.94

block-(T1-1) 56.20 47.22 34.20 2.07 54.32 43.85 42.30

isolated 81.10 71.28 68.25 51.83 19.89 7.41 -

IF (L1-1) 0.64 0.61 0.42 0.37 2.84 6.45 -

IF (T1-1) 0.69 0.66 0.50 0.04 2.73 5.92 -

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displacement along the Z-axis; which is perpendicular to the

wind direction. Whereas it is almost negligible in case of block

T1-1 and a similar block in an isolated position. Sidewall-B of

block L1-1 towards the re-entrant corner is subjected to signi-

ficant wind forces acting towards the wall surface due to the

blockage of wind flow by block L1-2, whereas the other sidewall-

F1 of block L1-1 is subjected to suction, hence wind forces on it,

are acting away from the wall. Due to this phenomenon, the

wind loads on both the sidewalls of block L1-1 are acting along

the same direction. Whereas sidewalls-B and H of block T1-1

are subjected to almost symmetrical forces acting away from the

walls and hence, net wind forces acting along the Z-axis on

sidewalls of block T1-1 are almost negligible. Therefore block

L1-1 is subjected to significantly higher displacements along the

Z-axis as compared to block T1-1 at wind incidence angle of 0o.

At skew wind incidence angle of 15o to 75o, displacement of

block-1 of both the arrangements along the Z-axis is almost range-

bound and considerably higher than the displacement of a similar

block in an isolated position. The mean wind loads/displace-

ments of block-1 of both L and T shape arrangements along

the Z-axis are increases up to 15% as compared to the maximum

mean wind loads/displacements of the corresponding block in an

isolation position. Interference factors (IFz) for displacement

along the Z-axis of block-1 at different wind incidence angles are

shown in Table 7.

5.2.3 Torsion

Figure 13(a) shows torsion moment developed at the base of

block-1 due to unevenly distributed wind pressures on the differ-

ent walls of block-1. At wind incidence angle of 0o, block L1-1 is

subjected to noticeable torsion, whereas the torsion on block T1-

1 is almost negligible due to almost symmetrical wind flow

around it. Torque on block-1 of both types of arrangements follows

a cyclic pattern. The torque cycle of block L1-1 has a phase dif-

ference of approximately 7.5o as compared to block T1-1. At wind

incidence angle of 7.5o to 40o and at 80o to 90o, block T1-1 is sub-

jected to higher torque as compared to block L1-1. Torsional mo-

ment on block-1 of both types of arrangements is substantially

reduced as compared to the corresponding block in an isolated

position for most wind orientations. The maximum mean torque

on block-1 of both L and T shape arrangements is reduced by

up to 44% and up to 32% respectively, as compared to the maxi-

mum mean torque developed on the corresponding block in an

isolated position. Fig. 13(b) shows the torque at the base of block-

1 as normalized eccentricity. The wind forces on block L1-1 and

Fig. 12. Displacements at the Top of Block-1 Along the Z-axis

Table 7. Displacements (mm) at the Top of Block-1 Along the Z-axis and IFz

Angle 0o 15o 30o 45o 60o 75o 90o

block-(L1-1) 145.53 167.10 172.50 173.20 178.23 163.66 150.10

block-(T1-1) 7.25 145.85 171.20 175.44 178.00 159.33 152.20

isolated - 15.70 75.55 112.43 136.12 155.98 163.09

IF (L1-1) - 10.64 2.28 1.54 1.31 1.05 0.92

IF (T1-1) - 9.29 2.27 1.56 1.31 1.02 0.93

Fig. 13. Variation of Torsion on Block-1 with Wind Incidence Angles;

(a) Torsion, (b) Torsion as a Normalized Eccentricity

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Vol. 16, No. 1 / January 2012 129

block T1-1 has a maximum eccentricity of 2.5% and 4% respec-

tively, whereas a similar block in an isolated position has a maxi-

mum eccentricity of 10% at wind incidence angle of 15o. The

torsion and Interference Factors (IFT) for torsion on block-1 at

different wind incidence angles are shown in Table 8.

5.2.4 Base Moment (Mxand Mz)

Figures 14(a) and 14(b) shows the base moment about global

X and Z-axes respectively on block-1 of both L and T shape

arrangements at different wind orientations. At 0o wind incidence

angle, block L1-1 of L shape arrangement is subjected to signi-

ficantly higher base moment Mx due to the higher blockage of

wind flow by block L1-2 as compared to the corresponding block

T1-1 of T shape arrangement. At wind incidence angle of 90o,

block T1-1 is subjected to slightly higher base moment Mx as

compared to block L1-1, whereas at skew wind incidence angles,

the base moment Mx on block-1 is not affected by its location

either at the center or at the edges of block-2. Block T1-1 and

L1-1 are subjected to higher base momentMx as compared to a

similar block in an isolated position at most wind incidence

angles. Table 9 shows the base moments developed on block-1

about the global X-axis and Interference Factors (IFMx) for block-

1. As expected, the IFMx for base moments are almost similar to

the IFz for displacements along the Z-axis.

The location of block-1 has almost slight influence on the

magnitude of base moment Mz on block-1 of both types of

arrangements at most wind orientations. Up to wind incidence

angle of 37.5o, block T1-1 is subjected to slightly higher base

momentMzas compared to block L1-1. Beyond wind incidence

angle of 45o, block L1-1 is subjected to slightly higher base

momentMzas compared to block T1-1.

5.2.5 Base Shear (Vbxand Vbz)

Figures 15(a) and 15(b) shows the variation of base shear

along the global X and Z-axes respectively on block-1 of both

L and T shape arrangements. The magnitude of base shearVbx

on block-1 is slightly influenced by its location either at the

edges or at the center of block-2. The wind forces are not acting

on the backside of block-1 of both L and T shape arrange-

ments due to the shielding effects of block-2. Therefore, up to

wind incidence angle of 45o, block L1-1 and block T1-1 is sub-

jected to considerably smaller base shearVbx. Whereas after wind

incidence angle of 60o, it is subjected to considerably higher base

Table 8. Torsion (MN-m) and IFT of Block-1

Angle 0o 15o 30o 45o 60o 75o 90o

block-(L1-2) 1.79 2.72 2.12 -0.57 1.15 3.45 -0.32

block-(T1-2) 0.05 4.20 2.77 0.63 0.30 2.69 2.42

isolated - 6.03 0.67 -2.67 -6.20 -3.22 -

IFT (L1-2) - 0.45 3.16 0.21 -0.19 -1.07 -

IFT (T1-2) - 0.70 4.14 -0.24 -0.05 -0.83 -

Table 9. Base Moments Mx(MN-m) and IFMx of Block-1

Angle 0o 15o 30o 45o 60o 75o 90o

block-(L1-1) -201.94 -231.36 -241.98 -242.97 -247.26 -226.67 -196.78

block-(T1-1) -0.02 -208.43 -239.18 -245.00 -247.19 -221.25 -212.71

isolated 0.00 -20.00 -105.21 -153.20 -188.98 -216.26 -222.55

IFMx (L1-1) - 11.57 2.30 1.59 1.31 1.05 0.88

IFMx (T1-1) - 10.42 2.27 1.60 1.31 1.02 0.96

Fig. 14. Variation of Base Moment on Block-1 with Wind Incidence

Angles: (a) Base Moment-Mx, (b)Base Moment-Mz

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shearVbx as compared to a similar block in an isolated position.

At 0o wind incidence angle, block L1-1 is subjected to signifi-

cantly higher base shearVbzas compared to block T1-1, whereas

at skew wind incidence angles, the base shearVbz on block-1 is

not influenced by its location either at the center or at the edges

of block-2.

6. Conclusions

Wind tunnel model tests showed that wind orientation could

induce different pressure distributions on walls of pair of rectan-

gular building models located in close proximity in a geometrical

configuration of L and T shape from those of similar isolated

models. Changing the position of block-1 from the edge of block-2

to the center of block-2 significantly affects the wind pressure

distributions and responses of both the blocks. The present study

has shown that, it is appropriate to define different interference

factors for displacements and torsion. The significant outcomes

are summarized as follows.

1. The wind pressure distribution and its magnitude on inner

walls-B and C are depends on the arrangement of building

models, wind orientation and their relative dimensions due to

the mutual interference of wind flow by both the models.

2. The presence of block-2 causes an increased in the mean dis-

placement of block-1 of both types of arrangements along the

Z-axis by as much as 15%, with respect to the maximum

mean displacement of a similar block in an isolated position.

3. The maximum mean torque on block-1 of both L and T

shape arrangements reduces up to 44%, and up to 32%

respectively, with respect to the maximum mean torque on a

similar block in an isolated position, due to the presence of

block-2.

4. Block-2 of L-shape arrangement is subjected to considerably

higher mean displacement along the X-axis as compared to

block-2 of T shape arrangement at all considered wind inci-

dence angles.

5. At wind incidence angle of 0o, presence of upstream block-1

reduces the mean alongwind wind displacements of block-2

of both L and T shape arrangements up to 25% and up to

71% respectively as compared to the displacement of a similar

block in an isolated position.

6. Block T1-2 of T shape arrangement is subjected to signifi-

cantly higher mean torque as compared to block L1-2 of L

shape arrangement at skew wind incidence angles.

7. The presence of block-1 increases the maximum mean torque

on block-2 of L and T shape arrangements up to 28%, and

up to 88% respectively, as compared to the maximum mean

torque developed on a similar building in an isolated position.

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