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SIESMIC POUNDING EFFECT IN BUILDING

Amruta Sadanand Tapashetti1, S Vijaya2, B Shivakumara Swamy3

¹PG Student, Department of Civil Engineering, Dr. A.I.T College, Bengaluru, VTU, Karnataka, India

²Professor, Department of Civil Engineering, Dr. A.I.T College, Bengaluru, VTU, Karnataka, India

³Head of Department, Department of Civil Engineering, Dr. A.I.T College Bengaluru, VTU, Karnataka, India

Abstract—: Investigations of past and recent earthquake damage have illustrated that the building structures are

vulnerable to severe damage and/or collapse during moderate to strong ground motion. Among the possible structural

damages, seismic induced pounding has been commonly observed in several earthquakes. Seismic pounding between

adjacent buildings can cause severe damage to the structures under earthquakes, when owing to their different dynamic

characteristics. During earthquake, the buildings vibrate out of phase and at rest separation is deficient to accommodate

their relative motions. Such buildings are usually separated by expansion joint which is insufficient to provide the lateral

movements of the buildings during earthquakes. It can be prevented by providing safe separation distances, sometimes

getting of required safe separations is not possible in metropolitan areas due to high land value and limited availability

of land space. If building separations is found to be deficient to prevent pounding, then there should be some secure and

cost effective methods to prevent structural pounding between adjacent buildings. There are many buildings which are

constructed very nearly to one another in Metropolitan cities, because everyone wants to construct up to their property

line due to high cost of land. This study covers the prevention techniques of pounding between adjacent buildings due to

earthquakes. Constructing new RC walls, cross bracing system and combined RC wall & bracing, dampers, combined

system of RC wall and dampers and combined system of bracing and dampers with proper placement are proposed as

possible prevention techniques for pounding between adjacent buildings.

Keywords— Seismic Pounding; Separation gap; SAP 2000; Prevention of Structural Pounding by addition of additional

stiffeners; RC wall; Bracings; Dampers.

I. INTRODUCTION

The Seismic Pounding is defined as collision of the adjacent buildings during the earthquakes. The

principle reason for the seismic pounding is insufficient separation in between the adjacent buildings. The

phenomenon is mostly observed in the old buildings that were constructed before the advent and

popularity of earthquake resistant design principles. Although many current codes specify a minimum

seismic gap, it is still inadequate as codes necessarily lag behind the current research and fail to include the

effect of other parameters that affect the structural deformation. The gap is often seen as the waste of

prime real estate by developers and has been reduced in some newer versions. Pounding damage was

observed during the 1985 Mexico earthquake, the 1988 Sequenay earthquake in Canada, the 1992 Cairo

earthquake, the 1994 Northridge earthquake, the 1995 Kobe earthquake and 1944 Elcentro earthquake.

Pounding of adjacent buildings could have worse damage as adjacent buildings with different dynamic

characteristics which vibrate out of phase and there is insufficient separation distance (1.Abdel and

Shehata, 2006). Past seismic codes did not give definite guidelines to preclude pounding, due to economic

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considerations including maximum and usage requirements, especially in the high density populated areas

of cities, there are many buildings worldwide which are already built in contact or extremely close to

another, that could suffer pounding damage in future earthquakes. A large separation is controversial from

both technical [difficulty in using expansion joint and economical loss of land usage] views (2. A Hameed

et.al, 2012).The highly congested building system in many metropolitan cities constitutes a major concern

for seismic pounding damage. The most simplest and effective way for pounding mitigation and reducing

damage due to pounding is to provide enough separation, but it is sometimes difficult to be implemented

due to high cost of land. An alternative to the seismic separation gap provision in the structure design is to

minimize the effect of pounding through decreasing lateral motion. The main objective and scope is to

evaluate the effects of structural pounding on the global response of building structures, to determine the

minimum seismic gap between buildings and provide engineers with practical analytical tools for

predicting pounding response and damage.

A. Required Seismic Separation Distance to Avoid Pounding

Bureau of Indian Standards clearly gives in its code IS 4326 that a Separation distance is to be provided

between buildings to avoid collision during an earthquake. The code is mentions in following Table 1.

TABLE I

The design seismic coefficient to be used shall be in accordance with

IS 1893:1984

NOTE — Minimum total gap shall be 25 mm. For any other value of αh the gap width

shall be determined proportionately.

II. METHODOLOGY

To observe pounding, a three dimensional reinforced concrete moment resisting frame buildings with

open ground floor is taken and analysed in SAP2000. The two buildings consist of eight stories (G+8) and

six stories (G+6). All columns in all models are to be fixed at the base. The height of all floors is 3.2m.

Slab of eight stories and six stories is modelled as rigid diaphragm element of 150mm thickness

respectively, for all stories considered. Live load on floor is taken as 3kN/m2 and on roof is 1.5kN/m2.

Floor finish on the floor is 1kN/m2 and weathering course on roof is 1kN/m2. The seismic weight is

calculated conforming to IS 1893-2002(part-I). The unit weights of concrete is taken as 24kN/m3 The

SL

No.

Type of Constructions

Gap Width/Storey, n mm for

Design Seismic Coefficient αh

=0.12

1. Box system or frames with

shear walls

15.0

2. Moment resistant reinforced concrete frame 20.0

3. Moment resistant steel frame 30.0

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grade of concrete for column is M-25 and for beam and slab is M-20.The building is special moment

resisting frame considered to be situated in seismic zone IV having medium soil and intended for

residential use. These buildings are separated by expansion joint of 100mm. Both buildings are analysed in

SAP2000 and are designed as per IS 456-2000. Both buildings are subjected to gravity and dynamic loads.

To observe pounding, Time History Analysis is carried out taking data of Elcentro.

Building-1 (G+8) has 4 bays in X and Y directions, having corner columns dimension of 0.4x0.8 m2,

whereas all inner columns are of 0.55x1.0 m2. Width of each bay in X direction is 4m, and that of in Y

direction is 5m, while the beam size is 0.35x0.6 m2 in both the direction. Building-2 (G+6) has 4 bays in X

and Y directions, width of each bay in X direction are 3m, in Y direction it is 5m, having outer column

dimension of 0.3x0.75 m2, whereas all inner column are of 0.3x0.9 m2. Beam size is 0.3x0.45 m2 in both

the direction. Pounding is considered in top floor of G+ 6 story i.e. at sixth floor, for observation Positive

displacement of six stories and negative displacement of eight stories is considered, as we are going for

worst condition due to its different dynamic characteristics.

Fig 1: Representation of the Position of Adjacent Buildings.

III. PREVENTION MEASURES TO VOID POUNDING

A. Providing Proper Safe Separation Gap

As pounding is observed in six storey (G+6) building due to positive displacement and negative

displacement at the sixth floor level of eight story buildings. To prevent this, FEMA-273(Federal

Emergency Management Agency) provides safe separation distances between adjacent buildings.

S = √ (Q12 + Q22) is a SRSS (Square Root of the Sum of the Squares) Method

Where,

Q1 = highest displacement of building -1

Q2 = highest displacement of building – 2

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S should not be greater than the distance between adjacent buildings.

S=Q1+Q2 is Absolute Method

Where,

Q1= highest displacement of building-1

Q2= highest displacement of building-2

After analysing these two buildings in SAP2000 under Time History data of Elcentro which is to be

known as above average earthquake, the buildings were observed displacement with respect to time. For

pounding observance we are considering worst condition by taking positive displacement of G+ 6 storey

and Negative displacement of G+ 8 storey due to different dynamic characteristics. This figure 2 shows

that maximum positive displacement of G+6 storey building 80.0 mm at 1.19 sec. The figure 3 also shows

that negative positive displacement for G+8 story building is 81.82 mm at 1.23 sec. From figure it is

noticed that maximum out of phase movement of both building at 1.23 sec is (80+81.82) - 100 =

61.82mm which is greater than expansion joint, hence the separation joint between the buildings is 100

mm which is unable to accommodate this out of phase movement, and adjacent buildings will strike or

collide at this time for about 38.18mm.

Fig 2. Time vs displacement graph for (G+6) storey building without any additional stiffness for Elcentro data

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Fig 3. Time vs displacement graph for (G+8) storey building without any additional stiffness for Elcentro data

B. Introducing New RC wall to Increase the Stiffness

Since gap between the buildings cannot be increased to accommodate the relative movement of both the

buildings, we can reduce the relative displacement by providing additional stiffness i.e. by bracings, shear

wall and by combined action, to accommodate out of phase movement under provided gap. Shear walls are

provided to reduce the lateral displacements in the buildings, here we are replacing masonry wall with RC

wall.

Initially beams are designed for (0.23x20x2.75=12.65 KN/m) and now it is replaced by 0.18m thickness

of RC wall, hence the load on beam will be the same (0.18x24x2.75=11.8 KN/m). RC wall needs to be

dowelled with the adjacent beams and columns to transfer lateral force safely to the ground.

Here in this both the buildings, New RC walls are introduced in the mid panel of buildings to reduce

lateral displacement of the buildings (11. Murthy, C.V.R, 2005). In G+ 8storey building, wall is of 4 m in

X direction and in G+ 6 storey building, wall is of 3.0m length in X direction. As we are interested to

reduce the the lateral displacement in X direction were pounding can occur. Time History Analysis is done

by taking Elcentro Earthquake data, to which time vs displacement graph plotted, to observe the

displacements of both the buildings at sixth floor level.

Fig 4. Shows the dowelling of New RC walls through existing structure.

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Fig 5. Time vs displacement graph for both buildings with additional stiffness as RC wall

From figure-5, it is noted that Maximum Positive displacement of six storey building is 38.23 mm at

2.45 sec and Maximum Negative displacement of eight storey is 38.39 mm at 2.7 sec. Maximum out of

phase movement is 38.23+38.39=76.62 mm, it is less than expansion joint hence, no chance of pounding at

any interval of time.

C. Provide Cross Bracings to Increase the Stiffness

Cross Bracings are provided at the end panels of the both buildings to reduce the relative displacements.

The connection of steel cross braces with concrete frame structure requires a very special consideration

and the strong connection should be there to transfer the load from concrete frame to cross braces safely.

Fig 6. ‘I’Section for the steel bracings

In the below figure-7, shows time vs displacement graph at 6th floor level, in this Maximum Positive

displacement of G+ 6 story is 41.42 mm at 2.45 sec and Maximum Negative displacement of G+ 8 story at

sixth floor level is 54.58 mm at 2.3 sec. It shows that Maximum out of phase movement is (54.58+41.42)

mm=96 mm which is lesser than the expansion joint i.e. 100 mm, hence No chance of pounding at any

interval, when these kinds of additional stiffness is provided.

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Fig 7. Time vs displacement graph for both buildings with additional stiffness as steel bracings for Elcentro data

Fig 8 . Shows the cross bracing of Eight story and Five story buildings

D. Increasing the Stiffness of Building Using Combined System of RC wall and Steel Bracings

RC walls are provided at extrior mid panels and inner panels are placed by cross bracings in x direction

for both the buildings, in order to prevent pounding between adjacent buildings.

From figure-9, it can be observed that Maximum Positive displacement of six (G+6) storey is 54.0 mm

at 5.05 sec and Maximum negative displacement of G+ 8 storey at sixth floor level is 40.81 mm at 2.45 sec.

The absolute sum of both is (54.0+40.81) mm= 94.81mm. Hence it is also less than Expansion joint which

is 100 mm, no chance of pounding at any interval of time.

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Fig 9. Shows time vs displacement graph for both buildings at 6th floor level when both RC wall and cross bracings provided

Fig 10. Shows the both RC wall are at exterior mid panel and cross bracing are in interior mid panels.

E. Provide Dampers to Increase the Stiffness

To improve seismic response FVD is provided as a diagonal brace. Analytical and experimental studies

have shown that FVD has an advantage over other dampers as it substantially increases the

damping within the structure. It also provides additional stiffness and strength and energy

dissipation capacity during strong winds and moderate earthquakes. In mathematical modeling

of FVD, energy dissipation capacity depends upon its damping coefficient & non-linearity is defined by

the damping exponent.

Damping Coefficient= ξ x 2√Stiffness x Mass

Where ‘ξ’ is Damping ratio i.e. damping ratio of 5% is considered.

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Fig 11. Pushover curve for (G+8) storey building in X direction Fig 12. Pushover curve for (G+6) storey building in X direction

From figures 11 & 12 , it can be seen that up to a base shear of 2030KN and displacement of around 27.0

mm for (G+8) storey building and base shear of 1320KN and displacement of around 28.2 mm it is in

Elastic range, after that building is in post elastic range,. Here we can get the initial stiffness of the

buildings which is required to get Damping Coefficient of a fluid viscous damper.

TABLE II

Showing initial stiffness, weight and damping coefficient obtained from pushover curve

NO OF

STORIES

WEIGHT (KN) STIFFNESS

(KN/M)

DAMPING COEFFICIENT

(KN-SEC/M)

(G+6) 18112 46808.51 929.63

(G+8) 30145 75185.18 1519.99

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Fig 13. Shows the dampers in Six storey and Eight storey at the exterior panels

Fig 14. Time vs displacement graph for both buildings with additional stiffness as Dampers

Above figure-14 shows time vs displacement graph at 6th

floor level, in this Maximum Negative

displacement of (G+8) stories at sixth floor is 26.13 mm, 2.65sec and Maximum Positive displacement of

(G+6) stories is 25.78 mm, 2.9 sec. It shows that Maximum out of phase movement is (26.13+25.78) mm=

51.91 mm which is lesser than the expansion joint i.e. 100 mm, hence No chance of pounding at any

interval, when these kinds of additional stiffness is provided.

F. Providing Combined System of RC Wall and Dampers

In this both the structures, New RC walls are introduced in the mid exterior panel and dampers are

provided in inner mid panels of structures diagnally to reduce lateral displacement of the buildings. In

(G+8) storey building, wall is of 4.0 m in X direction and in (G+6) storey building, wall is of 3.0 m length

in X direction.

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Fig 15. Combined system of RC wall and Damper

Fig 16. Time vs displacement graph for both buildings with additional stiffness as combined system of RC wall and Dampers

Above figure-16 shows time vs displacement graph at 6th

floor level, in this Maximum Negative

displacement of (G+8) stories at sixth floor is 38.63 mm, 4.95 sec and Maximum Positive displacement of

(G+6) stories is 32.58 mm, 2.9 sec. It shows that Maximum out of phase movement is (38.63+32.58) mm=

71.21 mm which is lesser than the expansion joint i.e. 100 mm, hence No chance of pounding at any

interval, when these kinds of additional stiffness is provided.

G. Providing Combined System of Steel Cross Bracing and Dampers

In this both the structures, Dampers are introduced in exterior panels for bottom four floors, in

exterior mid panel of bottom four floors steel bracings are provided. Steel bracings are provided for rest

floors at all exterior panels in X- direction to reduce lateral displacement of the buildings. In (G+8) storey

building, wall is of 4.0 m in X direction and in (G+6) storey building, wall is of 3.0 m length in x

direction.

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Fig 17. Combined system of Steel Cross Bracings and Damper

Fig 18. Time vs displacement graph for both buildings with additional stiffness as combined system of Bracings and Dampers

Above figure-18 shows time vs displacement graph at 6th

floor level, in this Maximum Negative

displacement of (G+8) stories at sixth floor is 49.41 mm, 4.95 sec and Maximum Positive displacement of

(G+6) stories is 38.61 mm, 2.45 sec. It shows that Maximum out of phase movement is (49.41+38.61)

mm= 88.02 mm which is lesser than the expansion joint i.e. 100 mm, hence No chance of pounding at any

interval, when these kinds of additional stiffness is provided.

IV. CONCLUSION

The overall conclusions for preventing seismic pounding in adjacent buildings are:

� At the time of design, Design Engineer have to ensure that there will be no pounding between

adjacent buildings.

� Necessary safe separation gap should be provided according to FEMA 273-1997.

� Absolute method should be used to leave set back between the buildings.

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� If buildings are old and are not in a stage to provide safe separation gap, then prevention measure

should be taken.

� Stiffness of the buildings increased by providing RC wall, Steel Bracings, combined system and

Dampers.

� All the prevention methods that are used in this study proved effective to prevent pounding

between adjacent buildings.

� All the additional stiffness’s should be fixed rigidly for the better performance.

� It is better to leave set back/safe separation gap according to FEMA 273-1997, when the buildings

are in early stage of design.

AKNOWLEDGEMENT

I would like to Thanks Almighty and especially to my father Mr. SADANAND TAPASHETTI , my

mother M/s BHARATI SADANAND TAPASHETTI and my respected guide Dr. S VIJAYA for

their continuous Guidance, Inspiration and Support, which are the main factors behind any work. I take

the pleasure of thanking all those who have helped, supported and gave constant encouragement

throughout my work without whom, this work would not have been completed in time.

REFERANCES

[1] Abdel R and E.Shehata “Seismic Pounding between Adjacent Building Structures” Electronic Journal of Structural

Engineering 66-74 (2006).

[2] A. Hameed, M. Saleem, A.U. Qazi, S. Saeed and M. A. Bashir “Mitigation of seismic pounding between adjacent buildings”

Pakistan journal of science vol.64, December 2012.

[3] Sudhir K Jain et.al, “A field report on structural and geotechnical damages sustained during the 26 January 2001 M7.9 Bhuj

Earthquake in Western India”

[4] IS 456:2000 “Indian Standard Plain and Reinforced Concrete Code of Practice”.

[5] FEMA±273 [1997] “NEHRP Guidelines for the seismic rehabilitation of buildings, Report No.FEMA±273,” Federal

Emergency Management Agency, October.

[6] IS 1893 (Part 1):2002 Indian Standard “Criteria for Earthquake Resistant Design of Structures” Part 1 General Provision

and Buildings, (Fifth Revision).

[7] ATC 40 [1996] “Seismic evaluation and retrofit of concrete buildings, Vol No.1 and“Applied Technology Council, Seismic

Safety Commission, State of California.

[8] Robert Jankowski” Non-linear Modeling of Earthquake Induced Pounding of buildings” published on ICTAM, 15-21

August 2004, Warsaw, Poland.

[9] Fabian R. Rojas, James C. Anderson and M.ASCE “Pounding of an 18-Story Building during recorded earthquakes”

American Society of Civil Engineers, 2012.

[10] Murty, C.V.R “Earthquake Tips Learning and Earthquake Design and Construction”. National Information Center of

Earthquake Engineering, IIT Kanpur, India, September, 2005.

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