FIRST STAGE PROJECT REPORT

ON

“SEISMIC ANALYSIS OF RC FRAME BUILDING WITH FIRST SOFT STOREY “

Presented By

MUNDE PRAVIN

Guided By

Prof. HEMANT MAGARPATIL

MAHARASHTRA INSTITUTE OF TECHNOLOGY, PUNE

INTODUCTION

First story become soft and weak relative to upper storey. Soft Storey A soft storey is one in which the lateral stiffness is less than 70 percent of that in the storey above or less than 80 percent of average lateral stiffness of the three stories above.

Open first storey is now a days unavoidable feature for most of the multistory buildings in urban areas for vehicle parking, shops etc.

BEHAVIOUR OF SOFT STOREY

building with discontinuities is subjected to concentration of forces and deformation at point of discontinuity which may bring to failure of member at junction and collapse of building.

soft storey consist of discontinuity of strength or stiffness which occurs at the second floor column connection. In buildings with soft first storey the inter-storey drift in the soft first storey is large.

COMPLEX LATERAL LOAD TRANSFER MECHANISM

------Discontinuing of masonry infills in ground storey

EFFECT OF MASONRY INFILLS

increase the strength and stiffness of RC frame and hence to decrease lateral drift.

reduces bending moment in beams and columns.

the infill system works as a

braced frame, with the wall forming compression.

Hence, frames with infills

Behave like trusses.

Arlekar, Jain, Murty “Seismic Response of RC Frame Buildings with Soft First Storeys” 1

Highlighted the importance of recognizing the presence of the open first storey in the analysis of the building. The error involved in modeling such buildings as complete bare frames, neglecting the presence of infill in the upper stories, is brought out through the study of an example building with different analytical models.

nine different model are studied and comparative study is done, two different analysis equivalent static analysis and multimodel dyanamic analysis

Suggested some measures as increase the size of column in open storey and introduction to concrete core to reduce the stiffness irregularity and to provide adequate lateral strength

LITERATURE REVIEW

Haque S., Khan “Seismic Vulnerability of Columns of RC Framed Buildings with Soft Ground Floor” 2

Finite element models of six, nine and twelve storied buildings are subjected to earthquake load in accordance with equivalent static force method as well as response spectrum method are analysed

Equivalent static force method produces same magnitude of earthquake force regardless of the infill present in the model

discussed the behavior of the columns at ground level of multistoried buildings with soft ground floor subjected to dynamic earthquake loading.

when the same buildings are subjected to response spectrum method, significant increase in column shear and moment as well as total base shear has been observed in presence of infill. In general, a two fold increase in base shear has been observed when infill is present on upper floors with ground floor open when compared to the base shear given by equivalent static force method.

Distribution of shear force in columns due to response spectrum earthquake load for no infill condition

Distribution of shear force in columns due to response spectrum earthquake load for 50 percent infill condition

Fardis, Panagiotakos “Seismic Design And Response of Bare And Masonry Infilled Reinforced Concrete Buildings3

If the infilling is uniform in all stories, drift and structural damage are dramatically reduced, without an increase in the seismic force demand

the soft storey effects due to the absence of infill in the bottom storey are not so important for seismic motion at the design intensity, but may be very large at the higher intensities, if the ultimate strength of infill amount to a large percentage of building weight.

4 storey structure was considered with various configurations of infill without opening including soft storey

Dolsek M., Fajfar “Soft Storey Effects in Uniformly Infilled Reinforced Concrete Frames”

The analyses were performed with a modified version of DRAIN-2DX..

The bare frame structure was designed according to Euro code 8.

Uniformly distributed infill walls considerably reduce deformations and related damage.

if the leve1 of ground motion intensity exceeded, a soft storey effect occurs.

A four-storey RC frame structure, with and without infills, was selected as a test example to represent a contemporary, earthquake-resistant building.

Dolsek M., Fajfar “The Effect of Masonry Infills on the Seismic Response of a Four-Storey Reinforced Concrete Frame-A Deterministic Assessment”

studied the effect of masonry infill on the seismic response of a four-storey reinforced concrete frame with masonry infill, with and without openings was considered. A comparison has been made with the behavior of the bare frame using pushover analysis and the inelastic spectrum approach.

The infill can have a beneficial effect on the structural response, provided that they are placed regularly throughout the structure, and that they do not cause shear failures of columns.

The infill can have a beneficial effect on the structural response, provided that they are placed regularly throughout the structure, and that they do not cause shear failures of columns.

Simple modeling with equivalent diagonal struts, which carry loads only in compression, is able to simulate the global seismic response of the infilled frames, and is suitable for practical applications.

Kaushik H. B., Rai, “Effectiveness of Some Strengthening Options for Masonry Infilled RC Frames with Open First Story”

studied several strengthening schemes for masonry infilled reinforced concrete frame buildings with an open first storey for their effectiveness in improving the performance during earthquakes

pushover analyses of typical RC frames were carried out using SAP 2000.

A typical four storey RC frame was designed for the most critical load combination using the relevant Indian Standards and using the prevalent design philosophy of not including strength and stiffness of infills in design process

concluded that (a) lateral load performance especially ductility of the open first storey RC frames cannot be improved by using code specified strengthening schemes, i.e., by designing the first storey members for higher forces; and (b) the performance of such frames can be significantly improved by providing additional columns and lateral buttresses in the open first storey.

Many buildings with open ground storeys – Collapsed in 2001 Bhuj earthquake

DETAILED ANALYSIS PROCEDURES

The analysis process can be categorized on the basis of three factors

a) The type of externally applied loads LINEAR STATIC LINEAR DYNAMIC NON LINEAR STATIC NONLINEAR DYNAMIC b) Behavior of structure or structural materials and c) Type of structural model selected

The building is modeled as an equivalent single degree of freedom system with a linear elastic stiffness and an equivalent viscous damping.

The seismic input is modeled by an equivalent lateral force with the objective to produce the same stresses and strains as the earthquake it represents.

Based on an estimation of the first fundamental frequency of the building, the spectral acceleration Sa is determined from the appropriate response spectrum which, multiplied by the mass of thebuilding, m , results in the equivalent lateral force, V

LINEAR STATIC

The lateral force is then distributed over the height of the building and the corresponding internal forces and displacements are determined using linear elastic analysis

ADVANTAGES:

Cost and time effective

DISADVATAGES:

Applicability is restricted to regular buildings for 

which the first mode of vibration is predominant

LINEAR DYANAMIC ANALYSIS Response spectrum method Building is modeled as multi-degree-of-freedom system with a linear

elastic stiffness matrix and an equivalent viscous damping matrix Each modal response is then determined from the spectral analysis of

single degree of freedom sys Model response may be combined either square root of sum of squares

(SRSS) or complete quadratic combination (CQC) method to obtain the resultant response

Dynamic analysis shall be performed for the following buildings:

a) Regular buildings - Those greater than 40m in height in zone IV and V, and those greater than 90 m in height in zone II and III.

b) Irregular buildings – All framed buildings higher than 12 m in zone IV and V, and those greater than 40 m in height in zone II and III.

For irregular buildings lesser than 40 m in height in zone II and III, dynamic analysis even though not mandatory, is recommended.

ADVANTAGES:

Higher modes can be consider which make them suitable for irregular building

DISADVATAGE:

Their applicability decreases with increase in non linear behaviour

Modelling of masonry infill

infill system works as a braced frame, with the wall forming compression “struts”.

The equivalent strut shall have the same thickness and modulus of elasticity as the infill panel it represents.  Where,

ww= ½ Sqrt (column2 x beam2 )

= length of contact for column and beam Ei = modulus of elasticity of masonry

Ec = modulus of elasticity of concrete

t= thickness of wallIc = MI of columnh = height of wall

Project work Objective To study the earthquake behavior reinforced concrete

moment resisting frame building with an open first storey & unreinforced masonry infill in the upper story’s

The main objective of the study is to increase the first storey stiffness, so that the stiffness irregularity can be minimize and inter storey drift can be reduced.

To compare behavior of building with soft first storey with the bare frame building.

To give immediate measures to prevent the indiscriminate use of soft story in building, which are designed without regard to the increase displacement, ductility, and force demands.

MODEL 1 Bare frame. However, masses of infill walls are included in the model.

MODEL 2 Soft first storeyBuilding has no walls in the first storey and external walls , internal walls in the upper stories

MODEL 3 Soft first storey with walls at specific locations in first storey.Building has external walls and internal wall in the upper stories. Further, masonry infill is provided in the first storey at specific location.

MODEL 4 Soft first storey with stiffer columns.Buildings has no walls in the first storey and external walls , internal walls in the upper stories. However, the columns in the first storey are stiffer than those in the upper stories to reduce the stiffness irregularity between the first storey and the storey above

METHODOLOGY

Following models are taken for analysis using equivalent static analysis and linear dynamic analysis

compare the results of Linear Static and Linear Dynamic Analysis of models as mentioned above for following criteria,

i) Base shear

ii) Fundamental natural period

iii) Lateral displacement

iv) Storey drift

v) Axial force

 

Scope for future work The effect of brick infills on the seismic performance of these

buildings needs to be well understood and based on that, design methodologies, which exploit the benefits of infills in a rational manner, need to be developed.

1)The effect of infill wall predominately changes the behavior of the structure and it is essential to consider infill walls for seismic evaluation of the structure.

2)Arrangement of infills may affect the post yield behavior and has an influence on distribution and sequence of damage formation. To generalize this, more infill arrangements should be investigated.

3)How performance of stilts buildings vary with the height of ground storey can be studied.

4)Opening in the infill panel of upper stories, provision of shear wall in the first storey may be considered in future work.

AUTOCAD MODELS OF BUILDINGSModel I : Bare frame

 

 

 

 

X-Z plane Y_Z plane

MODEL 2 Soft story moel

MODEL 3Soft first storey with MI walls at specific locations in first storey

MODEL 4

Soft first storey with stiffer columns

REFERENCES1). Arlekar J. N., Jain S. K., Murty C. V. R., 1997, “seismic Response of RC Frame Buildings with Soft First Storeys”, Proceeding of the CBRI Golden Jubilee Conference on National Hazards in Urban Habitat, New Delhi.2). Haque S., Khan M. A., 2008, “Seismic Vulnerability of Columns of RC Framed Buildings with Soft Ground Floor”, International Journal of mathematical Models And Methods in Applied Sciences, Vol. 2, No.3, pp. 364-3713). Fardis M. N., Panagiotakos T. B., 1997, “Seismic Design And Response of Bare And Masonry Infilled Reinforced Concrete Buildings Part II: Infilled Structures”, Journal of Earthquake Engineering, Imperial College Press, Vol. 1, No. 3, pp. 475-503.4). Dolsek M., Fajfar P., 2000, “Soft Storey Effects in Uniformly Infilled Reinforced Concrete Frames”, Journal of Earthquake Engineering, Imperial College Press, Vol. 5, No. 1, pp. 1-12.5). Dolsek M., Fajfar P., 2008, “The Effect of Masonry Infills on the Seismic Response of a Four-Storey Reinforced Concrete Frame-A Deterministic Assessment”, Science Direct, Engineering Structures, Vol. 30, pp. 1991-2001.

6). Kaushik H. B., Rai D. C., Jain S. K., 2009, “Effectiveness of Some Strengthening Options for Masonry Infilled RC Frames with Open First Story” Journal of Structural Engineering, Vol. 135, No. 8, pp. 925-937.

7). Tuladhar P., Kusunoki K, “Seismic Design Of The Masonry Infill RC Frame Buildings With First Soft Storey 8). Nagae T, Suita K, Nakashima M, 2006, “Performance Assessment for Reinforced Concrete Buildings with Soft First Stories”, Annuals of Disaster Research Institute, Kyoto University, No.49C, pp. 189-1969). Lee H. S., Woo S. W., 2002 “Seismic performance of a 3-Storey RC Frame in a Low Seismicity Region” Engineering Structures, Vol. 24, pp. 719-73410) Lee H. S., Woo S. W., 2002 “Seismic performance of a 3-Storey RC Frame in a Low Seismicity Region” Engineering Structures, Vol. 24, pp. 719-734.11) Su R.K.L., 2008, “Seismic Behaviour of Buildings with Transfer Structures in Low-to- Moderate Seismicity Regions”, Electronic Journal of Structural Engineering Special Issue, Earthquake Engineering in the low and moderate seismic regions of Southeast Asia and Australia, pp. 99-109.

1) The modal mass ( kM ) of mode k is given by

n

iiki

n

iiki

k

Wg

WM

1

2

1

2

)(

][

2) The modal participation factor ( kP ) of mode k is given by

n

iiki

n

iiki

k

W

WP

1

2

1

)(

3) The peak lateral force ( ikQ ) at floor i in mode k is given by

ikikkik WPAQ

4) The peak shear force ( ikV ) acting in the storey i in mode k is given by

n

ijikik QV

1

5) The peak storey shear force ( iV ) in storey i due to all modes considered is obtained by

combining those due to each mode in accordance with clause 7.8.4.4 of code.

6) The design lateral forces, roofF and iF , at roof and at floor i is given by

roofroof VF

1 iii VVF

1) The weight of all the floors and the roof is calculated and total seismic weight of the building

is found out.

iWW

2) The approximate fundamental natural period of vibration ( aT ), in seconds, of all buildings,

including moment-resisting frame buildings with brick infill panels, is estimated by the empirical

expression:

d

hTa

09.0

3) The design horizontal seismic coefficient hA for a structure is determined by the following

expression:

g

SaX

R

IX

ZAh 2

4) The total design lateral force or design seismic base shear is determined by the following

expression.

XWAV hB

5) The design base shear computed as above is distributed along the height of building as per the

following expression.

2

2

ii

iiBi

hW

hWXVQ