Seismic Vulnerability of Multistoried Buildings With Ground Soft Story and With Infills

Seismic Vulnerability of Multistoried Buildings With Ground Soft Story and With Infills

T. Sandeep1 S. V. Narsi Reddy2

Assistant Professor, Malla Reddy Engineering College (Autonomous), Secunderabad

Abstract Multistory buildings with open (soft story) ground floor are inherently vulnerable to collapse due to seismic loads, their constructions is still widespread in develop nations. Social and functional need to provide car parking space at ground level far out weights the warning against such buildings from engineering community. In this study, 3D analytical model of multistory buildings have been generating for different buildings models and analyzing using structural analysis tool ETABS. To study the effect of ground soft, infill, and models with ground soft during earthquake, seismic analysis both linear static, linear dynamic (response spectrum method) as well as nonlinear static (pushover) procedure have to be perform. The analytical model of building includes all important components that influence the mass, strength, stiffness of the structure. The deflections at each story have to be compare by performing equivalent static, response spectrum method as well as pushover have also be perform to determine capacity, demand and performance level of the considering models. Numerical results for the following seismic demands considering the inelastic behavior of the building, ductility coefficients of structures

Keywords Soft Story,Ductlity,Outweights,Stifness, linear static, linear dynamic (response spectrum method), nonlinear static(pushover)

1. INTRODUCTION

   The capacity of structural members to undergo inelastic deformations governs the structural behavior and damageability of multi-storey buildings during earthquake ground motions. From this point of view, the evaluation and design of buildings should be based on the inelastic deformations demanded by earthquakes, besides the stresses induced by the equivalent static forces as specified in several seismic regulations and codes. Although, the current practice for earthquake-resistant design is mainly governed by the principles of force-based seismic design, there have been significant attempts to incorporate the concepts of deformation-based seismic design and evaluation into the earthquake engineering practice. In general, the study of the inelastic seismic responses of buildings is not only useful to improve the guidelines and code provisions for minimizing the potential damage of buildings, but also important to provide economical design by making use of the reserved strength of the building as it experiences inelastic deformations. Pushover methods are

   becoming practical tools of analysis and evaluation of buildings considering the performance-based seismic philosophy. Pushover curve represents the lateral capacity of the building by plotting the nonlinear relation between the base shear and roof displacement of the building. The intersection of this pushover curve with the seismic demand curve determined by the design response spectrum represents the deformation state at which the performance of the building is evaluated.

2. NECESSITY OF THE STUDY

1. To study the effect of infill walls and without infill walls on structure.

2. To study of natural frequency of the structure.

3. To study the performance level of the structure

   3. DIFFERENT METHODS OF SEISMIC EVALUATION STUDIES

   1. LINEAR STATIC ANALYSIS

      In linear static procedures the building is modeled as an equivalent single-degree of freedom (SDOF) system with a linear static 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 estimate of the first fundamental frequency of the building using empirical relationships or Rayleighs method

   2. LINEAR DYNAMIC ANALYSIS

      In a linear dynamic procedure the building is modeled as a multi-degree-of-freedom (MDOF) system with a linear elastic stiffness matrix and an equivalent viscous damping matrix. The seismic input is modeled using either modal spectral analysis or time history analysis. Modal spectral analysis assumes that the dynamic response of a building can be found by considering the independent response of each natural mode of vibration using linear elastic response spectra. Only the modes contributing considerably to the response need to be considered. The modal responses are compared using schemes such as the square-root-sum-of-squares (SRSS). Time-history analysis involves a time step- by-step evaluation of building response, using recorded or synthetic earthquake records

      as a base motion input. In both cases the corresponding internal forces and displacements are determined using again linear elastic analyses.

   3. NONLINEAR STATIC ANALYSIS

Pushover Analysis is a nonlinear static method of analysis. This analysis technique, also known as sequential yield analysis or simply Pushover analysis has gained significant popularity during past few years. It is one of the three analysis techniques recommended by FEMA 273/274 and a main component of Capacity Spectrum Analysis method (ATC-40).

Pushover analysis provides information on many response characteristics that cannot be obtained from an elastic static or elastic dynamic analysis. These are [30];

1. ANALYSIS OF MULTISTORIED BUILDINGS WITH GROUND SOFT STORY AND WITH INFILLS

2. DESCRIPTION OF THE SAMPLE BUILDING

   The plan layout for all the building models are shown in figures

   SYMMETRIC BUILDING MODELS:

   Model 1: Twelve stoteyed Building with full infill masonry wall (230 mm thick) in all storeys.

   Model 2: Twelve storeyed Building (ground soft story) no walls in the first storey and full brick infill masonry walls (230 mm thick) in the upper storeys.

   • Estimates of inter story drifts and its distribution along the height.

   • Determination of force demands on brittle members, such as axial force demands on columns, moment demands on beam-column connections.

   • Determination of deformation demands for ductile members.

   • Identification of location of weak points in the structure (or potential failure modes).

   • Consequences of strength deterioration of individual members on the behavior of structural system.

   • Identification of strength discontinuities in plan or elevation that will lead to changes in dynamic characteristics in the inelastic range.

   • Verification of the completeness and adequacy of load path.

1. NON-LINEAR DYNAMIC ANALYSIS

   In nonlinear dynamic procedure the building model is similar to the one used in non-linear static procedures incorporating directly the inelastic material response using in general finite elements. The main difference is that seismic input is modeled using a time history analysis, which involves time-step-by-time-step evaluation of the building response.

2. ADVANTAGES OF INELASTIC PROCEDURE OVER ELASTIC PROCEDURES.

Although an elastic analysis gives a good understanding of the elastic capacity of structures and indicates where first yielding will occur, it cannot predict failure mechanisms and account for redistribution of forces during progressive yielding. Inelastic analyses procedures help demonstrate how buildings really work by identifying modes of failure and the potential for progressive collapse. he use of inelastic procedures for design and evaluation is an attempt to help engineers better understands how structures will behave when subjected to major earthquakes, where it is assumed that the elastic capacity of the structure will be exceeded. This resolves some of the uncertainties associated with code and elastic procedures.

Figure:4.1 Plan Layout

Fig:4.2 Elevation of twelve storeyed Building Model 1 (full infill)

Fig:4.3 Elevation of twelve storeyed Building Model 2 (ground soft)

1. DESIGN DATA: Material properties:

   IS: 1893-2002 Response Spectrum Method: Spectrum is applied from fig.2 of the code corresponding to medium soil sites. The spectrum is applied in the longitudinal and transverse directions.

2. Manual Calculation

Natural periods and average response acceleration coefficients:

For twelve-storeyed frame building:

Fundamental Natural period, longitudinal and transverse direction, Ta=0.075*360.75=1.102sec

For medium soil sites, Sa/g = 1.36/T=1.36/1.102=1.234

For twelve-storeyed brick infills buildings: Fundamental natural period longitudinal direction,

0.09×36 0.66

Ta= 25

sec

For medium soil sites, Sa/g = 1.36/0.66=2.060 Fundamental Natural period, transverse direction,

0.09×32 0.643

Ta= 20

sec

4.2 Design Data:

For medium soil sites, Sa/g = 1.36/0.643=2.11 Design horizontal seismic coefficient,

Material Properties:

Ah

Z x I

2 R

x Sa g

Ah= (0.36/2) x (1.5/5) x 2.060 =0.11124 in

longitudinal direction.

Ah= (0.36/2) x (1.5/5) x 2.11 =0.1139 in transverse direction

Table 4: Deign Seismic Based Shear for twelve storeyed buildings

Table 4.1: Distribution of Lateral Seismic Shear force for twelve storeyed building for Model 1

Table 4.2: Distribution of Lateral Seismic Shear force for twelve storeyed building for Model 2

Figure 4.4: Shear diagram for twelve storeyed Model 1 along longitudinal and transverse direction

Figure 4.5: Shear diagram for twelve storeyed Model 2 along longitudinal and transverse direction

5. RESULTS AND DISCUSSIONS

Equivalent Static Method:

As compared to Model 1, Model 2 has 3.68% of less displacement than Model 1, in longitudinal direction and 3.49% less in transverse direction.

Response Spectrum Method:

As compared to Model 1, Model 2 has 7.33% of less displacement than Model 1, in longitudinal direction and 5.42% less in transverse direction.

Pushover Analysis:

In Pushover Analysis different building Models have pushed to its failure and correspondingly displacement is noted.

From the displacement table 5.1 to 5.2 and graphs 5.1-5.6.As compared to Model 1, Model 2 have 62.033% of more displacement than Model 1, in longitudinal direction and 15.59% more in transverse direction.

TABLE 5.1 DISPLACEMENTS OF 12 STOREY INFILL STRUCTURE IN MM.

B

C

12

10

X Axis Title

8

6

4

2

0

0 2 4 6 8 10 12 14 16 18

Y Axis Title

Fig 5.2 displacement of linear static analysis of 12th storey buildings in y direction.

B

C

12

10

TABLE 5.2 DISPLACEMENTS OF 12 GROUND SOFT STOREY STRUCTURE IN MM.

B C

8

X Axis Title

6

4

2

0

0 2 4 6 8 10 12

Y Axis Title

12 Fig 5.3 displacement of linear dynamic analysis of 12th storey

buildings in x direction.

10

B

C

X Axis Title

8

6 12

4 10

X Axis Title

2 8

6

0

0 2 4 6 8 10 12 14 16

Y Axis Title 4

2

Fig 5.1 displacement of linear static analysis of 12th storey buildings in x direction.

0

0 2 4 6 8 10 12 14

Y Axis Title

Fig 5.4 displacement of linear dynamic analysis of 12th storey buildings in y direction.

B

7. REFERENCES

C

12

10

X Axis Title

8

6

4

2

0

0 10 20 30 40 50 60 70 80

Y Axis Title

Fig 5.5 displacement of linear non static analysis of 12th storey buildings in x directio.

B

C

12

10

X Axis Title

8

6

4

2

1. Arlekar, N.J., Jain K.S., and Murthy, C.V.R. Seismic Response of RC Frame Buildings with Soft First Storeys, Proceedings of the CBRI Golden Jubilee Conference on Natural Hazards in Urban Habitat, New Delhi, 1997.

2. Krawinkler, H., and Seneviratna, G.D.P.K. (1998): Pros & Cons of Pushover Analysis of Seismic Performance Evaluation.

3. Ashraf Habibullah, Stephen Pyle, Practical three-dimensional non-linear static pushover analysis Structure Magazsanine, Winter, 1998.

4. MOHAMED NOUR EL-DIN ABD-ALLA Application of recent techniques of pushover for evaluating seismic performance of multistory buildings, Faculty of Engineering, Cairo University Giza, Egypt September 2007.

5. Kasm Armagan KORKMAZ, Fuat DEM_R and Mustafa S_VR_ Earthquake assessment of R/C structures with masonry infill walls Suleyman Demirel University, Civil Engineering Department, Cunur, Isparta, TURKIYE armagan@mmf.sdu.edu.tr. (Received: 06.08.2007; Accepted: 03.10.2007

6. IS: 1893 (Part-I) 2002 (2002): Criteria for Earthquake Resistant Design of Structures, Part-I General Provisions and Buildings, Fifth Revision, Bureau of Indian Standards, New Delhi

7. Kanitkar, R., and Kanitkar, V., Seismic Performance of Conventional Multi-storey Buildings with Open Ground Storey for Vehicular Parking, Indian Concrete Journal, February 2004.

About The Authors:

0

0 5 10 15 20 25 30 35 40 45 50 55

Y Axis Title

Fig 5.6 displacement of linear non static analysis of 12th storey buildings in y direction.

6. SUMMARY AND CONCLUSION

The present work attempts to study the seismic response and performance level of different RC buildings located in seismic zone-V. In this study all important components of the building that influence the mass, strength, stiffness and deformability of the structure are included in the analytical model. To study the effect of infill and soft storey building models. The deflections at different storey levels and storey drifts are compared by performing response spectrum method as well as pushover method of analysis

It is essential to consider the effect of masonry infill for the seismic evaluation of movement resisting RC frames especially for the prediction of its ultimate state. Infills increase the lateral resistance and initial stiffness of the frames they appear to have a significant effect on the reduction of the global lateral displacement.

Infills having no irregularity in elevation having beneficial effects on buildings. In infilled frames with irregularities, such as ground soft storey, damage was found to concentrate in the level where the discontinuity occurs.