A COMPARITATIVE STUDY ON NONLINEAR BEHAVIOUR OF … · the two important guidelines availa ble for pushover analysis[13]. The pushover analysis can be done in two methods, they are

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International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 5, May 2017, pp. 509–520, Article ID: IJCIET_08_05_058 Available online at http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=5 ISSN Print: 0976-6308 and ISSN Online: 0976-6316 © IAEME Publication Scopus Indexed

A COMPARITATIVE STUDY ON NONLINEAR BEHAVIOUR OF BUILDING FRAMES WITH PROVISION OF VARIOUS ARRANGEMENTS

FOR MITIGATING THE SOFT STOREY EFFECT S. Kiran

P.G. Student, Department of Civil Engineering, National Institute of Technology, Raipur, India

G.D. Ramtekkar Professor, Department of Civil Engineering,

National Institute of Technology, Raipur, India

ABSTRACT This study focuses on mitigating the soft storey effect in buildings by using

different structural arrangements such as shear walls, diagonal steel bracings and cross steel bracings. Nonlinear static procedure (NSP) also known as push over analysis is conducted in different symmetrical building models such as low rise (G+6), medium rise (G+14) and high rise (G+24).The response of the buildings is analyzed in terms of performance point, ductility displacements, sequence of hinge formation, spectral acceleration and storey drift. A comparative analysis is then done for low rise, medium rise and high rise buildings. It can be effectively concluded that the performance point which signifies the performance of the building is high for building with shear wall configuration followed by cross steel bracing configuration. It is also observed that with increase in the height of the building, the shear wall configuration has more ductility displacement and least number of hinges in the early stage. The storey drift can also be reduced by incorporation of shear wall followed by provision of cross steel bracings.

Key words: Soft storey effect, Nonlinear static procedure, Pushover analysis, Performance point, Ductility displacement, Hinges.

Cite this Article: S. Kiran and G.D. Ramtekkar, A Comparitative Study on Nonlinear Behaviour of Building Frames with Provision of Various Arrangements for Mitigating the Soft Storey Effect. International Journal of Civil Engineering and Technology, 8(5), 2017, pp. 509–520. http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=5

S. Kiran and G.D. Ramtekkar


1. INTRODUCTION Parking has become a major problem in metro cities for the past few years. One of the main solution to this problem is to provide open ground storey configuration in the building. Soft storey construction is a modern technique which is most commonly adopted method to avoid this problem. According to IS: 1893-2002 (Part-1), a soft storey is one in which the lateral stiffness is less than 70% of that in the storey above it or less than 80% of the average lateral stiffness of the three storey above it[1], [2]. Due to the soft storey effect these types of building will show higher tendency of collapse during earthquake. Infill walls which can be mentioned by masonry wall which is provided in all other floors other than the ground floor increases the stiffness of the other floors and the overall stiffness of the building.

The soft storey effect is mainly due to the increased flexibility of soft storey as compared to other storey. The infill walls may create the soft storey effect which becomes a cause of destruction. The building stiffness will be reduced due to the presence of open ground storey and due to this large bending moment and shear force will act at the bottom storey of the building[3], [4]. Here, the brick masonry which are the non-structural components are attached to the columns and beams of the upper storeys and the bottom storey is left devoid of infill walls. Soft storey term is used to indicate having a storey much less rigid than the stories above or below, is particularly susceptible to earthquake damage because of large unreinforced openings on their specific floor and in their typically RC frame structure. Without a proper design these types of structures are not able to withstand the lateral forces that push the structure such as those earthquakes generates. In these types of buildings, the major percentage if the base shear and bending moment is resisted by the beam-column joint of the ground storey and it is very essential to mitigate this soft storey effect to greater extend.Thus it is very necessary to conduct an in-depth study on the nonlinear behaviour of the structure so that it gives the proper response of the structure during earthquakes. In order to achieve this, different structural arrangement are provided in buildings[5]–[8].

Different types of nonlinear analyses are available which were not performed before due to the lack of knowledge, technology and simplicity. Pushover analysis is the one of the above-mentioned analysis. Pushover analysis is a type of nonlinear static analysis, which deals with the static analysis considering nonlinear characteristics of the material. In this analysis, it considers the post elastic behaviour which has been developed over past twenty years and has become the preferred analysis procedure for design and seismic performance evaluation purpose[9]. It is an important tool which is used for assessing the inelastic strength and deformation demand of the buildings. In this pushover analysis, the vertical load is kept constant and the lateral load is increased in a predefined distribution pattern along the whole height of the building. This increase in lateral load is done until a collapse mechanism is developed. This is an important method to observe the successive damage of the building. The main output of the pushover analysis is the force-displacement curve also known as the pushover curve. The pushover analysis is done by the procedure described in FEMA-356 and ATC-40. This pushover analysis provides detailed information that cannot be obtained from linear static or linear dynamic procedures[10].

2. METHODOLOGY The soft storey effect can be mitigated by the inclusion of different types of structural arrangements in the building. The different adopted method to overcome this is (i) Providing Infill walls (ii) Providing shear walls (iii) Providing diagonal steel bracings (iv) Providing cross steel bracings (v) Providing stiffer columns. In this present study, a hypothetical R.C.C building of different varying heights (such as low rise (G+6), medium rise (G+14), high rise

A Comparitative Study on Nonlinear Behaviour of Building Frames with Provision of Various Arrangements for Mitigating the Soft Storey Effect


(G+24)) are considered and certain structural arrangements are incorporated in them and nonlinear analysis are performed in the following five models such as,

Bare frame building

Frame with open ground storey and infill walls at the upper storeys of the building

Frame with open ground storey and infill walls at the upper storey and with shear walls at corner locations of the building

Frame with open ground storey and infill walls at the upper storey and with diagonal steel bracing at ground storey of the building

Frame with open ground storey and infill walls at the upper storey and with cross steel bracing at ground storey of the building

Nonlinear static analysis also known as pushover analysis is carried out in the above-mentioned models and the performance of the building is evaluated in terms of performance point, displacement ductility, sequence of plastic hinge formation, spectral acceleration, storey drift and is compared for low rise (G+6), medium rise (G+14) and high rise (G+24) buildings. The nonlinear hinge properties are assigned in the ETABS model. Moment hinges (M3) are assigned to the both ends of the beams, Shear hinges (V2) are assigned at mid length of the beams, axial force and biaxial moment hinges (P-M-M) are assigned to both ends of the columns, axial hinges (P) are assigned to the steel bracings. Geometric nonlinearity (P-delta) and large displacement is considered.

3. MODELLING Hypothetical buildings of varying heights such as of G+6 (low rise), G+14 (medium rise) andG+24 (high rise) is taken for the seismic analysis located in zone III. The building is symmetric and regular in plan. The base of the structure is made rigid and all the degree of freedom is restrained.

Table 1 Building Description

Sl.No. Description Specifications 1. Building Frame System SMRF 2. Ground Storey height 3.5m 3. Typical Storey height 3.5m 4. Type of soil Medium (II) 5. Support Condition Fixed 6. Grade of concrete M30 7. Grade of steel Fe415 8. Live Load 3.5 kN/m2 9. Floor Finish 1 kN/m2 10. Steel section ISMB 600 11. Infill Panel Brick Masonry 12. Importance factor 1 13. Response Reduction Factor 5 14. Column Size 650mm x 650mm 15. Beam size 400mm x 400mm 16. Slab Thickness 125mm 17. Thickness of brick wall 230mm

The modelling of masonry infill wall in the building is done by equivalent diagonal strut method as given in FEMA-273. The thickness and material properties of diagonal strut are similar to that of infill wall. According to FEMA-273, width of diagonal strut is given by,



= 0.175 ( ℎ ) . (1)

where, =

(2)

and, ℎ = Column height between center lines of beams (in.), ℎ = Height of infill panes (in.),

= Expected Modulus of Elasticity of Frame material (psi), = Expected Modulus of Elasticity of Infill material (psi),

= Moment of inertia of column (in.4), = Length of infill panel (in.), = Diagonal length of infill panel (in.), = Thickness of infill panel and equivalent strut (in.),

= Angle whose tangent is the infill height to length aspect ratio (rad), & = Coefficient used to determine the equivalent width of infill strut The generated models of Low rise buildings in ETABS, with different suggested

structural arrangements are shown below in Fig. 1 (a) - (e). The models of medium rises and high rises are also provided with the same structural configurations but they are not shown here so as to minimize the usage of space[11].

(a) (b)

(c) (d)



(e)

Figure 1 Models of Low rise building with: (a) Bare frame configuration, (b) Soft storey configuration, (c) Shear wall configuration, (d) Diagonal steel bracings, (e) Cross steel bracings

4. PUSH OVER ANALYSIS Pushover analysis is a nonlinear static procedure (NSP) which is a performance based design approach in which the performance of the building can be clearly defined[12]. Nonlinear static procedure (NSP) is the most commonly used method to predict the nonlinear behaviour of structure under earthquake shaking. Pushover analysis also provides a wide range of application option for retrofit and seismic evaluation the structure. FEMA and ATC 40 are the two important guidelines available for pushover analysis[13]. The pushover analysis can be done in two methods, they are (i) force controlled and (ii) displacement controlled methods.

In this nonlinear static procedure, the structure is subjected to constant vertical loads and gradually increasing lateral loads[14]. The analysis is carried out till the failure of the structure. This analysis method helps one to know about the weakness of the structure such giving an idea about the approximate retrofitting. Demand and capacity are the main two components of performance based design, where demand is the representation of seismic ground motion and capacity is the representation of ability of the structure to resist seismic demand[15]–[17]. The main data which we get from the pushover analysis is the pushover curve which is also known as the capacity curve which shows a plot between roof displacement and base shear. Along with that the performance of the building is evaluated in terms of, base shear capacity, spectral accelerations, displacement ductility, sequence of plastic hinge formation, storey drift and displacement.

The structural performance level of the building can be classified into (i) Operational Level (OP) (ii) Immediate Occupancy (IO) (iii) Life Safety (LS) (iv) Collapse Prevention (CP). This definition of the performance level is provided by the FEMA 356 guidelines[10].



Figure 2 Force-Deformation for pushover analysis

A, B, C, D and E are the five points which are used to define the force-deflection behaviour of the hinges. In this, the region A-B represents the elastic state, B-IO represents the immediate occupancy state, IO-LS represents between immediate occupancy and life safety state, LS-CP represents between life safety and collapse point, CP-C represents between collapse point and ultimate capacity, C-D represents between collapse point and residual strength, D-E represents residual strength and collapse and >E shows collapse[18]–[20]. In ETABS 9.7.4 these states can be identified by color bands which shows the plastic hinge formation in each stage.

Figure 3 Performance based design approach

In the performance based design approach, the inelastic seismic demands are based on the inelastic seismic capacity of the structure. In the Capacity Spectrum Method, it gives the performance point at which the building shows the maximum performance due to seismic conditions and the design of the building is based on the displacement obtained from that performance point[21]–[23].

5. RESULTS AND DISCUSSIONS The structural models are analyzed by using static nonlinear analysis procedure (pushover analysis) and the structural responses of the different models are compared accordingly in terms of performance point, ductility displacement, number of hinge formation, spectral acceleration and storey drifts in low rise (G+6), medium rise (G+14) and high rise (G+24) buildings.



(a) (b) (c)

Figure 4 Performance point comparison for (a) Low rises (b) Medium rises (c) High rises

From the pushover analysis, the pushover curve (capacity curve) is obtained which is a plot between Base shear vs Roof displacement from which the maximum base shear capacity is obtained for which the structure has to properly designed. The capacity curve thus obtained is converted into capacity spectrum and demand spectrum and then the intersection of the capacity spectrum and demand spectrum gives the performance point which signifies the performance of the building during earthquake. From figure 4 it can be clearly seen that the building with shear wall configuration shows the best performance of the structure and also cross steel bracings also plays a good role in increasing the performance of the structure in low rise, medium rise and high rise cases during seismic conditions.

(a) (b) (c)

Figure 5 Ductility Displacement comparison for (a) Low rises (b) Medium rises (c) High rises

The ability of a structure to undergo inelastic deformation beyond the initial yield deformation is termed as ductility displacement. The ductility displacement demand a given earthquake load is obtained from the pushover curve. The more the ductility displacement the more ductile is the structure. From the figure, it can be clearly seen that building with shear wall configuration has the highest ductility displacement followed by that of building with cross steel bracings have the more ductility displacement in case of low rise, medium rise and high rise buildings.



Table 2 Comparison of hinge formation in different configurations

MODEL A-B B-IO IO-LS LS-CP CP-C C-D D-E >E TOTAL LOW RISE BUILDING

Bare Frame 1256 300 207 0 0 0 0 0 1763 Soft Storey 1338 361 28 36 0 0 0 0 1763 Shear Wall 1156 467 124 16 0 0 0 0 1763 Diagonal Steel 1235 250 205 67 0 0 3 4 1764 Cross Steel 1162 307 202 89 0 0 4 0 1764

MEDIUM RISE BUILDING Bare Frame 3250 134 60 319 0 1 16 0 3780 Soft Storey 3410 155 133 80 0 2 0 0 3780 Shear Wall 2707 824 233 16 0 0 0 0 3780 Diagonal Steel 3335 192 153 93 0 0 6 2 3796 Cross Steel 3300 321 155 0 0 1 4 0 3781

HIGH RISE BUILDING Bare Frame 5780 122 62 327 0 0 9 0 6300 Soft Storey 5855 171 178 93 0 1 2 0 6300 Shear Wall 5061 746 278 213 0 2 0 0 6300 Diagonal Steel 5592 272 249 190 0 0 2 4 6309 Cross Steel 5477 452 375 0 0 1 4 0 6309

The plastic hinges may be applied to the beams, columns and bracings to study the nonlinear behaviour as they show the structural conditions at different stages. Hinges will attain a collapsible condition after passing through some intermediate stages i.e. immediate occupancy(IO) and life safety (LS) levels. From the Table 2, it can be clear that as the height of the building increases the number of hinges formed is increased.

(a) (b)



(c) (d)

(e)

Figure 6 Failure hinge pattern for: (a) Bare frame (b) Soft storey (c) Shear wall (d) Diagonal steel bracing (e) Cross steel bracing configurations, for low rise buildings

The formation of maximum number of hinges in the early stage is not good for the structure as it signifies the early reaching of collapse of the structure. From the table 2, it is clear that the number of hinge formation in shear wall configuration is less compared to all other cases, and followed by cross steel bracing configuration in case of low rise, medium rise and high rise buildings.



(a) (b) (c)

Figure 7 Spectral Acceleration comparison for (a) Low rises (b) Medium rises (c) High rises

From the Fig.7, it can be seen that spectral acceleration of the building with shear wall configuration is the highest which means that the building can be withstand that acceleration. The low-rise building configuration can withstand the more amount of spectral acceleration as compared to medium and high rise buildings. As the height of the building increases the ability of the building to withstand spectral acceleration is reducing.

(a) (b) (c)

Figure 8 Storey Drift comparison for (a) Low rises (b) Medium rises (c) High rises

From Fig. 8, it can be found that the maximum storey drift is occurring in the case of open ground storey building. As the height of the building increases, the storey drift is considerably reducing, i.e. the bottom storey is experiencing a larger drift compared to other storey of open ground storey building. It can be seen that the effect of shear wall has terribly reduced the ground storey drift. As compared to diagonal steel bracing the cross-steel bracing reduces the storey drift to a greater extent



6. CONCLUSIONS In this paper studies have been carried out to mitigate the effect of soft storey in buildings using different structural arrangements. Nonlinear static analysis (pushover analysis) is carried out in different structural arrangements and the performance of the building is explained in terms of performance point, ductility displacement, sequence of formation of plastic hinges and storey drift in low rise(G+6), medium rise (G+14) and high rise (G+24) buildings. It can be concluded that-

The nonlinear behaviour of the building can be visualized through the pushover analysis.

As the height of the building increases the performance of the building has to be properly examined during seismic conditions. In the present study, it can be seen that as the height increases, the building is more susceptible to earthquake.

The performance point which signifies the performance of the building is high for building with shear wall configuration followed by cross steel bracing configuration. As the height of the building increase the performance point of the building is increasing by 55% in case of medium rise as compared to low rise and 16% increase in high rise as compared to medium rise by the provision of shear wall.

As the height of the building increases, the ductility displacement becomes a major governing factor. In the present study, it is seen that with increase in the height of the building, there is an increase of 52% in ductility displacement of high rise with respect to medium rise building with the provision of shear walls.

Formation of maximum number of hinges in the early stage is not good for the structure as it signifies the collapsible condition of structure. As the height of the building increases, the number of hinges formed is increased and it can be seen that the building with shear wall configuration has the least number of hinges in the early stage.

As the height of the building increases, the ability of the building to withstand spectral acceleration gets reduced. The spectral acceleration of the building with shear wall configuration is the highest which means that the building can withstand that acceleration and as the height of the building increases the spectral acceleration get reduced by 45% in case of medium rise as compared to low rise building and 30% reduction in case of high rise as compared to medium rise by the provision of shear walls.

As the height of the building increases, the storey drift can be reduced to a great extent by incorporation of shear wall followed by provision of cross steel bracings.

To summarize, by conducting the nonlinear static analysis in the above-mentioned configurations, the shear wall configuration shows the best suited results which is followed by the cross-steel bracing for the good seismic performance of the building.

REFERENCES [1] X.-K. Zou and C.-M. Chan, “Optimal seismic performance-based design of reinforced

concrete buildings using nonlinear pushover analysis,” Eng. Struct., vol. 27, no. 8, pp. 1289–1302, Jul. 2005.

[2] T. Rossetto and A. Elnashai, “Derivation of vulnerability functions for European-type RC structures based on observational data,” Eng. Struct., vol. 25, no. 10, pp. 1241–1263, Aug. 2003.

[3] F. Hejazi, S. Jilani, J. Noorzaei, C. Y. Chieng, M. S. Jaafar, and a a A. Ali, “Effect of Soft Story on Structural Response of High Rise Buildings,” IOP Conf. Ser. Mater. Sci. Eng., vol. 17, no. February, p. 12034, Feb. 2011.



[4] R. B. Kargal, S. B. Patil, and N. S. Kapse, “Capacity Based Earthquake Resistant Design of multi-storey structure - A Review,” Int. J. Eng. Res., vol. 3, pp. 231–238, 2015.

[5] R. S. Shekhawat, A. Sud, and P. Dhiman, “Economical Placement of Shear Walls in a Moment Resisting Frame for Earthquake Protection,” Int. J. Res. Eng. Technol., vol. 3, no. 9, pp. 346–352, 2014.

[6] Q. Wang, L. Wang, and Q. Liu, “Effect of shear wall height on earthquake response,” vol. 23, pp. 376–384, 2001.

[7] M. Surana, Y. Singh, and D. H. Lang, Seismic Performance of Shear-Wall and Shear-Wall Core Buildings Designed for Indian Codes. New Delhi: Springer India, 2015.

[8] T. Magendra, A. Titiksh, and A. A. Qureshi, “Optimum Positioning of Shear Walls in Multistorey-Buildings,” Int. J. Trend Res. Dev., vol. 3, no. 3, pp. 666–671, 2016.

[9] Y. U. Kulkarni and P. K. Joshi, “Analysys and Design of Various Bracing System in High Rise Steel Structures,” Int. J. Adv. Res. Sci. Eng., vol. 3, no. 11, pp. 11–17, 2014.

[10] “Eurocode 8: Design of structures for earthquake resistance - Part 1 : General rules, seismic actions and rules for buildings Eurocode,” vol. 1, no. 2004. 2011.

[11] S. Kiran, G. D. Ramtekkar, and A. Titiksh, “Comparative study for mitigating the soft storey efffect in multi storey buildings using different structural arrangements,” Int. J. Civ. Eng. Technol., vol. 8, no. 3, pp. 520–531, 2017.

[12] S. Themelis, “Pushover analysis for seismic assessment and design of structures,” Heriot-Watt University, 2008.

[13] A. Bhosale, “Seismic evaluation of R/C framed building using shear failure model,” National institute of Technology, Rourkela, 2012.

[14] A. Habibullah and S. Pyle, “Practical Three Dimensional Nonlinear Static Pushover Analysis,” Structure Magazine, vol. 2, 1998.

[15] R. Rana, L. Jin, and A. Zekioglu, “Pushover analysis of a 19 story concrete shear wall building,” in 13th World Conference on Earthquake Engineering, 2004.

[16] D. N. Shinde, V. Nair Veena, and M. Pudale Yojana, “Pushover analysis of multi story building,” Int. J. Res. Eng. Technol., vol. 3, no. 3, pp. 691–693, 2014.

[17] N. M. Kashid, “Performance based seismic analysis for buildings in India.pdf.” . [18] M. N. Aydinoglu, “Incremental response spectrum analysis (IRSA) procedure for multi-

mode pushover including P-delta efects,” in 13th World Conference on Earthquake Engineering, 2004, no. 1440.

[19] F. I. Parvari Rad, “Design and characterization of curved and spherical flexure hinges for planer and spatial compliant mechanisms,” AlmaMater Studiorum- Universita di Bologna, 2014.

[20] R. Leslie, “The Pushover Analysis, explained in its Simplicity.” 2002. [21] M. J. N. Priestley, “Performance based seismic design,” in 12th World Conference on

Earthquake Engineering, 2000, vol. 1, no. 1, pp. 1–22. [22] T. J. SULLIVAN, M. J. N. PRIESTLEY, and G. M. CALVI, “Direct Displacement-Based

Design of Frame-Wall Structures,” J. Earthq. Eng., vol. 10, no. sup001, pp. 91–124, 2006.

[23] Q. Xue and C.-C. Chen, “Performance-based seismic design of structures: a direct displacement-based approach,” Eng. Struct., vol. 25, no. 14, pp. 1803–1813, Dec. 2003.

[24] S. Kiran, G.D. Ramtekkar and A. Titiksh, Comparative Study for Mitigating the Soft Storey Effect in Multi Storey Buildings Using Different Structural Arrangements. International Journal of Civil Engineering and Technology, 8(3), 2017, pp. 520–531

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A COMPARITATIVE STUDY ON NONLINEAR BEHAVIOUR OF … · the two important guidelines availa ble for pushover analysis[13]. The pushover analysis can be done in two methods, they are