JECETV3I2C_4

E-ISSN: 2278179X

JECET; March 2014 May 2014; Vol.3.No.2, 1035-1047.

Journal of Environmental Science, Computer Science and Engineering & Technology An International Peer Review E-3 Journal of Sciences and Technology

Available online atwww.jecet.org

Section C: Engineering & Tecnology

Research Article

JECET; March 2014 May 2014; Sec. C, Vol.3.No.2, 1035-1047. 1035

Seismic performance assessment of infill reinforced Concrete frame buildings with soft storey

M.Mouzzoun* ,O.Moustachi, A.Taleb *Mohammadia School of engineers, Rabat, Morocco,

Received: 3 March 2014; Revised: 11April 2014; Accepted: 16 April 2014

Abstract: Recent earthquakes that have occurred in the world, Northridge 1994, Kobe 1995, Izmit1999 and Alhoceima 2004 have shown the seismic vulnerability of multi- storey reinforced concrete buildings with soft storey. In Morocco a large number of buildings with soft storey have been built in recent years. In the soft storey, the inter-storey drift and seismic demands of the columns are excessive that causes heavy damage or collapse of the buildings during a severe earthquake. Moroccan seismic design code for buildings RPS2000 do not contain enough criteria to predict the real behaviour of such buildings, the analysis is conducted without regard to the effect of soft storey on seismic response. This paper focuses on evaluating the seismic behaviour of infill reinforced concrete buildings with soft storey. Four infill reinforced concrete buildings were analyzed. Equivalent diagonal strut were provided, as suggested in FEMA308 [2] to capture the global effects of the infill. Non linear pushover analysis has been used to evaluate to seismic response. Results show a general changing pattern in lateral drift irrespective to building height and maximum inter-storey drift was obtained where the soft storey was located.

Keywords: Earthquakes, infill, soft storey, analysis, pushover, reinforced concrete, strut model, drift, seismic

INTRODUCTION

Social and functional needs for vehicle parking, shops, reception etc, are compelling to provide a soft storey in multi- storey reinforced concrete buildings. In buildings with soft first storey, the upper stories being stiff, under go smaller inter-storey drifts. However, in the soft storey, the inter-storey drift and the strength demands on the columns are excessive that causes heavy damage or collapse of

Seismic... Mouzzoun et al.


the buildings during a severe earthquake. According to (UBC-1997, IBC-2003 and ASCE-2002) a soft story is the one in which the lateral stiffness is less than 70% of that in the story above or less than 80% of the average stiffness of the three stores above. Soft story creates a major weak point in an earthquake, and since soft stories are classically associated with retail spaces and parking garages, they are often on the lower stories of a building, which means that when they collapse, they can take the whole building down with them, causing serious structural damage or collapse of the building. The common practice of building design considers infill as non-structural elements and building is designed as framed structures without regard to structural action of masonry infill walls. The soft storey effect and presence of infill in any building changes the behavior of frame action due to the relative changes of stiffness and lateral load distribution mechanism and thus may induce changes in phenomenon like lateral displacement and inter-storey drift ratio. Engineers believe that analysis without considering infill stiffness leads to a conservative design. But this may not be always true, especially for vertically irregular buildings with discontinuous infill walls. Hence, the modelling of infill walls in the seismic analysis of framed buildings is imperative.

2. MODELING PARAMETERS

2.1. Nonlinear behaviour: The buildings were idealized and analysed using a two-dimensional finite element model consisting of a series of frame elements using SAP2000 computer code. The column and beam elements are modulated as elastic elements with pairs of plastic zones at each end. In these zones, material nonlinearity was introduced in the model using Takeda-type plastic hinges (or nonlinear rotational springs) with a moment-rotation relationship (Figure 2). This relationship also included stiffness degradation over successive cycles of the hysteresis (of particular use for studying the effects of strong motion duration).

2.2. Modelling of masonry infill: In the case of an infill wall located in a lateral load resisting frame the stiffness and strength contribution of the infill are considered by modelling the infill as an equivalent compression strut. Because of its simplicity, several investigators have recommended the equivalent strut concept. In the present study, a trussed frame model is considered. This type of model does not neglect the bending moment in beams and columns. Rigid joints connect the beams and columns, but pin joints at the beam-to column junctions connect the equivalent struts. Infill parameters (effective width, elastic modulus and strength) are calculated using the method recommended by FEMA308 [2]. The length of the strut is given by the diagonal distance dof the

Figure 1: Collapse of ground storey due to reduction of infills



panel (Figure 3) and its thickness is given by the thickness of the infill wall. The initial elastic modulus of the strut Eiis equated to Emthe elastic modulus of masonry. Emis given as 750fm, where fmis the compressive strength of masonry in MPa. The width W of the strut is given by:

t is the thickness of infill, d is the length of the strut,

cc IE is the bending stiffness of the columns,

mH is the height of the frame, mL is the width of the frame, is the angle between the diagonal and horizontal of the infill (figure 3).

The initial rigidity of infill is: 2220

cosmm

m

LHWtEK

The initial rigidity of the frame without infill is:

Figure 3: Equivalent diagonal strut model

m

m

LHarct04)(175.0 cHdW 4 4

2sinmcc

m

HIEtE

b

cccf IH

ILwithH

IEK )21(

123

Figure 2:Building model, potential plastic hinge locations and moment-rotation relationship



3. STRUCTURAL DESCRIPTION

The structures considered for numerical analysis in the present study are located in Morocco seismic zone 3 with medium soil conditions, site S2. The design peak ground acceleration (PGA) of this zone is specified as 0.16g. These frames are designed as per prevailing practice in Morocco, ignoring the soft-storey effect. The characteristic strength of concrete and steel were taken as 25MPa and 500MPa.The dead load is taken as 2.4 kN/m2 and live load as 1.5kN/m2. The compressive strength of masonry infill is taken as 4MPa, the thickness is 15cm. The buildings are deliberately kept symmetrically in both orthogonal directions in plan to avoid torsional response under pure lateral forces and hence a single plane frame may be considered to be representative of the building along one direction. The design of the RC elements is carried out according to the Moroccan seismic code RPS2000 [1].The pushover analysis is conducted by SAP2000 software analysis. The superstructure was modelled as a spatial frame, considered fixed at the base of the ground floor. The reinforced concrete floor has substantial stiffness and resistance to take over the stresses produced by the lateral forces, and due to the regularity and homogeneity of the structure, it can be considered non-deformable in its plan. The beam and column elements are modelled as nonlinear frame elements with lumped plasticity by defining plastic hinges at both ends of beams and columns. For seismic performance assessment of the buildings, according to FEMA356 [5] three discrete damage levels are considered, immediate occupation (IO), life safety (LS) and collapse prevention (CP) (table 1).

Table-1:.Damage limits associated with various structural

performance levels

Figure 4: Response spectra for different sites according to RPS2000

0

0,5

1

1,5

2

2,5

3

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2

Period (s)

Ampl

ifica

tion

Fact

or

Site 1 Site 2 Site 3



4. RESULTS

Structure.S1 Structure. S2

Structure.S3 Structure. S4 Figure 5: Details of building frames considered in the study

3@3m

6@3m

3@3m

6@3m

3@3m

6@3m

3@3m 6@

3m



Fundamental period T=0.25s



Figure 6: Mode shape of deformation and fundamental periods




Figure 7: displacement versus storey. IO state

Figure 11: Inter-storey drifts at LS performance level

Figure 8: displacement versus storey. LS state

Figure 9: displacement versus storey. CP state Figure 10: Inter-storey drifts at IO performance level

Figure 12: Inter-storey drifts at CP performance level

Stor

ey d

ispla

cem

ent (

cm)

Storey number

Stor

ey d

ispla

cem

ent (

cm)

Storey number

Stor

ey d

ispl

acem

ent (

cm)

Storey number

Inte

r-sto

rey

drift

(%)

Storeynumber

Inte

r-sto

rey

drift

(%)

Storeynumber

Inte

r-sto

rey

drift

(%)

Storeynumber



Roof drift=16cm Roof drift= 18.03cm Roof drift=22cm Figure 13: Roof drift at various damage states. Structure S1

Figure 14.Roof displacementvs seismic intensity. B2 Roof drift=6.16cm Roof drift= 10.74cm Roof drift=14.03cm Figure 14.Roof drift at various damage states. Structure S2

Roof drift=7.19cm Roof drift=11.19cm Roof drift= 17.83cm Figure 15: Roof drift at various damage states. Structure S3

Roof drift=6.16cm Roof drift= 10.74cm Roof drift=14.03cm Figure 14: Roof drift at various damage states. Structure S2



Roof drift=6.58cm Roof drift=9.62cm Roof drift= 10.76cm Figure 16: Roof drift at various damage states. Structure S4

Figure 17: Axial forces. Structure S1 versus structure S4

Figure 18: Shear forces. Structure S1 versus structure S4



Figure 19: Bending moments. Structure S1 versus structure S4

Figure 20:Fill diagrams of axial force,shear force and bending moment in the fully infill frame axial forces shear forces bending moments

Figure 21: Fill diagrams of axial force, shear force and bending moment in the frame with soft ground storey axial forces shear forces bending moments



02468

1012141618

Bas

e sh

ear (

KN

)

column1 column2 column3 column4

0

10

20

30

40

50

60

Bas

e sh

ear (

KN

)


0

5

10

15

20

25

30

35

Bend

ing

mom

ent (

KN,m

)


6264666870727476788082

Bend

ing

mom

ent (

KN,m

)


0

100

200

300

400

500

600

axia

l for

ce (K

N)


0

100

200

300

400

500

600

700

axia

l for

ce (K

N)


Figure 22: Seismic demands of the columns at first storey in the fully infill and soft ground frames

Fully infill frame Soft ground frame



Table -2: Seismic demand plastic rotations of columns in the ground storey



5. CONCLUSION

In this study, numerical investigations were performed to assess the seismic behaviour of infilled reinforced concrete frame buildings with soft storey. The main conclusions that can be drawn from this study are:

Figures 7, 8 and 9, show a general changing pattern in lateral drift irrespective to building height. For structure S4, the first storey displacement is 2.11, 2.52 and 249 times higher than that for structure S1 at performance levels IO, LS, CP respectively. For structure S2, the displacement at second level is 1.78, 1.53 and 1.13 times higher than that for structure S1 at performance levels IO, LS, CP respectively. For structure S3, the displacement at third level is 1.42, 1.13 and 1.12 times higher than that in the case of structure S1 at performance levels IO, LS, CP respectively. It is shown from figure 6 that infill configuration in the frames change the deformation mode, it was found that the fundamental period of the fully infill frame is less compared to that of frames having soft storey. It is observed from figures 10, 11 and 12 that maximum inter-storey drift was obtained where the soft storey was located. For structure S4, the inter-storey drift of the first storey is 2.18, 2.53 and 2.4 times higher than that in the case of structure S1 at performance levels IO, LS, CP respectively. Figures 13, 14, 15 and16 show that the presence of soft storey changes the plastic hinges distribution. Plastic hinges in Structures S2, S3 and S4 were concentrated on the soft story compared to those of structure S1 (fully infilled frame) as shown in these figures. It was found that the fully infilled structure is more ductile than the others structures. For structure S1, the top displacement is 2.4, 1.90 and 2 times higher than that of structure S4 having first soft storey, at immediate occupation, life safety and collapse prevention performance levels respectively.

It is observed from figures 18, 19 and 22 that there is a drastic change in the internal forces in members in the first soft storey.Seismic demands are severely higher for first soft storey columns. For frame with first soft storey, the shear forces in the columns of the first storey are about 2.84 to 5.05 times higher than those in the columns of fully infilled frame, while the bending moments in the same columns are about 2.78 to 3.76 times higher than those in the case of fully infilled frame. This increase in the forces is due to the presence of infill, because in the upper stories, the relative drift between adjacent floors is restricted causing mass of the upper floors to act together as a single mass. In such a case, the total inertia of the all upper floors causes a significant increase in shear forces and bending moments in the ground floor columns.

Table 2 shows that seismic demand plastic rotations of columns in the soft ground storey are too large compared to those of columns in the fully infilled frame. It is difficult to provide such capacities in the

Figure 23: Pushover curves for different structures



columns, hence this leads to the collapse under severe earthquake. Finally, open ground storey is an important functional requirement of almost all the urban multi-story buildings, and hence cannot be eliminated. Hence alternative measures need to be adopted for this specific situation. To avoid soft story effect, it is suggested to increase the stiffness of the soft storey columns, or to find some frames where we can provide shear walls or bracings that can fulfil the stiffness deficiency in ground soft storey.

REFERENCES

1. RPS2000 Moroccan Seismic Provisions. Published by ministry of housing 2000. 2. FEMA308 Federal Emergency Management Agency. Evaluation of earthquake damaged

concrete and masonry wall buildings. 3. M. Mouzzoun, O. Moustachi and A.Taleb, fragility curves for seismic vulnerability

assessment of reinforced concrete buildings, International Journal of materials and environmental science, 2012, 3, 6, 1037.

4. M. Mouzzoun, O. Moustachi and A.Taleb, Seismic damage prediction of reinforced concrete buildings using pushover analysis, International Journal of Computational Engineering Research, 2013, 3, 1,137.

5. FEMA356 Federal Emergency Management Agency. NEHRP recommended Provisions for Seismic Regulations for New Buildings and Other Structures.

6. ATC Seismic Evaluation and Retrofit of Concrete Buildings, Volume 1, ATC-40 Report, Applied Technology Council, Redwood City, California, 1996.

7. CSI, Analysis Reference Manual for SAP2000, ETABS, and SAFE Computers and Structures, Inc. Berkeley, California, USA, 2005.

8. R. Mainstone, on the stiffness and strengths of infilled frames, Proceedings Institution of Civil Engineers London, 1971, 6, 57.

9. M. Fardis, T.Panagiotakos, seismic design and response of bare and infilled reinforced concrete buildings, Journal of Earthquake Engineering, 1997, 1, 3, 475.

10. M. Holmes, Combined Loading on lnfilled frames, Proceedings of the Institution of the Civil Engineers, 1963, 25, 31.

11. A. Chopra, Dynamics of Structures, Theory and Applications to Earthquake Engineering. Pearson.Prentice Hall, third edition, 2007

12. EN 1998, Eurocode 8, Design of structures for earthquake resistance, 2005. 13. HAZUS, Earthquake Loss Estimation Methodology, Technical manual, Washington D.C.,

UnitedStates, 1999.

*CorrespondingAuthor:M.Mouzzoun;Mohammadia School of engineers,

Rabat, Morocco.

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