Effect of Soft Story on Structural Response of High Rise Buildings

Effect of Soft Story on Structural Response of High Rise Buildings

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2011 IOP Conf. Ser.: Mater. Sci. Eng. 17 012034

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Effect of Soft Story on Structural Response of High Rise Buildings

F. Hejazi11,2 S. Jilani,1,2, J. Noorzaei1,2, C. Y. Chieng1, M. S. Jaafar1,2, A. A. Abang Ali1 1Department of Civil, Engineering faculty, University Putra Malaysia, 43300 Serdang, Selangor, Malaysia 2Institute of Advanced Technology, University Putra Malaysia, 43300 Serdang, Selangor, Malaysia Email: [email protected]

Abstract Severe structural damage suffered by several modern buildings during recent earthquakes illustrates the importance of avoiding sudden changes in lateral stiffness and strength. Recent earthquakes that occurred have shown that a large number of existing reinforced concrete buildings are vulnerable to damage or even collapse during a strong earthquake. While damage and collapse due to soft story are most often observed in buildings, they can also be developed in other types of structures. The lower level containing the concrete columns behaved as a soft story in that the columns were unable to provide adequate shear resistance during the earthquake. Usually the most economical way of retrofitting such as a building is by adding proper bracing to soft stories. So, in this paper occurring of soft at the lower level of high rise buildings subjected to earthquake has been studied. Also has been tired to investigate on adding of bracing in various arrangements to structure in order to reduce soft story effect on seismic response of building. It is lead to assess the vulnerability level of existing multi-storied buildings so that they can be retrofitted to possess the minimum requirements. This will help in minimizing the impending damages and catastrophes.

1. Introduction: A soft story known as weak story is defined as a story in a building that has substantially less resistance or stiffness or inadequate ductility (energy absorption capacity) to resist the earthquake-induced building stresses. Soft story buildings are characterized by having a story which has a lot of open space (Figure. 1). Parking garages, for example, are often soft stories, as are large retail spaces or floors with a lot of windows. Figure. 2 shows the image of a soft story.

Figure.1. Cross Section of a Soft Story Building

CAMAN IOP PublishingIOP Conf. Series: Materials Science and Engineering 17 (2011) 012034 doi:10.1088/1757-899X/17/1/012034

Published under licence by IOP Publishing Ltd 1

If a building has a floor which is 70% less stiff than the floor above it, it is considered a soft story building (UBC-1997, IBC-2003 and ASCE-2002). This 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 which may render the structure totally unusable. The concept of the soft story has been recognized long back by Fintel and Khan (1969). This concept is an attempt to reduce acceleration in a building by allowing the first-storey column to yield during an earthquake and produce energy-dissipation action. However, excessive drifts in the first story coupled with P- effects on the yielded columns make buildings collapse. A new approach was proposed by Mo and Chang (1995) for soft story building which in this system, Teflon sliders are placed on the top of the first story reinforced concrete framed shear walls. These shear walls are framed by columns and beams, and are designed to carry a portion of the weight of the superstructure and the lateral load determined by the frictional characteristics of the Teflon sliders. The remaining first-story columns are designed for ductile behavior in order to accommodate large drifts. Arlekar (1997) highlighted the importance of explicitly recognizing the presence of the open first storey in the seismic analysis of buildings. The error involved in modeling such buildings as complete bare frames, neglecting the presence of infills in the upper storeys, was brought out with different analytical models. Kanitkar and Kanitkar (2004) studied a five-storey building with soft storey with the intent of reviewing the new provisions for the earthquake resistant design of structures addressed in the code IS 1893:2002. Lee and Co (2007) quantified the effect of the shear walls located in the lower soft stories on the general seismic response characteristics. Mastrandrea and Piluso (2009) proposed a design methodology for development of a collapse mechanism of a global type for eccentrically braced frames in soft story buildings. Tesfamariam and Liu (2010) used special index for soft story for seismic risk assessment of reinforced concrete buildings as classification vulnerability techniques.

Figure.2. Example of a soft story at the ground floor


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It was observed that although the new soft storey provisions of IS 1893 are a step in the right direction, more investigations are required to completely define the resulting buildings in terms of their ductility capacities and stable inelastic action under the expected seismic loads. The objective of this paper is to study the effect of soft story on structural behavior of high rise buildings and retrofitting and seismic rehabilitation of soft story structures through bracing devices. Also compare the soft story structural response of high rise building with various type of bracing arrangement on building and finding of optimum design of earthquake resistance soft story buildings by considering of required performance level. 2. Building Analyzed The intention to study a 12 reinforced concrete framed storey building is to see the effect of high rise building on the earthquake and the effect of soft story which are designed at the bottom floor of the building. Application of building is consider as Hotel and three dimension view of structure are showed in Figure. 3.

Figure.3. Three dimensional view of considered model

Stories height is in 3.3m except first and top floors. First story height is 5m and for top floor, the storey height is only 2.8m. The column size is increasing from top floor to bottom due to the increase in loading that the columns have to support. The frame members are modeled with rigid end zones, and the floors are modeled as diaphragms rigid in-plane. 3. Loading The loading calculation was made based on BS6399 CODE of practice. Table (1) shows the detail calculation of weight of building due dead and live load. The dead load and live load and calculation of weight of floors are tabulate on Table (1). Based on the code just 20% of live load is considered for calculation of weight of building in earthquake excitation.


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Table 1. Total weight for each floor

Describe

Unit

area

m2

No

Dead load kN/m2

Live load

kN/m2

Total weight kN

Bedroom 36

14

4.7651

20.2= 0.4

2603.2

Corridor 90 1

4.7651

40.2= 0.8 500.8

staircase 18 2 6.376

40.2= 0.8 258.3

roof 18 4 4.605 0.750.2= 0.15

342.3

External wall

3.0* -- 2.6 --

internal wall

3.0* -- 1.373 --

1012.9

*height of wall So total weight of building is determined as follow:

Total weight for each floor:

w1 = 2603.2 + 500.8 + 258.3 + 1012.9

= 4375.3 kN

Total weight for 11th floor:

w11 = (3614)4.755+ 500.8 + 258.3 + 012.9

= 4168.6

Total weight for whole building:

W = 10 w1+ w11+ Roof load

= 10 4375.3 + 4168.6+342.3= 48264 kN

The condition of earthquake load calculation in this design is assumed that the location of building is in medium zone of risk for earthquake to happen. Also the soil condition is assumed as beds of gravel and sand with weak cementation (soil type III). Equivalent static base shear force due to earthquake load is calculated as follow:

Vb = c w Where Vb is shear force due to earthquake load, c is earthquake coefficient and w is total weight of building with consider of dead load and 20% of live load. Base on the code, earthquake coefficient for considered building is 0.0821 in mention condition. So earthquake equivalent shear force will be determined as:


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Vb = 0.0821 48264 = 3962 kN After calculate the base shear, by using equation below, the lateral force due earthquake imposed on each floor are calculated and tabulated in table (2).

Table 2. Lateral force for each storey

Floor Storey height

m

Height above base

m

Weight of

storey kN

Lateral force kN

1 5 5 4375.3 83.27139

2 3.3 8.3 4375.3 138.2305

3 3.3 11.6 4375.3 193.1896

4 3.3 14.9 4375.3 248.1487

5 3.3 18.2 4375.3 303.1079

6 3.3 21.5 4375.3 358.067

7 3.3 24.8 4375.3 413.0261

8 3.3 28.1 4375.3 467.9852

9 3.3 31.4 4375.3 522.9443

10 3.3 34.7 4375.3 577.9035

11 3.3 38 4168.6 602.9646

12 2.8 40.8 342.3 53.16001

4. Retrofit models Usually the most economical way of retrofitting of soft story building is by adding proper bracing to soft stories. For investigation on effect of different bracing installation arrangement on building in seismic response of structure with soft story at bottom, 6 models are designed with different condition: 4.1. Model 1 The structure with out any lateral load resistance system is called model 1 in this paper and elevation view of the building is showed in Figure. 4.


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Figure. 1. Model 1 - Soft storey at bottom floor of structure 4.2. Model 2 The model 1 (Soft story at bottom) is modified into this model with adding the bracing at the lowest floor. The bracing is added at all the first floor as shown in Figure. 5. So Model 2 is a Model that tries to eliminate the effect of soft story at the lowest floor and reduce soft story effect on seismic response of retrofitted building. 4.3. Model 3 Model 3 is also a bracing building. The bracing is only added at the centre bay of the building. In this Model, the soft story at the lowest floor has been added the bracing in center bay too. Elevation view of this model is shown in Figure. 6.

Figure 2. Model 2 Add bracing in first floor


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Figure. 3. Model 3 Bracing at the center bay 4.4. Model 4 As shown in Figure. 7, in this model, bracings are added from second floor to top floor but not all the members are braced. This model has a soft story at the bottom floor too. Include this model; there are a total number of 5 models of bracing building.

Figure. 4. Model 4 Bracings are added to some bays from second floor to top floor 4.5. Model 5 The difference of model 5 with model 4 is bracing added in all bay from second floor to the uppest floor of the building. So as expected the soft story will happened at first floor. This model is shown in Figure. 8.

Figure. 5. Model 5 All bays bracing from second floor 4.6. Model 6


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In order to see the effect and the use of bracing in the whole bay, model 6 is prepared and all bays in all floors are braced in both direction. However, this type of model is seldom seen in real live as it might affect the aesthetically of the whole building.

Figure. 6. Model 6 Bracing installed at all bays and all floors 5. Concept analysis of building Linear and nonlinear static analyses are performed for the all models of the building which described before, by SAP2000 software package. After preparing the physical model of building and define of material properties and beam, columns sections, live and dead load applied to the structure and earthquake static equivalent coefficient have been determined and imposed to the model. Then different load combination cases are considered in order to get the critical result. Stiffness of each stories are calculated and tabulated in order to recognize of soft story in different models. The response of structures in term of displacement of model in height of structure is evaluated and plotted into graph for comparison. 6. Result and discussion Table (3) and Table (4) show the stiffness in X and Y direction corresponding for different type of bracing buildings. Generally, the stiffness is decrease while increase the number of floors. That is because for upper floor, the loading that should be carried by the column is lower as at upper floor, only few floors of weight need to be carried. However, the stiffness at the bottom floor is still lower than second although bracing is added on the lowest floor for Model 2, Model 3 and Model 6. The stiffness of floors for model 1 to 6 are calculated and tabulated in Table (3) for X direction and Table (4) for Y direction. All models except model 1 are bracing building which braces are added in different arrangement. As seen in the table, from out of 6 models, 5 models have the soft story at the lowest floor. Relative stiffness (stiffness of each story compare of above story stiffness) of first story in model 1 is below 70% and it is shown that the soft story happened in this floor. For solving of soft story problem, bracings are added to this floor in model 2 on both side of building. Although the relative stiffness is still below 1 in X and Y direction but it is not consider as a soft story as its relative stiffness is greater than 70%. In model 3, where bracing is only added at the center bay of the building, base on the relative stiffness values for this building, soft story is happened in first floor. Model 4 and Model 5 shows a very soft story in the lowest floor as the relative stiffness to the storey above is less than 25% in X direction and 20% in Y direction.


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Table 3. Relative stiffness of stories X direction for Model 1 to Model 6

Relative stiffness of stories in X

direction No. of floors Model 1 Model 2 Model 3

1 0.335682 0.775756 0.362097 2 1.135028 1.135028 1.123185 3 1.189777 1.189777 1.170297 4 1.530209 1.530209 1.451228 5 1.085898 1.085898 1.072179 6 1.222282 1.222282 1.180376 7 1.348433 1.348433 1.265316 8 1.53476 1.534759 1.361131 9 1.839861 1.839863 1.445646

10 6.244613 6.244577 1.803904 11 1.221694 1.221701 0.941562

No. of floors Model 4 Model 5 Model 6

1 0.250729 0.210731 0.461593 2 1.097524 1.080711 1.080711 3 1.130203 1.10539 1.10539 4 1.311861 1.238269 1.238269 5 1.048798 1.036858 1.036858 6 1.115215 1.084639 1.084639 7 1.154647 1.109128 1 8 1.182938 1.122495 1.244991 9 1.185058 1.116775 1.11684

10 1.227082 1.132214 1.132148 11 0.914329 1.570384 1.570384

Because in this models by adding the bracing on all floors except bottom floor, effect of soft story is intensified. All bays in all floors are equipped bracing in model 6 on both side. Although the stiffness of floors is so high, but as seen in Table (3) and (4) soft story happened in first floor on both direction again because of first story height. Displacement of different type of bracing buildings against storey height in horizontal direction (horizontal movement) under earthquake excitation is showed in Figureure 10. As seen in the plot displacements are increasing when the storey height increased. This graph is plotted base on nonlinear response of reinforced structure response and that it is easy to make the comparison and the effect of using different type of bracing in building.


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Table 4. Relative stiffness of stories Y direction for Model 1 to Model 6

Relative stiffness of stories in Y direction No. of floors

Model 1 Model 2 Model 3 1 0.301 0.958 0.338 2 1.219 1.219 1.155 3 1.099 1.099 1.068 4 1.147 1.147 1.097 5 1.023 1.023 1.015 6 1.065 1.065 1.042 7 1.316 1.316 1.181 8 1.461 1.461 1.221 9 1.642 1.642 1.230

10 2.791 2.791 1.299 11 1.222 1.222 7.329

No. of floors Model 4 Model 5 Model 6

1 0.172 0.144 0.459 2 1.114 1.094 1.094 3 1.050 1.040 1.040 4 1.068 1.055 1.055 5 1.011 1.008 1.008 6 1.029 1.023 1.023 7 1.119 1.093 1.093 8 1.135 1.102 1.102 9 1.129 1.095 1.095

10 1.149 1.105 1.105 11 14.719 2.356 2.356

Since all the models are fixed at the ground floor, there is no displacement at ground floor. Model 1, 4 and 5 states a maximum displacement in horizontal direction at the soft story (First story) about 11.085, 9.504 and 8.822mm respectively. It is because there is no bracing at the soft story of these models and the lateral strength become very weak in compare of other models. The maximum displacement at top floor is cause by Model 1 with a displacement of 148.57 mm. Also, model 2 have a maximum displacement in top story (139.131mm) in compare of other bracing buildings. As describe before only in this model soft story problem is solved by adding bracing. For this reason horizontal movement of this model at first story is less than model 3 to 5. But because of install bracing just at first floor of model 2, stiffness of other floors are less in compare of other models, so relevant displacements is so more. Stiffness of structure stories in Model 3, 4, 5, 6 respectively are increased because of bracing arrange format on buildings.


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Bracing are added just in 1 bay in model 3, 4 bay in model 4, 7 bay from second floor up to top in model 5 and in all bays on model 6, therefore number of using braces and stiffness of buildings are increasing respectively in models. Top floor horizontal displacements of model 3, 4, 5, 6 are evaluated about 101.59, 59.775, 28.068 and 21.422 respectively.

Figure. 7 Displacement of building in X direction against Storey height for different type of bracing

buildings

0

5

10

15

20

25

30

35

40

45

0 20 40 60 80 100 120 140 160

Stor

ey h

eigh

t (m

)

Displacement (mm)

Model 1 Model 2 Model 3 Model 4 Model 5 Model 6

Figure. 8 Displacement of building in Y direction against Storey height for different type of bracing

buildings

0

5

10

15

20

25

30

35

40

45

0 50 100 150 200 250

Stor

ey h

eigh

t (m

)

Displacement (mm)

Model 1 Model 2 Model 3 Model 4 Model 5 Model 6


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From this graph, the best model to resist earthquake is Model 6 with the minimum displacement for overall buildings although it is a soft story model. In actual buildings, it is impossible to add bracing in all bays but from the result, it is observed that effect of soft story can reduced by adding some earthquake resistant method such as bracing. Figure. 11 shows vertical displacement of buildings against the storey height for various modeled which explained as above. As seen in this plot, although Model 2 is not a soft storey, it has a maximum displacement in bracing buildings at each floor unless the first floor. This is because the bracing is only be added at the first floor, so the displacement in the first floor is relatively small compare with others. After the first floor, it starts to deviate much as the no bracing at all start from second floor. Model 3 also has great effect during earthquake although the entire stories are braced. This is due the bracing Model 3 only added at the centre of the building. Earthquake is a vibration in the whole building so that if the braced is only added at the centre of the building, it will not give good effect on earthquake resistant. Model 4 and Model 5 state a maximum displacement at the lowest floor as the relative stiffness is too low compare with others. Model 6 shows a most ideal condition during earthquake, the maximum displacement is only 20mm while other gives a larger value for this. From these few Models, bracing is one of the method that can be used to resist earthquake compare to moment resisting frame. It is because beside increase the strength in the member, it also increases the overall stiffness in the building. However, the bracing must be added at the correct position in order to get good effect on earthquake resistance. With evaluation of various bracing arrangements on structure, vulnerability level of existing multi-storied buildings is assess and it helped for design new and exist building retrofit plan on consider level of operation and safety with minimum requirements. 6. Conclusion RC frame buildings with open first storeys are known to perform poorly during in strong earthquake shaking. The large opening on the lowest floor causing the stiffness is relative low compare to the stiffness at the storey above. The stiffness at lower floor is 70% lesser than stiffness at storey above it causing the soft storey to happen. The lateral strength of the building is related to the stiffness. The lateral strength of a building is sum of all the stiffness from column, shear wall and bracing are added at each storey. So the low strength in the lowest floor causing the failure occurs especially during earthquake. For a building that is not provided any lateral load resistance component such as bracing or shear wall, the strength is consider very weak and easy fail during earthquake. In this paper it has been tried to investigate on adding bracing to the building in different arrangement in order to reduce soft story effect on structural seismic response. It was found that location and numbering of bracing acts an important factor for the soft story structures to displace during earthquake. Also the soft story has been eliminated as the bracing is added to the consider floor, although the displacement at the top floor is still very high because there is no bracing at the top floor. So, result show that a bracing will only makes a different result for the storey that equipped with bracing. The horizontal and vertical movements of building which bracing installed in most bays are much reduced during earthquake compare with other models. So it shows that the use of bracing is effectively reduced effect of soft story on structure response in earthquake excitation. In the other hand, vulnerability level of existing multi-storied buildings is assess by analysis of different arrangement of bracing on building and it helped for retrofitting of structure on consider


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level of operation and safety with minimum requirements. Acknowledgment This work has received financial support from Ministry of Science, Technology and Innovation of Malaysia under Research Project No. 5450366 and supported by Institute of Advance Technology of University Putra Malaysia and gratefully acknowledged. References [1] M. Fintel and F. Khan (1969) " Shock-absorbing soft story concept for multistory earthquake structures ". Proceeding. [2] Farzad Naeim (2001), The seismic design handbook, 2nd Ed. Springer, 304p-306p [3] James E. Ambrose, Dimitry Vergun (1995), Simplified Building Design for Wind and Earthquake Forces, John Wiley and Sons. [4] Brayan Stafford Smith (1991), Tall Building Structures: Analysis And Design, John Wiley & Sons, Inc. [5] Anil K. Chopra (1995) Dynamics of Structurs, Theory and Application to Earthquake Engineering. Prentice Hall, Englewood Cliffs, New Jersey. [6] Y. Mo and Y. Chang (1995). " Application of base isolation concept to soft first story buildings ", journal of Computers and Structures, Vol. 55, pp 883-896 [7] Uniform Building Code (UBC) (1997). International Council of Building [8] American Society of Civil Engineers (2002). Seismic Evaluation of Existing Buildings, American Society of Civil Engineers, Reston, Virginia. [9] J. N. Alekar, S. Jain and C. V. R. Murty (1997)" Seismic Response of RC frame buildings with soft first storeys". Proceeding of Central Building Research Institute Golden Jubilee Year Conference on " Natural Hazard in the Urban Habitat", Edited by: Iyengar, R. N., New Dehli, India, pp. 13-24 [10] International Building Code (2003). International Code Council. [11] R. Kanitkar and V. Kanitkar (2004), "Seismic performance of conventional multi-story building with open ground floors for vehicular parking". The Indian Concrete Journal, pp 99-104 [12] H. S. Lee and D. W. Ko (1997), " Seismic response characteristics of high-rise RC wall buildings having different irregularities in lower stories". Journal of Engineering Structures, Vol. 29, pp. 31493167 [13] L. Mastrandrea, V. Piluso (2009), "Plastic design of eccentrically braced frames, II: Failure mode control". Journal of Constructional Steel Research, Vol. 65, pp. 1015-1028 [14] S. Tesfamariam, Z. Liu (2010), "Earthquake induced damage classification for reinforced concrete buildings". Journal of Structural Safety, Vol. 32. pp 154164


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Effect of Soft Story on Structural Response of High Rise Buildings

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