Behaviour of Flat Slab Rcc Structure Under Earthquake Loading

Behaviour of Flat Slab Rcc Structure Under Earthquake Loading

Sanjay P N1, Mahesh Prabhu K2, Umesh S S3

1. Post Graduate Student, Department of civil Engineering, SCEM, Mangalore Karnataka, India, 575007

   1. Associate Professor, Department of civil Engineering, GEC, Ramnagara Karnataka, India, 571511

   2. Associate Professor, Department of civil Engineering, SCEM, Mangalore Karnataka, India, 575007

AbstractAs flat slab building structures are significantly more flexible than traditional concrete frame/wall or frame structures, thus becoming more vulnerable to seismic loading. Therefore, the characteristics of the seismic behavior of flat slab buildings suggest that additional measures for guiding the conception and design of these structures in seismic regions are needed. To improve the performance of building having flat slabs under seismic loading, provision of flat slab with drop and without drop is proposed in the present work. The object of the present work is to compare the behaviour of multi-storey buildings having flat slabs with drops and without drop on the performance of these two types of buildings under seismic forces. And different types of zones and different type of soils condition as per IS code Provision Present work provides a good source of information on the parameters storey shear, base shear, storey drift, and maximum bending moment at column

Keywords Flat slab with drop and without drop, Storey drift, Storey shear, Base shear, Maximum bending moment.ETABS

1. INTRODUCTION

   Reinforced concrete has been used for building construction since the middle of the 19th century, first for some parts of buildings, and then for the entire building structure. Reinforced concrete is a major construction material for civil infrastructure in current society. Construction has always preceded the development of structural design methodology. Dramatic collapse of buildings has been observed after each disastrous earthquake, resulting in loss of life.

   A flat slab is a reinforced concrete slab supported directly by concrete columns without the use of beams. Reinforced concrete flat slabs are one of the most popular floor systems used in residential buildings, car parks and many other structures. They represent elegant and easy-to-construct floor systems. Flat slabs are favored by both architects and clients because of their aesthetic appeal and economic advantage.

   Reinforced concrete flat slabs are commonly used in construction as they provide a number of benefits to the designer including: 1.Thin sections allowing for greater roof heights and lighter floors, 2.Exposed ceilings, 3.Flexible column arrangements, this is more difficult to achieve for a

   beam-column design,4.Fast and cheap construction using simple formwork.

   However, flat slabs have a lower stiffness in comparison to a beam-column floor plan which can lead to relatively large deflections. In addition to this, the shear capacity can also be reduced in particular around the column head where large shear forces can develop. There are two main failure modes of flat slabs: 1.Flexural Failure and 2.Punching Shear Failure.

   Slabs are designed to fail by flexural failure, the failure mode is ductile therefore giving relatively large deflections under excessive loading, also cracks will appear on the bottom surface before failure occurs. These signs allow the problem to be addressed before failure occurs.

   Punching shear failure by comparison is a brittle failure mode when shear reinforcement is not added, meaning failure will occur before significant deflections take place, in addition to this any cracks that will develop before failure will propagate from the top surface. Since this surface is typically covered, it is unlikely that there will be sufficient warning available before failure occurs.

2. PROBLEM FORMULATION

   In our study we are focusing on the behaviour of flat slab rcc structure of two different types one is with drops and another one is without drop which involves its behaviour for earthquake condition. As it is clear from previous literature that flat slab structure are unstable for seismic forces, we are analytically investigating the effect of flat slab generally with various site condition and in various earthquake zones. The analysis method considering for the Response spectrum method as per IS provision. And by using ETABS software also

3. PARAMETERS CONSIDERED

   The parameters considered in the performance evaluation are as under

   1. Flat Slab Framed Structures subjected to seismic forces.

   2. Flat Slab Framed Structures subjected to different zones and different soil condition as per IS 1893 (Part-1): 2002

   3. Analysis method Equivalent static analysis (ESA) and Response spectrum analysis (RSA).

   4. Maximum Bending moment and Storey Drift in the buildings with and without Drop.

   5. Storey Shear and Design base shear in the buildings with and without Drop.

4. MODELING AND ANALYSIS

   Description of building:

   Type of structure: Multi-storey Flat slab RCC structure with and without Drops

   Occupancy: Office Building Number of stories: 6(G+5) Ground storey height: 3.5m Intermediate storey height: 3.5m

   Model design: One of the objectives of this model designing is to ensure that the models represent the characteristics of commercial buildings. These days, high-rise buildings are different in shape, height and functions. This makes each building characteristics different from each others. There are some standards for each kind of high-rise buildings, such as residential, official and commercials. However, for model designing, main factors such as grid spacing, floor shape, floor height and column section were considered. Two buildings with equal number of stories, with 6(G+5) story having same floor plan of 77 m × 55m dimensions were considered for this study. The floor plans were divided into seven by five bays in such a way that center to center distance between two grids is 11 meters and 11 meters respectively.

   Model 1: Building having flat slab without drops

   Model 2: Building having flat slab with drops

   The modeling of the structure has been done using the structural software ETABS as

   per the data given below:

   Design Parameters

   Type of soil: Soil type 1 (Hard soil) and Soil type 2 (Medium soil)

   Zones: Zone-2 and Zone-3 and Zone-4

   Materials

   Grade of Concrete: M25 Density of Concrete: 25kN/m2

   Modulus of Elasticity of concrete: 5000fck (As per IS 456:2000[11], pp16)

   Member dimensions CornerBeamSizes:BM300X900mm,BM1000X750mm,BM15 00X900mm,BM200X900mm,BM350X1000mm,BM200X100

   0mm,BM300X750mm,BM200X750mm,BM350X750mm,BM

   1000X900mm,BM600X900mm

   Column Sizes: 1000X1000mm and 1000X1500mm Slab Thickness: 275mm

   Wall Thickness: 250mm

   Shear Wall Thickness: 250mm Drop Thickness: 500mm

   Method of dynamic analysis:

   Building with regular or nominally irregular plan configuration may be modeled as a system of masses lumped at floor levels with each mass having one degree of freedom, that of lateral displacement in the direction under consideration. Undamped free vibration analysis of entire building modeled as spring mass model shall be performed using appropriate masses and elastic stiffness of the structural system to obtain natural periods (T) and mode shapes {f} of those of its modes of vibration that needs to be considered. The number of modes to be used should be such that thesum of total of modal masses of all modes considered is at least 90% of total seismic mass

   Plan for Flat slab without drop and with drop

   Plan of flat slab without drop

   Plan of flat slab with drop

   Dynamic analysis

   1. Response spectrum method is used for analysis, Importance factor & response reduction factor are considered as 1 & 5

      respectively. By considering 12 modes mass participation of flat slab building is achieved up to 96%(following table)

   2. Therefore for all buildings 12 modes are considered.

   3. Ritz Vector analyses are used for analysis. Rigid diaphragm action is considered for analysis.

      Center of mass & centre of rigidity coincides, due to regularity in the plan, mass and stiffness of the building.

      Details of time period and mass participation for flat slab without drop building

      Mode	Time Period	% mass participation	Cumulative Mass participation
      1	0.44001	35.8179	35.8179
      2	0.315	0	35.8179
      3	0.25786	39.6196	75.4376
      4	0.1222	8.0967	83.5343
      5	0.08876	0.0001	83.5344
      6	0.07349	7.8684	91.4028
      7	0.06484	0	91.4028
      8	0.06474	0.0022	91.405
      9	0.06272	2.3058	93.7107
      10	0.06265	0.0033	93.714
      11	0.06228	0.2934	94.0074
      12	0.05939	0	94.0074

      Mode	Time Period	% mass participation	Cumulative Mass participation
      1	0.462062	37.0061	37.0061
      2	0.335397	0.0018	37.0079
      3	0.27648	38.92	75.9279
      4	0.131079	7.9922	83.9201
      5	0.095838	0.0001	83.9202
      6	0.079527	7.7426	91.6628
      7	0.067701	2.779	94.4419
      8	0.064879	0	94.4419
      9	0.064798	0.012	94.4539
      10	0.062733	0	94.4539
      11	0.062593	0.0161	94.47
      12	0.059419	0	94.47

      Details of time period and mass participation for flat slab with drop building

5. RESULTS:

Results for flat slab without drop building.

Table 5.1 Design Storey shear for Flat slab without drop building in zone-2 different type of soils

Figure5.1 Design Storey Shear for flat slab without drop building in zone-2, soil type-1 and soil type-2 (Hard soil and Medium soil).

Table 5.2 Storey Drift for Flat slab without drop building in zone-2 different type of soils

Figure5.2 Storey Drift in X-direction for flat slab without drop building in zone-2, soil type-1 and soil type-2 (Hard Soil and Medium soil).

Table 5.3 Maximum bending moment at Column No.45 in Storey No.4 for Flat slab without drop building in zone-2, soil type-1 (Hard soil)

Table 5.4 Maximum bending moment at Column No.45 in Storey No.4 for Flat slab without drop building in zone-2, soil type-2 (Medium soil)

Figure5.3 and 5.4 Maximum bending moment at Column No.45 in Storey No.4 for flat slab without drop building in zone-2, soil type-1 and soil type-2 (Hard soil and Medium soil).

Table 5.5 Design Storey shear for Flat slab without drop building in zone-3 different type of soils

Figure5.5 Design Storey Shear for flat slab without drop building in zone-3, soil type-1 and soil type-2 (Hard soil and Medium soil).

Table 5.6 Storey Drift for Flat slab without drop building in zone-3 different type of soils

Figure5.6 Storey Drift in X-direction for flat slab without drop building in zone-3, soil type-1 and soil type-2 (Hard Soil and Medium soil).

Table 5.7 Maximum bending moment at Column No.45 in Storey No.4 for Flat slab without drop building in zone-3, soil type-1 (Hard soil)

Table 5.8 Maximum bending moment at Column No.45 in Storey No.4 for Flat slab without drop building in zone-3, soil type-2 (Medium soil)

Figure5.7 and 5.8 Maximum bending moment at Column No.45 in Storey No.4 for flat slab without drop building in zone-3, soil type-1 and soil type-2 (Hard soil and Medium soil).

Table 5.9 Design Storey shear for Flat slab without drop building in zone-4 different type of soils

Figure5.9 Design Storey Shear for flat slab without drop building in zone-4, soil type-1 and soil type-2 (Hard soil and Medium soil).

Table 6.0 Storey Drift for Flat slab without drop building in zone-4 different type of soils

Figure6.0 Storey Drift in X-direction for flat slab without drop building in zone-4, soil type-1 and soil type-2 (Hard Soil and Medium soil).

Table 7.1 Maximum bending moment at Column No.45 in Storey No.4 for Flat slab without drop building in zone-4, soil type-1 (Hard soil)

Table 7.2 Maximum bending moment at Column No.45 in Storey No.4 for Flat slab without drop building in zone-4, soil type-2 (Medium soil)

Figure7.1 and 7.2 Maximum bending moment at Column No.45 in Storey No.4 for flat slab without drop building in zone-4, soil type-1 and soil type-2 (Hard soil and Medium soil).

Results for flat slab with drop building

Table 7.3 Design Storey shear for Flat slab with drop building in zone-2 different type of soils

Figure7.3 Design Storey Shear for flat slab with drop building in zone-2, soil type-1 and soil type-2 (Hard soil and Medium soil)

Table 7.4 Storey Drift for Flat slab with drop building in zone- 2 different type of soils

Figure7.4 Storey Drift in X-direction for flat slab with drop building in zone- 2, soil type-1 and soil type-2 (Hard Soil and Medium soil).

Table 7.5 Maximum bending moment at Column No.44 in Storey No.4 for Flat slab with drop building in zone-2, soil type-1 (Hard soil)

Table 7.6 Maximum bending moment at Column No.44 in Storey No.4 for Flat slab with drop building in zone-2, soil type-2 (Medium soil)

Figure7.5 and 7.6 Maximum bending moment at Column No.44 in Storey No.4 for flat slab with drop building in zone-2, soil type-1 and soil type-2 (Hard soil and Medium soil).

Table 7.7 Design Storey shear for Flat slab with drop building in zone-3 different type of soils

Figure7.7 Design Storey Shear for flat slab with drop building in zone-3, soil type-1 and soil type-2 (Hard soil and Medium soil)

Table 7.8 Storey Drift for Flat slab with drop building in zone- 3 different type of soils

Figure7.8 Storey Drift in X-direction for flat slab with drop building in zone- 3, soil type-1 and soil type-2 (Hard Soil and Medium soil).

Table 7.9 Maximum bending moment at Column No.44 in Storey No.4 for Flat slab with drop building in zone-3, soil type-1 (Hard soil)

Table 8.0 Maximum bending moment at Column No.44 in Storey No.4 for Flat slab with drop building in zone-3, soil type-2 (Medium soil)

Figure7.9 and 8.0 Maximum bending moment at Column No.44 in Storey No.4 for flat slab with drop building in zone-3, soil type-1 and soil type-2 (Hard soil and Medium soil).

Table 8.1 Design Storey shear for Flat slab with drop building in zone-4 different type of soils

Figure8.1 Design Storey Shear for flat slab with drop building in zone-4, soil type-1 and soil type-2 (Hard soil and Medium soil)

Table 8.2 Storey Drift for Flat slab with drop building in zone- 4 different type of soils

Figure8.2 Storey Drift in X-direction for flat slab with drop building in zone- 4, soil type-1 and soil type-2 (Hard Soil and Medium soil).

Table 8.3 Maximum bending moment at Column No.44 in Storey No.4 for Flat slab with drop building in zone-4, soil type-1 (Hard soil)

Table 8.4 Maximum bending moment at Column No.44 in Storey No.4 for Flat slab with drop building in zone-4, soil type-2 (Medium soil)

Figure8.3 and 8.4 Maximum bending moment at Column No.44 in Storey No.4 for flat slab with drop building in zone- 4, soil type-1 and soil type-2 (Hard soil and Medium soil).

CONCLUSIONS:

Within the scope of present work following conclusions are drown

1. For all the cases considered drift values follow a parabolic path along storey height with maximum value laying the fourth storey.

2. Use of flat slab with drop and without drop results in drift value is marginally in a range of 0.5mm to 3mm. still all drift values are within permissible limits, even with shear walls.

3. The fundamental natural period value is much higher in flat slab with drop buildings compared to flat slab without drop building.

4. For all the structure, design base shear increases as the number of stories increases. This increases design base shear is gradual up to 6th storey. The design base shear of soil type-1 is less than the soil type-2 for all type of zones.

5. The design base shear of zone-4 is higher compare to other zones (i.e., zone-3 and zone-2) for all type of structures.

6. In flat slab without drop building the maximum bending moment at column no.45 in storey no.4 for all type of soils and zone. In such that the flat slab with drop building the maximum bending moment at column no.44 in storey no.4 for all type of soils and zones.

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