MenuISSN (0970-2083)
ISSN (0970-2083)
1M.Tech Structural Engineering, SMBS, VIT University - Chennai Campus, Chennai
2Associate Professor, SMBS, VIT University - Chennai Campus, Chennai
Received 17 June, 2017; Accepted 24 November, 2017
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In today’s world of modern construction tall structures are taking place to overcome land problem. The use of flat slab is becoming very popular because it increases the construction speed, reduces dead weight. But it has poor performance under lateral load, due to thesis reason flat slab are generally avoided in India for high seismic zone. To improve the performance of structure and to make flat slab stiffer, drops are provided under flat slab. This study is to check performance of flat slab under seismic load situated in high seismic zone according to guideline of IS-1893(part 1):2002 and UBC-97. The results of maximum story displacement are compared for both codes.
Flat slab with drop, Response spectrum analysis, Maximum story displacement
In this modern era of infrastructure development the main issue is the shortage of land. To overcome this problem the tall building construction has been taking place very quickly. To make the construction quick and economical some elements are modified and flat slab is introduced in construction. It makes the beam invisible, reduces dead load and increases floor area. Its main advantages that it provides more head room so for the same building number of floors increases. Due to absence of shear walls and deep beams this system is more flexible to lateral loads. Because of this type of behavior flat slab are mostly avoided by people to use in high seismic zones. The present work is to examine the behavior of flat slab with drop for seismic loads in high seismic zone using Indian Standards and to compare it with standards of universal building code.
Farhy, et al., 1993 check the lateral load effect for RC flat slab and column sub assemblages and get failure shape. (Quian, et al., 2013) did the retrofitting and strengthening of flat slab using CFRP laminates. (Micallef, et al., 2014) check the punching shear failure because of impact loading on RC flat slab. Increasing in stiffness rather than enhancement of material property is more dominant. (Soni and Raval, 2014) examine the behavior of flat slab with drop when subjected to different types of loading. (Askar, 2015) used prestressed bolts for retrofitting of flat slab failed in punching shear.
(Chan, 1987) did Response Spectrum Analysis to study offshore platforms. (Asthana, et al., 1989) performed random vibration analysis with the use of response spectrum analysis method to study behavior of flexible base building. In this method spectral approach is used to fine mode shapes and natural frequencies. (Chandak, 2012; Coelho, et al., 2004; Sable, et al., 2012) used response spectrum analysis to check the behavior of reinforced concrete structure. Design spectrum of Indian code, UBC code and Euro code 8 is compared. Results said that base shear from the Indian code is higher.
Two models of flat slab with drop structures are prepared and analyzed in ETABS 15.0.0 software. Total height of building is 31 m. first model is prepared according to IS-1893 (part 1):2002. According to IS code thickness of flat slab shall be generally considered based on span to depth ration. Minimum thickness should be at least 125 mm. second model is prepared for UBC 97 (Figures 1 and 2).
Figure 1: Plan of flat slab with drop model.
Figure 2: Elevation of model
As the height increases, the structure gets more flexible to lateral loads. If the height of the structure is not enough for wind load effect than the major lateral load effect takes place due to earthquake load. So it becomes necessary to analyze the structure for seismic force in high seismic zone and to provide an alternative of conventional slab system.
Response Spectrum Analysis: For the response spectrum analysis first the response spectrum function is defined using both the code. For defining model case, Ritz vector is defined because it gives better participation factor. Initially scale factor is taken as 1 for both models. After analyzing the model base reaction is checked because it should be 80% of the response spectrum base reaction. For that the scale factor is being modified as per given formula Tables 1 and 2.
| Earthquake load property | IS-1893 (part 1):2002 | UBC 97 |
|---|---|---|
| Importance factor I | 1.5 | 1.5 |
| Response reduction factor R | 4 | 3.5 |
| Type of soil | Medium soil (Type II) | SD |
| Seismic zone | V | 4 |
| Seismic zone factor Z | 0.36 | 0.40 |
Table 1: Seismic load input
| No. of story | G+8 |
| Beam size | 300 × 800 mm |
| Column size | 900 × 900 mm |
| Bottom story height | 4 m |
| Story height | 3 m |
| Thickness of flat slab | 225 mm |
| Thickness of drop | 450 mm |
Table 2: Structural property of model
For IS Code: Scale factor=
For UBC code: Scale factor=
Performance of both models is been analyzed and behavior is observed for maximum story displacement in both X and Y direction for load case EQXP and EQYP. Analysis says that flat slab with drop system reduces the effect of lateral loads on high rise building. It also reduces the construction time.
In the (Figures 3-6) shows that the displacment according to UBC 97 code is high compare to IS-1893 (part 1):2002. The main reason for that is, the seismic zone factor value is more in UBC code than Indian code. The consideration of risk accesment factor from the past earthquake is taken accuratly. From this analysis it can be said that in the indian standard code, the effect of the seismic load is underestimated compare to UBC code. From the past dissaster occurred due to eathquake, Maximum Considered Earthquake should be taken properly (Figures 3-6).
Figure 3: Displacement in X-direction.
Figure 4: Displacement in Y-direction
Figure 5: Displacement in X-direction.
Figure 6: Displacement in Y-direction.
Two models of flat slab with drop in high seismic zone are analyszed according to the standared of UBC 97 and IS-1893 (part 1):2002 using Response Spectrum Analysis Method. Analysis concluded that it is better to use flat slab with hdrop system in high rise buildings because it reduces the maximum story displacent and also increase story height and deceases construction time. IS code gives lesser value of displacment cmapre to UBC code. In UBC code the seismic risk characteristics by maximum Considered Earthquake in the Zone where the steucture is taken high as 0.40, where in the IS code it is taken as 0.36 which is less than UBC code. From this study, it can be said the the disaster from the paster earthquake is not considred properly in indian standared code.
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