Seismic Performance of Tall Reinforced Concrete Buildings under P-Delta Analysis

DOI : 10.17577/IJERTCONV5IS08013

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Seismic Performance of Tall Reinforced Concrete Buildings under P-Delta Analysis

Lakshmi Subash

PG Student, Department of Civil Engineering,

Sreepathy Institute of Management And Technology, Vavanoor, Kerala, India

Sruthi K Chandran

Assistant Professor, Department of Civil Engineering,

Sreepathy Institute of Management And Technology, Vavanoor, Kerala, India

Abstract Rapid growth of urbanization world over indicates that there is growing movement of population which result in high concentration of building construction activities in urban areas. This leads to ever increasing demand for land for building construction. As land is a scarce resource for developments in any urban area, there is heavy pressure on the available land. For economic reasons people tend to construct buildings with maximum floor area on the available land and consequently, construction of tall buildings is increasingly becoming inevitable in urban areas. Tall building structure is so designed to get optimum built up area to the requirements and comfort of the occupants in such a way that the structural system is economically feasible and sufficiently safe to resist all natural forces including lateral forces due to wind or earthquakes. Structures subjected to lateral loads often experience secondary forces due to the movement of the point of application of vertical loads. This secondary effect, commonly known as P-Delta effect plays an important role in the analysis of the structure. In the above context, this thesis work describes Seismic analysis of a multi-storey RC building by using ETABS structural analysis software. Building models with different number of storeys have been analyzed and the maximum response values in buildings are determined in terms of displacements and storey Drift.

KeywordsSecond order effect, P-Delta effect, Storey drift

  1. INTRODUCTION

    Earthquake is considered to be one of the most destructive or damaging natural calamities in the past decades. It pause serious threat to lives and damage to properties. Mostly, the causes of deaths reported after the incidence of earthquakes are eventually due to the collapse of buildings/structures. This is an indication of the necessity of developing earthquake resistant building construction techniques. Truly, there has been a growing interest among the scientific community in acquiring better knowledge on the seismic behaviour of reinforced concrete buildings in recent past. Various study reports on earthquake have revealed that under an earthquake, tall buildings will tend to have a higher magnitude of vibration as compared to short buildings. Also the buildings constructed without considering the second order P-Delta effect and appropriate characteristic seismic resistance constitute the main source of risk during an earthquake. Structures often experience secondary forces due to the movement of the point of application of vertical loads and when it is subjected to lateral loads. P-Delta effect holds an important role in the analysis of the structure. The P-Delta effect is mainly dependent on the applied loads and the building

    characteristics. Studying the seismic behaviour of buildings under P-Delta effect may be of growing interest to researchers.

  2. P-DELTA EFFECT

      1. elta effect refers to the abrupt changes in ground shear, overturning moment and the axial force distribution at the base of a sufficiently tall structure or structural component when it is subject to a critical lateral displacement. The P- Delta effect is a destabilizing moment equal to the force of gravity multiplied by the horizontal displacement of a structure undergoes when loaded laterally.

        Fig.1. P-Delta effect

  3. BACKGROUND AND MOTIVATION

    The inappropriate analysis and design of structures may lead to collapse of the structure. Usually the multi-storied buildings are analyzed using general one step linear static analysis assuming the full loads to be applied on the structure. But there may be lot of differences in the outcomes obtained from the analysis against practical aspect. Therefore, to briefly understand and to overcome this problem a non-linear P-Delta analysis is performed for the structure.

  4. OBJECTIVE OF STUDY

    The objective of the present work is to determine in what way the P-delta analysis influence the variation of responses of the structure such displacements and storey drift against linear static analysis. To perform the analysis ETABS 2016 software is used for all models of each case. In order to understand the trend of P-delta effects, the height of the building is gradually increased from story 5 to story 30 in 5 story intervals.

  5. MODEL DESCRIPTION

    To study the effects of P-delta, six different storey cases are taken where storey variation starts from storey 5 to storey 30.

    Fig.2. Typical Plan of the models

    Fig.3. 3D view of 30 storey model

    The detail basic specifications of the building are:

    Table 1 Material Properties

    a) Properties of concrete

    Grade of concrete

    M25

    Elasticity Modulus, Ec

    25000Mpa

    Poissons Ratio

    0.2

    Density of concrete

    25 KN/m3

    b) Properties of reinforcement steel

    Grade of steel

    Fe415

    Modulus of elasticity, Es

    210000Mpa

    Poissons ratio

    c) Properties of masonry

    Density of brick wall including plaster

    20 KN/m3

    Poissons ratio

    Table 2 Geometric Properties

    Slab thickness

    150mm

    Beam size

    300mm x 475mm

    Column size

    600mm x 800mm

    Brick wall on external beams

    230mm thick

    Brick wall on internal beams

    150mm thick

    Parapet wall on roof

    150mm thick

    Storey height

    3m

    A.Loads on Structure

        • Live load on roof and floor : 3kN/m3

        • Roof/floor finish : 1.5kN/m3

        • Load from brick wall on external beams : 13.8kN/m3

        • Load from brick wall on internal beams : 9kN/m3

        • Load from parapet wall on roof : 3kN/m3

  6. RESULTS AND DISCUSSION

    To study the Second order or P-delta effects, with the increase in height of the structure, different building models of 5 storey, 10 storey, 15 storey, 20 storey 25 storey and 30 storey were analyzed in the ETABS software. The lateral loads, dead load and live load are considered for the design of structure as per the Indian standard code of practice for Seismic Zone V.

    The comparison of maximum displacements and maximum storey drift for different models with and without P-delta effect is as shown in table-3 below.

    1. Maximum Displacement

      Table 3 Maximum Displacement

      Model

      Maximum Displacement (mm)

      Average %Increase in Displacements

      Without P- Delta

      With P-Delta

      5 strey

      model

      35.96

      37.17

      3.26

      10 storey

      model

      92.23

      98

      5.89

      15 storey model

      137.73

      149.93

      8.14

      20 storey

      model

      219.6

      242.31

      9.37

      25 storey

      model

      267.45

      305.13

      12.35

      30 storey

      model

      321.82

      385.1

      16.43

      Fig.4 Variation of horizontal displacement in top

    2. Maximum Storey Drift

    Table 4 Maximum Storey Drift

    Model

    Maximum

    Storey drift

    Average %Increase in Storey Drift

    Without P- Delta

    With P-Delta

    5 storey model

    0.001729

    0.001738

    0.52

    10 storey

    model

    0.002162

    0.002202

    1.82

    15 storey model

    0.002319

    0.002409

    3.74

    20 storey model

    0.002338

    0.00249

    6.10

    25 storey model

    0.002481

    0.002706

    8.31

    30 storey

    model

    0.002949

    0.003419

    13.75

    Fig.5 Variation of maximum storey drift

    The second order effect in the 5 storey model is around 3%, which increases to 16% in the 30 storey model. In case of storey drift, the percentage of increase in storey drift due to P- Delta effect in the 5 storey model is about 0.52% which increased to 13.75% in the 30 storey model. Therefore 5 story models are least affected while the 30 story models are most affected by second order effect indicating the second order effects need to be considered for tall structures. P-Delta effect is negligible up to 10 storey buildings. While increasing height from 10th storey, there is considerable increase in displacement with P-Delta effect.

  7. CONCLUSION

The structural performance of buildings with different heights, 5, 10, 15, 20, 25 and 30 stories is studied. The buildings are modelled and analysed using standard ETABS 2016 software. On the bases of results of analysis, the following conclusions are drawn:

  • It is seen that the seismic parameter of building models without P-Delta is less than corresponding building models with P-Delta effect.

  • Displacement value increases with increase in height, but it is more severe for P-Delta analysis.

  • The change in displacement, and storey drifts due to P- Delta effect increases with increase in number of storey or building height.

  • Both Llinear static and P-Delta both are necessary for RC structures.

  • Due to wide displacement variation with increase in slenderness P-Delta analysis is required for the structures higher than 10 storeys.

REFERENCES

  1. IS 1893 (part 1) (2002) Indian Standard Criteria for Practice for Earthquake Resistant Design of Structures General Provisions and Buildings (Fifth Revision).

  2. IS 456:2000, Indian standardPlain and reinforced concrete Code of Practice, Bureau of Indian standard, 2000, New Delhi.

  3. Regina Gaiotti, P-Delta analysis of building structures, Journal of Structural Engineering ASCE, Vol.115, No4, April 1989

  4. M.A.A.Mollick, Experimental study on P-Delta effect in RC high-rise building, Journal of Civil Engineering The institution of Engineering Bagladesh, Vol CE 25, No.2 1997

  5. Ashraf Uddin, P-Delta effect in Reinforced Concrete Structures of Rigid Joint, IOSR Journal of Mechanical and Civil Engineering,

    Volumn 10, Issue 4, Nov 2013

  6. Deepak Soni,Dynamic Behaviour of Reinforced Concrete Framed Buildings under Nonlinear Analysis, International Journal of Engineering Development and Research, Vol-2, Issue-4, 2014

  7. B N Mallikarjuna, Stability analysis of Steel frame structures: P-Delta analysis, International of research in Engineering and Technilogy,

    Vol.3, Issue 8 2014

  8. P V Dhanshetti, Effect of P-Delta action on multi-story buildings, International Journal of Engineering Research and Technology, Vol-4,

    Issue-1, 2015

  9. N Anvesh, Effect of Mass Irregularity on Reinforced Concrete Structures using Etabs, International Journal of Innovative Research in Science, Engineering and Technology Vol-4, Issue-10, October 2015.

  10. R Saranraj, Comparison of Displacement for Regular and Irregular Building Due to Seismic Forces, Imperial Journal of Interdisciplinary Research, Vol-2, Issue-6 , 2016.

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