Second Order or P-Delta Effects in Tall Structures

IJSRD - International Journal for Scientific Research & Development| Vol. 4, Issue 05, 2016 | ISSN (online): 2321-0613

Second Order or P-Delta Effects in Tall Structures Chetan Pramod Gull1 Vinod I. Hosur2 1 P.G. Student 2Assistant Professor 1,2 Department of Civil Engineering 1,2 KLS Gogte Institute of Technology Belgaum, Karnataka, India Abstract— Tall building structures are in practice now a days. Due to the frequent earthquakes happening around the world, these buildings need to be designed Earthquake resistant. The second order effect or P-delta effect has to be included in the analysis since the deflections are generally large for tall building structures. The objective of the current work is to study is to the influence of second order or P-Delta effect in the structural behaviour of tall building structures. Alternative lateral load resisting systems i.e. OMRF (Ordinary moment resisting system), SMRF (Special moment resisting system), Shear wall system and Bracing system for the tall building structures are considered. For this purpose 5, 10, 15 and 20 story structures are modelled and analysed using ETABS software and using the Indian Standard Codes for analysis and design namely, IS 1893:2002, IS 456:2000. The buildings modelled are with floor area of (20m x 20m) with 4 bays of 5m span along both directions. Key words: Second Order Effect, P-Delta Effect, SMRF, OMRF, Bracing System, Shear Wall System

the structure that is equal to the total vertical load (P), times the lateral displacement (Δ).

Fig. 2: P- “Little” Delta (P-δ) Effect P- “Little” delta (P-δ) effect has reference to the effects of the axial in an individual members of the structure, subjected to a deflection (curvature) between its endpoints due to buckling of member. For example, column load (P), due to gravity load, wind load or seismic forces act on a column, that has curvature induced in it by buckling which depends on the type of the beam-column connection. Moments are induced in member equal to the axial load (P), times the member deflection (δ).

I. INTRODUCTION Definition: “ When the horizontal loading acts on a building causing it to deflect, the resulting eccentricity of the gravity loading from the inclined axes of structure’s vertical members causes the lateral displacements of the structure and the moment in the member to increase. This secondorder effect is termed as the P-Delta effect”. P- Delta effect is the non-linear (Second order) effect that occurs in every structure, where the elements are subjected to axial loads. D-delta effect is the genuine effect that is associated with the Magnitude of the applied axial Load (P) and the lateral displacement (Delta). II. TYPES OF P-DELTA EFFECTS  

P- “BIG” Delta (P-Δ) effect: Structural Instability Effect. P- “Little” Delta (P-δ) effect: Member Instability Effect.

Fig. 1: P- “BIG” delta (P- Δ) effect P- “BIG” delta (P- Δ) effect has references to the effects of vertical load acting on the laterally displaced structures. For Example, Seismic force or wind force (V), Causes a horizontal displacement (Δ), of the structure, while the gravity load (P), simultaneously act vertically on this displaced structure. Due to this moments are introduced into

III. METHODS OF Ρ-DELTA ANALYSIS OF BUILDING STRUCTURES

    

Amplification Factor Method Iterative Method Iterative Gravity Load Method Negative Property Fictitious Member Method Moment Amplification Factor Method IV. LITERATURE REVIEW

Regina Gaiotti and Bryan Stafford Smith (1989) inferred that, the demand for the high rise slender and lighter buildings has led the structures possibly more likely to be effected by the p-delta effect. Several methods estimating second order effects have been compared in terms of their accuracy, effectiveness and ease of calculation. The main factor that differentiate these different method of p-delta analysis are the ease of use or calculation, accuracy, time taken for the analysis and whether they fit for the hand calculation or the computer calculation only and whether they can be applied to all type of structures or only specific type of structure. The American institute of steel construction, load and resistance factor design (AISC-LFRD), indicates that the P-delta or the second order effect should be accounted in the design of structural frames. To consider this second order or p-delta effect for the design requirements, the AISC-LFRD provided the approximate method, based on first order analysis with Moment Amplification Factor (Sheng-Jin Chen and Wu-ChyuanWang, 1999).

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Second Order or P-Delta Effects in Tall Structures

Published on Apr 13, 2017

International Journal for Scientific Research and Development - IJSRD

Tall building structures are in practice now a days. Due to the frequent earthquakes happening around the world, these buildings need to be designed Earthquake resistant. The second order effect or P-delta effect has to be included in the analysis since the deflections are generally large for tall building structures. The objective of the current work is to study is to the influence of second order or P-Delta effect in the structural behaviour of tall building structures. Alternative lateral load resisting systems i.e. OMRF (Ordinary moment resisting system), SMRF (Special moment resisting system), Shear wall system and Bracing system for the tall building structures are considered. For this purpose 5, 10, 15 and 20 story structures are modelled and analysed using ETABS software and using the Indian Standard Codes for analysis and design namely, IS 1893:2002, IS 456:2000. The buildings modelled are with floor area of (20m x 20m) with 4 bays of 5m span along both directions.