SSRG International Journal of Civil Engineering (SSRG-IJCE) volume 4 Issue 7 July 2017

Transcription

1 Study of Torsional Irregularity in Irregular Structure Provided with Lead Rubber Bearing Isolator Nandini K.G 1, Pranathi Reddy.B 2 1 Post Graduate Student, Department of Civil Engineering, The ACS College of Engineering, Bangalore, India 2 Assistant Professor, Department of Civil Engineering, The ACS College of Engineering, Bangalore, India Abstract-Structure collapse occurs due to the torsional motions causing significant damage to the building. Analysis using the provisions of IS and with the aid of time history and response spectrum given for soft, medium and hard soils in foundations, the basis of multi-storey buildings are taken to be fixed. In the seismic response, type of soil present in and around the foundation affects in the seismic response of the structure. Comparative study on the structures with plan irregularity is done. Base isolation is usually carried out for soft and medium soil. This study is mainly concentrated on medium soil conditions. The RC framed building of different models of storey height G+10 models is considered to analyze the fixed base condition and base isolation condition with torsional irregularity. The base isolation is assigned for the fixed base model condition by calculating the design procedure for base isolator and the results are compared for fixed and base isolation model for earthquake and response spectrum parameter and torsional irregularity for the earthquake parameter for fixed and base isolation results are compared. The analysis of the structural models is carried out using ETABS 2015 software, version 0.2 Keywords- Response spectrum analyses, torsion irregularity, lead rubber bearing isolator I. Introduction Earthquake causes great destruction by sudden violent ground shaking leading to huge loss of life and property. Sudden release of energy from the lithosphere is known as earthquake. Interaction between interior of the crust and the earth is the main cause for earthquake. Earthquake Ground Motions are the effective natural hazards which tend to loss of life & property. Most of the earthquake losses are structure collapse. It is most considerable step to be considered while designing the structures to resist against earthquake ground motion. If the existing building is not designed for earthquake then some alternative measures should be consider in terms of retrofitting. Consequential damages are caused in the nonuniform structures due to disparate distribution of mass, strength and stiffness improperly and also leads to tensional motions. Such buildings may go through tensional motions. But it s difficult to attain ideal condition because of architectural requirements and practical needs. The earthquake structures are designed as per Indian standard code IS1893:2002. For structures having height less than 40m the seismic forces are determined by equivalent static force method. Hence study deals with medium rise buildings. Generally the buildings are almost irregular, absolute regularity is an idealization which occurs infrequently. In order to identify the torsionally irregular structures, IS 1893 (Part 1): 2002 has given the clear definitions of irregular buildings in Clause 7.1. An expression for the design eccentricity, which is very much needed for the analysis of torsionally unbalanced structures is given in Clause 7.9 of the IS In the codal provisions, it is also suggested that, the method of analysis to be used for a structure, depends on its irregularity, in addition to total height of the structure and the seismic zone where it is situated (Clause 7.8.1). To understand the importance of codal provisions, which are especially meant for plan asymmetric buildings, an attempt is made in the present study considering various parameters, which are contributing torsional irregularity. The structure configuration has been defined as uniform or non-uniform in terms of size and shapes of structures, positioning of structural elements and mass. Regular building configurations are almost symmetrical (in plan and elevation) about the axis and have uniform distribution of the lateral force-resisting structure. A building that lack in symmetry and has a lack of continuity in geometry, mass or load resisting elements is called irregular building. These irregularities cause interruption of stress concentrations and force flow. Asymmetrical arrangement of mass and stiffness of elements may cause a large torsional force. Torsion in buildings is caused due to seismic motion because of non-symmetric distributions of mass and stiffness. An eccentricity in building leads to torsional response during earthquake. Modern codes such as EC8, UBC97and Greek code are referred for the torsional design in the building. ISSN: Page 55

2 distribution of mass and stiffness in plan and in preferment lead to less harmful than irregular configurations as per Indian Standard IS 1893 (Part 1): 2002 Fig : Torsion irregularity. The different buildings with different shaped plans that are shown below in figure Base Isolation Base Isolation concept is introduced in the USA and New Zealand in 1980.Base isolators are most often installed at the base level of a building and it is called base isolation. In order to reduce the vibrations leading to damage of the building. Base isolation is defined as an easily bending without breaking material which is fixed at base of the structure to reduce earthquake forces. Isolators are provided at the base of the structure which results in reduction of earth motion transmitted to the superstructure above isolator, minimizing the reaction of a typical structure and corresponding loading. They are detected between the foundation and the building structure and are designed to minimize the magnitude and frequency of seismic shock permitted to enter the building. They are provided with both spring and energy absorbing characteristics. Fig 1.1: Plan View of the buildings with different shapes. Asymmetry cause substantial unreliability in the capacity of the buildings to meet the design justification of the code these requirements are studied only to make designers aware of the existence and potential harmful effects of irregularities and to provide minimum necessity for their accommodation. The type of irregularities considered includes: i) Vertical mass irregularities. ii) Vertical stiffness: strength irregularities for buildings with constant interstorey heights. iii) Horizontal stiffness/strength asymmetry on rigid floor diaphragm buildings which leads to tensional deformation. iv) Horizontal floors diaphragm plasticity which affects the structural behaviour. 1.2 Regular Configuration and Irregular Configuration. To resist against earthquake a building should have simple and systematic configuration, sufficient lateral strength, stiffness and ductility. Structures which have systematic regular geometry and uniform Fig1.2 Fixed base v/s Base-isolated building (Nelson 2010) 1.4 Classification of Base Isolators: The most common types of base isolator used in building are 1) Laminated Rubber (Elastomeric) Bearing. a. Natural rubber bearing (high damping). b. Natural and synthetic rubber (low damping). 2) Friction Pendu1um Bearing Isolator. 3) Lead Rubber Bearing (LRB) 1.5 Lead Rubber Bearing Isolator The LRB was first used in New Zealand in 1995 and later used in large scale in New Zealand, ISSN: Page 56

3 Japan and United states. LRB are laminated same as low-damping rubber bearings. Lead rubber bearing consists of fine layer of low damping natural rubber and steel plates built in alternate layers and a lead cylinder plug firmly fitted in a hole at its centre to deform in pure shear as shown in figure (E). Functions of Lead Rubber Bearing Base Isolator are as follows Load Supporting Functions: Structure is stable by the support of steel plates which is reinforced by rubber. Single-1ayer rubber pad construction gives accurate vertical rigidity for supporting a building. Horizonta1-Elasticity Function: By providing LRB seismic motions are converted to lowspeed motions. The horizontal stiffness of multi-layer rubber bearing is low, severe seismic vibrations are controlled and To and Fro motion of the building is increased. Restoration function: Horizonta1-e1asticity of LRB returns the building to its original state. Fig 1.4 Lead Rubber Bearing 1.7 Advantages of Base Isolation Base isolators resists against structural damage during earthquake The base Isolation will protect the building by preventing plastic deformation of structural elements. Secondary damage and injury like falling of furniture s and other materials will be avoided. Even after the earthquake super-structure is designed to be remaining elastic. Evacuation routes and corridors are secured with base-isolators in building. Reduces weight of building & cost of construction. Base isolator reduces cost of repair of structure after earthquake and reduces loss of life. 1.8 Limitations of Base Isolation It is suitable only for low to medium-rise buildings on hard soil. As the period of vibration in building increased with increase in height. Fig1.3: Lead Rubber Bearing 1.6 Design of Lead Rubber Bearing Base-Isolator: The isolator considered in this work is LRB base isolator and design follows the Earthquake Engineering Handbook by W.H Chen and Charles Scawthorn. The mechanical properties of the LRB base-isolator system follow the Earthquake handbook. The design parameters considered are verticalstiffness (Kv), horizontal-stiffness (Kh), Pre yield stiffness (Ku), Yield force of lead plug (Q d ) and Post Yield Stiffness Ratio. Cost involved in constructing a new building is higher than the cost of conventional earthquake resistant structural system, seismic isolation bearings are expensive. II.AIM AND SCOPE 2.1 Aim of the Study This thesis work is a comparative study on the reinforced concrete structure with different types of irregularities. These different irregular types models are subjected to dynamic analysis.dynamic parameters such as base shear, displacement, storey drifts, mode shapes and torsional irregularity are obtained and comparisons are shown and drawn. The scope of this study is to compare the performance of an irregular structure with different types of irregularities to conclude the effectiveness of STUDY OF TORSIONAL IRREGULARITY IN IRREGULAR STRUCTURES PROVIDED WITH LEAD RUBBER BEARING ISOLATOR. ISSN: Page 57

4 2.2 Parametric Study These building models are analyzed for the following case Using equivalent static lateral force method as per IS 1893(part 1):2002. Displacements, Storey-drift, base-shear and mode shape. Torsional irregularity. III. METHODOLOGY 1) A detailed introduction regarding Earthquake, torsion irregularities, regular and irregular shaped structures and their configurations plan and vertical irregularities, and base isolation. 2) A complete literature survey is carried out on study of torsional irregularities in irregular structures. 3) The RC frame was modelled in software ETABS ) The analysis of the prescribed RC frame was carried out to evaluate the Vibration Parameters (Natural Frequency, mode shapes and corresponding model participating mass ratios) of a RC building frame by carrying out model analysis method using computer program ETABS Maximum axial load (P) on column was noted down after modal analysis for a load combination of 1.5(DL+LL+FF) in case of fixed base modal. The measured value of axial load an isolator i.e. Lead Rubber Bearing was designed as per the formulations provided in Earthquake Engineering Handbook by W. F. Chen. 5) By removing the restraints of fixed base modal LRB parameters are calculated. 6) Equivalent static and linear response spectrum analysis were carried out to compare the performance of fixed base model and LRB base isolated model. 7) The dynamic analysis of reinforced concrete frame is compared to get the displacement, base shears, storey drifts and mode shapes and torsional irregularities of the structure. The reinforced concrete frames models are with different plan irregularities are analyzed. 8) Results thus obtained from the dynamic analysis are tabulated, discussed and graphs plotted for mode period, disp1acement, storeydrift, and baseshear and torsional irregularities. 3.2 Salient Features of the Building The Constant Parameters of Building Model is studied using ETABS software of version The modal considered for this analysis is G+ 10 storey s. 3.3 Other Parameter In this work comparative study is carried out on irregular buildings fixed base modal and LRB base modal. The buildings consist fixed RC framed building with plan irregularities such as (I SHAPE AND L SHAPE) and base isolator has been assigned for the fixed base building. The study is carried out for G+10 storeys and for zone that is for zone V respectively and typical storey height considered is 3m. The column size is 230X450 and Plinth Beam size is 230X450 for G+10storey building respectively. Types of structure considered for the study is 3.1 Objective of the Work The main objectives are as follows: 1) To evaluate the response spectrum of a reinforced concrete frame structure (i.e., displacement, base shear, storey drift, mode shapes and torsional irregularities) by Equivalent static lateral force method using software ETABS ) Response spectrum method is used as per Indian Standard-1893 Part-1: ) All the models are studied and analysed using response spectrum method. ISSN: Page 58

5 Fig 3.1: Irregular plan view of I Shape model Fig 3.2: 3D View of I Shape model Fig 3.3: L R B Provided For L&I Shape In 3d View 3.4 Loading of Structure Structures loads vary from loads of lowrise0building in to much larger structural forces, with the increase in the loading in the greater importance of dynamic0effects. Three0types of loads are considered for the analysis of structure and for design purposes. The loads which is considered as gravity loads they are dead0load, live load, wind and earthquake0loads. 3.5 Gravity Loads on Structure Dead0loads are called as gravity0loads that0will be applied laterally on the structural frame with consideration of earthquake0motion. Live0loads are also called as gravity0loads that do not apply laterally at0the same rate as the structural0frame when the Structure0undergoes with the consideration of earthquake motion. 3.6 Lateral Loads The loads which are applied almost horizontally and those loads must be taken consideration for the structural0analysis and0design. Such loads are defined as lateral0loads. Lateral0loads are important for0structures are earthquake and wind loads. 3.7 Earthquake Load Seismic load contains inertial forces of structures mass that results in vibration of its substructure by a seismic interference. Other severe seismic forces may occur due to land sliding, collapse, active faulting below the foundation or liquefaction of the local sub grade as a result of vibration. Whereas earthquakes occur, their intensity is inversely proportion to their frequency of occurrence like severe0earthquakes are rare, moderate ones are often and minor ones are relatively frequent. ISSN: Page 59

6 IV.ANALYSIS RESULTS OF FIXED BASE AND BASE ISOLATOR. When analysis is completed, the comparison of the fixed base and base isolation storey drifts, displacement, base shears and mode shapes values and torsional irregularities values are taken and graphs are plotted. 4.1 Torsional Irregularity Torsional irregularity is one such horizontal irregularity which has to be taken care while designing a structure. A building is certified to be stable during its design process by inspecting the entire structure for both vertical and horizonta1 irregularities which is mentioned according to various international codes. The ill effects of irregu1arities are mainly experienced when a building is subjected to seismic motion which leads to damage and even the collapse of the structure. Before explaining the term torsional irregularity, one has to be acquainted with the terms drift, storey drift, eccentricity and torsion. Torsional irregularity exists if, dmax/davg >1.2 Recent codes have defined torsional irregularity as the condition where the maximum storey drift inc1uding accidental torsion at end of the structure transverse to an axis is more than 1.2 times the average of the storey drifts at the two ends of the structure. When the earthquake forces occurred in X-dir then the total drift will be more in the opposite ends of the structure and in Y-dir also the total drift will be more in the opposite directions. The below pictures shows that the deformed shape for the earthquake forces occurred in both directions for the fixed base model. If the earthquake occurred in the X-dir then the opposite corners of the building points has been named as R1 and R2, similarly in the Y-dir earthquake occurred, the opposite corners of the building are named as R3 and R4. But the total drift should not be more than the permissible drift which is the height of the structure by 250. To determine the torsional irregularity of the building the column sizes has been reduced and earthquake forces has been considered for the same model and the total drifts has been determined and the cross check of the total drift has been done. I Shape Fixed Base Deformed Shape for Earthquake in Both Directions Calculations: Earthquake in X-dir: R1 = 31.04mm, R2 = mm. Earthquake in Y-dir: R3 = mm, R4 = mm. Permissible Drift = H/250 = 31.5/250 = 126mm So our storey is within permissible drift. Check for Torsional irregularity in X-dir. Dmax = Davg = (R1+R2)/2 = ( )/2 = (Dmax/Davg) = (32.109/ ) = 1.01 < 1.2 Check for Torsional irregularity in Y-dir. Dmax = Davg = (R1+R2)/2 = ( )/2 = (Dmax/Davg) = (47.899/47.78) = 1.0 < 1.2 So the torsional irregularity for the fixed base in both directions does not exist. To check the torsional irregularity of the structure when the column section has been revised for the same model and the earthquake forces has been considered and then the comparison has been done. Earthquake in X-dir: R1 = mm, R2 = mm. Earthquake in Y-dir: R3 = mm, R4 = mm. Check for Torsional irregularity in X-dir. Dmax = ISSN: Page 60

7 Davg = (R1+R2)/2 = ( )/2 = (Dmax/Davg) = (46.412/37.564) = 1.24 > 1.2. So torsional irregularity exist. Reconsider the design of the structure. Check for Torsional irregularity in Y-dir. Dmax = Davg = (R1+R2)/2 = ( )/2 = (Dmax/Davg) = (50.931/51.02) = 0.99 > 1.2. I Shape 3d View Fixed Torsion Irregularity I Shape Plan View of Fixed Torsion Irregularity ISSN: Page 61

8 Earthquake in Y-dir: R3 = mm, R4 = mm. Check for Torsional irregularity in X-dir. Dmax = Davg = (R1+R2)/2 = ( )/2 = (Dmax/Davg) = (40.783/ ) = 1.23 > 1.2. So torsional irregularity exist. Reconsider the design of the structure. Check for Torsional irregularity in Y-dir. Dmax = Davg = (R1+R2)/2 = ( )/2 = (Dmax/Davg) = (44.977/45.054) = 0.99 > 1.2. I Shape LRB Base Deformed Shape for Earthquake in Both Directions Calculations: Earthquake in X-dir: R1 = mm, R2 = mm. Earthquake in Y-dir: R3 = mm, R4 = mm. Permissible Drift = H/250 = 31.5/250 = 126mm So our storey is within permissible drift. Check for Torsional irregularity in X-dir. Dmax = Davg = (R1+R2)/2 = ( )/2 = mm. (Dmax/Davg) = (28.971/28.055) = 1.03 < 1.2 Check for Torsional irregularity in Y-dir. Dmax = Davg = (R1+R2)/2 = ( )/2 = (Dmax/Davg) = (42.894/ ) = 1.0 < 1.2 So the torsional irregularity for the LRB base for both directions does not exist. To check the torsional irregularity of the structure when the column section has been revised for the same model and the earthquake forces has been considered and then the comparison has been done. Earthquake in X-dir: R1 = mm, R2 = mm. I Shape Plan View LRB Torsion Irregularity ISSN: Page 62

9 REFERENCES I Shape 3d View LRB Torsion Irregularity V.CONCLUSION Torsional irregularity exists in the X-dir when the column size reduces in the fixed and LRB base model. So we must increases the lateral stiffness along the column with maximum storey drift by increasing the column sizes, retaining walls or bracings. So LRB and Torsional irregularity is important parameters for the design of the tall structures. ISSN: Page 63