Comparative Study of Shear Walls and Bracings for A Multistoried Structure Under Seismic Loading

Comparative Study of Shear Walls and Bracings for A Multistoried Structure Under Seismic Loading

Karnati Vijetha1 , Dr. B. Panduranga Rao2

1M. Tech Stuructural Engineering

2 Professor, M. Tech Stuructural Engineering Velagapudi Rama Krishna Siddhartha Engineering College

Abstract:- Earthquake is the natural calamity, it produce strong ground motions which affect the structure. Small or weak motions that can or cannot be felt by the humans. Provision of shear walls and bracings are installed to enhance the lateral stiffness, ductility, minimum lateral displacements and safety of the structure. Storey drift and lateral displacements are the critical issues in seismic design of buildings.

Three types of frame models are developed and evaluated by static analysis by ETABS . In the present work G+15 multi Storey building is analysed by using shear wall and braced frame at outer most of the structure and Comparison with multistoried structure . Main purpose of this study is to compare the seismic response of the structure.

BACK GROUND:

Lateral forces on buildings such as wind, earthquake and blast forces can be produced critical stresses in the buildings that it cause excessive lateral sway of the buildings and undesirable stresses and vibrations in the buildings. Design and structural evaluation of the building systems subjected to lateral loads form the important task of the present generation and the designers are faced with problems of providing adequate strength and stability of buildings against lateral loads. Different lateral loads resisting systems are used in high-rise building as the lateral loads due to earthquakes are a matter of concern. Steel plate shear walls system and steel bracings system are used in steel structures buildings and their effect shows unequal variations and behaviour against seismic loads. Recently, laminated composite plate shear walls are used as a lateral loads resisting system where the laminated composite plates are used as infill plate in shear walls. The laminated composite plates are created by constructing plates of two or more thin bonded layers of materials and it can be either cross-ply laminates or angle-ply laminates.

The major criteria now-a-days in designing RCC structures in seismic zones is control of lateral displacement resulting from lateral forces. In this thesis effort has been made to investigate the effect of Shear Wall position on lateral displacement in RCC Frames. Eight models of various shapes of shear walls with varying thickness are arranged at different locations of eleven(11) storeyed framed building and performed linear static analysis, obtained displacements is compared with the corresponding frame without shear wall.

In this project shear wall systems are taken consideration and executed for lateral forces like wind and earthquake.

DEFINITION OF SHEAR WALL:

Shear walls are vertical elements of the horizontal force resisting system. Shear walls are constructed to counter the effects of lateral load acting on a structure. In residential construction, shear walls are straight external walls that typically form a box which provides all of the lateral support for the building. When shear walls are designed and constructed properly, and they will have the strength and stiffness to resist the horizontal forces.

SCOPE OF THE WORK

The scope is to analyse the constructed shear wall that is to be constructed. Firstly the different thickness of shear wall is placed at different locations are implemented into known computer software and then it is analysed based on the investigation of strength based on design codes. The strength of shear walls tested are compared with the strength of frame correspondingly of without shear wall.

• Only multi-storey frames are considered.

• Plan irregularities are not considered.

• Shear walls are considered for the frame at different position for the study of linear static analysis.

• Linear static analysis is used to predict the actual performance of the RC Frames under lateral loadings.

  OBJECTIVE

  The walls are structurally integrated with roofs / floors (diaphragms) and other lateral walls running across at right angles, thereby giving the three dimensional stability for the building structures. Shear wall structural systems are more stable.

  Walls have to resist the uplift forces caused by the pull of the wind. Walls have to resist the shear forces that try to push the walls over. Walls have to resist the lateral force of the wind that tries to push the walls in and pull them away from the building.

  PROJECT OBJECTIVES:

• To determine the effective location of shear wall and bracings on the basis of storey displacement under lateral loading.

• To determine percentage reduction in storey displacement with different places of shear wall and Bracings at different locations on different models when compared to without shear wall.

METHODOLOGY

For the purpose of study a plan of eleven storeyed frame were considered. For linear elastic study, RC plane frames with and without shear wall were analysed and designed for gravity loads as per IS 456:2000 and lateral loads (earthquake loads) as per IS 1893 (part-1):2002.

SHEAR WALLS

Shear walls are vertical elements of the horizontal force resisting system. Shear walls are constructed to counter the effects of lateral load acting on a structure. In residential construction, shear walls are straight external walls that typically form a box which provides all of the lateral support for the building. When shear walls are designed and constructed properly, and they will have the strength and stiffness to resist the horizontal forces. In building construction, a rigid vertical diaphragm capable of transferring lateral forces from exterior walls, floors, and roofs to the ground foundation in a direction parallel to their planes. Examples are the reinforced-concrete wall or vertical truss. Lateral forces caused by wind, earthquake, and uneven settlement loads, in addition to the weight of structure and occupants; create powerful twisting (torsion) forces. These forces can literally tear (shear) a building apart. Reinforcing a frame by attaching or placing a rigid wall inside it maintains the shape of the frame and prevents rotation at the joints. Shear walls are especially important in high-rise buildings subjected to lateral wind and seismic forces. In the last two decades, shear walls became an important part of mid and high-rise residential buildings. As part of an earthquake resistant building design, these walls are placed in building plans reducing lateral displacements under earthquake loads. So shear-wall frame structures are obtained.

Shear wall buildings are usually regular in plan and in elevation. However, in some buildings, lower floors are used for commercial purposes and the buildings are characterized with larger plan dimensions at those floors. In other cases, there are setbacks at higher floor levels. Shear wall buildings are commonly used for residential purposes and can house from 100 to 500 inhabitants per building.

PURPOSE OF CONSTRUCTING SHEAR WALLS

Shear walls are not only designed to resist gravity / vertical loads (due to its self-weight and other living / moving loads), but they are also designed for lateral loads of earthquakes / wind. The walls are structurally integrated with roofs / floors (diaphragms) and other lateral walls running across at right angles, thereby giving the three dimensional stability for the building structures. Shear wall structural systems are more stable. Because, their supporting area (total cross-sectional area of all shear walls) with reference to total plans area of building, is comparatively more, unlike in the case of RCC framed structures. Walls have to resist the uplift forces caused by the pull of the wind. Walls have to resist the shear forces that try to push the walls over. Walls have to resist the lateral force of the wind that tries to push the walls in and pull them away from the building.

Shear walls are quick in construction, as the method adopted to construct is concreting the members using formwork.

Shear walls doesnt need any extra plastering or finishing as the wall itself gives such a high level of precision, that it doesnt require plastering.

BRACINGS

INTRO

Bracing is a highly efficient and economical method to laterally stiffen the frame structures against wind loads. A braced bent consists of usual columns and girders whose primary purpose is to support the gravity loading, and diagonal bracing members that are connected so that total set of members forms a vertical cantilever truss to resist the horizontal forces. Bracing is efficient because the diagonals work in axial stress

Tall Buildings and Structural Form

Tallness is considered as relative term which cannot be defined in terms of height. From structural engineers point of view, tall building is defined as the building whose structural design is governed by the lateral forces induced due to wind and earthquake.

The structural form of a tall building depends on a number of factors, some are given below;

• Internal planning

• Material and method of construction

• External architectural treatment

• Location and routing of service system

• Nature and magnitude of horizontal loading

• Height and proportion of building

Following are some structural forms of the tall buildings:

E-TABS SOFTWARE

History of E-Tabs

A well-known and established across the world a structural & earthquake engineering Software Company, Computers and Structures, Inc. (CSI) established in 1975 and located in Walnut Creek, California with further office position in New York. The structural analysis and design software CSI is a developer a lots of software including CSiBridge, SAFE, and CSiCOL, ETABS and SAP2000. The most useful structural analysis and design software developed by Computers and Structures, Inc, is ETABS which at firs utilized to develop the mathematical complete model of the Burj Khalifa, right now the highest building of the world that has been developed and designed by Chicago, Illinois-based Skidmore, Owings & Merrill LLP (SOM). Taking position of the Design and Construction of the worlds highest building in their Structural Engineering magazine article on December 2009 in the Structural analysis part: The Burj Dubai next time renamed as Burj Khalifa, William F. Baker,

S.E. and James J. Pawlikowski, S.E. mentioned that the gravity as well a wind and seismic behaviour were everything considered using ETABS. Further, ETABS geometric nonlinear capability provided for P-Delta Effect consideration. Since the launches of this program, now is being used across the world by the Civil Engineering profession who are practicing structural analysis and design.

About ETABS

It is an engineering software product that caters to multi- story building analysis and design. Modelling tools and templates, code-based load prescriptions, analysis methods and solution techniques, all coordinate with the grid-like geometry unique to this class of structure. Basic or advanced systems under static or dynamic conditions may be evaluated using ETABS. For a sophisticated assessment of seismic performance, modal and direct-integration time- history analyses may couple with P-Delta and Large Displacement effects. Nonlinear links and concentrated PMM or fibre hinges may capture material nonlinearity under monotonic or hysteretic behaviour. Intuitive and integrated features make applications of any complexity practical to implement. Interoperability with a series of design and documentation platforms makes ETABS a coordinated and productive tool for designs which range from simple 2D frames to elaborate modern high-rises.

Use of E-TABS

Generally, ETABS is used for the analysis of concrete shear walls and concrete moment frames. Once we are able to limit the drift, we can output the forces from ETABS into a spreadsheet for design. We design the piers of the shears

walls for flexure and axial loads using PCA Column. We design the moment frames using PCA Slab. For the gravity design of the concrete floor system, we use RAM Advance and SAFE. ETABS is also very useful in designing complex steel braced frame and moment frame lateral system. It is especially useful in seismic applications. However, RAM is more suitable for the gravity design of steel floor systems, in my opinion. Often we have two models; a RAM gravity model and an ETABS lateral model.

EQUIVALENT STATIC ANALYSIS (LINEAR STATIC)

This method is also called linear static method. The equivalent static lateral force method is a simplified technique to substitute the effect of dynamic loading of an expected earthquake by a static force distributed laterally on a structure for design purposes. The total applied seismic force V is generally evaluated in two horizontal directions parallel to the main axes of the building (Fig. 8). It assumes that the building responds in its fundamental lateral mode. For this to be true, the building must be low rise and must be fairly symmetric to avoid torsional movement underground motions. The structure must be able to resist effects caused by seismic forces in either direction, but not in both directions simultaneously.

Fig. Lateral force on two axis

Linear elastic seismic analysis of structures is dependent on many assumptions. First assumption is, all the buildings will behave more or less the same under a particular sets of rules. For example, all shear wall buildings will have similar amount of ductility demand, all moment frame buildings will also have similar behaviour. Which in reality is not true. There are some building codes which understand this scenario. They assign additional factors to be incorporated on the building forces, which amplifies the seismic demands. There are some limitations to this method as well but it is still better in predicting the seismic demands.

The seismic demands on the structure is estimated by different methods. Elastic method and inelastic method. In elastic method, we consider that the structure remains elastic and the seismic demands on the structure is reduced using response reduction factor which assumes that the structure has the capacity to go beyond elastic limit but the performance is never checked. While in inelastic method, unreduced seismic demands are tested against the force resisting capacity and inelastic deformation capacity of the structure. In this scenario a true performance of the structure is tested and made sure it meets all the criteria of collapse prevention under maximum considered earthquake.

Fig.Equivalent SDOF

ELF analysis is based on an assumption of static cantilever beam. There is a slight effect of second mode of the structure taken into consideration in the story shear distribution but nothing more than that. The amount of seismic base shear consider for designing the building is on the basis of approximate period of the building, site specific ground acceleration and response spectrum curve, site class of the site and type of building system used to resist the lateral forces. This type of analysis should only be used when the building is symmetric, torsion is minimal, no vertical or horizontal irregularities and no discontinuities in the system and where the primary mode of the structure governs the structural dynamics. This means that ELF analysis leads to a fairly accurate resuls for short and very symmetric and regular buildings.

ELF analysis, story force is generated as per the height at which the story is located from the seismic base. Higher the story up in the building, more loads will be generated by that story. But the drawback of this method is, the if there is a significantly heavier story near the base of the building, let us say Level 2 or 3, the contribution from this heavier story

present close to the bottom of the building is significantly less. The only reason is because the equations are developed in such a way that it gives more weight to the height of the story from the base as compared to the story weight itself. This was just an example of drawback of ELF analysis. And so, it is always recommended that ELF analysis should be carried out on structures that are very regular and symmetric and have do not exhibit any complex behaviour.

MODELLING OF MULTI-STOREY FRAME USING E- TABS

For this study, 11-story building with a 3.5 meters bottom storey height and 3 meters typical for each storey, regular in plan is modelled. These buildings were designed in compliance to the Indian Code of Practice for Seismic Resistant Design of Buildings. The buildings are assumed to be fixed at the base. The sections of structural elements are square and rectangular. Storey heights of buildings are assumed to be constant except the ground storey. The buildings are modelled using software ETABS linear v 9.6.0-2015.

Fig. Dimensional details of model-1

Fig. Defining materials, beams, columns, slab sections and base joint restraints.

Fig. Model-1 before assigning beams, column and slab

1. MATERIALS

   Fig.Model-1 after assigning beams, column and slab

   dimensions, reinforcement details and the type of material

   The modulus of elasticity of reinforced concrete as per IS 456:2000 is given by

   For the steel rebar, the necessary information is yield stress, modulus of elasticity and ultimate strength. High yield strength deformed bars (HYSD) having yield strength

   500 N/mm2 is widely used in design practice and is adopted for the present study. Cement used is of grade M35.

2. STRUCTURAL ELEMENTS

   In this section, the details of the modelling adopted for various elements of the frame are given below.

   1. Beams and Columns

      Beams and columns were modelled as frame elements. The elements represent the strength, stiffness and deformation capacity of the members. While modelling the beams and columns, the properties to be assigned are cross sectional

      used.

      All the Beams in the frame were sized to 250mm*300mm All the columns in the frame were sized to 300mm*300mm

   2. Beam-Column Joints

      The beam-column joints are assumed to be rigid.

   3. Wall

      A periphery of 300mm wall is taken into consideration.

   4. Slab

      A slab of 150mm is considered as per IS 456:2000.

   5. Foundation Modelling

Fixed supports were provided at the ends of supporting columns.

6.3 Loads

Loads that are considered for the frame analysis are follows

1. Dead Load

2. Live Load

3. Lateral Load due to Earthquake and

4. . Lateral Load due to wind

1. Load cases

   As dead load and wall load are permanent loads on building so we take self weight multiplier as one for those cases.

   Fig.14 Load Cases

2. Load patterns

   1. Seismic pattern

   2. Wind pattern

      Fig.15 Seismic load pattern

      Note:

      Fig.16 Wind load pattern

      with Special Moment resisting Frame) with response

      Terrain category I Open sea; free lakes with at least 5 km surface wind direction; smooth, flat land without barriers. Terrain category II Terrain with hedges, individual farmsteads, houses or trees, for example agricultural areas Terrain category III Suburbs, industrial or commercial areas, forests

      Terrain category IV Urban areas in which at least 15% of the surface is built with buildings whose average height exceeds 15 m

      Site type I Hard soil

      Site type II Medium soil

      Site type III Soft soil

      We assumed Vijayawada as location of framed building. It is under seismic zone III with zone factor 0.16 and taking importance factor as 1.5 and with SMRF (Steel Building

      reduction factor as 5.

3. Member Loading

   All the members were assigned the following loadings.

   • Dead Load = 3 KN/m2

   • Wall load = 12 KN/m

   • Live Load = 4 KN/m2

   • Earthquake Loading—— as per IS-code:1983- 2002

   • Wind Loading—— as per IS875:1987

1. LOAD COMBINATIONS

The load combinations considered in the analysis according to IS 1893:2002 are given below.

COMB1 = 1.2(DL+LL) + EQx + Wx + 0.3(EQy + Wy)

MODEL 1-(G+15) STRUCTURAL MODEL

Structure Data

This chapter provides model geometry information, including items such as story levels, point coordinates, and element connectivity.

Story Data

Table – Story Data

Loads

This chapter provides loading information as applied to the model.

Load Patterns

Table – Load Patterns

Load Cases

– Story Stiffness

Table – Load Cases Summary

RESULTS

MAXIMUM STORY DISPLACEMENT EQX

TABLE- MAXIMUM STORY DISPLACEMENT EQX

MAXIMUM STORY DISPLACEMENT EQY

TABLE- MAXIMUM STORY DISPLACEMENT EQY

MAXIMUM STORY DISPLACEMENT WIND1 CASE

MAXIMUM STORY DISPLACEMENT WIND2 CASE

MAXIMUM STORY DRIFT EQX CASE

MAXIMUM STORY DRIFT EQY

MAXIMUM STORY DRIFT WIND1

MAXIMUM STORY DRIFT WIND2

MODEL2 STRUCTURAL MODAL WITH SHEAR WALL

MAXIMUM STORY DISPLACEMENT EQX

TABLE- MAXIMUM STORY DISPLACEMENT EQX

MAXIMUM STORY DISPLACEMENT EQY

TABLE-MAXIMUM STORY DISPLACEMENT EQY

TABLE- MAXIMUM STORY DISPLACEMENT WIND 1

TABLE-MAXIMUM STORY DISPLACEMENT WIND 2

TABLE- MAXIMUM STORY DRIFT EQX

TABLE-MAXIMUM STORY DRIFT EQY

TABLE- MAXIMUM STORY DRIFT WIND1

TABLE-MAXIMUM STORY DRIFT WIND2

MODEL3 STRUCTURAL MODAL WITH BRACINGS

MAXMIMUM STOREY DISPLACEMENT EQX

TABLE- MAXMIMUM STOREY DISPLACEMENT EQX

MAXMIMUM STOREY DISPLACEMENT EQY

TABLE-MAXMIMUM STOREY DISPLACEMENT EQY

MAXMIMUM STOREY DISPLACEMENT WIND1

TABLE–MAXMIMUM STOREY DISPLACEMENT WIND1

MAXMIMUM STOREY DRIFT EQX

TABLE-MAXMIMUM STOREY DRIFT EQX

MAXMIMUM STOREY DRIFT EQY

TABLE-MAXMIMUM STOREY DRIFT EQY

MAXMIMUM STOREY DRIFT WIND 1

TABLE-MAXMIMUM STOREY DRIFT WIND1

CONCLUSIONS

From the above results introducing shear walls reduces the sway or displacement

Providing shearwalls at adequate locations substantially reduces the displacements due to earthquake

Base Shear Of The above Mentioned Structures Heavily Increases And makes the Structure stable against siesmic loading.

The Natural Time period of the above designed Structures are highly reduced after placing of bracings and Shear walls with comparison to Normal structure.

The lateral forces are resisting capacity is highly incresed after the placement of Shear wall.

When Comparing the above Structures Lateral displacements are minimal when Shear wall are applied.

From the above Comparison of structures and through discussion it is concluded that Shear wall could improve the lateral Stability of the structures.

FURTHER IMPROVEMENT OF WORK

Different locations of Shear wall can be placed and analysed. Different types of bracings such as Vshape, Y shape, Inverted V shape, can also be used for further analysis.

Different Multistoried structures can be used for further analysis

REFERENCES

1. Significance of Shear Wall in Multi-Storey Structure With Seismic Analysis .( Rajat Bongilwar, V R Harne and Aditya Chopade)

2. Comparative study of multi-storied rcc building with and without shear wall ( Axay thapa & sajal sarkar )

3. Effective location of shear wall on performance of building frame subjected to lateral loading ( 1.md. rokanuzzaman, 2.farjana khanam, 3.anik das, s. reza chowdhury)

4. Study of Structural RC Shear Wall System in a 56-Story RC Tall Building(O. Esmaili ,S. Epackachi ,M. Samadzad ,and S.R. Mirghaderi)

5. Optimal location of shear wall in high rise building subjected to seismic loading (Rakshit Patil, Avinash S Deshpande, Shrishail Sambanni)

6. Effect of placement and openings in shear wall on the displacement at various levels in a building subjected to earthquake loads (Pooja R. Gupta and A.M. Pande)

7. Comparative study of behavior of shear wall with different percentage of opening for different aspect ratios ( ganesh n, jaybhave k,k. tolani)

8. 8.Comparitive study on behaviour of shear walls at different locations in a high rise structures (Sahithya Boyina, Gorantla Yaswanth).

9. 9.Comparison study of Shear wall and bracings under seismic loading in Multi-storey residential building(Dharanya,Gayatri,Deepika)