Seismic Performance of Asymmetrical Steel Structure using Lateral Load Resisting Systems

Seismic Performance of Asymmetrical Steel Structure using Lateral Load Resisting Systems

Mohmmad Younes Fazly1

1Post Graduate Student,

School of Civil Engineering, REVA University, Bengaluru.

Mr.Venkatesh Wadki 2

2Assistant Professor,

School of Civil Engineering, REVA University, Bengaluru.

Abstract The demand for high rise structure in commercial, residential and industrial areas are increasing all over the world. Such types of structure are flexible and constructed as light as possible, which have low value of damping that makes them unsafe to unwanted vibration. These vibrations generate some problem to the serviceability requirement of the structure and also decrease structural integrity with possibility of failure. In this study steel structures are taken for seismic performance evaluation. The steel buildings are modeled with different structural control system such as base isolator, damper and bracing with use of ETABS software. After that to evaluate structural response of building various ground motion data is applied. Equivalent static analysis is carried out for building model with each control system and the result of the seismic response of each control system is compared with other control system. Equivalent static analysis result shows that building with cross bracing shows lesser displacement than building with FVD and building with LRB.

Key Words: Seismic Performance, Conventional Bare Frame, Cross Bracing, Lead Rubber Bearing, Damper, Equivalent Static Analysis

1. INTRODUCTION

   Generally the reason for elevated structure is to exchange the primary gravity load securely. The common gravity loads are dead, live load. Likewise the structure should withstand the lateral load brought about by earthquake, blasting, and wind depending upon terrain categories. The lateral load decreases stability of structure by creating sway moments and induces high stresses. So in such cases stiffness could easily compare to strength to resist lateral loads.

   There are various ways of providing lateral load resisting system, for example, bracing, base isolation, damper, to improve seismic performance of structures. Base isolations is a passive vibration control system that does not require any outer power sources for its task and uses the movement of the structure to build up the control force. The upside of this method is to keep the structure basically versatile and along these lines guarantees security among enormous earthquake. Viscous damper are hydraulic devices that disseminate the kinetic energy of seismic occasions and pad the effect between the structures. They are flexible and can be intended to permit free movements just as controlled damping of a structure to protect from wind load, thermal motion or seismic event. The improvement of bracing made the construction of high rise structure possible.

   Bracing are strong in compression. At the point when bracings are put in steel outline it acts as diagonal compression strut and transmits compression force to another joint. Variety in the column stiffness can impact the method of failure and lateral stiffness of the bracing

   Fig. 1 A: Typical Floor Plan

   Fig.2: 3D Modeling steel structure with Bare frame ofu10, 15, 20 storey.

   Fig.3: 3D Modeling steel structure with Brace frame ofu10, 15, 20 storey.

   Fig.4: 3D Modeling of steel structure with friction damper of 10, 15, 20 storey.

   Fig.5: 3D Modeling of steel structure with Lead Rubber Bearing of 10, 15, 20 storey.

   Table 1: Data of Structure

   SECTION MODEL	DIMENSIONS
   Beam	ISMB 600
   Column	ISMC 400
   Plan	10, 15, 20 storey model
   Column Spacing	4m in both direction
   Floor height	3 m
   Steel section	Fe345
   Slab thickness	100mm M25 grade
   Shear wall thickness	200 mm
   Bracing (X)	ISMB 450
   Damper type	Friction Damper
   Base isolation	Lead Rubber Bearing
   Live Load	3.5 KN/ m^ 2
   Superimposed Dead Load	1.5 KN/ m^ 2
   Live Loads on Roof	1.5 KN/ m^ 2
   Seismic Zone	V
   Seismic Factor	0.36
   Soil Type	Medium type 2
   Importance Factor	1.5
   Reduction Factor	5
   Earthquake Load	X and Y Direction
   Floor Finish	1 KN/ m^ 2
   Unit Weight of Steel	78 KN/ m^ 3

2. DETAILS OF LEAD RUBBER BEARING (LRB)

   Lead rubber bearing are made up of a standard elastomeric laminated rubber bearing the rubber compound can be natural or chloroprene rubber. The shape can be round or

   rectangular. The calculations for the design of LRB are as per the provisions of UBC-97.

   Table 1: Detail of LRB Base isolator

   Effective Stiffness	1065 KN/ m
   Horizontal stiffness	350
   Vertical Stiffness	180
   Yield Force	20 KN
   Stiffness Ratio	0.1
   Damping	0.05

3. DETAILS OF FRICTION DAMPER

   In these kinds of damper the energy is consumed by surfaces with frictions between them scouring against one another.

   Table 2: Detail of friction damper

   Link Type	Plastic (Wen)
   Mass (Kg)	222.07
   weight (KN)	2.18
   Effective Stiffness (KN/m)	152500
   Yield Strength (KN)	450
   Post Yield Stiffness Ratio	0.0001
   Yield Exponent	10
   Effective Damping (KNs/m)	0

4. RESULTS AND DISCUSSIONS

Lateral loads resisting systems are used to reduce the seismic effect of the structure which is subjected to the earthquake load. The frames with base isolation, LRB and cross bracing are modeled according to the properties of structure which are explained in the work. The model is subjected to analysis for gravity load i.e. dead load and live load and seismic loads. The seismic behavior of the steel structure is judged by observing the roof displacement.

Roof Displacement:

Displacement is the parameter of maximum importance as it governs the failure pattern of the structure. From this present study, the displacement of the model with cross bracing, base isolation (LRB) and FVD damper and without lateral load resisting systems are is observed. By providing the cross bracing to the structure we observed that the displacement of the structure is reduced.

Table 3: Roof Displacement value for G+10

Graph 1: Roof Displacement value for G+10 for bracing, damper, isolator, and conventional bare frame

Table 4: Roof Displacement value for G+15

Graph 2: Roof Displacement value for G+15 for bracing, damper, isolator, and conventional bare frame.

Graph 3: Roof Displacement value for G+20 for bracing, damper, isolator, and conventional bare frame

Table 5: Roof Displacement value for G+20

From the graph it is shown that maximum roof displacement of building with bracing, damper and isolator are very less compared to normal conventional building.

CONCLUSION

After carrying out results by using ETABS software for buildings with various heights, the parameters like roof displacement for different lateral load resisting systems are compared. Following conclusion is made.

1. By comparing the results from (Graph 1, 2, 3 ) it is concluded that the roof displacement for steel structure with bracing is less as compared to the other lateral load resisting system.

2. Lead rubber isolator generally increases the maximum displacement of the structures in high rise building compared to fixed base structures, but in middle rise buildings the difference is negligible.

3. In low rise, simple base isolation has good performance and there is no need to modify the superstructure characteristics. Indeed, these modifications cannot have positive effect on isolation performance.

4. In middle rise building we can reach better isolation by assigning additional base mass and increasing the damping of the structure.

5. In high rise building, stiffening superstructure and increasing the damping will cause an effective base isolation.

6. Comparison between building with damper and building with braces showed that bracing are more significant to reduce seismic quantities with same direction of placement as brace.

REFERENCES

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CODES OF PRACTICES:

1. IS: 456-2000 Plain & Reinforced Concrete code of practice. Bureau of Indian Standard, New Delhi.

2. IS: 800 2007 General Construction in Steel code of practices. Bureau of Indian Standard, New Delhi.

3. IS: 1893 (part 1) General provisions on building & Dynamic Analysis of structure, criteria for earthquake Resistance Design. Bureau of Indian Standard, New Delhi.

4. International building code 2000 Edition by International code council.