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ANALYSIS OF SEISMIC STRENGTHENING OF MULTI STOREY BUILDING
MD WAHIDUL HAQUE1, P. C. DIWAN2
1M. Tech Scholar Dept. of Civil Engineering, Swami Vivekanand University Sagar (M.P.)
2Head of Dept. Of Civil Engineering, Swami Vivekanand University Sagar (M.P.)
Abstract:- In recent years great developments have been made in the assessment of existing buildings and their performance in resistance to earthquake loading, potential seismic risk, vulnerability and lateral loads.
Existing buildings can be repaired and strengthened to include new developments and methods to resist earthquake and seismic loads, which is the most economical way to safeguard against the economic and social catastrophe affected by severe seismic activity in urban environments.
Traditional buildings in the 20th century were mostly constructed without sufficient protection, considering only the gravity loads of the structure. On the other hand, steel bars in the concrete may also corrode depending on construction age and environmental factors and effect structure performance against earthquakes. The current work is one step towards understanding the complex effects of this dynamic force particularly on low rise RC structures which are found in almost all parts of the world. During 2001 Bhuj earthquake of India, a major damage was observed in RC framed structures at Ahmadabad which were in the range of G+3 to G+7 storey.
Most of the buildings were having a normal grid of 3m x 3m column spacing with a storey height of 3m
1 INTRODUCTION
Earthquake is one of the greatest natural disasters in the world, causing immense damage to human lives and property. In addition, economic losses occur due to moderate and severe ground motions. Thousands of people die due to earthquakes every year throughout the world [1]. This is a long standing phenomenon in high seismic active zones such as US, Turkey, Japan, Italy, Indonesia, China and Iran.
The inhabitants of these areas must always consider their own specifications in order to design earthquake resistant structures, such as hospitals, fire stations, schools, and ordinary buildings.
Maximum safety of these
structures must be ensured during and after an earthquake.
In order to withstand such a scenario, it is desirable opt for performance based engineering and performance based design as far as seismic strengthening is concerned. Using the static pushover analysis, the structural performance can be restricted to a predefined level 9 say - Immediate Occupancy, Life Safety or Collapse Prevention. Hence, it is required to divide the newly planned city into zones those are having specific seismic performance.
Thus, a zone of the city may be reserved for all the buildings meeting the requirement of immediate occupancy as per push over analysis. Thus, in the event of an earthquake, all the buildings in that zone will be in a state of
2 immediate occupancy. This will ensure that there is no disturbance from other buildings to damage or collapse in the event of an earthquake. This states that the zone will not suffer from any loss of man.
Thus, the new technology and research may help us in minimizing the earthquake risk up to an extent. It is expected that the concept of push over analysis for framed structures will become a general practice in future, to identify the seismic performance of a building.
It is a well known fact that when a building is subjected to duration dependant force, it is said to be subjected to dynamic force. In order to analyze a structure which is subjected to dynamic forces, the structure may be assumed to be in a state of dynamic equilibrium. In such a state it will have it's mass M changing it's position as duration adjustments. It is usual to assume the mass as lumped at a point in structural dynamics and if one is competent to specify the location of the mass at various times with reference to a datum, one can say that the difficulties of dynamics is solved. In order to achieve this, the equation of motion may be considered for a Single Degree of Freedom (SDF) system with specific mass, stiffness and damping.
2 METHOD OF ANALYSIS FOR EARTHQUAKE FORCES
Earthquake & Seismic Analysis Seismic analysis is related to observe the response of building.
The analysis process of structural design which includes earthquake or structural assessment and retrofitting and brick infill are required. While, earthquake many of the buildings collapse lack of understanding of the inelastic behavior of structure. Elastic analysis states only elastic capacity of the structure and indicates where the first yielding occurs.
In order to study the inelastic behavior of structure the nonlinear analysis is required. The development of rational methodology that is applicable to the seismic design of new structures using available ground motion information and
3 engineering knowledge, and yet is flexible abundance to permit the inception of new technology became available. The major effort in the development are occurring day by day.
Earthquake and Sesimic analysis 2.1 Methods for Linear Static Analysis
Linear static analysis concludes a series of forces acting based on building to represent the effect of earthquake and is ground motion, which defined by a seismic design response spectrum. It assumed about building responds idle mode.
The building must be of lesser hieght and must not twist when the ground shakes.
The response is from a design spectrum, as per the natural frequency of the building. The application of this method is in many building codes by applying parameters to account for higher buildings with some higher modes, and for low levels of twisting. To account for effects attributable to
"yielding" of the structure, many codes apply modification factors that alleviate the design forces.
2.2 Methods for Linear Dynamic Analysis
2.2.1 Linear Dynamic Analysis Procedures are appropriate when higher mode effects are not
significant. This is generally true for short, regular buildings.
Therefore, for tall buildings, buildings with the tensional irregularities, and non-orthogonal systems, a good procedure is required. The linear procedure, the building is modeled as a multi degree- of-freedom (MDOF) system with elastic stiffness. The seismic input is modeled using the spectral analysis and history analysis. The corresponding internal forces and displacements are determined using linear elastic analysis. The advantage of these linear dynamic procedures with respect to linear static procedures is that higher modes can be considered.
2.2.2 Method for Nonlinear Analysis
The nonlinear static procedures includes an inelastic analysis that is considered when anticipating, what is happening to buildings after they begin to crack and yield in response to realistic earthquake motion. This approach is different from traditional static linear procedure that ensures the collapse of structure.
3.EVALUATION OF PERFORMANCE
Many analysis methods, both linear and nonlinear, are there for the analyzing the reframed structures. Elastic analysis methods include code static lateral force procedures, code dynamic lateral force procedures and elastic procedures using demand capacity ratios. The most basic inelastic analysis method is the complete nonlinear duration history analysis. Simplified nonlinear analysis methods, referred to as nonlinear static analysis
4 procedures, include the Capacity Spectrum Method (CSM) that uses the intersection of the capacity (pushover) curve and a alleviated response spectrum to estimate maximum displacement; the displacement coefficient method that uses pushover analysis and a modified version of the equal displacement approximation to estimate maximum displacement;
and the secant method that uses a substitute structure and secant stiffness’s
Although an elastic analysis bestows a good indication of the elastic capacity of structures and indicates where first yielding will occur, it cannot predict failure mechanism and account for.
Inelastic analysis shows building behaving by identifying the failure and potential for progression collapse. The application of the procedures for designing is used to help engineers to understand the structures will behave when subjected to major earthquakes, where it is assumed that the elastic capacity of the structure will be exceeded.
The capacity spectrum method, a nonlinear static procedure that provides a graphical representation of the global force-displacement capacity curve of the structure (i.e., pushover) and compares it to the response spectra representations of the earthquake demands, is a very beneficial tool in the evaluation and retrofit design of existing concrete buildings. The graphical representation provides a clear picture of how a building responds to earthquake ground motion, and, as illustrated in this chapter, it provides an immediate
and clear picture of how various retrofit or safeguard strategies, such as adding stiffness or strength, will affect the building's response to earthquake demands 4. SEISMIC PERFORMANCE OF RC FRAMES WITHOUT INFILL WALLS
The 2001 Bhuj earthquake of India was an eye opener. It made thousands of people lose their lives and rendered millions to lose their houses. The effect was consequently wide disseminate that it not only affected the people in the vicinity of the epicenter but those living in a metro city Ahmedabad, approximately 250 km away from the epicenter were badly affected. One additional duration it revealed the inherent weakness lying in the concrete building which are not detailed as per ductile detailing. A major damage was observed in RC framed structures which were in the range of ground + three storey G+3) to (G+7) storey. Further, these buildings were having a normal grid of 3m x 3m column spacing with a normal storey height of 3m. Attributable to aesthetic considerations, the columns were generally 230mm wide in order to be flush with the 230 mm thick brick wall which is a standard building material used in India. Hence, keeping all these factors in mind, it is calculated to study a typical RC building frame having these dimensions under pushover analysis and to report the findings in a systematic manner. It is also calculated to study another structure having a panel size to be 3m x 4.5m to
5 incorporate the effect of asymmetry.
Steps for Analysis Following are general steps are followed to perform nonlinear pushover analysis using \ software:
1. Create a model that material non-linearily and pushover analysis results are restricted to frame elements, although other element types may be present in the model.
2. Define the static load cases, like dead, live, etc. that are needed for application in the pushover analysis (Define > Static Load Cases).
3. Define any other static and dynamic analysis cases, like quake, response spectrum, etc, that may be needed for steel or concrete design of frame elements.
4. Define the pushover load cases (Define > Static Pushover Cases).
5. Define hinge properties (Define >
Hinge Properties).
6. Assign hinge properties to frame elements (Assign > Frame >
Hinges (Pushover)).
7. Run the basic linear and dynamic analyses (Analyze > Run).
8. If any concrete hinge properties are based on default values to be computed by the program, one must perform concrete design consequently that reinforcing steel is determined (Design > Concrete Frame Design > Commence Design/Check of Structure).
9. Run the pushover analysis (Analyze > Run Static Pushover).
10. Review the pushover results (Display > Demonstrate Static Pushover Curve), (Display >
Demonstrate Deformed Shape), (Display > Demonstrate Element Forces/Accentuatees > Frames...), and (File > Print Output Tables...
Frame forces in disseminatesheet format).
11. Revise the model as necessary and repeat.
5. CONCLUSION
1. For a G+6 storey RC frame having an overall plan dimension of 6m x 6m and a panel size of 3m x 3m, the seismic performance of frame having rectangular shaped columns is found inferior to the same frame having equivalent square columns.
2. The results of the push over analysis for G+6 storey RC space frame indicates that the storey drift for model with rectangular columns demonstrates a much higher storey drift at first storey level as compared to the model having equivalent square columns.
3. The number and intensity of plastic hinges developed in a G+6 storey RC space frame with rectangular columns at performance point is found much higher compared to the same model having equivalent square columns. This fact indicates a better seismic performance of the square shaped columns.
4. For an overall plan dimension of 6m x 9m for a G+6 storey building, the push over analysis indicates that the seismic performance of both rectangular and square columns is practically similar.
However, the maximum storey drift for model with square columns is less than that with rectangular columns.
5. When brick infill walls are considered in the form of struts in the push over analysis, the number of plastic hinges decreases but severity of plastic hinges developed at performance point
6 increases for both G+6 storey models having square and rectangular columns as compared to the same without considering infill walls.
6. In case of G+6 storey RC frames, looking at the effective damping and base shear at performance point, it can be stated that square columns perform better for overall square plan (3m x 3m panel) whereas rectangular columns perform better for rectangular overall plan (3m x 4.5m panel).
This is true for push over analysis with infill walls modeled as struts and even without infill walls.
7. For a G+6 storey model, T shaped columns demonstrate a better seismic performance as compared to the rectangular columns in terms of plastic hinges developed at performance point as well as storey drift which is observed. It is also clear that the rectangular column demonstrate better performance when pushed in the direction of it's strong axis and inferior performance when pushed in the direction of it's weak axis as compared to T shaped columns. This behaviour is found additional pronounced when infill walls are considered in the form of compression struts.
8. The seismic performance of frames with rectangular columns as compared to Tshaped columns is better in one direction and inferior in the other direction push from the pointof view of roof displacement and base shear observed at performance point.
Thiss also found true when infill walls are considered for the models
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