Keywords : pushover analysis, performance evaluation, inelastic behaviour. Necessity of Non-Linear Static Pushover Analysis(nlsa)



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Abstract

The static pushover analysis is becoming a popular tool for seismic performance evaluation of existing and new structures. The expectation is that the pushover analysis will provide adequate information on seismic demands imposed by the design ground motion on the structural system and its components. The purpose of the paper is to summarize the basic concepts on which the pushover analysis can be based, assess the accuracy of pushover predictions, identify conditions under which the pushover will provide adequate information and, perhaps more importantly, identify cases in which the pushover predictions will be inadequate or even misleading.



Keywords : - pushover analysis, performance evaluation, inelastic behaviour.

Necessity of Non-Linear Static Pushover Analysis(NLSA):
The existing building can become seismically deficient since seismic design code requirements are constantly upgraded and advancement in engineering knowledge. Further, Indian buildings built over past two decades are seismically deficient because of lack of awareness regarding seismic behaviour of structures. The widespread damage especially to RC buildings during Bhuj earthquake exposed the construction practices being adopted in India, and generated a great demand for seismic evaluation and retrofitting of existing building stocks.
Introduction

The pushover analysis of a structure is a static non-linear analysis under permanent vertical loads and gradually increasing lateral loads. The equivalent static lateral loads approximately represent earthquake induced forces. A plot of the total base shear versus top displacement in a structure is obtained by this analysis that would indicate any premature failure or weakness. The analysis is carried out upto failure, thus it enables determination of collapse load and ductility capacity. On a building frame, and plastic rotation is monitored, and lateral inelastic forces versus displacement response for the complete structure is analytically computed. This type of analysis enables weakness in the structure to be identified. The decision to retrofit can be taken in such studies.

The seismic design can be viewed as a two step process. The first, and usually most important one, is the conception of an effective structural system that needs to be configured with due regard to all important seismic performance objectives, ranging from serviceability considerations. This step comprises the art of seismic engineering. The rules of thumb for the strength and stiffness targets, based on fundamental knowledge of ground motion and elastic and inelastic dynamic response characteristics, should suffice to configure and rough-size an effective structural system.

Elaborate mathematical/physical models can only be built once a structural system has been created. Such models are needed to evaluate seismic performance of an existing system and to modify component behavior characteristics (strength, stiffness, deformation capacity) to better suit the specified performance criteria.

The second step consists of the design process that involves demand/capacity evaluation at all important capacity parameters, as well as the prediction of demands imposed by ground motions. Suitable capacity parameters and their acceptable values, as well as suitable methods for demand prediction will depend on the performance level to be evaluated.

The implementation of this solution requires the availability of as set of ground motion records (each with three components) that account for the uncertainties and differences in severity, frequency characteristics, and duration due rapture characteristics distances of the various faults that may cause motions at the site. It requires further the capability to model adequately the cyclic load-deformation characteristics of all important elements of the three dimensional soil foundation structure system, and the availability of efficient tools to implement the solution process within the time and financial constraints on an engineering.



Purpose of pushover analysis
The purpose of pushover analysis is to evaluate the expected performance of structural systems by estimating performance of a structural system by estimating its strength and deformation demands in design earthquakes by means of static inelastic analysis, and comparing these demands to available capacities at the performance levels of interest. The evaluation is based on an assessment of important performance parameters, including global drift, interstory drift, inelastic element deformations (either absolute or normalized with respect to a yield value), deformations between elements, and element connection forces (for elements and connections that cannot sustain inelastic deformations), The inelastic static pushover analysis can be viewed as a method for predicting seismic force and deformation demands, which accounts in an approximate manner for the redistribution of internal forces that no longer can be resisted within the elastic range of structural behavior.

The pushover is expected to provide information on many response characteristics that cannot be obtained from an elastic static or dynamic analysis. The following are the examples of such response characteristics:




  • The realistic force demands on potentially brittle elements, such as axial force demands on columns, force demands on brace connections, moment demands on beam to column connections, shear force demands in deep reinforced concrete spandrel beams, shear force demands in unreinforced masonry wall piers, etc.

  • Estimates of the deformations demands for elements that have to form inelastically in order to dissipate the energy imparted to the structure.
  • Consequences of the strength deterioration of individual elements on behavior of structural system.


  • Consequences of the strength detoriation of the individual elements on the behaviour of the structural system.

  • Identification of the critical regions in which the deformation demands are expected to be high and that have to become the focus through detailing.

  • Identification of the strength discontinuities in plan elevation that will lead to changes in the dynamic characteristics in elastic range.

  • Estimates of the interstory drifts that account for strength or stiffness discontinuities and that may be used to control the damages and to evaluate P-Delta effects.

  • Verification of the completeness and adequacy of load path, considering all the elements of the structural system, all the connections, the stiff nonstructural elements of significant strength, and the foundation system.

The last item is the most relevant one as the analytical model incorporates all elements, whether structural or non structural, that contribute significantly to the lateral load distribution. Load transfer through across the connections through the ductile elements can be checked with realistic forces; the effects of stiff partial-height infill walls on shear forces in columns can be evaluated; and the maximum overturning moment in walls, which is often limited by the uplift capacity of foundation elements can be estimated.

These benefits come at the cost of the additional analysis effort, associated with incorporating all important elements, modeling their inelastic load-deformation characteristics, and executing incremental inelastic analysis, preferably with three dimensional analytical models.
Target displacement :

The fundamental question in the execution of the pushover analysis is the magnitude of the target displacement at which seismic performance evaluation of the structure is to be performed. The target displacement serves as an estimate of the global displacement of the structure is expected to experience in a design earthquake. It is the roof displacement at the center of mass of the structure. In the pushover analysis it is assumed that the target displacement for the MDOF structure can be estimated as the displacement demand for the corresponding equivalent SDOF system transformed to the SDOF domain through the use of a shape factor. This assumption, which is always an approximation, can only be accepted within limitations and only be accepted within limitations and only if great care is taken in incorporating in the predicted SDOF displacement demand all the important ground motion and structural response characteristics that significantly affect the maximum displacement of the MDOF structure. Inherent in this approach is the assumption that the maximum MDOF displacement is controlled by a single shape factor without regards to higher mode effects.

Under the Non-linear Static Procedure, a model directly incorporating inelastic material response is displaced to a target displacement, and resulting internal deformations and forces are determined. The mathematical model of the building is subjected to monotonically increasing lateral forces or displacements until either a target displacement is exceeded, or the building collapses. The target displacement is intended to represent the maximum displacement likely to be experienced during the design earthquake.
Adaptabitility of computer programs :
It is well known fact the distribution of mass and rigidity is one of the major considerations in the seismic design of moderate to high rise buildings. Invariably these factors introduce coupling effects and non-linearities in the system, hence it is imperative to use non-linear static analysis approach by using specialized programs viz., SAP2000,STAADPRO2005,ETABS,IDARC,NISA-CIVIL,etc., for cost-effective seismic evaluation and retrofitting of buildings.

Non-Linear Static Analysis (NLSA) for buildings :

Seismic analysis of buildings can be categorized depending upon the sophistication of modeling adopted for the analysis. Buildings loaded beyond the elastic range can be analyzed using Non-Linear static analysis, but in this method one would not be able to capture the dynamic response, especially the higher mode effects. This is pushover analysis. There is no specific code for NLSA. This procedure leads to the capacity curve which can be compared with design spectrum/DCR of members and one can determine whether the building is safe or needs strengthening and its extent.

The capacity of structure is represented by pushover curve. The most convenient way to plot he load deformation curve is by tracking the bas shear and the roof displacement. The pushover procedure can be presented in various forms can be used in a variety of forms for the use in a variety of methodologies. As the name implies it is a process of pushing horizontally, with a prescribed loading pattern, incrementally, until the structure reaches the limit state. There are several types of sophistication that can be used over for pushover curve analysis.

Level-1: It is generally used for single storey building, whereat a single concentrated horizontal force equal to base shear applied at the top of the structure and displacement is obtained.

Level-2: In this level, lateral force in proportion to storey mass is applied at different floor levels in accordance with IS:1893-2002 (Part-I) procedure, and story drift is obtained.

Level-3: In this method lateral force is applied in proportion to the product of storey masses and first mode shape elastic model of the structure. The pushover curve is constructed to represent the first mode response of structure based on the assumption that the fundamental mode of vibration is the predominant response of the structure. This procedure is valid for tall buildings with fundamental period of vibration upto 1 sec.

Level-4: This procedure is applied to soft storey buildings, wherein lateral force in proportion to product of storey masses and first mode of shape of elastic model of the structure, until first yielding, the forces are adjusted with the changing the deflected shape.

Level-5: This procedure is similar to level 3 and level 4 but the effect of higher mode of vibration in determining yielding in individual structural element are included while plotting the pushover curve for the building in terms of the first mode lateral forces and displacements. The higher mode effects can be determined by doing higher mode pushover analysis. For the higher modes, structure is pushed and pulled concurrently to maintain the mode shape.

The algorithm of the pushover procedure is given below:-


















Case Study of Non Linear Static Analysis using SAP2000 :

The recent advent of performance based design has brought the nonlinear static pushover analysis procedure to the forefront. Pushover analysis is a static, nonlinear procedure in which the magnitude of the structural loading is incrementally increased in accordance with a certain predefined pattern. With the increase in the magnitude of the loading, weak links and failure modes of the structure are found. The loading is monotonic with the effects of the cyclic behavior and load reversals being estimated by using a modified monotonic force-deformation criteria and with damping approximations. Static pushover analysis is an attempt by the structural engineering profession to evaluate the real strength of the structure and it promises to be a useful and effective tool for performance based design.

The ATC-40 and FEMA-273 documents have developed modeling procedures, acceptance criteria and analysis procedures for pushover analysis. These documents define force-deformation criteria for hinges used in pushover analysis. As shown in Figure 1, five points labeled A, B, C, D, and E are used to define the force deflection behavior of the hinge and three points labeled IO, LS and CP are used to define the acceptance criteria for the hinge. (IO, LS and CP stand for Immediate Occupancy, Life Safety and Collapse Prevention respectively.) The values assigned to each of these points vary depending on the type of member as well as many other parameters defined in the ATC-40 and FEMA-273 documents.

This article presents the steps used in performing a pushover analysis of a simple three-dimensional building. SAP2000, a state-of-the-art, general-purpose, three-dimensional structural analysis program, is used as a tool for performing the pushover. The SAP2000 static pushover analysis capabilities, which are fully integrated into the program, allow quick and easy implementation of the pushover procedures prescribed in the ATC-40 and FEMA-273 documents for both two and three-dimensional buildings.




Deformation

Force-Deformation For Pushover Hinge
The following steps are included in the pushover analysis. Steps 1 through 4 discuss creating the computer model, step 5 runs the analysis, and steps 6 through 10 review the pushover analysis results.
  1. Create the basic computer model (without the pushover data) in the usual manner using the graphical interface of SAP2000 makes this a quick and easy task.





  1. Define properties and acceptance criteria for the pushover hinges as shown in Figure.



  1. The program includes several built-in default hinge properties that are based on average values from ATC-40 for concrete members and average values from
    FEMA-273 for steel members. These built in properties can be useful for
    preliminary analyses, but user-defined properties are recommended for final analyses. This example uses default properties.

  2. Locate the pushover hinges on the model by selecting one or more frame members and assigning them one or more hinge properties and hinge locations.

  3. Define the pushover load cases. In SAP2000 more than one pushover load case can be run in the same analysis. Also a pushover load case can start from the final conditions of another pushover load case that was previously run in the same analysis.
    Typically the first pushover load case is used to apply gravity load and then subsequent lateral pushover load cases are specified to start from the final conditions of the gravity pushover. Pushover load cases can be force controlled, that is, pushed to a certain defined force level, or they can be displacement controlled, that is, pushed to a specified displacement. Typically a gravity load pushover is force controlled and
    lateral pushovers are displacement controlled. SAP2000 allows the distribution of lateral force used in the pushover to be based on a uniform acceleration in a specified

    direction, a specified mode shape, or a user-defined static load case.Here how the displacement controlled lateral pushover case that is based on a user-defined static lateral load pattern named PUSH is defined for this example.





  1. Run the basic static analysis and,if desired, dynamic analysis.Then run the static nonlinear pushover analysis.

  2. Display the pushover curve . The File menu shown in this display window allows you to view and if desired, print to either a printer
    or an ASCII file, a table which gives the coordinates of each step of the pushover curve and summarizes the number of hinges in each state as defined in Figure
    1 (for example, between IO and LS, or between D and E).

  3. Display the capacity spectrum curve. Note that you can interactively modify the magnitude of the earthquake and the damping information on
    this form and immediately see the new capacity spectrum plot. The performance point for a given set of values is defined by the intersection of the capacity curve (green) and the single demand spectrum curve (yellow). Also, the file menu in this
    display allows you to print the coordinates of the capacity curve and the demand curve as well as other information used to convert the pushover curve to Acceleration-Displacement Response Spectrum format.

  4. Review the pushover displaced shape and sequence of hinge formation on a step-by-step basis . The arrows in the bottom right-hand corner of the screen allow you to move through the pushover step-by- step. Hinges appear when they yield and are color coded based on their state (see legend at
    bottom of screen).




  1. Review member forces on a step-by-step basis . Often it is useful to view the model in two side-by-side windows with the step-by-step displaced shape in one window and the step-by-step member forces in the other. These windows can be synchronized to the same step, and can thus greatly enhance the understanding of the pushover
    results.

  1. Output for the pushover analysis can be printed in a tabular form for the entire model or for selected elements of the model. The types of output available in
    this form include joint displacements at each step of the pushover, frame member forces at each step of the pushover, and hinge force, displacement and state at each step of the pushover.


For buildings that are being rehabilitated it is easy to investigate the effect of different strengthening schemes. The effect of added damping can be immediately seen on the capacity spectrum form. You can easily stiffen or strengthen the building by changing member properties and rerunning the analysis. Finally you can easily change the assumed detailing of the building by modifying the hinge acceptance criteria and rerunning the analysis.

LIMITATIONS :-

There are many unsolved issues that need to be addressed through more research and development. Examples of the important issues that need to be investigated are :



  • Incorporation of torsional effects( due to mass, stiffness and strength irregularities).

  • 3-D problems (orthogonality effects, direction of loading, semi-rigid diaphragms.etc)

  • Use of site specific spectra.

  • Cumulative damage issues.

  • Most importantly, the consideration of higher mode effects once a local mechanism has formed.

Since the pushover analysis is approximate in nature and is based on static loading, as such it cannot represent dynamic phenomena with a large degree of accuracy. It may not detect some important deformation modes that occur in a structure subjected to severe earthquakes, and it may significantly from predictions based on invariant or adaptive static load patterns, particularly if higher mode effects become important.



Conclusion

From the study of above model example discussed following conclusions can be obtained:



  1. There are good reasons for advocating the use of the inelastic pushover analysis for demand prediction, since in many cases it will provide much more relevant information that an elastic static or even dynamic analysis, but it would be counterproductive to advocate this method as a general solution technique for all cases;

  2. The pushover analysis is a useful, but not in fallible, tool for accessing inelastic strength and deformation demands and for exposing design weaknesses.

  3. Its foremost advantage is that it encourages the design engineer to recognize important seismic response quantities and to use sound judgment concerning the force and deformation demands and capacities that control the seismic response close to failure, but it needs to be recognized that in some cases it may be provide a false feeling of security if its shortcomings and pitfalls are not recognized.

  4. It must be emphasized that the pushover analysis is approximate in nature and is based on static loading. As such it cannot represent dynamic phenomena with a large degree of accuracy. It may not detect some important deformation modes that may occur in a structure subjected to severe earthquakes, and it may exaggerate others. Inelastic dynamic response may differ significantly from predictions based on invariant or adaptive static load patterns, particularly if higher mode effects become important.

  5. Thus performance of pushover analysis primarily depends upon choice of material models included in the study.



REFERENCES :
  1. Engineering structures journal, Vol. 20,1998 :- Pros and cons of a pushover analysis of seismic performance evaluation by Helmut Krawinkler and G.D.P.K. Seneviratna.


  2. Workshop on Retrofitting of Structures(2003),IIT ROORKEE :- Non-Linear static analysis for seismic evaluation and retrofit of RC buildings by Shailesh Agarwal and Ajay Chourasia.

  3. CSI Reference Manual on SAP 2000 software.

  4. Earthquake Resistant Design of Structures By Pankaj Agarwal and Manish Shrikhande.






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