Pusover Analysis

STAAD.Pro 2006 Build 1001 Release Report AD.2006.3.1 Pushover Analysis

Purpose This is a set of procedures to implement a Pushover Analysis as defined in the document FEMA 356:2000. STAAD Pushover analysis in STAAD is a static, non-linear procedure in accordance with FEMA 356 specification. Basically, in this method, the magnitude of the lateral push load is increased progressively according to a predefined loading pattern until either loading or the deflection reaches the described level. Description There are three stages to utilizing this new feature, firstly the model must be correctly defined with the appropriate commands in the Modeling Mode. The second stage is then to analyze the structure to generate a set of results. The final, third stage is to see the resulting data in the Results Mode. The steps can be summarized thus:- A) Modeling Mode 1) Create a suitable STAAD.Pro model using any existing STAAD modeling techniques. The model should contain the geometry, properties and any required specifications that would result in the successful creation of a STAAD.Pro analysis stiffness matrix. 2) The Pushover analysis is complex and requires a significant amount of detail to be defined for a successful analysis to take place. These parameters are defined in the Definitions of the Load dialog box. To set up these parameters, enter the General > Load Page and expand the Definitions shown in the Load box to display the Pushover Definitions. Click on the description ‘Pushover Definitions’ and then click on the New… (or Add…) button to start entering the required data.

Figure 35

of 17Pushover Analysis

11/8/2012mk:@MSITStore:C:\staad%202008\STAAD\Help\Release_Report_2006.chm::/Pushover_...

There are 5 parts to setting up the pushover parameters which are:-- 1: Define Input - 2: Load Patterns - 3: Spectrum Details - 4: Hinge Properties - 5: Solution Control

- 1: Define Input The first part of defining the pushover parameters is to define the following in the Define Input sheet:-

Figure 36

The input can either be as a General Input or as Member Specific Parameters. If the option of General Input is selected then the following data must be defined. Type of frame Chose whether the structure is a fixed rigid moment frame or concentric braced frame. Geometric Non-Linearity Effect Ignore or include Geometric Non-Linearity Effect. If the Geometric Non-Linearity effect is included then Displacement Tolerance and No. Of Iterations under Convergence Criteria for Geometric Non-Linearity will appear highlighted. Convergence Criteria for Geometric Non-Linearity Displacement Tolerance and No. of Iterations can be specified by checking the boxes and editing the value in the edit box. If nothing is specified, analysis engine will assume the default value. Input value for No. of Iterations for KG Matrix and Maximum No. of Analysis Cycles can be specified by checking the respective boxes. If nothing is specified, analysis engine will assume the default value. Save Output Results for Multiple Steps



Incremental value of displacement at roof or at control joint in current unit system is to be given. Also incremental value of Base Shear can be specified after checking the box. If nothing is specified, analysis engine will assume the default value. Print Output Results Select the Print Results check box for printing final analysis results (joint displacements, member end forces and support reactions) in the output file. Click the Add button as shown in the figure below to add the inputs specified under Pushover Definitions in Load dialog box. If the alternative option of Member Specific Parameters is selected, then a number of specific values of thlisted with a check box active are added to the Loads dialog each time the Add… button is clicked.

Figure 37

These values need to assigned to a range of members in the model, e.g. to assign an Expected Yield Stress of 36 ksi to a range of members, the following steps need to be taken:-



Figure 38

- 2: Load Patterns As the name suggests, this sets out how the loading pattern is applied.

Figure 39

If the loading pattern is set to ‘Auto’, then Vertical Distribution of Base Shear must be defined. To do this check the Method for Lateral Load calculation check box and select the method from the drop-list of Method 1, Method 2 or Method 3. If nothing specified, STAAD will use the default method. Additionally, with the ‘Auto’ loading pattern selected, then it is possible to define the Total Base Shear to Be Distributed. If selected, then the Direction and amount of Total Base Shear to be distributed must be specified. If base shear to be distributed is not defined, the analysis will assume 10 percent of the gravity loading as a lateral load.



The maximum number of load step increments is defined with the Number of Push Load Steps. Once defined, the data from is added to the model by clicking on the Add… button. - 3: Spectrum Details This is used to specify the parameters of the spectrums that are to be used.

Figure 40

The inputs on this sheet are the percentage of Critical Damping for 1st, 2nd, 3rd and 4th spectrums. Note that the Values for 2nd, 3rd and 4th spectrums are optional. The class of the Site Category should be selected from the drop-list of Class A, B, C, D or E as defined in the FEMA document. The Mapped Spectral Acceleration as defined in the FEMA document should also be specified. Once defined, the data from is added to the model by clicking on the Add… button. - 4: Hinge Properties There are three options on the creation of hinges which is set up on this sheet, either use the FEMA specification, Ignore or create a User Defined hinge. If either the option of FEMA or Ignore is selected, then no additional data input is required. Simply clickon Add… to add this setting to the pushover parameter settings.



Figure 41

To define a User Defined hinge, select User Defined from the drop-list and carry out the following:- Enter a Type Identifier (Hinge Property number). Specify the co-ordinates (A, B, C, D and E) of Load Deformation Curve, Yield Moment, Yield Rotation and Acceptance Criteria (Immediate Occupancy, Life Safety and Collapse Prevention). Once defined, the hinge is added to the parameter list by clicking on the Add… button. All the hinge types now need to be assigned to the model. Defined hinges will appear in the Load dialog box, for example if a single user hinge, Type 1 has been defined, it will appear thus:-

Figure 42



The hinges should be assigned to members as applying loading thus to assign a User Defined Hinge, defined as Type 1, the following steps should be performed:-

Figure 43

Note that different hinges cannot be assigned to different ends of a beam. - 5: Solution Control The final sheet of the pushover definition defines the limits for the pushover analysis.

Figure 44

The pushover can be terminated by the defined Base Shear limit and/or Displacement limit. If the Base Shear limit is selected, then the direction, X or Z should be selected and the limiting base shear in the current force units.



If the Displacement limit is selected, then the direction, X or Z should be selected and the limiting displacements for a specified joint. 3) Create the primary load cases as for any other analysis, and add the analysis command 'PERFORM PUSHOVER ANALYSIS'.

Figure 45

Click on the Add button followed by the Close button. Perform Pushover Analysis will be added as follows:

Figure 46

Note that a pushover analysis will be performed on ALL load cases defined from any previous analysis commands and the COMBINATION load cases are NOT analyzed. B) Perform Analysis 1) Run the analysis as for any other STAAD model using the menu command, Analyze>Run Analysis... CTRL+F5. Note that the Pushover analysis should only be used with the STAAD engine option. Note that this is not a feature available in the Stardyne engine. C) Results Mode



1) The results from the pushover analysis result in a new set of pages being available in the Post-Processing Mode. The pages are:-

Figure 47 Each page offer the following information:- - Loads Page

- Loads - Capacity Curve - Node Results - Beam Results



Figure 48

The table on the right hand side of the figure shows forces Fx, Fy and Fz at the nodes and maximum base shear corresponding to different load steps. If the load steps are changed up to the Max. Load Steps, Base Shear and Forces at different nodes will also change as shown in the figure below.

Figure 49

- Capacity Curve The Capacity Curve page is used to display the Capacity Curve and a table of the relationship between displacement and base shear for each load step.



Figure 50

This page displays a graph of the Capacity Curve above a table of the data points. The Capacity Curve displays the relationship of Base Shear to displacement of the Control Joint.

Figure 51

Right click on the Capacity Curve graph to obtain a pop-up menu, which allows a Target Displacementto be set. If selected from the pop-up menu then a wizard of three steps is presented in the dialog box. Step 1, Define Modification factor C0



Figure 52

This sets the Modification Factor C0 which can be selected from the table or calculated based on entering the data of the Building Type, Loading Pattern and Number of Stories. The resulting modification factor, whether selected from the table or calculated is displayed in the lower right corner of the dialog box. To accept the value of C0 and move on to the next page of the settings for the Target Displacements, click the ‘Next>’ button. Step 2, Define Modification Factor C1

Figure 53



This sets the Modification Factor C1which is selected from the table.The graphs show the Demand Spectrum for all the spectrums defined in the pushover definition specified for the analysis. Recall that up-to 4 spectrums could be defined. The value of Cm is defined according to the selected Spectrum damping and item in the Cm table (FEMA 356:2000, table 3-1). To accept the value of Cm and move on to the next page of the settings for the Target Displacements, click the ‘Next>’ button. Step 3, Define Modification Factors C2 and C3

Figure 54

This sets the Modification Factor C2 which is selected from the table. To accept the value of C2 and move on to the complete the settings for the Target Displacements, click the ‘Finish’ button. The value of the calculated Target Displacement is added to the Capacity Curve Graph, thus:-



Figure 55

The Capacity Curve Graph can also be enhanced by indicating the Idealized Capacity Curve. To show this right click on the Capacity Graph diagram and select the option ‘Show Idealized Capacity Curve’. The graph is then added and colored in blue, thus:-

Figure 56

By right clicking on the Capacity Curve Graph there is also an option to ‘Show Data Points’. If this is selected, the points displayed in the Capacity Curve table are marked directly onto the curve, thus:-



Figure 57

As with other graphical windows in STAAD.Pro, the Capacity Curve window can be recorded as a picture to be included in a User Report. To take a picture of the Capacity Curve, either right click on the window and select ‘Take Picture’ from the pop-up menu. Alternatively select the option from the Edit menu or Toolbar icon as elsewhere in the program. - Node Results

Figure 58

The two tables on the right of the screen display the Node Displacements and the Support Reactions for a given load step. Both tables and the displacement diagram are updated if the selected Load Step is changed on either table. - Beam Results



Figure 59

There are two tables displayed along the right side. The upper table, called Beam Hinge Results table, shows the Status (Linear or Non-linear) of the beam, Location of hinge formation in a beam and Status of the hinges (with respect to Acceptance criteria) formed against each Load Step. In the hinge status column, status of hinges is displayed if hinges are formed i.e. whether the hinges lie between Linear and Immediate Occupancy (<=IO), Immediate Occupancy and Life Safety (IO-LS) or Life Safety and Collapse Prevention (LS-CP) or exceeds Collapse Prevention (>=CP) levels. If we keep on increasing the load steps more beams will become nonlinear due to formation of hinges. If the member crosses Collapse Prevention level it becomes inactive.

Figure 60

The location of hinges at beam ends is highlighted on the drawing as the number of selected load step is increased. The color of the hinge itself gives the indication at what level it is. Level : <= IO , Color : Green



Level : <= IO-LS , Color : BlueLevel : <= LS-CP , Color : Magenta Level : >= CP , Color : Red When the member is inactive, the color of the member changes to red at the load step at which it is deactivated.

Figure 61

Restrictions Pushover analysis, currently implemented in STAAD, is limited in application to buildings that are regular and do not have adverse torsional or multimode effects. The capacity curve is generally constructed to represent the first mode response of the structure based on the assumption that the fundamental mode of vibration is the predominant response of the structure. · PERFORM PUSHOVER ANALYSIS cannot be repeated more than once. · PERFORM PUSHOVER ANALYSIS PRINT is not a valid command. · CHANGE command cannot be used in conjunction with a Pushover analysis. · LOAD COMBINATION command is not accepted. · SET Z UP command should not be used. The pushover analysis should only be used on models

defined with the Y up co-ordinate system. · SET RESTART 1 command will not work i.e. <filename>.L17 file will not form. · SURFACE objects are not considered. · Only steel structures should be considered. · Non-prismatic sections should not be used. · Curved beam specification is not considered. · Cable specification on members is not considered. · Plate and solid elements are not considered. · Beta angles other than 0 and 90 degree are not valid. Please refer to the example files ExampPush01.std, ExampPush02.std, ExampPush03.std and ExampPush04.std in the folder X:\SPro2006\STAAD\Examp\Pushover\ , where X: is the default drive where you have installed STAAD.Pro 2006.



Pusover Analysis

Documents

Transcript of Pusover Analysis