SEISMIC EVALUATION OF BUILDING WITH POST TENSIONED FLOORS BY PUSHOVER ANALYSIS Page 66 BASIC STEPS TO PERFORM PUSHOVER ANALYSIS IN SAP 2000 5.1 OVERVIEW SAP 2000 is the finite element method based commercial software. The analysis in SAP 2000 involves the following four steps: 1. Modeling 2. Static analysis 3. Designing 4. Pushover analysis 1. Modeling Modeling is the primary task of any analytical study and the result obtained to a large extent depends on the simplification taken during this step. Modeling involves creation of geometry of overall structure by including elements of various components representing respective structural behavior including boundary conditions. In the considered problem, material properties of various elements and loads on various elements and its combinations are defined. The various steps involved in the modeling are as follows: 1. Units are set for the convenience. 2. Define the properties of various materials used in the models. 3. Define the section properties of various structural elements of the model. 4. Draw the model in the graphical environment.
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SEISMIC EVALUATION OF BUILDING WITH POST TENSIONED FLOORS BY PUSHOVER ANALYSIS Page66
BASIC STEPS TO PERFORM PUSHOVER
ANALYSIS IN SAP 2000
5.1 OVERVIEW
SAP 2000 is the finite element method based commercial software.
The analysis in SAP 2000 involves the following four steps:
1. Modeling
2. Static analysis
3. Designing
4. Pushover analysis
1. Modeling
Modeling is the primary task of any analytical study and the
result obtained to a large extent depends on the simplification taken
during this step. Modeling involves creation of geometry of overall
structure by including elements of various components representing
respective structural behavior including boundary conditions. In the
considered problem, material properties of various elements and loads
on various elements and its combinations are defined. The various
steps involved in the modeling are as follows:
1. Units are set for the convenience.
2. Define the properties of various materials used in the
models.
3. Define the section properties of various structural elements
of the model.
4. Draw the model in the graphical environment.
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5. Define different loads and their combination. Including
cases of pushover analysis.
6. Assign section properties to the model and boundary
condition.
7. Assign the various loads on the structure.
8. Draw tendon elements in PT beam elements.
9. Assign prestressed loads and post tensioning parameters to
tendon elements.
10. Assign nonlinear hinges on beams and columns.
2. Static Analysis
Once the model is built, the static analysis is performed after
defining the various loads and their combinations.
3. Design
In RC frame sections, properties of nonlinear hinges are mainly
based on the outcome of the designed section. So, prior to pushover
analysis it is necessary to do design. Using appropriate code
recommendations, model is first designed for the response to the static
analysis.
4. Pushover Analysis
Many nonlinear static analyses are possible. But it is usual to
consider only three primary cases that are Push1- gravitational push,
Push2- push staring from the end of gravitational push and it is in X-
direction, and the last one is Push3- this also starts from end of the
gravitational push but in Y-direction.
The following general sequences of steps are involved in performing
a static nonlinear analysis:
1.Create a model just like any other analysis.
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2.Define frame hinge properties and assign them to the frame
elements.
3.Define any Load Cases and static and dynamic Analysis Cases
that may be needed for steel or concrete design of the frame
elements, particularly if default hinges are used.
4.Run the Analysis Cases needed for design.
5.If any concrete hinge properties are based on default values to
be computed by the program, you must perform concrete
design so that reinforcing steel is determined.
6.If any steel hinge properties are based on default values to
be computed by the program for Auto-Select frame section
properties, you must perform steel design and accept the
sections chosen by the program.
7.Define the Load Cases that are needed for use in the
pushover analysis, including:
Gravity loads and other loads that may be acting on the
structure before the lateral seismic loads are applied.
You may have already defined these Load Cases above
for design.
Lateral loads that will be used to push the structure. If
you are going to use Acceleration Loads or modal loads,
you don’t need any new Load Cases, although modal
loads are required to define a Modal Analysis Case.
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8.Define the nonlinear static Analysis Cases to be used for
pushover analysis, including:
A sequence of one or more cases that start from zero and
apply gravity and other fixed loads using load control.
These cases can include staged construction and geometric
nonlinearity.
One or more pushover cases that start from this sequence
and apply lateral pushover loads. These loads should be
applied under displacement control.
The monitored displacement is usually at the roof level of
the structure and will be used to plot the pushover curve.
9. Run the push over Analysis Cases.
10. Review the pushover results: Plot the pushover curve,
the Deflected shape showing the hinge states, force and
moment plots, and print or display any other results you
need.
11. Revise the model as necessary and repeat.
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5.2 BASIC STEPS
5.2.1 Create a model
1. Create the Basic Grid System
In this step, creating the basic grid system. The structural objects
are set relative to the grid system.
Begin creating the grid system by clicking the File menu > New
Model command or the New Model button , the form shown in
Fig.5.1 will be displayed.
Fig.5.1 The New Model Initialization form
Select the Grid Only button on the form shown in Fig.5.1,
form shown in Fig.5.2 will be displayed and in this form, define Grid
Dimensions, Story Dimensions and Units.
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Fig.5.2 Building Plan Grid System and Story Data Definition form
2. Define material properties
Begin defining various material properties used in the model by
clicking the Define menu > Material Properties command, the form
shown in Fig.5.3 will be displayed.
Fig.5.3 Define Materials form
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Fig.5.4 Material Property Data form
Select the Add New Material Or Modify/Show Material
button on the form shown in Fig.5.3, form shown in Fig.5.4 will be
displayed and in this form, Define Material Property data.
3. Define section properties
Begin defining various section properties used in the model by
clicking the Define menu > Frame Sections command, the form
shown in Fig.5.5 will be displayed.
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Fig.5.5 Define Frame Properties form
Select the Add New Property button on the form shown in
Fig.5.5, form shown in Fig.5.6 will be displayed and in this form, Add
Frame Section Property.
Fig.5.6 Frame Section Properties form
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Select the Concrete from Frame Section Property Type and
Select Rectangular button on the form shown in Fig.5.6, form shown
in Fig.5.7 will be displayed and in this form, Define Section Property
data.
Fig. 5.7 Rectangular Property form
4. Add structural objects
Objects, such as columns, beams, and floors, can be drawn
manually as follows:
Draw Frame Elements:
Make sure that in case of drawing columns elevation view is
active and in case of drawing beams plan view is active. Click the Draw
Frame/Tendon Elements or Click button , or use the Draw
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menu > Draw Frame/Tendon Elements > Create Columns in
Region. The Properties of Object pop-up box for frame elements
shown in Fig.5.8 will be displayed and using this command Draw
Frame/Tendon Elements.
Fig.5.8 Properties Of Frame/Tendon Elements
Select type of section as Column in case of drawing columns in
model and select type of section as Beam in case of drawing beam
elements in model.
Draw the Floor:
Make sure that the Plan View is active. Click the Draw Poly
Areas button , or select the Draw menu > Draw Poly Area
Objects > Draw Areas command. The Properties of Object pop-up
box for areas shown in Fig.5.9 will be displayed and using this
command Draw Area Objects.
Fig.5.9 Properties Of Object Box For Areas
Similarly, using Draw menu you can construct other objects like secondary beams, point objects etc.
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Draw Tendons
First we need to define Tendon properties, click Define menu >
Materials > Add New Material, so the Form as shown in Fig. 5.10
will be displayed. Select Tendon from Material Type and in this form
define Tendon Material property.
Fig. 5.10 Tendon property data form
Click the Draw Frame/Tendon Elements or Click button , or use
the Draw menu > Frame/Cable/Tendons command to access the
Properties of Object form. The Properties of Object pop-up box for
frame elements shown in Fig.5.11 will be displayed,
1. Click the Line Object Type drop-down list and select the Tendon
option.
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Click on the joint at one end of the beam and then click on the
joint at the other end of the beam to draw the tendon. The
Tendon Data for Line Object 2 form will display when you
release the mouse button.
Fig. 5.11 Properties of Object box for Tendons
Fig. 5.12 Tendon data for Line Object form
With the Tendon Data for Line Object form as displayed in Fig.
5.12, click the Parabolic Calculator button to access the Define
Parabolic Tendon Layout for Line Object 2 form as shown in Fig.
5.13. In that form,
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Select 1 from the Quick Start drop-down list in the lower
left corner of the form.
Click the Quick Start button to update the Tendon Layout
Data spreadsheet. In the spreadsheet area,
o Enter the values of elevation of cable from center of
beam to generate the parabolic profile in the beam.
o The values of elevation are defined at a distance of
L/20 for each span.
Click the Refresh button to update the spreadsheet area
and the graphical display area.
Click the Done button to close the Define Parabolic
Tendon Layout for Line Object form and redisplay the
Tendon Data for Line Object form.
Fig. 5.13 Define Parabolic Tendon Layout for line object form
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With the Tendon Data for Line Object form redisplayed,
click the Add button in the Tendon Loads area of the form to
access the Tendon Load form. This form is displayed in Fig.
5.14 as Tendon Load Assignment Data form. In that form,
Select the PRESTRESS load case from the Load Case Name
drop-down list.
Enter value of Tendon End Force edit box.
Ensure that all of the Friction and Anchorage Losses and
Other Loss Parameters are zero. Type the values of the
Curvature Coefficient, Wobble Coefficient, Anchorage Set
Slip as specified in the technical note of ADAPT Corporation.