Steel Pushover Analysis

Gen Training Series Pushover Analysis as per EC8:2004

MIDAS Information Technology Co., Ltd.

Program Version V7.4.1

Program License Registered, Trial

Revision Date 2008.12.26

Pushover Analysis of steel structure

as per EC8:2004

DL SD NC


MIDAS Information Technology Co., Ltd.2

Pushover analysis is one of the performance-based design

methods, recently attracting practicing structural engineers

engaged in the field of seismic design. The objective of a

performance-based design is achieved after the owner and the

designer collectively select a target performance for the

structure in question. The engineer carries out the conventional

design and subsequently performs a pushover (elasto-plastic)

analysis to evaluate if the selected performance objective has

been met.

In midas Gen V741, pushover analysis as per EN1998:2004

is newly added, and analysis performance and usability are

significantly improved. This tutorial explains the method and

procedure for pushover analysis of 3-dimensional Steel

structures as per EN1998:2004. For this reason, the procedure

for modeling and analysis were not explained in detail. For the

users who are not familiar with the basic functions for modeling

and analysis, it is recommended to review “Application 1”

tutorial before following this tutorial.

The pushover analysis procedure is as follows:

Modeling & Design

- Details of Building

- Perform analysis

- Perform steel code checking

Pushover Analysis

- Pushover Global Control

- Pushover Load Cases

- Define Hinge Properties

- Assign Hinge Properties

- Perform Pushover Analysis

- Pushover Curve

- Pushover Hinge Status Results

- Safety Verification Table

Optimal Design ProcedureOverview



Details of the example structure

Figure 3. Elevation

Figure 1. Three-dimensional structural model

6,000

10,000

2,500 2,500 2,500 2,500

A

B

1 2 3

G2

G2

G2G2

C2 C2

C2C2 C1

C1

G1

G1

G1

BR

BR

3,000

3,000

3,000

9,000

BR1

BR1

BR2

Figure 2. Structural plan

Section Name Section ID Section DB Section Size

C1 1 UNI HEA240

C2 2 UNI HEA300

G1 21 UNI HEA280

G2 22 UNI IPE240

Brace1 31 UNI HEA160

Brace2 32 UNI HEA120



Materials

•Eurocode3:2005

Gravity loads

•Dead load: 4.9kN/m2

•Live load : 2.5kN/m2

Static Wind Loads

•Applied code: Eurocode1:2005

•Terrain Category : II

• Fundamental Basic Wind Velocity (Vb,o): 26m/s

Static Seismic Loads

•Applied code: Eurocode8:2004

•Ground Type: B

•Design Ground Acceleration: 0.08g

•Behavior Factor (q): 1.5

•Lower Bound Factor (b): 0.2

•Importance Factor (I) : 1

Applied Loads

Load Name Details

Static

Load

Cases

1 DL Dead Load

2 LL Live Load

3 WXWind Load

(X-direction in the global coordinates)

4 XYWind Load

(Y-direction in the global coordinates)

5 EXSeismic Load

(X-direction in the global coordinates)

6 EYSeismic Load

(Y-direction in the global coordinates)

Unit Load Cases

•Column: S235

•Beam: S235

•Brace: S235

Applied Design Code



Step 1. Open the model file and perform analysis

1. Open “Steel pushover analysis.mgb”

2. Select Works tab in the Tree Menu.

3. Check the entered section data and boundary condition in

Properties and Boundaries.

4. In the tree menu, select Static Loads>Static Load Case 1

and then right click Floor Loads:3.

5. In the context Menu, select Display and then check the

applied loads.

6. By this way, check the applied seismic loads and wind

loads.

7. Click icon to perform analysis.

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Pushover analysis is carried out in the post-processing mode

after completing elastic analysis.

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Step 2: Check Steel Design Code

1. Design > Steel Design Parameter > Design Code

2. Specify the code as „Eurocode3:05‟.

3. Check [OK] button.

Design Code specified in the Steel Design Code dialog is

applied to calculate the capacity of members in pushover

analysis.



Step 3: Steel Code Checking

1. Design > Steel Code Checking

Rotation capacity at the end of steel beams

or columns depends on the class of cross

section. In order for the program to

automatically determine the class of cross

section for the pushover analysis, select

„Auto‟. For the automatic classification

Steel Code Checking should be performed

first.



Step 4-1: Pushover Global Control

1. Design > Pushover analysis > Pushover Global Control

2. Select DL in the combo box and click [Add] button.

3. Select LL in the combo box and enter the Scale Factor as 0.25.

4. Click [Add] button.

5. Click [OK] button.

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In this step, we will define the initial load cases, which

will be applied prior to pushover analysis such as dead

load and live load.



Step 4-2: Pushover Global Control„Reference Design code (Eurocode 8:2004)‟ option is displayed when

the design code (in the main menu, Design > Concrete Design

Parameter or Steel Design Parameter > Design code) is specified as

Eurocode and Design code in preferences (in the main menu, Tools >

Preferences) is specified as Eurocode.

Scale Factor for Ultimate Rotation

1) Wall: In calculating the total chord rotation capacity at ultimate , θu, for wall , the value is divided

by 1.6 as per EN1998-3:2004 A.3.1.1.

2) Cold-worked brittle steel: If cold-worked brittle steel is used the total chord rotation capacity is

divided by 1.6 as per EN1998-3:2004 A.3.1.1.

3) Without Detailing for earthquake resistance: In members without detailing for earthquake

resistance the total chord rotation capacity is multiplied by 0.85 as per EN1998-3:2004 .

4) Smooth longitudinal bars: in members with smooth (plain) longitudinal bars without lapping in

the vicinity of the end region where yielding is expected, the total chord rotation capacity may be

multiplied by 0.575 as per EN1998-3:2004.



Secondary Seismic Elements

In order to calculate the total chord rotation capacity at ultimate, θu, the factor „γel’ is used. Since γel is differently applied for primary and

secondary seismic elements (γel = 1.5 for primary seismic elements, γel = 1.0 for secondary seismic elements as per EN1998-3:2004

A.3.1.1), the user can define Secondary Seismic Elements group. If Secondary Seismic Elements are not defined, all the elements are

considered as Primary Seismic Elements.

In this tutorial, Secondary Seismic Elements are not defined since pushover hinge properties are assigned to primary elements only.

Step 4-3: Pushover Global Control


1. Design > Pushover analysis > Pushover Load Cases


3. Enter the pushover load case name as „Accel_X‟.

4. Enter the Increment Steps as „30‟.

5. Check on „Use Initial Load‟ option.

6. Check on „Consider P-Delta Effect‟ option.

7. Select „Displacement Control‟ in the Increment Method.

8. Select „Mater Node‟ option.

9. Click the entry field and click the node no. 20 with the

mouse in the model view.

10. Enter the Max. Displacement as 0.12m.

11. Check off „Limit inter-Story Deformation Angle‟

option.

12. Specify the Load Pattern as „Uniform Acceleration‟.

13. Specify the Direction as „DX‟ in the combo box and click

[Add] button.



Step 5-1: Pushover Load Case

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1. Design > Pushover analysis > Pushover Load Cases


3. Enter the pushover load case name as „EY_Y‟.

4. Enter the Increment Steps as „30‟.

5. Check on „Use Initial Load‟ option.

6. Check on „Consider P-Delta Effect‟ option.

7. Select „Displacement Control‟ in the Increment Method.

8. Select „Mater Node‟ option.

9. Click the entry field and click the node no. 20 with the

mouse in the model view.

10. Enter the Max. Displacement as 0.3m.

11. Check off „Limit inter-Story Deformation Angle‟ option.

12. Specify the Load Pattern as „Static Load Cases‟.

13. Specify the Load Case as „EY‟ in the combo box and

click [Add] button.



Step 5-2: Pushover Load Case

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Step 6-1: Define Pushover Hinge Properties for beams 2

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In order to check or modify the hinge properties, click [Properties…]

button of the desired component.

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1. Design > Pushover analysis > Define Pushover Hinge Properties


3. Enter the pushover hinge properties name as „Beam‟.

4. Select the Material Type as „Steel / SRC(Filled)‟

5. Check on „Fz‟ & „Mz‟ components.

6. Click [Properties…] button for My component.

7. Select the Class of cross section as „Auto‟.

8. Click [Apply] button.

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Step 6-2: Define Pushover Hinge Properties for Columns

1. Enter the pushover hinge properties name as „Column‟.


3. Select „P-M-M in Status Determination‟ option for Interaction Type.

4. Check on „Fy‟, „Fz‟ & „My‟ components.

5. Click [Yield Surface Properties…] button.

6. Click [Y-Axis…] button for My component.



9. Click [z-Axis…] button for My component.





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Coupled axial force-biaxial moment behavior is reflected by

calculating the flexural yield strength of a hinge considering the

effect of axial force.

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Step 6-3: Define Pushover Hinge Properties for braces

1. Enter the pushover hinge properties name as „Brace‟.

2. Select the Element Type as „Truss‟.


4. Check on „Fx‟ component.

5. Click [Properties…] button for Fx component.




9. Click [Close] button.

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Pushover Hinge Properties

Shear

HingeEurocode3:2005, equation (6.18)

*θy= MyL/6EI

Where, My: Yield moment, L: Length of a member, E: Elasticity of Modulus, I: moment of inertia

Yield strength of Steel structures

Flexural

Hinge

Class Class 1 Class 2

DL 1.0θy 0.25 θy

SD 6.0 θy 2.0 θy

NC 8.0 θy 3.0 θy

Yield rotation of Steel structures



Step 7-1: Assign Pushover Hinge Properties for columns

1. Select column members (Material ID 1:Column) from the Tree Menu.

2. Design > Pushover analysis > Assign Pushover Hinge Properties

3. Select Hinge Properties Type as „Column‟ in the combo box.



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Right-click



Step 7-2: Assign Pushover Hinge Properties for Beams

1. Select beam members (Material ID 2:Girder) from the Tree Menu.

2. Drag and drop the „Beam’ hinge property from the Tree Menu to the

Model Window .

Right-click

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Pushover hinge properties can be simply assigned to the

selected elements by Drag & Drop.



Step 7-3: Assign Pushover Hinge Properties for braces

1. Select brace members from the Tree Menu.

2. Drag and drop the „Brace‟ hinge property from the Tree Menu to the

Model Window .

Right-click1

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Pushover hinge properties can be simply assigned to the

selected elements by Drag & Drop.



Step 8: Check assigned hinge properties

1. Right-click „B1_Column‟ from the Tree Menu.

2. Select „Properties‟ from the context menu.

3. Check the calculated yield strength and yield strain.

4. Click [Detail] button to check the calculated values in detail.

1Right-click

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Before assigning hinge properties

Before assigning hinge properties (while defining hinge

properties), Yield Strength and Yield Strain are displayed

as „1‟. After assigning hinge properties, calculated values

for each element are displayed.

By clicking [Detail] button, the user can check the

detailed equation and values.

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Step 9: Perform pushover analysis

1. Click „Task Pane‟.

2. Click „▼‟ icon and select „Analysis‟.

3. Click „Perform Pushover Analysis’.

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Task Pane displays work procedure for

advanced analysis functions and

description on input items so as to enable

the user to work more easily.

midas program provides 4 types of

advanced analysis features - Pushover

Analysis, Nonlinear Time History

Analysis, Time History Analysis and

Material Nonlinear Analysis.

In addition, Task Pane data can be

saved in the html format in the User

Folder, so that the user can directly write

or add the required input items for

analysis.


For the detailed formula of the Target Displacement, refer to „ANNEX B DETERMINATION OF THE

TARGET DISPLACEMENT FOR NONLINEAR STATIC (PUSHOVER) ANALYSIS, EN 1998-1:2004‟. The

target displacement, which is obtained from the above, corresponds to the seismic demand of the Limit State of

Significant Damage (SD). Target displacement of the Limit State of Near Collapse (NC) is taken equal to that

of SD multiplied by 1.5. Target displacement of the Limit State of Damage Limitation (DL) is taken equal to

that of SD divided by 2.5.


Step 10-1: Pushover Curve

1. Click „Pushover Curve‟ in the Task Pane.

2. Select the Pushover Load Case as „Accel_X‟.

3. Select „For Target Displacement

(EC8/Masonry)‟.

4. Select the Spectrum Type as „Horizontal Design

Spectrum‟.

5. Enter the Design Ground Acc.(Ag) as „0.5‟.

6. Enter the Behavior Factor (b) as „1.5‟.

7. Click [Draw] button.

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For the detailed formula of the Target Displacement, refer to „ANNEX B DETERMINATION OF THE

TARGET DISPLACEMENT FOR NONLINEAR STATIC (PUSHOVER) ANALYSIS, EN 1998-1:2004‟. The

target displacement, which is obtained from the above, corresponds to the seismic demand of the Limit State of

Significant Damage (SD). Target displacement of the Limit State of Near Collapse (NC) is taken equal to that

of SD multiplied by 1.5. Target displacement of the Limit State of Damage Limitation (DL) is taken equal to

that of SD divided by 2.5.


Step 10-2: Pushover Curve

1. Select the Pushover Load Case as „EY_Y‟.

2. Select „For Target Displacement

(EC8/Masonry)‟.

3. Select the Spectrum Type as „Horizontal Design

Spectrum‟.

4. Enter the Design Ground Acc.(Ag) as „0.5‟.

5. Enter the Behavior Factor (b) as „1.5‟.

6. Click [Draw] button.


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Step 11-1: Hinge Status Results

1. Click „Hinge Status Results‟ in the Task Pane.

2. Select the Pushover Load Cases Name as

„Accel_X‟.

3. Select „Status of Yielding (EC8:2004)‟.

4. Select the Components as „Ry‟.

5. Check on „Legend‟ and „Deform‟.

6. Specify the desired step in the combo box.

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The user can also check hinge status results for

each step by clicking on the step box and scroll

or clicking on the pushover graph which is

displayed in the black background.

Clicking and scrolling

Clicking on the graph

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Step 11-2: Hinge Status Results

1. Select the Pushover Load Cases Name as

„EY_Y‟.

2. Select „Status of Yielding (EC8:2004)‟.

3. Select the Components as „Ry‟.

4. Check on „Legend‟ and „Deform‟.

5. Specify the desire step in the combo box.

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Step 12-1: Safety Verification Table

1. Click „Safety Verification Table‟ in the Task Pane.

2. Select „Show All Elements’.


4. Select Pushover Load Case as „Accel_x‟.

5. Select „Significant Damage (SD)‟.

6. Check on „My‟ and „Fz‟.


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Safety Verification Table displays the comparison results between the demand and

capacities of the elements as per EN1998-3:2004 Table 4.3. Safety verification shall be

conducted for both ductile and brittle elements respectively. For ductile elements,

verification will be conducted in terms of deformation using mean values of properties

divided by CF. For brittle elements, verification shall be conducted in terms of strength

using mean values of properties divided by CF and by partial factor.

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Step 12-2: Safety Verification Table

1. Right-click on the table and select „Set Safety

Verification‟ in the Context menu.

2. Select Pushover Load Case as „EY_Y‟.

3. Select „Significant Damage (SD)‟.

4. Check on „My‟ and „Fz‟.


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Safety Verification Table displays the comparison results between the demand and

capacities of the elements as per EN1998-3:2004 Table 4.3. Safety verification shall be

conducted for both ductile and brittle elements respectively. For ductile elements,

verification will be conducted in terms of deformation using mean values of properties

divided by CF. For brittle elements, verification shall be conducted in terms of strength

using mean values of properties divided by CF and by partial factor.

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*For ductile elements, mean values of properties divided by CF are used. For brittle members, mean values of properties

divided by CF and by partial factor.

*θy= MyL/6EI

Where, My: Yield moment, L: Length of a member, E: Elasticity of Modulus, I: moment of inertia

Capacity of Steel structures for assessment in the Safety Verification Table (Eurocode8-3:2004, Annex A.3.1)

Plastic rotation

capacity

Class Class 1 Class 2

DL 1.0θy 0.25 θy

SD 6.0 θy 2.0 θy

NC 8.0 θy 3.0 θy

Shear capacity - Eurocode3:2005, equation (6.18)

Steel Pushover Analysis

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