Pushover Analysis Procedure_part2

8/13/2019 Pushover Analysis Procedure_part2

1/50

Pushover Analysis Procedure

using SAP2000

PART 2

PUSHOVER EXAMPLE


2/50

Pushover Analysis in SAP2000:

Steps Sequence

1. Create the model.2. Define frame hinges properties and assign them to the frame

elements.

3. Define the parameters to calculate the demand for each performancelevel (f.e.: acceleration response spectra, ATC-40, FEMA; MRSA).

4. Define the load patterns which are needed for pushover: gravity loadsand any other load acting on the structure before lateral seismicloading.

5. Define the Non-Linear Static load cases and the Modal load case to beused for pushover analysis.

6. Run the Pushover load cases and check the results (Pushover curve).

7. Determine the target displacement by an appropriate method (ATC-40, FEMA, other).

8. Evaluate the number and state of plastic hinges in the structure, andthen check the maximum strains for the most critical hinge.

9. Change structural configuration (additional piles, modified pile layout)

if needed and repeat the process.


3/50

Model Description

Trestle 105 m length.

7 spans 15 m length.

Bents with three steel

pipe (= 1.42 m) piles. Steel box section cap

beam (H=1.20).

5 steel I-Wide section

longitudinal beams. Concrete slab thickness =

0.30 m.


4/50

Model Description


5/50

Discretization

Elements discretizationSprings for soil-

structure interaction


6/50

Mass Source Definition

DEAD load.

Piping load (permanently

attached equipment).

5KPa (10% of uniform live

load)

Vehicle load (part of the

crane above the deck)

MASS_ADD_MG

(additional mass due to

marine growth)

MASS_ADD_WATER

(Hydrodynamic mass

internal and external to

the pile)


7/50

Marine Growth Load

Pile diameter Dp = 1.42 m

MG thk = 0.20 m

MG = 12.75 KN/m3

Distrib. Load = (/4) [(Dp + 2*thk)2- Dp2] = 13 KN/m


8/50

Hydrodynamic Added Mass

Pile diameter Dp = 1.42 m

Water = 9.81 KN/m3

Distrib. Load = 2**(/4)*Dp2

= 31 KN/m


9/50

Pile Cross-Section Geometry


10/50

Nonlinear material definition


11/50

Pile Moment-Curvature Analysis

Three levels of axial load: P1 = -10000 KN (max compression),

P2 = -1700 KN (DEAD load), P3 = 5000 (max tension).

One bending angle: 0 (symmetrical section)


12/50


Determination of the bilinear M-Phi

diagram for:

P = -1700 KN

Angle = 0


13/50


Bilinear M-Phi diagram for: P = -1700 KN, Angle = 0

Mne = 18700 KN-m

Mp = 21000 KN-m

Mp/Mne = 1.12

Y= 0.012

u= 0.237

p = uY= 0.225

The same procedure should be repeated for the other two levels of

axial force!


14/50

Pile Hinge Definition 1


15/50



16/50


Assign the defined hinge to the frame elements


17/50


Display the names of the generated hinges


18/50


Show generated hinge property 11H1


19/50


Calculating the SF of the hinge PILE-ST52

To define the Moment-Curvature Curve of hinge PILE-ST52, it is

required to calculate the yielding curvature (SF) used by the program:

SF = y / Lp = 5.016E-3 / 1.5 = 3.344E-3


20/50


M-Phi curve and Interaction surface definition for PILE-ST52


21/50


M-Phi curve definition for PILE-ST52


22/50


M-Phi curve definition for PILE-ST52

From bilinear M-Phi:

Mp/Mne = 1.12

0.20 Mp/Mne = 0.22

p = 0.225

SF = 3.344E-3

p / SF = 67

Notice that only plastic curvatures over SF have to be specified in the right column.

In the left column the Mp/Mne (obtained from M-Phi bilinear diagram) is assigned

to point C and a 20% of that value to point D.

Due to numerical stability it is highly recommendable that line CD has a negative

slope (not vertical) and line DE a positive slope (not horizontal).


23/50


Interaction surface definition for PILE-ST52

Automatically defined for each generated hinge.

It is only used to determine the exact yielding moment (Mne2, Mne3)

and the angle of bending (tg = Mne3 / Mne2).


24/50

1. The described procedure has to be repeated for the

definition of hinges in the cap beam and longitudinalbeam.

2. Due to the lower importance of beam hinges, they canbe defined faster by means of the Automatic Hinges if

the conditions for type of material and cross-sectiongeometry are met (See Part 1 of this guide).

3. It is very likely that in the hinge length for beams is notthe same as the total length of the frame element but a

fraction. This should be taken into account when definingthe hinge length (Lp) relative to the frame elementlength. This is an important parameter for the correctdetermination of the SF (yield curvature) from the yield

rotation automatically calculated by the program.

Beam Hinge Definition


25/50

1. Sudden strength loss is not recommended (segment C-D)2. Reduce the mesh size helps when using sudden strength

loss.

3. It is possible to define hinges along the whole frame

length (e.g.: 10 hinges spaced every tenth part of thetotal length).

4. Repeat the described process for all the combinations ofbending angles and axial loads.

5. For bi-axial or asymmetrical cross-sections, define M-Phifor intermediate bending angles (e.g.: 45).

6. Once the plastic hinge definition is concluded (SFcalulated by the program determined), the property has

to be newly assigned to all the corresponding frames.

Hinge Definition: Final Recommendations


26/50

Parameters for ATC-40 Capacity-Spectrum

Method

Definition of response spectrumfunction (without scaling) for

each demand level (OLE-D1, CLE-

D2, DE-D3)


27/50

Parameters for ATC-40 Capacity-Spectrum

Method


28/50

DEAD Load Case Definition

Constitutes the starting point of pushover analysis.

Non-linear geometric P-Delta analysis is performed for all the

gravity loads present before earthquake


29/50

MODAL Load Case Definition

The results of MODAL load case are used in the definition of the ATC-40

capacity spectrum (ADRS format) and the equivalent period required to

obtain the performance point (target displacement).

Besides, the mode shapes are used in the definition of the MODE load

pattern for pushover analysis.


30/50

Pushover MODE Load Pattern

1. More critical pushover load pattern. Preferred

over uniform ACCEL load pattern.

2. Modal analysis has to be performed first.

3. The two fundamental modes with higher modal

mass participation in directions X and Y,

respectively, are selected for the definition of

MODE load pattern.


31/50


Vibration modes selection for X and Y directions


32/50


Vibration modes selection for X and Y directions


33/50


Pushover Load Case Definition in Y direction


34/50

Pushover Load control application

P h N b f d t f


35/50

Pushover: Number of saved steps for

results analysis


36/50

Pushover Non-Linear Analysis Parameters


37/50

1. Keep using the same type of geometric non-linearity (e.g.: P-Delta) through all the non-linear load case defined in themodel.

2. Start the model with as few hinges as possible. then graduallyincrement the number of hinges as necessary.

3. The first run may no include any type of geometric non-linearity. Then, after checking results and analysisperformance, add P-Delta and may be Large Displacements.

4. Start with modest target displacements and limited numberof steps (saved and total). The idea is always have thepossibility of first perform a quickly analysis. Afterwards thenon-linear behavior could be incremented.

5. Consider more than two loading directions (or loadingmodes) to evaluate the structure under different loadingsituations.

Pushover Analysis:

Final Important Considerations


38/50

Pushover Results Analysis

Pushover in Y direction (Mode 1 load pattern).

ATC-40 Capacity Spectrum display parameters.D2 Performance Level.

h l l


39/50



Target displacement. D2 Performance Level.

h l l


40/50



ATC-40 Capacity Spectrum display parameters.D3 Performance Level.

Besides the elastic

period and damping

ratio, the effective

inelastic parameters

are also displayed.

h l A l i


41/50



Target displacement. D3 Performance Level.

P h R l A l i


42/50



Load steps corresponding to target displacements.

Level D2: Step 34 Level D3: Step 49

P h R l A l i


43/50



Identification of first hinge that yields (hinge with maximumdeformations through all the pushover analysis load history.)

First plastic in-ground hinge appears in the bottom part of the pile

at load step 32

P h R lt A l i


44/50



Results display of first plastic hinge at yielding point (step 32).

P h R lt A l i


45/50



Extract information for yielding (step 32), level D2 (step 34) and levelD3 (step 49)

P h R lt A l i


46/50



Extract information for yielding (step 32), level D2 (step 34) and levelD3 (step 49)

P h R lt A l i


47/50



Yield curvature calculation

My = 11085 KN.m

y = 2.282E-3 m-1(aprox.)

P h R lt A l i


48/50



Steel strain calculation for level D2

P h R lt A l i


49/50



Steel strain calculation for level D3



50/50



Tensile steel strain for level D2 and D3 are smaller than Strain limitsfor CLE and DE performance levels, respectively. Therefore, the

seismic capacity of the piles is verified.

Table 4-1 POLB

Documents

Pushover Analysis Procedure_part2