Midas Civil Webinar

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Pushover and Response Spectrum Analysis

Demand & Capacity Evaluation


1. Introduction

midas Civil Demand & Capacity Evaluation

Case Study: Seismic Design Check of a Skewed Steel Bridge


Force Based Design :


F

D

D

F

u

FE

y

Elastic Analysis

Inelastic Response

FP

Design Load FP = FE / R

R

1

R = Response Modification Factor R = u / y, represent the ductility capacity of the ERS


Displacement-Based Design:


Displacement-Based Design: F D

F

u

y

Elastic Analysis

Inelastic Response

FP Equal Displacement Assumption: Displacements resulted from inelastic response is approximately equal to displacement obtained from linear elastic response spectrum analysis.

Design Load is simply FP. What to be checked:

D u

DD


Seismic Design


Comparison of two Design Approaches:

Force

AASHTO LRFD Bridge Design Specification Complete design for STR, SERV limit state first Elastic demand forces divided by

Ductile response is assumed to be adequate without verification Capacity protection assumed

Displacement

AASHTO Guide Specification for LRFD Seismic Bridge Design Complete design for STR, SERV limit state first Displacement demands checked against displacement capacity Ductile response is assured with limitations prescribed for each SDC Capacity protection assured

Capacity Protection:

Column Shear Capacity Pier Cap Foundation Joint


2. Code Provisions


Code Provisions:


3. Determination of Demand


SDC D:

Step 1: Displacement Analysis




SDC D: Step 1: Displacement Analysis

Engineer Considerations:





Consideration 2:

The periods can be obtained from the Eigenvalue

Analysis.

The user can perform the eigenvalue analysis in

Midas civil and can obtain the periods in the

Longitudinal and the transverse direction.





Analysis Procedure

We will follow Elastic Dynamic Analysis




Elastic Dynamic Analysis: Code Considerations: Section 5.4.3 1. At least 90 % Mass Participation in Longitudinal and Transverse

direction 2. Damping : 5% 3. EDA Mode results must be combined by CQC method




Elastic Dynamic Analysis: Factors Affecting Elastic Dynamic Analysis: 1. Design Spectra: Site Conditions, return period etc. 2. Behavior of Structure: Elastically Linear and Damped 3. Foundation Behavior: Linear




Elastic Dynamic Analysis: Factors Affecting Elastic Dynamic Analysis: 1. Design Spectra: Site Conditions, return period etc.




Elastic Dynamic Analysis: Factors Affecting Elastic Dynamic Analysis: 2. Behavior of Structure: Elastically Linear and Damped

> The structure is supposed to behaves linearly. > The Design Spectra Changes with Damping. Damping can be

different in different modes.




Elastic Dynamic Analysis: Factors Affecting Elastic Dynamic Analysis: 3. Foundation Behavior: Linear

Load vs. Displacement curve for Soil Springs




Elastic Dynamic Analysis: Step1: Generation of Response Spectra: AASHTO LRFD specifies detailed guidelines on the generation of the Response Spectra based on the notional Ground Motion maps given and site factors described in Article 3.4.2




Elastic Dynamic Analysis: Step1: Generation of Response Spectra:




Elastic Dynamic Analysis: Step1: Generation of Response Spectra: Midas Civil




Elastic Dynamic Analysis: Step 2 : Mode Analysis for obtaining the Mode shapes and Mass Participation Factor. Midas Civil Definition of Eigen value analysis with 20 Modes. Mass Participation factor < 90 %

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