Midas Civil

Steel I-Girder Design with special attention to Eurocode provisions

Vidish A. Iyer

Structural Engineer and CAE consultant at Midas IT

Bridging Your Innovations to Realities

Bridging Your Innovations to RealitiesClick to edit Master title style Bridging Your Innovations to Realities

2

midas Civil

• About Midas IT

• About Midas Civil

• Modeling Philosophy

• Design Philosophy and Eurocode specifications

• BS vs EC design for Composite structures.

CONTENTS

Bridging Your Innovations to RealitiesClick to edit Master title style

Click to edit Master subtitle style

Bridging Your Innovations to Realitiesmidas Civil

CONTENTS

MIDAS Programs were being developed since 1989 and have been used commercially since 1996.

With our headquarters in South Korea , we currently have corporate offices in Beijing, Shanghai, Detroit, Dallas, Europe, India and Japan and are ever expanding .

One of the Largest civil analysis software developers

Proven Reliability with over 5,000 project applications

Intensive quality control system

Analyses verified by various institutions

CONTENTS




ABOUT MIDAS IT

We shall soon be opening a new branch in Singapore

Integrated Solution System for Bridge and Civil Engineeringmidas Civil

What is midas Civil?

General Purpose

Special Purpose

FEM FBM BEM

Structural Engineer

Geotechnical Engineer

Bridge Underground Structure Building

Plant Tunnel Dam

Why midas Civil

CONTENTS




WHAT IS MIDAS CIVIL ?

2-D 3-D


What kind of bridge type can midas Civil handle?

Conventional Bridge

Staged Segmental Bridge

Cable-stayed Bridge & Suspension Bridge

Why midas Civil

CONTENTS




WHAT TYPES OF BRIDGES CAN IT HANDLE ?


CONTENTS




MODELING PHILOSOPHY


7

midas Civil

Three main modeling methods

• 2D Grillage models

• 3D Grillage models

• Meshed Finite Element model

MODELING


8

midas Civil

• Most common modeling method

• Modeled as orthogonal or skewed grillage depending on site requirements

2D MODELING


9

midas Civil

• Usual grillage modeling principles apply

• For multi-girder bridges , shear lag is unlikely to reduce the effective slab width below the slab actual width . Usually models for bare steel condition , short term composite condition and long term composite condition are required.

• Section properties for the composite main beams should use the full composite second moment of inertia. Intermediate longitudinal elements should be given properties of slab only.

• Torsional stiffness of the slab should be divided equally between transverse and longitudinal beams. ( bt3/6 in each direction )

• Intermediate bracings should be modeled

2D MODELING


10

midas Civil

• 3D Grillages are quite useful when dealing with ladder deck bridges

3D GRILLAGE MODELING


11

midas Civil

• Vertical Bending is assigned wholly to the upper members while bottom flange elements represent only the plan bending of these flanges.

• Although this model captures the local effects in a better fashion , it is still not possible to separate the global and local effects .

3D GRILLAGE MODELING


12

midas Civil

• More realistic structural response. Accurate representation of local and global responses.

• Models can be built using combination of plate and beam elements .

FINITE ELEMENT MODELING


13

midas Civil

There are several actions on any structure that are not normally accounted for without construction stage analysis . These include :

• Creep , Shrinkage and Time dependent strength variation effects

• Locked in stresses arising from staged construction , material defects etc.

• Prestress Losses

• Accounting for the pouring sequence of the deck slab.

• Accurate deflections – these directly affect the erection process and camber

For composite structures in particular the pouring sequence, creep , shrinkage , strength variation and locked in stresses are of great import since these factors can significantly affect the overall design.

Construction Stage Analysis


14

midas Civil

DESIGN PHILOSOPHY


15

midas Civil

• Required Codes :

1) EN 1990 – Load combinations

2) EN 1991-2 – Moving loads

3) EN 1991-1-1 – Densities of materials

4) EN 1991-1-4 – Wind Actions

5) EN 1991-1-5 – Temperature actions

6) EN 1993-1-1 – Design of steel structures

7) EN 1993-1-5 – Plated Structural Elements ( for LTB )

8) EN 1993-1-9 - Fatigue

9) EN 1994-2 – Design of composite structures ( bridges )

10) EN 1997 – Geotechnical Design

11) EN 1998 – Seismic Design

12) National Annexes to above codes

DESIGN CODES


16

midas Civil

Ultimate Limit State :

• Bending Resistance

• Shear Resistance

• Lateral Torsional Buckling

• Fatigue Resistance

Serviceability Limit State :

• Deformation

• Crack Control

• Stress Checks

DESIGN REQUIREMENTS


17

midas Civil

• Classification of Sections – 4 classes as per EC3 -1-1

1) Class 1- can form plastic hinge with rotation capacity

2) Class 2 – can form plastic hinge but limited rotation capacity

3) Class 3 – can fully develop elastic resistance across section

4) Class 4- buckles before elastic limit is reached

DESIGN PROCEDURE OVERVIEW


18

midas Civil

• Plastic bending resistance for Classes 1&2

• Resistance is for Effective Cross Section ( allowances for Shear lag & local buckling for class 4)

BENDING RESISTANCE


19

midas Civil

The elastic resistance for classes 3&4 can be calculated from the equation :

Where Ma,Ed is design moment in steel section alone (during constn. Stage)

Mc,Ed is design moment in composite section (after construction)

k is an amplifying factor that causes the stress limit to be reached in

steel or reinforcement (whichever is first )

ELASTIC MOMENT OF RESISTANCE


20

midas Civil

• In composite beam sections, shear resistance is simply taken as that of the steel section.

• There are two basic components : Vertical shear resistance and buckling shear resistance ( Both taken from EC3)

• For shear buckling , contributions from web and flange are dealt separately ( refer EC 3-1-1 , cl. 5.1,2,3,8)

• For contribution from the flange , in case of composite beams the bottom flange should be used for shear resistance calculation – even if it is larger .

SHEAR RESISTANCE IN BEAM WEBS


21

midas Civil

BENDING AND SHEAR INTERACTION


22

midas Civil

• For composite beams , buckling usually happens in the bottom flanges when they are in compression .

• Here buckling is not true lateral torsional buckling but rather a distortional buckling

• Nevertheless , EC 3 & 4 prescribe rules for LTB based on non dimensional slenderness

BUCKLING RESISTANCE


23

midas Civil

• The following sets of equations are used :

The relationship between χ LT and λLT can be seen from EC3-1-1

BUCKLING RESISTANCE


24

midas Civil

• A simplified method , as outlined in EN 1993-1-1 is often used for calculating the buckling resistance .

BUCKLING RESISTANCE


25

midas Civil

• Restraint effect in Integral Bridges

• Beams curved in plan – presence of radial force necessitates provision of lateral restraints at intervals

• Flange curved in elevation – presence of vertical radial force which results in transverse plan bending of flange and vertical stresses in web.

• Plan bending from interaction with cross girders – of special concern in ladder decks where vehicle loading may induce lateral actions.

OTHER EFFECTS IN MAIN GIRDERS


26

midas Civil

• Fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading

• For road bridges , the Eurocode advises the use of EN 1992-2 and EN 1992-3 by using fatigue load model LM 3 (basically it’s a moving load analysis)

• EN 1993-2 and EN 1993-1-9 should be referred to for detailed provisions regarding fatigue

• Per these codes :

a) Determine the stress range Δσp due to the passage of the fatigue load model 3 vehicle

b) Determine damage equivalence factor λ.

c) Determine the design value of the stress range

Fatigue Analysis


27

midas Civil

• Basic Check for fatigue :

Δσc is the reference value of fatigue strength at 2 x 106 cycles, which is numerically the same as the relevant detail category according to BS EN 1993-1-9 Tables 8.1 to 8.10.

Fatigue Analysis


28

midas Civil

Three categories of combinations of actions are proposed in EN:

• characteristic (normally used for irreversible limit states, e.g. for exceeding of some cracking limits in concrete)

• frequent (is normally used for reversible limit states) and

• quasi-permanent (is normally used for assessment of long-term effects)

Serviceability Limit State – Load Combinations


29

midas Civil

• Stress checks are done as per SLS combinations for unfactored values of characteristic actions.

• Basically there should be no inelastic behavior.

• Stress limits in Concrete , steel , reinforcement and studs are reduced by certain values ( k factors )

Serviceability Limit State - Stress


30

midas Civil

• EC 4 refers us to EC 3-2 for deflection limits.

• Basically deformations are calculated from the Frequent load combinations

• EC 3 is silent on any actual limits for deformation . Normal practices for deflection limits can apply . National annexes should also be referred to .

Serviceability Limit State - Deflection


31

midas Civil

• The Eurocode advises section 7.4.2 of EC4-2 which prescribes minimum reinforcement in lieu of more accurate method and describes this as a conservative approach.

Serviceability Limit State – Crack Control


32

midas Civil

• For direct loading, limitation of crack widths can be achieved by limiting bar spacing /bar diameter as per the following tables

Serviceability Limit State – Crack Control


33

midas Civil

• Although it would seem that the Eurocodes represent a significant departure from the earlier BS code practices , the two are closer than they appear .

• The differences are not that numerous and most of the design practices and methods are quite similar in both codes.

• The next few slides highlight some major points of difference between EC4 and BS-5950-3 provisions for composite design.

DESIGN PROCEDURE – BS VS EC


34

midas Civil

EC 4 – concrete strength is taken from cylinder

BS – concrete strength is taken from Cube

Sample : C20/25 ( cube str = 25 , cyl str =20 )

CONCRETE STRENGTH


35

midas Civil

• BS 5950-3: characteristic resistance of studs in solid slabs is given for various combinations of height, diameter and concrete strength.

• EC4 calculates the resistance as the minimum of two equations – one for failure of concrete by crushing and one for shearing of the stud

SHEAR CONNECTION


36

midas Civil

The graph below shows a comparison for stud resistance between EC4 and BS 5950-3.

SHEAR CONNECTION


37

midas Civil

Minimum Shear Connection Requirements – BS code simply states these as a function of span length but Eurocodes consider asymmetry of the section as well.

SHEAR CONNECTION


38

midas Civil

• As per British code , Eff. Width = Span/8 subject to conditions

• For Eurocodes it varies along the length of the beam

EFFECTIVE WIDTH


39

midas Civil

• Different Shear Areas for BS and EC ( slightly larger for EC than for BS )

VERTICAL SHEAR


40

midas Civil

THANK YOU


41

midas Civil

http://en.midasuser.com

Contact : [email protected]@midasit.com

http://en.midasuser.com/

mailto:[email protected]

mailto:[email protected]

Midas Civil

Documents

Transcript of Midas Civil