steel i girder

7/27/2019 Steel I Girder

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Steel I-Girder Designwith special attention to Eurocode provisions

Vidish A. Iyer

Structural Engineer and CAE consultant at Midas IT

Bridging Your Innovat ions to Real i t ies


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Bridging Your Innovations to RealitiesClick to edit Master title style

Bridging Your Innovations to Realities

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midas Civil

About Midas IT

About Midas Civil

Modeling Philosophy

Design Philosophy and Eurocode specifications

BS vs EC design for Composite structures.

CONTENTS


Click to edit Master subtitle style

Bridging Your Innovations to Realitiesmidas Civil

CONTENTS


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MIDAS Programs were being developed since 1989 and have been usedcommercially 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 shal l soon be opening a new branch in Singapore


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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 BuildingPlant Tunnel Dam

Why midas Civil

CONTENTS




WHAT IS MIDAS CIVIL ?

2-D 3-D


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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 ?


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CONTENTS




MODELING PHILOSOPHY


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Three main modeling methods

2D Grillage models

3D Grillage models

Meshed Finite Element model

MODELING


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Most common modeling method

Modeled as orthogonal or skewed grillage depending on site

requirements

2D MODELING


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Usual grillage modeling principles apply

For multi-girder bridges , shear lag is unlikely to reduce the effective slabwidth 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 fullcomposite second moment of inertia. Intermediate longitudinal elementsshould be given properties of slab only.

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

Intermediate bracings should be modeled

2D MODELING


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3D Grillages are quite useful when dealing with ladder deck

bridges

3D GRILLAGE MODELING


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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


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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


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There are several actions on any structure that are not normally accountedfor 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 deflectionsthese directly affect the erection process andcamber

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

Construction Stage Analysis


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DESIGN PHILOSOPHY


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Required Codes :

1) EN 1990Load combinations

2) EN 1991-2Moving loads

3) EN 1991-1-1Densities of materials

4) EN 1991-1-4Wind Actions5) EN 1991-1-5Temperature actions

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

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

8) EN 1993-1-9 - Fatigue

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

10) EN 1997Geotechnical Design

11) EN 1998Seismic Design

12) National Annexes to above codes

DESIGN CODES


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Ultimate Limit State :

Bending Resistance

Shear Resistance

Lateral Torsional Buckling

Fatigue Resistance

Serviceability Limit State :

Deformation

Crack Control

Stress Checks

DESIGN REQUIREMENTS


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Classification of Sections4 classes as per EC3 -1-1

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

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

3) Class 3can fully develop elastic resistance across section

4) Class 4- buckles before elastic limit is reached

DESIGN PROCEDURE OVERVIEW


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Plastic bending resistance for Classes 1&2

Resistance is for Effective Cross Section ( allowances for Shear

lag & local buckling for class 4)

BENDING RESISTANCE


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

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

Mc,Edis 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


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In composite beam sections, shear resistance is simply takenas 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

calculationeven if it is larger .

SHEAR RESISTANCE IN BEAM WEBS


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BENDING AND SHEAR INTERACTION


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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 dimensionalslenderness

BUCKLING RESISTANCE


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The following sets of equations are used :

The relationship between LT and LTcan be seen from EC3-1-1

BUCKLING RESISTANCE


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A simplified method , as outlined in EN 1993-1-1 is often used for

calculating the buckling resistance .

BUCKLING RESISTANCE


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Restraint effect in Integral Bridges

Beams curved in planpresence of radial force necessitates provision oflateral restraints at intervals

Flange curved in elevationpresence of vertical radial force which results

in transverse plan bending of flange and vertical stresses in web.

Plan bending from interaction with cross girdersof special concern in

ladder decks where vehicle loading may induce lateral actions.

OTHER EFFECTS IN MAIN GIRDERS


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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 its 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 pdue to the passage of the fatigue load model 3 vehicle

b) Determine damage equivalence factor .

c) Determine the design value of the stress range

Fatigue Analysis


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Basic Check for fatigue :

cis the reference value of fatigue strength at 2 x 106cycles, 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


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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 StateLoad Combinations


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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


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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


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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 StateCrack Control


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For direct loading, limitation of crack widths can be achieved by limiting bar

spacing /bar diameter as per the following tables

Serviceability Limit StateCrack Control


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

The differences are not that numerous and most of thedesign practices and methods are quite similar in bothcodes.

The next few slides highlight some major points ofdifference between EC4 and BS-5950-3 provisions forcomposite design.

DESIGN PROCEDUREBS VS EC


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EC 4concrete strength is taken from cylinder

BSconcrete strength is taken from Cube

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

CONCRETE STRENGTH


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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 equationsone for

failure of concrete by crushing and one for shearing of the stud

SHEAR CONNECTION


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The graph below shows a comparison for stud

resistance between EC4 and BS 5950-3.

SHEAR CONNECTION

li k di i l l


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Minimum Shear Connection RequirementsBS code simply states these as afunction of span length but Eurocodes consider asymmetry of the section as

well.

SHEAR CONNECTION

id i Y I i li iCli k di M i l l

id i Y I i li iid Ci il


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

For Eurocodes it varies along the length of the beam

EFFECTIVE WIDTH

B id i Y I ti t R litiCli k t dit M t titl t l

B id i Y I ti t R litiid Ci il


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Different Shear Areas for BS and EC ( slightly larger

for EC than for BS )

VERTICAL SHEAR

B id i Y I ti t R litiCli k t dit M t titl t l

B id i Y I ti t R litiid Ci il


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THANK YOU




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