str654: inspection, maintenance and repair of steel ... - cu

Faculty of Engineering - Cairo

University STR654

Fall 2020

Sherif A. Mourad - Lecture 10 1

STR654:

INSPECTION, MAINTENANCE and

REPAIR of STEEL STRUCTURES

Lecture 10, December 22, 2020

Sherif A. Mourad

Professor of Steel Structures and Bridges

Faculty of Engineering,

Cairo University

Introduction

Learning From Failure: Case Studies

Strengthening Steel Elements

Strengthening Calculations

Strengthening by Heat Treatment

Strengthening with FRP


University STR654

Fall 2020


Introduction

Why Strengthen?

Inadequate design.

(Design loads – modeling - …)

Heavier live loads.

(Changes in usage of building – Upgrading - …)

Local deterioration.

(aging – lack of maintenance - …)

Introduction

Assessing Building Condition:

Visual Inspection.

Tests.

Simple analysis methods to examine the system.

Detailed assessment.


University STR654

Fall 2020


Introduction

Condition Rating of Structure:

Failed Condition: the structure must be demolished and

re-built.

Critical Condition: the structure requires major

strengthening. Closed till corrective action is taken.

Serious Condition: the structure requires strengthening.

Fair Condition: the structure requires regular

maintenance.

Sound Condition.

Introduction

Performance of building material, building component or

structure in time:

(a) no corrective or preventive maintenance required, no

damage

(b) corrective maintenance was necessary, damage.


University STR654

Fall 2020


Introduction

Strengthen or Replace?

Age of the structure.

Quality of material.

Fatigue effect on operation.

Estimated cost.

Added length of life.

Possible future increase in load.

Introduction


Typically, strengthening is undertaken only to

improve certain weak parts, not the entire member.

If it is found that the entire member (the entire span

of a bridge, for example), it may be more

economical to replace the member, rather than

strengthen it.


University STR654

Fall 2020





University STR654

Fall 2020


Introduction

Steps Taken Before Implementation of

Strengthening:

Physical condition.

Evaluation of stresses.

Effect of eccentricity.

Use of welding.

Introduction

Physical Condition

Physical condition shall be determined by a

detailed inspection.

Establish the type of steel and its physical

and chemical properties by examining

original documents and/or taking specimens

for testing.

Carry out load test to check actual stresses,

deflections, and vibrations.


University STR654

Fall 2020


Introduction

Physical Condition

Table A. This table is a guide that lists the most probable

yield strength for W shapes. Actual yield strength of

existing framing members should be defined from

existing drawings or verified by testing.

2.11 t/cm2

2.54 t/cm2

Introduction

Evaluation of Stresses

Allowable stresses of the added materials

under the load for which they are

strengthened shall conform to the Egyptian

Code.


University STR654

Fall 2020


Introduction


New material shall be considered effective in

carrying additional (live) load only, unless

dead load stresses are removed temporarily

from the members.

The stresses of the original section shall be

checked under dead load and any additional

live loads.

Introduction


Members shall be investigated for any loss

of strength resulting from the temporary

removal of rivets, cover plates or any other

parts.


University STR654

Fall 2020


Introduction


Strengthening shall not be confined to main

members only, but shall include bracing,

lacing, battens, …

Introduction


Span

Height

1

Current Condition: The current Loads case

Fo = No/Ao


University STR654

Fall 2020


Introduction


Span

Height

1

During Repair: Decrease applied loads

Fi = (No – N1)/Ao = Ni/Ao

Introduction


Span

Height

1

Final Condition:

Ff = Ni/Ao + (Nf – Ni) / Af

Ff = Nf / Af Only true in case of complete shoring.


University STR654

Fall 2020


Introduction

Introduction

Eccentricity

The added material shall preferably be

applied so as to obtain a symmetric section.

If a symmetric section cannot be achieved,

the eccentricity of the member shall be taken

into consideration in the stress analysis.

Added plates

at two sides


University STR654

Fall 2020


Introduction

Welding

Electric arc welding may be used subject to

approval.

Welds and rivets shall not be assumed to

work together.

Welding shall be in accordance with the

Egyptian Code.

Introduction

When welds are added to existing rivets:

Welds shall be designed to transmit the entire

stresses, except where the original member

shall sustain the entire DL, then the welds

shall be designed to carry only the LL.

When a few of the rivets are loose or

defective, HSB may be used to carry DL

provided they are installed prior to welding.


University STR654

Fall 2020


Main Beams

Main Beams

Flanges and web.

Stiffeners.

Initial tension.

Effective span.

Wind bracing.

Increasing the number of girders.

Substitution.


University STR654

Fall 2020


Flanges & Web

Flange section may be increased by adding cover plates

or by replacing the existing cover plate by a new one.

Flanges & Web

If the exposed surface of old cover plates is rough or

uneven from the effect of corrosion, they shall be

discarded and new plates provided.

If more than one plate shall be replaced, it is better to

replace them with a single plate with adequate size.


University STR654

Fall 2020


Flanges & Web

Cover plates added to plate girders without existing

cover plates shall be full length, and shall be connected

with rivets, HSB, or continuous fillet weld (if material is

weldable).

Flanges & Web

If it is difficult or expensive to remove the R.C. floor on

top of beams, the flange section may be increased by adding

full-length angles or plates just below the flange angles.

Bottom flanges may be similarly reinforced to keep

symmetry.


University STR654

Fall 2020


Flanges & Web

Flanges & Web


University STR654

Fall 2020


Flanges & Web

Flanges & Web

If the web cannot carry the loads, a bolted reinforcing

web should be introduced.


University STR654

Fall 2020


Stiffeners

Bearing stiffeners may be strengthened by adding

angles or plates, grinding the new plates so that they fit

tightly, or by welding them to the flanges.

Intermediate stiffeners may be added by riveting, HSB,

or welding. Ensure that they are not welded to the tension

flange.

Stiffeners


University STR654

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Initial Tension

Initial tension may be introduced to a new cover

plate by:

Welding one end of the flange angles.

Heating the plate till it expands to a predetermined

length.

Initial Tension

Initial tension may be introduced to a new cover

plate by:

Weld the other end of the plate and allow it to cool.

Complete welding of the plate in the longitudinal

direction.


University STR654

Fall 2020


Effective Span

The effective span of the main beams may be

reduced by:

Moving the bearings nearer to the edge of the

support (provided the seat is wide enough, and that

it is checked for the new eccentricity).

Introducing new knee bracing (provided the

columns / abutment can sustain the lateral

component.

Effective Span

Adding columns or girders Adding bracing


University STR654

Fall 2020


Number of Main Girders

Strengthening may be done by doubling the main

girders (reducing the spacing by one-half) and

providing the necessary lateral bracing for the new

beams.

Spacing should consider allowing for inspection,

maintenance, and painting of the interior surface of

the beams.

Connections


University STR654

Fall 2020


Main Beams

Beam-to-beam connections.

Lateral bracing connections.

Riveted joints.

Stringer Bracket

Adding brackets

Adding weld lines


University STR654

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Stringer Bracket

Adding seat angle

Remove existing

rivets and add HSB

Truss Systems


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Truss Systems

Addition of new trusses:

New truss should be of stiffness comparable to

the old trusses.

New truss should be joined with the bracing

system to ensure uniform deflection.

Truss Systems

Auxiliary Support System:

New intermediate supports may be introduced

to shorten the span.

A complete re-analysis is necessary as straining

actions may reverse direction.

Members having insufficient capacity shall be

strengthened prior to installation of the new

supports.


University STR654

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Truss Systems

Additional Chords:

Convert the existing truss by adding new

members to increase the depth.

For the new system to be fully effective for DL

and LL, initial stresses must be provided into

the new chord.

Truss Systems

Additional Support:


University STR654

Fall 2020


Truss Systems

Additional Chords:

Truss Systems

Inserting Additional Members:


University STR654

Fall 2020


Truss Systems

Truss Connections:

str654: inspection, maintenance and repair of steel ... - cu

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