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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
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 2
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.
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 3
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.
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 4
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
Strengthen or Replace?
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.
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 5
Strengthen or Replace?
Strengthen or Replace?
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 6
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.
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 7
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.
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 8
Introduction
Evaluation of Stresses
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
Evaluation of Stresses
Members shall be investigated for any loss
of strength resulting from the temporary
removal of rivets, cover plates or any other
parts.
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 9
Introduction
Evaluation of Stresses
Strengthening shall not be confined to main
members only, but shall include bracing,
lacing, battens, …
Introduction
Evaluation of Stresses
Span
Height
1
Current Condition: The current Loads case
Fo = No/Ao
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 10
Introduction
Evaluation of Stresses
Span
Height
1
During Repair: Decrease applied loads
Fi = (No – N1)/Ao = Ni/Ao
Introduction
Evaluation of Stresses
Span
Height
1
Final Condition:
Ff = Ni/Ao + (Nf – Ni) / Af
Ff = Nf / Af Only true in case of complete shoring.
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 11
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
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 12
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.
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 13
Main Beams
Main Beams
Flanges and web.
Stiffeners.
Initial tension.
Effective span.
Wind bracing.
Increasing the number of girders.
Substitution.
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 14
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.
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 15
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.
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 16
Flanges & Web
Flanges & Web
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 17
Flanges & Web
Flanges & Web
If the web cannot carry the loads, a bolted reinforcing
web should be introduced.
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 18
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
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 19
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.
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 20
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
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 21
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
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 22
Main Beams
Beam-to-beam connections.
Lateral bracing connections.
Riveted joints.
Stringer Bracket
Adding brackets
Adding weld lines
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 23
Stringer Bracket
Adding seat angle
Remove existing
rivets and add HSB
Truss Systems
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 24
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.
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 25
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:
Faculty of Engineering - Cairo
University STR654
Fall 2020
Sherif A. Mourad - Lecture 10 26
Truss Systems
Additional Chords:
Truss Systems
Inserting Additional Members: