Transcript steel.fsv.cvut.cz

Rehabilitation and maintenance
of buildings - 01
Karel Mikeš
List of lessons
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Errors in the design of structures and modern reconstruction
Mechanical properties of cast iron, mild iron and mild steel
Causes and analysis of steel structural failures
Assessment of bearing struct. and reasons for refurbishment
Overview of codes for design and actions on structures
Inspections and material testing
Introduction of basic methods of reinforcing steel structures
Strengthening of individual members subjected to axial load 9.
Strengthening of individual members subjected to bending
10. Strengthening of members subjected to combinations
11. Strengthening of riveted/bolted/welded connections
12. Repair and reconstruction of civil structures
Objectives of the lecture
Errors in the design of structures and modern
reconstruction
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Introduction
History
Basic terms
Principles of the design
Assessment of structures
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Causes of failures of steel structures - phases
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Errors in design
errors in the conceptional design of the structure
errors in the documentation (calculations…)
wrong selection of constructional materials
erros in solution of constructional details
wrong calculation model (different from the real behaviour)
Fabrication, erection
Operation
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corrosion
fatigue
high temperature
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Additional temperature loading
Fire
accidental events
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Causes of failures of steel structures - phenomenons
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Underestimation of loading
Discrepancy of model and reality
Defective or inadequate material
Stability of compression members (or beams)
Stability of plates
Brittle fracture
Weak joints
Aerodynamics
Fatigue
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Typically Failure = more than one cause
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Causes of failures of steel structures - phenomenons
Discrepancy of model and reality
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Wrong selection of details, not correspondng to
assumption (fixed/hinged)
Unconsidered eccentricity in joints
Different load application points
Omitted effects (torsion, secondary moments)
Non-considered reduction of cross-section
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Tay bridge
1879
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Underestimation of load: wind load not considered
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Bad material: piers – cast iron, bracing – wrought iron with slag
Train speed 60 km/h instead of 40 km/h
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Tay bridge
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1879
Collapse in wind storm with train
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75 died
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St. Lawrence, Quebec 1907
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Flexural buckling of compression member
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Underestimation of dead load
Errors in the design of joints
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St. Lawrence, Quebec 1907
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Collapse in construction stage
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86 died
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Hasselt 1937
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Brittle fracture
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Bad selection of steel
Wrong welding process  large residual stresses
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Hasselt 1937
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Collapse when tram crossed
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Tacoma Narrows 1940
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Aerodynamics
Suspension bridge, span 853 m
New bridge in 1950
Nowadays 2 bridges (2007)
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Tacoma Narrows
Assembly
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Collapse
http://www.youtube.com/watch?v=AsCBK-fRNRk
http://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge_Collapse
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Collapse due to plate buckling
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Vienna
1968
Milford Haven (Wales)
1970
West Gate Bridge (Melbourne) 1970
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35 died
Koblenz (Germany)
1971
 Extensive
research in 1970‘s
 New codes with new procedures
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Milford Haven (Wales) 1970
Eccentric load of diaphragm
 Imperfections
 Insufficient stiffening of
diaphragm
 capacity  50% of actions
 4 died
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Koblenz
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1971
Buckling of unstiffened plate
9 died
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Failure of roof at Opatovice power station
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Structure from 1957
Main frame:
fixed columns + truss girder,
27,5 m span
Collapse: 11/2002
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during reconstruction of roof
snow load
Original documentation:
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Just part was found
Calculations missing
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Failure of roof at Opatovice power station
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Failure of roof at Opatovice power station
Causes
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Overloading by dead load
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Additional layers of concrete, water-proofing layers
Originally under-dimensioned structure
Very poor quality of welds
Not-functional dilatation detail
 collapse
of whole roof
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Reasons for refurbishment of steel
structures
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Malfunction of structure
Need of change
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Increased loading
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Change of use
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Bridges
Buildings
Need of free space
Bridges – new clear profile
Other reasons, e.g.:
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local situation (neighbour buildings)
war
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Refurbishment of steel structures can be
divided into:
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Reinforcement
Renovation
Extension
Replacement
Relocation
Special arrangements
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Utilization of reserve of structure
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Utilization of capacity reserves of structure
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Detection and improvement of loading
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Pernament loading
Climatic loading
Service loading
Real material properties
More precise calculation
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Utilization of capacity reserves of structure
Material properties
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Tensile tests
 Real fy, fu
Plastic reserve
 Bi-linear stress-strain relation
 MNA – plastic hinges
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Stress, MPa
300
360 MPa
235
200
0,03
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E = 2,1 *E5 MPa
Strain
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0,05
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0,15
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0,2
0,25
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Utilization of capacity reserves of structure
More precise calculation
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Calculation in accordance with
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 present (valid) codes
3D complex models
Shell elements
 Joints
 Shell structures (silos, pipelines ...)
Interaction of elements
Connections
 Semi-rigid connections – new standards enable to determine joint
stiffness
 Column bases
Stochastic methods of the reliability verification
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Reinforcement
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Reinforcement = adaptation to recover operational
features; increase loading capacity, reliability and
serviceability of the structure
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Direct reinforcement – reinforced and reinforcing part
constitute one load bearing element
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Indirect reinforcement – reinforced and reinforcing part are
constituted by separate load bearing elements
Renovation
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= arrangement of structural parts or the whole structure
after extraordinary events and accidents
Extension
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= extension of an existing section of the structure
Replacement
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= replaces the original structure or a section by a new
structure linked to an existing layout
Relocation
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= a part of the original structure or entire original
structure are moved to another place
Special arrangements
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Indirect reinforcing
Changes to the static system
Prestressing
Coupling with concrete
Reinstalation of the strusture for other uses
Assesment of steel structures for
refurbishment
In designing the refurbisment of an existing structure we
should follow the present valid standards and regulations
 Previously valid standards can serve only as informative
background
 Must be provided examination of the reliability of the
original structure
 First step = substandard analysis of the suitability of
unsuitability of the structure for the reconstruction;
structures unsuitable for the reconstruction when
replacement by the new structure is recomended include:
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Structures significantly damaged by impact or explosion
Structures totally damamged or distorted by high teperature
during a fire
Structures totally corroded
Structures for which the condition of stresses or the history
of acting variable loading cannot be reliably determined
Structures of wrought iron and other low-quality material
(exept of rare historical structuires)
Structures for which is not possible to obtain necessary
results of assessing examinations (taking into account the
level of acceptable costs for examining the existing load
bearing system
Proposal for reconstruction – structure
must be examined to establish:
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The condition of the structure as a whole
Properties of used materials
Loads that affect the structure or have affected structure
in the past
The influence of the environment on the structure
Documentation of the structure
Introduction
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Steel structures have an important role in civil engineering
Since the end of the 18th century, first cast iron, then
wrought steel and finally steel has increasingly been used as
a construction material.
Gradually, as industrial processes progressed, various steel
products became available, (rolled members, cold-formed
elements…).
From the beginning, the fields of application of structural steel
material included structures such as:
- Buildings,
- Bridges (first bridge made of cast iron and built 1777-1779
near Coalbrookdale -UK)
- Industrial plants.
Introduction – cont.
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Increasing economic and ecological pressure influences
the need for reconstruction works and maintenance
Use of appropriate method of recontruction is key point
of the whole process
It is usually complicated to obtain background
information about the structure (material properties,
static scheme, type of used elements, joints, bracing
system…)
This increases the financing and design responsibilities
Steel structures provide the widest range of
reconstruction possibilities than the other materials
History of using iron and steel
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Cast iron
Wrought iron
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since 1785
until 1892 – 1905
after 1905 only exceptionally
Mild steel
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since 1905
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Characteristic and design strength values for steel
Cast iron
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Fragile
Suitable for compression, worse for bending
High contents of C (2,1%)
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Mechanical properties:
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E ~ 100 000 MPa (N/mm2)
fu ~ 120 ÷ 140 MPa
Cast iron bridges
The use of cast iron for structural purposes began in the
late 1770s, when Abraham Darby III built the Iron Bridge
in the village Ironbridge /renamed by the bridge/
(Shropshire, England)
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Ironbridge
Just a few years after the construction
of the bridge, cracks were appearing in
the masonry abutments, caused by
ground movement. Some of the
present-day cracks in the cast iron may
date from this time, although others
are probably casting cracks
Cracked supports
Crack and repairs in bridge
Coalport – another old cast iron bridge was built in 1818
Wrought iron
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Production
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Temperature  1000oC  doughy state
Low charge – 200-600 kg
Mechanical reduction of undesirable elements
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Large scatter of mechanical properties
Layered anisotropic structure
Local defects
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Wrought iron
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Chemical composition
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Large scatter
Lower contents of C
High contents of P (phosphorus) – could be problem
Problems
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Uncertain weldeability
Low strength through thickness  Lamelar tearing
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Wrought iron
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Mechanical properties in rolling direction
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E = 180 000 ÷ 200 000 MPa (N/mm2)
fy ~ 230 MPa (mean)
fu ~ 340 ÷ 370 MPa
Lower ductility but still sufficient
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Eiffel Tower (designer Gustave Eiffel)
The tower was built as the entrance arch to
the 1889 World's Fair.
Iron pillar of Delhi built at the time of
Chandragupta Vikramaditya (375–413 n.l)
Mild steel
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Production
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Liquid state
Larger charges
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Since 1905 properties similar to present steel
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E = 210 000 MPa
fy , fu similar to present S235 (Fe360)
called also Carbon steel (≤2.1% carbon; low alloy)
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Properties of material
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Time of construction  Type of material
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How to determine:
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from documentation (rarely)
verification by tests is recommended
using tests
Mechanical properties of iron/steel are NOT time
depending
(except fatigue)
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FIRST STEEL STRUCTURE - Forth Bridge
The Forth Bridge is a cantilever railway bridge over the river named Firth of Forth in the
east of Scotland, to the east of the Forth Road Bridge, and 14 kilometres west of central
Edinburgh. It was opened on 4 March 1890.
References
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Agócs Z.,Ziolko J.,Vičan J., Brodniansky J.: Assessment and Refurbishment of
Steel Structures, Spon Press, 2005
Spal L.: Rekonstrukce ocelových konstrukcí (Refurbishment of Steel
Structures), SNTL, Praha, 1968
Refurbishment by steelwork, ArcelorMittal, Luxembourg
Vašek M.: Zesilování ocelových konstrukcí (Strengthening of steel
structures), DOS T 3, No. 04, ČKAIT, 2000
Lectures of prof. Macháček to subject YSMK, CTU in Prague, 2009
Háša P., Jeřábek L., Rosenkranz B.,Vašek M.: Havárie střechy kotelny
elektrárny Opatovice nad Labem (Collapse of boiler house roof of the
power station in Opatovice), Konstrukce No.3, 2004
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