Seismic Design Steel Structures 2005
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Transcript of Seismic Design Steel Structures 2005
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Canadian Seismic Design of Steel Structures
An Organized Overview
By: Alfredo Bohl
University of British Columbia
Department of Civil Engineering
March, 2005
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Parameters:
V: Design shear force.
S(Ta): Design spectral response
acceleration.
Mv: Factor for the higher mode
effects on the shear base.
IE: Earthquake importance factor of
the structure.
W: Expected weight of the structure.
Rd: Ductility-related force
modification factor.
Ro: Overstrength-related forcemodification factor.
od
Eva
RR
WIMTSV
Lateral seismic force at the baseaccording to the 2005 NBCC
Introduction
The overview given in this report is
based on the provisions contained
in:
The upcoming 2005 Edition of the
National Building Code of Canada.
Clause 27 of the 8th. Edition of theHandbook of Steel Construction.
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Overstrength-related force
modification factor (Ro):
This factor is related to the
calibration factor U used in the
previous code.
It takes into account in a more
explicit way the overstrength in
structures, by identifying the sources
of it and assigning factors that
consider each of these sources, like
the actual strength of the material,
rounding up of dimensions of the
elements, and redistribution ofinternal forces.
Ductility-related force modification
factor (Rd):
This factor corresponds to the R
factor used in the previous 1995
edition.
For steel structures, these have
been increased for ductile and
moderately ductile systems to 3.5
and 5.0, compared to 3.0 and 4.0 in
the previous code.
The design forces for these systems
are now lower; however, the details
requirements to ensure adequateductility according to these factors
are more demanding.
New force reduction factors in the 2005 NBCC
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Steel seismic force resisting systems (SFRS)
Classification of SFRS according tothere ductile behavior:
Ductile or Type D: They can sustainsevere inelastic deformations. Theyhave a force reduction factorbetween 4.0 and 5.0.
Moderately ductile or Type MD:Inelastic deformations are morelimited, members are designed toresist greater loads. They have aforce reduction factor between 3.0and 3.5.
Limited ductile or Type LD: These
are newly introduced types offrames. Inelastic deformations areeven more limited and design loadsare greater than in type MDelements. They have a forcereduction factor of 2.0.
Characteristics of SFRS:
The 2005 NBCC recognizes different
types of SFRS, their corresponding
Rd and Ro factors, and the design
and detail requirements for each of
them according to the CSA standard
CSA-S16-01.
In each of these SFRS, there are
certain structural elements which are
designed to dissipate energy by
inelastic deformation; these must be
able to sustain various cycles of
inelastic loading with a minimumreduction of strength and stiffness.
The other elements and connections
must respond elastically to loads
induced by yielding elements.
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SFRS with Rd = 5.0
Ductile moment-resisting frames:
The energy dissipating elements are
the beams.
Beams must be capable of plastic
hinging without connection failures.
Plastic hinges in columns is only
permitted at their base, except for
single-storey buildings.
Maximum axial load in columns
limited to 0.3AFy for all load
combinations, since their flexural
resistance deteriorates fast when
high axial loads are applied.
Ductile members must be class 1
and capable to undergo inelastic
response without stability failures.
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SFRS with Rd = 5.0
Ductile moment-resisting frames:
Limited inelastic deformations are
permitted in column joint panel
zones if they are properly detailed.
The beam-to-column connections
must be capable to develop an inter-
storey drift angle of 0.04 rad undercyclic loading.
Advantages: They absorb less shear
forces due to their flexibility and
have high energy dissipation
capacity.
Disadvantages: Large inter-storeydrifts may cause severe P-delta
effects and non-structural damage.
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SFRS with Rd = 5.0
Ductile plate walls:
Newly introduced system in the
CAN/CSA S16-01.
The main energy dissipating element
is the web plate, framing elements
also dissipate energy once the plate
has yielded.
Same requirements for beams,
columns, panel zones and
connections; except that columns
must always be class 1.
Elements are proportioned so that
yielding occurs first in the web plate(principle of capacity design).
The top and bottom web plates must
also be anchored to stiff elements.
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SFRS with Rd = 5.0
Ductile plate walls:
Columns must be stiffened at the
base, so that the plastic hinges form
at a certain distance above the base
plate.
Advantages: They have very large
stiffness, reducing the amount ofnon-structural damage during an
earthquake.
Disadvantages: They may be more
expensive; and calculating the
tension fields in the plate web and
determining the yielding sequence ofthe plate and the framing system is
still a problem, due to the limitations
of the strip model.
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SFRS with Rd = 4.0
Ductile eccentrically braced frames:
The energy dissipating elements are
the links, which are the beam
segments between the brace
connections and the beam.
Links must be class 1.
Link rotation limits depend on if it
yields in shear or flexure.
Full-depth stiffeners at both ends of
the link and intermediate stiffeners
are required to make sure that it will
have a ductile behavior.
Beams outside the link, braces andcolumns must be stronger than the
link.
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SFRS with Rd = 4.0
Ductile eccentrically braced frames:
If the link is directly connected to the
column, the link beam-to-column
connection must be able to develop
anticipated plastic deformation.
The columns must be designed for
secondary moment effects due tothe frame drift.
Advantages: Combines the ductile
behavior of the moment-resisting
frame and the stiffness of the
concentrically braced frame.
Disadvantages: Since all the energydissipation is restricted to the link,
the collapse mechanism forms once
this element has yielded; while other
SFRS are more redundant.
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SFRS with Rd = 3.5
Moderately ductile moment-resisting
frames:
Requirements are the same as for
ductile moment-resisting frames,
except for the following:
Beams must be class 1 or 2.
Maximum axial load in columns
limited to 0.5AFy for all load
combinations, since their flexural
resistance deteriorates fast when
high axial loads are applied.
The beam-to-column connections
must be capable to develop an inter-storey drift angle of 0.03 rad under
cyclic loading.
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SFRS with Rd = 3.0
Moderately ductile concentrically
braced frames:
The energy dissipating elements are
the diagonal braces.
Only configurations that allow
inelastic response without losing
stability are permitted, like tension-
compression, chevron or tension-
only bracing systems.
Because ground motions may occur
in any direction, the dimensions of
the diagonal braces must be such
that the shear resistance in eachstorey provided by the tension forces
developed in these elements is
similar for storey shears acting in
opposite directions.
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SFRS with Rd = 3.0
Moderately ductile concentrically
braced frames:
In tension-compression systems,
braces must be class 2 to delay local
buckling, but they must yield before
the other elements.
In chevron systems, the beams must
be strong enough to resist yielding
and buckling forces from the braces
together with gravity loads, without
considering the support from the
braces.
In tension-only systems, the energydissipation capacity is very limited,
the braces must be able to carry all
the seismic loads.
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SFRS with Rd = 3.0
Moderately ductile concentricallybraced frames:
Brace connections must have aductile rotational performance if highinelastic response is expected.
Beams, columns and connections
must resist forces induced byyielding of the braces.
The columns must be designed forsecondary moment effects due tothe frame drift.
Advantages: They have very largestiffness.
Disadvantages: They tend to have asoft-storey response, so heightrestrictions apply depending on thebracing system used.
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SFRS with Rd = 2.0
Moment-resisting frames with limited
ductility:
Requirements are the same as for
moderately ductile moment-resisting
frames, except for the following:
Beams must be class 1 or 2, while
columns must be class 1 and I-
shaped.
In high seismic areas, buildings with
this system cannot exceed 12
storeys.
The beam-to-column connections
must be capable to develop an inter-storey drift angle of 0.02 rad under
cyclic loading.
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SFRS with Rd = 2.0
Limited ductility plate walls:
Requirements are the same as for
ductile plate walls, except for the
following:
The energy dissipating element is
the web plate, not the framing
elements.
Beams, columns and their
connections do not have any special
requirements, since they are not
expected to yield.
Buildings with this type of system
cannot exceed 12 storeys.
Limited ductility concentrically
braced frames:
Requirements are the same as for
moderately ductile concentrically
braced frames, except for the
following:
Height restrictions are relaxed, since
elements are designed for higher
forces.
Diagonal braces can be class 2 or
lower in low seismic areas.
Brace connections do not need to
have a ductile rotationalperformance in low seismic areas if
braces are slender.
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Characteristics:
Connections in type D and MD
frames must be tested to ensure that
they satisfy certain deformation
criteria under cyclic loading.
Testing procedures are described in
the FEMA 350 document.
The test assemblies must represent
the prototype characteristics, and the
test loading the deformation
magnitude and cyclic nature.
For each given combination of beam
and column size, tests of at least twospecimens must be performed. The
results obtained must be able to
predict the mean value of the drift
angle capacity of the connection.
Physical tests for connections in moment-resisting frames
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Characteristics:
The size of the beam used in the
specimen must be at least the
largest depth and heaviest weight
used in the structure.
The column must provide a flexural
strength consistent with therequirements of strong-column-
weak-beam connections, and must
have a height similar to the real
column, so that the drift angles
obtained are representative of the
real structure. The mean drift angle capacity must
not be less than a certain limit.
Tests results must be supported with
analytical design procedures.
Physical tests for connections in moment-resisting frames
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Bolted unstiffened end plateconnection:
The beam is welded to an end plate,
extended above and below the
flanges. The beam flange-to-plate
joints have complete-penetration-
groove welds, and the beam web isconnected to the plate with fillet or
complete-joint-penetration-groove
welds. Then, the end plate is bolted
to the column using eight bolts.
Design principle: Member sizes are
selected to preclude brittle failuremodes, and for yielding to occur as a
combination of beam flexure and
panel zone yielding. This applies
also for the other connections.
Moment-resisting connections for seismic applications
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Bolted stiffened end plateconnection:
The beam is welded to an end plate.
The beam flange-to-plate joints have
complete-penetration-groove welds,
and the beam web is connected to
the plate with fillet or complete-joint-penetration-groove welds. The end
plate extensions at the top and
bottom of the beam are stiffened
with vertical stiffeners that extend
outward from beam flanges. Then,
the end plate is bolted to the column
using 16 bolts.
Elements must have similar sizes
and details to those that were tested,
to predict their behavior. This applies
also for the other connections.
Moment-resisting connections for seismic applications
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Reduced beam section connection:
The flexural resistance of the beam
is reduced at a certain distance from
the connection, so that yielding and
plastic hinging occurs in the beam.
The top and bottom beam flanges
have circular radius cuts for thispurpose. The flanges of the beam
are connected to the columns only
with complete joint penetration
groove welds. A shear tab, that can
be bolted or welded, is used for the
web connection.
This type of connection cannot be
used for type LD frames, but the
previous two connections may be
used for these cases.
Moment-resisting connections for seismic applications
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Special truss moment frames:
This system is designed in such a
way that the inelastic deformation is
moved to some segments of the
truss that are specially designed.
This truss has several diagonal
members in a segment at themidspan designed for this purpose,
they absorb most of the energy and
dissipate it by yielding. After the
earthquake, the diagonal members
that were damaged can easily be
repaired or replaced.
This system has the advantage that
it weighs less than common framing
systems. Also, it provides substantial
cost and time savings compared to
these systems.
Special seismic steel framing systems
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Friction-damped steel frames:
Friction dampers are designed in
such a way they have moving parts
that will slide over each other during
a strong earthquake. Friction is
created between these sliding
elements, which dissipates energybuilt up in the structure. Examples of
these are the basic sliding joint, the
rotation sliding joint, the dual level
joint, the Pall friction device and the
Sumitomo friction device.
They have the advantage that theirbehavior is not seriously affected by
repeated cycles of displacement, the
friction force between surfaces can
be controlled, and they are not
affected by fatigue.
Special seismic steel framing systems
Basic sliding joint
Pall friction device
Rotating sliding joint
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Conclusions
An overall overview of the seismicdesign of steel structures in Canada
has been carried out. The design
procedures for SFRS, moment-
resisting connections, and some
special framing systems, which are
spread in various documents andpublications, have all been organized
in this report.
Each of the SFRS presented have
their own advantages and
disadvantages.
There may be cases in which itmight not be possible to use the
prequalified connections, and
physical tests are usually very
expensive and cannot be afforded by
small engineering companies. More
research is needed to developdesign procedures for various types
of connections with different element
sections.
Systems like the special truss
moment frame and the friction-
damped steel frame provide safe
and economic solutions compared toconventional framing systems, and
may be implemented in future codes.
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End of the presentation
Thanks