SEISMIC BEHAVIOR AND DESIGN OF
FRICTION CONCENTRICALLY BRACED FRAMES
FOR STEEL BUILDINGS
By
Robert Tremblay
October 1993
Summary by
Govinder S. Dhesi
November 2001
The University of British Columbia
CIVL 510 Behavior of Steel Structures
Introduction:
This comprehensive study investigates the improvement of the
Seismic Behavior and Design of FCBFs for Steel Buildings
Background:
Efficiency of Concentrically Braced Frames
seismic performance of concentrically braced frames (CBFs) through
Concentrically braced frames (CBFs) are an efficient system for
the use of friction connections at the ends of the bracing members.
resisting lateral loads on a structure. Behaving as vertically
These friction connections slip at set load levels and dissipate the
cantilevered trusses anchored to the foundation, CBFs possess
energy input by ground motion through friction, preventing overload
several advantages that make them suitable for use in low to medium
of the bracing members.
rise structures (up to 20 stories). The use of a braced frame in a steel
structure is illustrated below in Figure 1.
Friction concentrically braced frames (FCBFs) were proposed that
had bolted brace connections using gusset plates with slotted holes.
Dynamic load testing of sample connections confirmed that a stable
response could be achieved using an appropriate sliding material and
bolt clamping force. Tests results of full scale braced frame
assemblies undergoing severe interstorey drifts showed that the
FCBF system responds in a satisfactory and predictable manner.
Analytical studies of single and multi-storey FCBFs were then
performed to develop design guidelines to assess ductility demands
and instability thresholds. Values for column strength and stiffness
were proposed for multi-storey FCBFs to prevent the formation of
collapse mechanisms that were identified in the study.
November 14, 2001
Figure 1: Typical Braced Steel Frame
(From Seismic Behavior and Design of FCBFs for Steel Buildings, R. Tremblay)
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CIVL 510 Behavior of Steel Structures
Seismic Behavior and Design of FCBFs for Steel Buildings
The truss type behavior exhibited by CBFs provides a relatively high
Figure 2, below, illustrates typical bracing configurations used in
lateral stiffness thus requiring only a few bracing bents within the
concentrically braced frames.
structure. For optimal resistance to torsional effects these bents are
typically located at the periphery of the structure. Since CBFs are
typically modeled as pin ended structures with continuity only at the
columns of multi story frames, connection detailing is of the simple
type requiring only bolted field connections between the members.
In addition, since the lateral loads induce primarily axial forces in the
bracing members, the member and connection designs are also
simplified. Further benefits of CBFs include the ability to
accommodate architectural features such as doors and windows
between the braces, the use of standard construction and fabrication
procedures familiar to the industry, and the application of standard
design procedures with accepted design rules. The use of dedicated
braced bays also allows the separation of the gravity load resisting
elements from the lateral load resisting system, thereby requiring
only the braced bays and their foundations to be designed for the
combined effects of the lateral and gravity loads. CBFs are also
suitable for the upgrade of existing structures. By the addition of
diagonal bracing members between existing beams and columns, the
strength and lateral stiffness of these structural systems may be
readily increased.
November 14, 2001
Figure 2: Typical CBF Configurations
(From Seismic Behavior and Design of FCBFs for Steel Buildings, R. Tremblay)
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CIVL 510 Behavior of Steel Structures
Efficiency of Other Lateral Load Resisting Systems
In contrast to CBFs, moment resistant frames (MRFs) are less
efficient because of their lower lateral stiffness, increased complexity
of member and connection design, and the requirement for field
welding at critical areas. In addition the gravity load resisting system
and the lateral load resisting system is combined adding further
complexity to the design. With cast-in-place concrete shear walls the
advantages of having a prebuilt structure, higher speed of installation
Seismic Behavior and Design of FCBFs for Steel Buildings
seismic behavior of concentrically braced frames. This research has
resulted in more comprehensive design codes requiring increasingly
complex design and detailing requirements. Overall, with reliability
of the braces still a matter of concern and the need to consider high
brace overstrength generated by these design procedures, the
economical benefit, originally offered by CBFs, becomes
outweighed. As a result the CBF system has become less attractive
for earthquake resistance.
and guaranteed strength are lost.
Improving the Seismic Performance of CBFs
Seismic Performance of CBFs
The high lateral stiffness of CBFs has a beneficial effect during
seismic loading as well by limiting story drifts and therefore
structural damage. Furthermore, by localizing the inelastic response
within the bracing members the gravity load resisting elements are
not impaired. Despite these advantages and those mentioned earlier,
the CBFs exhibit poor energy absorption and dissipation through
inelastic response. Progressive slackening of braces, degradation of
To address the shortcomings of the seismic performance of CBFs,
research has focused mainly on either improving the inelastic
response of the bracing members or by the introduction of substitute
energy dissipating mechanisms within the assembly. Methods for
improving the inelastic behavior of the braces have been discussed in
the thesis. For example, encasing the braces in pre cast concrete with
the steel member free to slide has been shown to improve the
efficiency and reliability of systems.
compressive strength and premature fracture render this system
inefficient and unreliable for seismic applications. However,
recognizing the advantages of CBFs much research has been
undertaken over the last two decades towards the improvement of the
November 14, 2001
Other studies have looked at the use of alternate energy dissipation
mechanisms within the assembly. The underlying concept is to
dedicate alternate members or devices as the critical elements
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CIVL 510 Behavior of Steel Structures
Seismic Behavior and Design of FCBFs for Steel Buildings
allowing the braces to be classified as non-critical elements. Several
The eccentrically braced frame (EBF) utilizes a short segment of the
alternate mechanisms that have been mentioned in the thesis are
beam as the critical element. This segment, called the shear or
illustrated below in Figure 3.
flexural link, yields either in flexure or shear. The ductile link braced
frame (DLCBF) is similar to the EBF where existing beams act as
the flexural links. The behavior of this system is similar to that of
coupled shear walls. The disposable knee bracing (DKB) and the
system with energy absorbing devices are similar in principle in that
they dissipate energy through inelastic bending in these members or
devices. The Y braced frames (YBF) and the ADAS systems also
incorporate dedicated devices where energy dissipation is localized.
Previous studies have indicated that viscous dampers are also
effective in reducing relative displacements and absolute
accelerations. In general viscous dampers are likely to be used in
parallel with other systems.
For the alternate systems mentioned above, studies have shown that
theses critical elements exhibit stable hysteretic behavior. The
advantage of these systems compared to CBFs is that post
earthquake damage can be limited to the devices. Secondly, since
the story shear strength and stiffness have been partly decoupled, the
brace size and critical element strengths may be selected
Figure 3: Alternate Energy Dissipation Mechanisms
(From Seismic Behavior and Design of FCBFs for Steel Buildings, R. Tremblay)
November 14, 2001
independently allowing increased design flexibility.
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CIVL 510 Behavior of Steel Structures
Seismic Behavior and Design of FCBFs for Steel Buildings
In contrast to concentrically braced frames, the author notes several
disadvantages of the alternate energy dissipation mechanisms
illustrated in Figure 3. The main drawback is the increased design
and detailing effort involved. In addition, the systems are not as
efficient in resisting lateral as CBFs for both strength and stiffness.
In all systems except for the CBFs with energy absorbing devices
within the braces, heavier members will be required because of the
induced bending moments.
Friction Concentrically Braced Frame System
Another recently proposed system involves the use of friction
Figure 4: Conventional CBF versus Proposed FCBF System
(From Seismic Behavior and Design of FCBFs for Steel Buildings, R. Tremblay)
devices at the ends of bracing members. The friction concentrically
braced frame (FCBF) is similar to the viscously damped frame
shown in Figure 3 except that energy dissipation is through a friction
device. This friction device is designed to slip at predetermined load
Proposed FCBF System:
levels and then dissipate ground motion energy through friction at
Based on the findings and recommendations of previous research an
the sustained slip loads. The motivation for the use of this system as
FCBF, as illustrated in Figure 5, was selected for this study. The
an alternate system was that it maintained the inherent advantages of
friction mechanism at one end of the tension-compression diagonal
CBFs as discussed earlier while improving the CBFs’ poor inelastic
brace was selected to consist of steel side plates on the bracing
response under cyclic loading. The proposed system along with the
members connecting to a slotted gusset plate with high strength bolts
ideal load deformation response is illustrated in Figure 4.
providing the normal clamping force. The connection is illustrated in
Figure 6.
November 14, 2001
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CIVL 510 Behavior of Steel Structures
Seismic Behavior and Design of FCBFs for Steel Buildings
The proposed system was selected for study because of previous
research findings illustrating the stable and predictable hysteretic
response of slotted gusset plate slip-critical friction connections. In
addition the detailing and design of such a system would resemble
that of conventional CBFs. This system would offer the advantages
of CBFs as discussed earlier but with improved seismic behavior.
From a fabricator and contractor’s perspective, the proposed system
utilizes standard materials and construction procedures without
intensive detailing demands.
Objectives:
Figure 5: Proposed FCBF System
(From Seismic Behavior and Design of FCBFs for Steel Buildings, R. Tremblay)
Although research has been conducted on friction type connections
in the past, a limited number of studies have addressed the behavior
of FCBF systems. Most research has been conducted on friction type
connections acting in parallel with other systems under relatively low
slip loads. This study addresses the use of such an FCBF system in
series with the lateral load resisting system under loading which is
more typical of seismic events. An experimental and analytical
study was undertaken to assess the behavior and stability of FCBF
systems under dynamic loads and to propose simple design
Figure 6: Typical Sliding Friction Connections
(From Seismic Behavior and Design of FCBFs for Steel Buildings, R. Tremblay)
November 14, 2001
guidelines for such structures. The study was limited to normal
buildings on stiff soils with uniform mass, stiffness and geometry.
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CIVL 510 Behavior of Steel Structures
Experimental Study:
Seismic Behavior and Design of FCBFs for Steel Buildings
materials, loading frequencies and whether disk spring washers were
used in the connection assembly were considered in this part of the
Experimental studies were performed under static loads and dynamic
study. The five studied faying surface materials were:
cyclic loads to assess the behavior of different connections and the
performance of a full scale braced bay assembly as a whole.
1. Standard weldable steel conforming to CAN/CSA G40.21-M87
2. Quenched & tempered alloy steel conforming to ASTM 514 Grade B
3. Quenched & tempered abrasion resisting steel alloy
Three series of tests were conducted to study the response of the
proposed FCBFs. Static slip tests were conducted on two bolted
4. Air-hardened Nickel-Chromium-Molybdenum alloy
5. Corrosion-Wear resisting alloy (Cobalt based alloy)
connections, dynamic cyclic tests were carried out on single bolted
connections and on a full scale braced assembly. Additional tests
Full-scale tests of braced frame assemblies were then performed to
were performed to test the properties of different wearing surface
observe the behavior of the entire system. A schematic of the tested
materials, the behavior of the disk spring washers and high strength
frame is shown below in Figure 6.
bolts.
Static slip tests on 12 specimens were first performed to study the
gross slip behavior of the slip-critical connections. Although
extensive studies have demonstrated the efficiency of these
connections, limited knowledge existed on the behavior of these
connections under gross slip.
Dynamic cyclic tests were then performed on a shake table to assess
the behavior of 42 connections under severe slip conditions. The
effects of using different clamping force levels, faying surface
November 14, 2001
Figure 7: Full Scale Test Frame Setup.
(From Seismic Behavior and Design of FCBFs for Steel Buildings, R. Tremblay)
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CIVL 510 Behavior of Steel Structures
Seismic Behavior and Design of FCBFs for Steel Buildings
As can be seen from Figure 7, the test model used pinned joints to
Based on the results of the first two series of tests (refer to “Results”
reduce the affects of other parameters on the test results. The friction
section) the chosen connection assembly consisted of cobalt based
connection, installed on one end of the HSS brace, used a 12 bolt
alloy (Material 5) inserts at the wearing surface. ½” diameter high
symmetrical arrangement. Gusset slot lengths were determined based
strength bolts without disc spring washers, torqued to their ultimate
on the maximum expected slip plus an additional allowance for
capacity, were used to provide the clamping force. All testing was
fabrication and erection tolerances. The maximum expected slip was
conducted in the Structure Laboratories in the Department of Civil
determined from brace elongation and ductility demand calculations.
Engineering of the University of British Columbia between
The brace connection details are shown below in Figure 8.
December 1991 and November 1992.
Analytical Study:
To assess the stability of the proposed system under the influence of
P-delta effects during earthquake induced storey drifts and to
propose guidelines for the prevention of such instabilities, nonlinear
time step dynamic analyses of SDOF and MDOF systems were
performed. Fo“gçæe analysis of the SDOF systems a computer
program was written and for the analysis of MDOF systems existing
programs, DRAIN-2D and DRAIN-2DX, were used. At rest initial
conditions and damping equal to 5% of critical were assumed in the
studies. In addition, only the horizontal components of ground
motion were considered in the analysis.
Figure 8: Details at Brace Connections.
(From Seismic Behavior and Design of FCBFs for Steel Buildings, R. Tremblay)
November 14, 2001
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CIVL 510 Behavior of Steel Structures
Seismic Behavior and Design of FCBFs for Steel Buildings
Results:
however during the initial cycles. The use of disc spring washers did
Static Slip Tests
not appear to influence the behavior of this configuration.
These tests indicated that the use of slotted holes did not adversely
affect the initial slip resistance of the joint. Secondly, under gross
Full Scale Braced Frame Tests
slip behavior, extensive overstrength development and material wear
For these tests the friction connection assembly recommended based
was observed. These tests indicated that a more suitable material
on the dynamic slip tests results was used to assess the behavior of a
needed to be considered at the interface or a reduction in the bolt
full-scale frame subjected to severe cyclic interstorey drifts. Two
tension was required to improve the response of the connection.
frames were subjected to three loading runs each. The effects of
These changes were considered in the dynamic testing phase.
using two different gusset plate materials (Material 1 and Material 2
as listed on page 7) were also addressed.
Dynamic Cyclic Tests
A significant increase in temperature, wear and resistance to slip was
These tests indicated that a stable hysteretic response could be
observed during these tests. The increased resistance to slip was
achieved with an FCBF system undergoing severe interstorey drifts
evident near the ends of the tests. A reduced bolt tension force alone
without structural damage to the components, provided these
was found to be insufficient to achieve the desired response. The use
components are designed to accommodate the expected deformations
of dissimilar materials at the sliding interface was found to improve
and the associated forces. Inspection of the insert plates following
the performance especially when the difference in material strengths
the tests indicated that high bearing stresses were occurring at the
was significant. Of the different configurations studied, the one with
bolts. The test results also indicated that there was no obvious benefit
cobalt based inserts and ½” diameter bolts torqued to their ultimate
of using higher strength gusset plates. The hysteretic response for
capacity appeared to be the most feasible, since it was simple,
specimen F-01 during run #3, along with the storey shear time
efficient and it didn’t display the increase in slip resistance at the end
history plots is shown in Figure 9 on the following page.
of the loadings. A gradual increase in slip resistance was noted
November 14, 2001
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CIVL 510 Behavior of Steel Structures
Seismic Behavior and Design of FCBFs for Steel Buildings
Conclusions and Recommendations:
Although efficient in many regards conventional CBFs were found to
be unreliable and inefficient in dissipating ground motion energy
through inelastic response. After a review of alternate lateral load
resisting systems and methods of improving the performance of
CBFs, an FCBF system was proposed since it maintained the
Figure 9: Storey Shear Time History and Hysteresis (Specimen F-01)
(From Seismic Behavior and Design of FCBFs for Steel Buildings, R. Tremblay
Stability of Single and Multi-Storey FCBF Systems.
simplicity and efficiency of CBFs but with improved seismic
behavior. This study, both experimentally and analytical,
demonstrates that FCBFs offer excellent potential for use as lateral
load resisting systems in seismic zones. In addition, existing design
The results of the nonlinear dynamic analyses of SDOF and MDOF
systems indicate that FCBF systems, with slip loads determined in
procedures can be extended with some modifications for the design
of these FCBF systems.
accordance with Part 4 of NBCC and using a force modification
factor of 4, can respond in a stable manner with respect to the storey
drifts and extent of damage to the structural elements. To achieve a
stable response the study indicates that sufficient post-slip column
Further research was recommended on the performance of the sliding
connections and the system as whole to develop higher confidence
levels and to develop complete design guidelines.
stiffness and strength are required to prevent the onset of collapse
mechanisms identified in the analyses. It was proposed that
References:
additional column shear stiffness be provided to offset the effects of
gravity on that stiffness. In addition column continuity is suggested,
preferably for the full height of the frame. Design spectra were
developed for predicting ductility demands and instability thresholds.
November 14, 2001
Tremblay, R. 1993. PhD Thesis, Department of Civil Engineering,
Faculty of Graduate Studies, University of British Columbia,
Vancouver, B.C.
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