SEAOC 2011 CONVENTION PROCEEDINGS
High-Strength Brace Connectors for use in SCBF and OCBF
Carlos de Oliveira, President and CEO
Cast Connex Corporation
Toronto, Canada
Constantin Christopoulos, Associate Professor
Dept. of Civil Engineering, University of Toronto
Toronto, Canada
Jeffrey A. Packer, Professor
Dept. of Civil Engineering, University of Toronto
Toronto, Canada
Abstract
Innovative cast steel high-strength connectors for round
hollow structural section brace members have been
developed at the University of Toronto for use in Special and
Ordinary Concentrically Braced Frames (SCBF and OCBF).
The standardized connectors, now available through Cast
Connex Corporation under their High-Strength Connector
line of products, have been applied in the construction of
various seismic-resistant steel braced frame buildings in the
United States and Canada. At one end, the connectors are
designed with a circular shape and preparation to allow for
complete joint penetration shop welding to a range of round
tubular braces of a given outer diameter for the full
development of their expected strength. At the other end, the
connectors are shaped such that a bolted double shear
connection or longitudinal fillet welds can be used for
connecting the shop-welded brace-connector assembly to
conventional gusset plates in the field. This paper describes
the mechanical properties of the cast steel material
comprising the connectors, provides an outline of the design
rationale for the connectors meeting the AISC Seismic
Provisions’ requirements for SCBF and OCBF brace
connections, and provides a summary of the full-scale
structural testing that has been conducted to validate the
connector design philosophy.
Introduction
Special and Ordinary Concentrically Braced Frames (SCBF
and OCBF) are an efficient and popular choice for the lateral
force resisting systems of mid- to low-rise seismic-resistant
steel building frames. When lateral forces are applied to these
structures, the diagonal bracing members of the braced frame
are subjected to predominately axial loading. As Hollow
Structural Section (HSS) members are the most economical
structural shapes for carrying compressive loading, and given
the wide range of section sizes that are readily available in
the United States, it is not surprising that HSS are the typical
brace member of choice in these buildings.
Statically loaded HSS braces are typically connected to the
beam, column, or beam-column intersection via slotted HSSto-gusset connections, and the braces are typically welded in
the field to the single gusset plate at each end of the brace.
Design of these connections for static axial loading
accounting for shear lag effects has been studied by a number
of researchers (British Steel, 1992; Korol et al., 1994; Zhao
and Hancock, 1995; Cheng et al., 1996; Zhao et al., 1999;
Wilkinson et al., 2002; Ling, 2005; Willibald et al., 2006;
Martinez-Saucedo et al., 2006; Martinez-Saucedo et al.,
2008; Martinez-Saucedo and Packer, 2009); these statically
loaded connections can be readily detailed according to
prevailing design standards. However, during a design level
seismic event, the brace members in SCBF and OCBF are
intended to cyclically yield in tension and buckle in
compression, and as such the connections at each end of the
brace must be appropriately capacity designed and detailed to
ensure this behavior. If brace end connections are
inappropriately designed, detailed, or fabricated, the
connections can be prone to premature failure, as witnessed
during post-earthquake reconnaissance and in laboratory
testing (AIJ, 1995; Bonneville and Bartoletti, 1996; Tremblay
et al., 1996; Yang and Mahin, 2005; Fell et al. 2006).
Failures in these types of connections in earthquakes typically
occur due to a concentration of inelastic strain at the reduced
section of the HSS-to-gusset connection. As a result, AISC
1
Seismic Provisions (AISC, 2005a) now require the
reinforcement of the net-section of the brace at the
connection whenever slotted HSS-to-gusset connections are
used. As reinforcement is more readily fabricated for flat
surfaces, the industry has tended toward the specification of
square or rectangular HSS rather than round HSS for seismicresistant braces. Preference has also been given to rectangular
HSS braces as the specified minimum yield strength of round
HSS is somewhat lower than for rectangular sections in
ASTM A500 grade tubulars (ASTM, 2010a). This inclination
to rectangular HSS over round is unfortunate, as recent
studies have again shown that round HSS braces provide an
improved cyclic inelastic braced frame response over
rectangular HSS (Fell et al., 2009). Besides the reasons
associated with a curved cross-section’s enhanced ability to
resist local buckling, round HSS braces outperform their
rectangular counterpart as a result of the additional cold
working required to form rectangular HSS. Specifically, the
manufacturing process for rectangular HSS reduces the
available ductility in the corner radii of the section, which in
turn increases the likelihood of premature brace fracture
during cyclic inelastic buckling. An additional reason to
select round HSS over rectangular HSS for seismic-resistant
braces is that North American rectangular HSS have been
shown to have inherently poor notch toughness (Kosteski et
al., 2005), which limits their ability to resist crack
propagation.
To address some of the shortcomings of existing standards
for HSS, the AISC HSS Committee has been working on
developing a new, higher performance manufacturing
specification for HSS. A draft has been submitted to ASTM
as an alternate production standard, in addition to ASTM
A500. This manufacturing standard will hopefully feature
tighter control of geometric tolerances and improved
mechanical properties, including a notch toughness
requirement and a restricted yield stress range, thus making it
ideal for seismic design. Nevertheless, the difficulties
surrounding the adequate detailing of seismic-resistant HSS
bracing connections will remain.
To offer a simple solution for connecting round HSS in SCBF
and OCBF, researchers at the University of Toronto
developed standardized, capacity-designed, cast steel
connectors which accommodate bolted double-shear
connection between round HSS braces and a typical corner
gusset plate and which also eliminate shear lag in the
connection (de Oliveira et al., 2008b). Thus, use of the
connectors vastly simplifies connection design by eliminating
the need for connection reinforcement and, as the connectors
allow bolted connection to the gusset plates, the need for field
welding the brace to the gusset is also eliminated when the
connectors are employed. The standardized connectors, now
available through Cast Connex Corporation under their High-
2
Figure 1: Seismic-resistant concentrically braced frame with braces
equipped with Cast ConneX High-Strength Connectors used in both
architecturally exposed and non-exposed braced bays (photograph courtesy
of Cast Connex Corporation – photographer: Jake Estok, GENIVAR)
Strength Connector line of products, have been applied in the
construction of various seismic-resistant steel braced frame
buildings in the United States and Canada. Given the
compactness of the connections their use delivers and the
smooth, curving appearance of the connectors themselves, the
connectors are commonly used in both architecturally
exposed and non-exposed braced frames (Figure 1). Each
standardized connector is capacity designed to accommodate
all of the typical wall-thicknesses of a given outer diameter of
round HSS produced to ASTM A500 and ASTM A53 pipe
(2010b), amongst other grades of HSS and pipe.
Mechanical Properties of Cast Steel Comprising the
Standardized Brace Connectors
Steel casting manufacturing offers improved geometric
freedom over conventional steel fabrication, and the casting
process is predisposed to the high-quality, economical mass
production of complex geometric forms. A variety of cast
steel grades can be readily produced having a wide range of
mechanical properties and characteristics, including enhanced
strength, ductility, resilience, toughness, corrosion resistance,
and wear resistance. Further, the mechanical properties of
castings are far closer to being isotropic than those in rolled
steel products, and, owing to the improved geometric
freedom offered by casting manufacturing, steel material
need only be placed where it is necessary as determined from
engineering analysis. It was for these reasons that the
University of Toronto researchers opted to produce the brace
connectors they conceived using steel casting technologies.
There are a number of ASTM cast steel grades that are
suitable for structural use – AISC 360 (2010a) currently lists
ASTM A216 (2008) as a cast steel grade useful for steel
structures. Another ASTM cast steel grade, ASTM A958
Grade SC8620 Class 80/50 (ASTM, 2010c), was evaluated
and approved by the AISC Connection Prequalification
Review Panel in the course of approving the Kaiser Bolted
Bracket for use in seismic-resistant moment resisting steel
frames (Adan and Gibb, 2009; AISC, 2005b). This same
grade, including the additional Charpy V-Notch (CVN)
requirements outlined for the material in Appendix A of
Supplement 1 to AISC 358 (2009), is also used in the
production of the standardized brace connectors. This casting
material has a chemical composition similar to that of
standard wrought steel grades and provides a minimum yield
strength of 50 ksi, a minimum ultimate tensile strength of 80
ksi, a minimum elongation in 2-inches of 22% with a
reduction of area of 35%, and a CVN toughness of 20 ft-lb at
70ºF. To ensure the connectors are sound, production
components are subjected to the following acceptance
criteria, which mirror or exceed those set for the Kaiser
Bolted Bracket: visual examination per ASTM A802 (2010d)
with a surface acceptance Level I, ultrasonic testing per
ASTM A609 (2007a) with an acceptance Level 3, and
magnetic particle testing with an acceptance Level III in
accordance with ASTM A903 (2007b).
strength of the bracing member, determined as RyFyAg
(LRFD) or RyFyAg/1.5 (ASD), or the maximum load effect
that can be transferred to the brace by the system as indicated
by analysis. The standardized cast steel connectors were thus
designed to have a tensile strength at all cross-sections and
joints exceeding the expected yield strength of all brace
members that the connector is intended to accommodate.
Specifically, the connector includes a weld prep bevel on one
end which is designed to provide a 60-degree included angle
between the end of the wall of the round HSS member and
the connector’s bevel, around the entire circumference of the
tube. The geometry is such that a complete joint penetration
(CJP) groove weld can be produced between the tubular
member and the connector, regardless of the HSS member’s
wall thickness (Figure 2). With an appropriately selected
electrode, and with properly executed welding to a Welding
Procedure Specification approved per AWS requirements
(AWS, 2010), a CJP provides a resistance equal to the
expected tensile strength of the brace.
Connector Design Rationale per AISC Seismic
Provisions
As presented above, when SCBF or OCBF are inelastically
loaded, their brace members undergo tensile yielding and
compressive inelastic buckling. The AISC Seismic Provisions
contain prescriptive requirements for bracing connections
which are intended to accommodate the forces and
deformations associated with brace yielding and buckling. As
the requirements for SCBF connections are more stringent
than those for OCBF connections, the standardized cast steel
connectors were designed to exceed the SCBF brace
connection requirements, as described in the following
sections.
Required Tensile Strength
The required tensile strength of an SCBF brace connection
must be greater than the lesser of either the expected yield
Figure 2: Brace connection configuration and illustration showing how a
single connector size can accommodate a range of wall thickness sizes for
HSS of a given outer diameter
3
Beyond the beveled end, the connector is solid steel and
transitions gradually and smoothly from a round shape into
two parallel plates. The transitional geometry is gradual
enough to eliminate shear lag in the connection to the tubular
member – refer to Section K1.6 of AISC 360 and to page 83
of AISC Design Guide 24 (AISC, 2010b) for the cap-plate-toround HSS case – and to the plates. A section through the
plates is the clearly governing gross section of the connector
(Figure 3); the plates are sized such that their tensile yield
strength exceeds the expected yield strength of all of the
brace members that each connector is intended to
accommodate. Beyond the governing gross section of the
connector, the bolted double shear connection to the gusset
plate can be readily detailed by the Engineer of Record or
connection designer to have the requisite tensile capacity at
all net sections (in the connector and gusset plate) and at the
gross section of the gusset. Because the connector’s two
plates have Fy and Fu greater than or equal to that of the
gusset plate and have a combined thickness well over that of
the gusset (note that the gusset plate’s maximum thickness is
limited by the connector’s standardized geometry,
specifically the size of the gap provided between the
connector’s two plates), the resistance of the bolted joint at
the gusset always governs the resistance of the overall bolted
double shear connection.
to accommodate brace-end rotations. In the first case, where
the rotation is to be confined to the bracing member, the
required flexural strength of the brace connection must be
greater than 1.1RyMp (LRFD) or (1.1/1.5) RyMp (ASD) of the
brace about its critical buckling axis. An alternative to this
requirement is to detail the brace connection such that it has
sufficient flexural stiffness and strength to force the
formation of the plastic hinge within the single gusset plate
that is employed between the brace member end and the
beam-column intersection. As described in the commentary
of the AISC Seismic Provisions, this is typically achieved in
conventional SCBF connections by separating the brace end
from the restraint provided by the beam, column, or other
brace joints by a free length of gusset plate that is at least
twice the thickness of the gusset. The same brace buckling
response can be achieved when employing the cast steel
connectors by providing the free length of gusset beyond the
ends of the connectors. The connectors are designed to have a
flexural stiffness at all cross-sections and joints exceeding the
flexural stiffness of the gusset plate at the plastic hinge, and a
flexural strength at all cross-sections and joints exceeding
1.1RyMp (LRFD) or (1.1/1.5) RyMp (ASD) of the gusset plate
at the plastic hinge. The latter flexural strength requirement is
conservative, as the bending moment induced in the
connector during inelastic buckling of the brace will be lower
than the plastic moment capacity of the gusset, as illustrated
in Figure 4.
Mp brace
Mp gusset
Figure 4: Deformed shape and the associated bending moments induced
through the brace and brace connection during out-of-plane inelastic
buckling of the brace – plotted on tension side (adapted from de Oliveira et
al., 2008b)
Figure 3: Top and side views of a typical cast steel connector showing the
governing gross section of the connector for use in determining the tensile
resistance of the connector
Required Flexural Strength
Braces in SCBFs are expected to undergo cyclic inelastic
buckling under severe ground motions, forming plastic hinges
at their center and at each end. Therefore, and as described in
the commentary of the AISC Seismic Provisions, bracing
connections must either have sufficient strength to confine
inelastic rotation to the bracing member or sufficient ductility
4
As noted above, the gusset plate’s maximum thickness is
limited by the connector’s standardized geometry,
specifically the size of the gap provided between the
connector’s two plates. It can be readily shown from
engineering first principles that the flexural stiffness and
strength of the connector at its governing cross section would
greatly exceed those of the single gusset plate at the plastic
hinge for all reasonable gusset plate widths. Note that
connection designers would always try to minimize the width
of the gusset plate at the plastic hinge to reduce the overall
size of the connection, particularly since the Whitmore width
would limit the effectiveness of any plate material beyond a
30-degree projection from the first bolt line of the bolted joint
at the gusset.
Required Compressive Strength
Pursuant to the AISC Seismic Provisions, the compressive
strength of SCBF brace connections must be at least 1.1RyPn
(LRFD) or (1.1/1.5)RyPn (ASD) where Pn is the nominal
compressive strength of the brace. This design force is based
on the limit state of overall buckling of the brace, amplified
by the brace material’s overstrength. In addition to the typical
gusset plate design procedures, the Engineer of Record or
connection designer making use of the cast steel brace
connectors need only confirm that the sum of the
compressive strengths of the two plates of the connector,
acting individually in compression, exceed the required
compressive strength indicated above. The free length of the
plates would be equal to the gap between the first bolt line of
the bolted joint at the gusset plate and the location where the
two plates merge together at the heavier portion of the
connector, as illustrated in Figure 5. For most practical (and
correctly capacity-designed) bolted joint details, the
compressive resistance of the two plates is equal to or
exceeds even the expected yield strength of the brace, RyFyAg
(LRFD) or RyFyAg/1.5 (ASD), so this design provision will
rarely, if ever, govern the connection design for all practical
brace lengths. Additionally, the connection designer can opt
to increase the longitudinal spacing between the bolts to
reduce the unbraced length of the connector’s plates.
results. The original publications can be referenced for the
full description of each of the tests and for the full suite of
test results.
Static Monotonic Tensile Testing
As described in de Oliveira et al. (2008b), a static monotonic
tensile test was conducted at the University of Toronto’s
Structural Testing Facilities on a full-scale brace-connector
assembly consisting of an HSS 6.625 x 0.500 member
produced to ASTM A500 Grade C with cast steel connectors
welded on either end. The cast connectors were in turn bolted
to gusset plates that were designed to fit the wedge
anchorages of the testing machine. During this test,
displacement was monotonically increased beyond the peak
load of 670 kip up to rupture of the specimen, which
occurred in the tube a short distance above the midpoint of
the round HSS member and only after significant necking of
the tube’s cross section was observed. Figure 6 shows the
fractured test specimen after completion of the tensile test.
This test validated the effectiveness of the CJP weld under
monotonic tensile loading and illustrated the significant
stiffness that the connectors possess, which forced tensile
yielding and necking toward the center of the HSS segment,
Figure 5: Top and side views of a typical cast steel connector showing the
unbraced length of the connector’s plates for use in determining the
compressive resistance of the connector
Validation of Design Philosophy through Full-Scale
Testing
To validate the overall connector design philosophy
presented above, a variety of full-scale destructive tests have
been conducted on various sizes of round HSS braces fitted
with cast steel connectors designed as described above. The
following subsections summarize each of the types of test
that have been conducted and provide representative test
Figure 6: Photograph of brace-connector assembly fractured during
monotonic tensile testing
5
Quasi-Static Cyclic Testing
Connector Assemblies
of
Short
Brace-
As described in de Oliveira et al. (2008b), full-scale, short
brace-connector assemblies were tested at the University of
Toronto’s Structural Testing Facilities under a specially
prepared quasi-static cyclic inelastic loading protocol that
was designed to stress the brace end connections to a higher
degree than would be expected in a typical braced frame.
Specifically, the loading protocol was tension-biased – as
large compressive displacements in short braces result in
premature local buckling and failure in the brace assemblies
– but as the braces were stocky (L/r in the range of 45), the
compressive loads attained in each compressive cycle were
significantly higher than would be expected in a brace of a
more typical slenderness. In this way, the brace-connector
assemblies (and thus their welded CJP joints) were subjected
to repeated loading, cycling between tensile brace yielding
500
400
300
200
Load [kip]
away from the welded joint. Also of note is that the
connectors employed in this test had already been used in one
of the short brace quasi-static cyclic tests described in the
following section. After that testing, the spent tube segments
were removed from the connectors, the connectors’ noses
were machined back to their original shape, and the
connectors were welded to the HSS 6.625 x 0.500 tube
segment for the tensile test pictured above.
100
0
-100
-200
-300
-400
0.00
0.05
0.10
0.15
0.20
Axial Strain [ - ]
Figure 8: Hysteretic response generated during a short brace-connector
assembly test – HSS 6.625 x 0.375 produced to ASTM A500 Grade C
and elevated compressive loading. In all tests, failure of the
brace occurred at the midsection of the brace, away from the
connections. Figure 7 shows one of the short brace
assemblies during a compression loading cycle and Figure 8
shows the hysteretic response of the short brace-connector
assembly tests. In the plot, the force transmitted axially
through the brace is plotted against the imparted axial strain
in the HSS segment.
The short brace-connector assembly tests provided further
verification of the strength and robustness of the welded CJP
joints between the connector and the round HSS elements.
Quasi-Static Cyclic Testing of Mid-Slenderness
Brace-Connector Assemblies
As described in de Oliveira et al. (2008a), full-scale, midslenderness brace-connector assemblies (L/r’s ranging
between 85 and 141) were tested under quasi-static cyclic
inelastic loading at the Hydro-Québec Structural Engineering
Laboratory at École Polytechnique de Montréal as part of a
collaborative research effort between University of Toronto,
École Polytechnique, and Cast Connex Corporation. The four
brace-connector specimens tested were fabricated using the
following round HSS sections produced to ASTM A500
Grade C: HSS 4.000 x 0.313, HSS 5.563 x 0.375, HSS 6.625
x 0.500, and HSS 8.625 x 0.625. These are the heaviest
walled sections available in ASTM A500 grade material in
North America for these outer diameters. The full-scale tests
were conducted in a 2,700 kip capacity MTS load frame
fitted with specially designed grips which provided realistic
representation of the out-of-plane restraint of the gusset plate
at the beam-column intersection, as illustrated in Figure 9.
Figure 7: Photograph of short brace-connector assembly during a
compressive cycle of cyclic inelastic testing
6
600
500
a
400
b
Load [kip]
35º
2tp
300
200
100
0
(a)
-100
-200
-0.05 -0.04 -0.03 -0.02 -0.01 0.00
2tp
a
b
0.01
0.02
0.03
0.04
0.05
Axial Strain [ - ]
(c)
(b)
Figure 9: (a) Brace connection detail at beam-column intersection; (b)
laboratory setup producing similar boundary conditions as those in the field;
(c) laboratory test setup
Symmetric quasi-static cyclic loading was applied to the
brace-connector assembly at increasing displacement
amplitudes until failure occurred. All four of the braceconnector assemblies behaved as would be expected of
ductile buckling braces during the cyclic quasi-static testing.
Large compressive excursions resulted in out-of-plane
inelastic buckling of the bracing element (Figure 10b), with
significant tensile yielding occurring over the majority of the
HSS member’s length during the higher amplitude tensile
excursions. As expected, fan-shaped plastic hinges formed
Figure 11: Hysteretic response generated during a mid-slenderness braceconnector assembly test – HSS 6.625 x 0.500 produced to ASTM A500
Grade C
beyond the ends of the connectors within the free length of
the gusset plates as a result of overall member buckling
(Figure 10c). At higher amplitude compressive excursions, a
discrete plastic hinge formed at the mid-length of the brace
during inelastic buckling (Figure 10d) which eventually
resulted in local buckling of the HSS member’s wall. In 3 out
of the 4 tests, brace fracture occurred prior to reaching the
displacement limit of the test frame, and in all of these cases
failure occurred at the midpoint of the brace, away from the
connections (Figure 10e). Figure 11 shows the hysteretic
response generated during one of the brace-assembly tests. In
the plot, the force transmitted axially through the brace is
plotted against the imparted axial strain in the HSS segment.
Figure 10: HSS 8.625 x 0.625 brace-connector assembly: (a) undeformed; (b) experiencing inelastic buckling; (c) plastic hinge formation in free length of gusset
plate; (d) localization of plastic hinge in HSS member; (e) fracture of brace at its mid-length
7
The mid-slenderness brace-connector assembly tests
provided further validation of the design philosophy for the
connectors, as the braces in these tests underwent all of the
expected limit states of typical SCBF and OCBF.
Quasi-Static Full-Scale Braced Frame Testing
Recently, a full-scale concentric braced frame with a round
HSS brace (HSS 5.563 x 0.375 produced to ASTM A500
Grade C) fitted with the cast steel connectors was tested
under quasi-static cyclic loading at the Hydro-Québec
Structural Engineering Laboratory at École Polytechnique de
Montréal (Figure 12). The beams (W21x93) and columns
(W14x233) of the frame were designed to remain fully
elastic during the test (and during subsequent frame tests)
and the frame was positioned vertically, loaded with twin 225
kip actuators connected on either side of the web of the
frame’s upper beam. The supports at the base of the columns,
located at the working point formed by the centerlines of the
column, beam, and brace, were rotational pins and the beams
were fastened to the columns using double angles. The
center-to-center bay width and height of the frame were
approximately 295-inches and 160-inches, respectively.
Given the relatively shallow angle of the frame, the brace
was connected to the beams only, rather than to both the
beams and columns. As the beams were to be reused in
subsequent frame tests, the braces were connected to the
beams via shaped WT18x97 sections bolted to the beam
flanges. To reduce the load transferred through the column
base pin connections, axial force in the bottom beam was
taken out of the frame via a horizontal reaction block that
was pretensioned to the laboratory’s strong floor.
Symmetric quasi-static cyclic loading was applied to the
frame at increasing story drift levels up to 4-percent drift. The
frame exhibited the typical behavior expected of a ductile
concentrically braced frame undergoing cyclic inelastic
loading. The first low amplitude cycles showed only minor
inelasticity in the system; elastic brace buckling was observed
early in the protocol. Subsequent larger compressive
excursions resulted in out-of-plane inelastic buckling of the
bracing, with significant tensile yielding occurring over the
majority of the HSS segment’s length during the higher
amplitude tensile excursions, as evidenced by the loss of
white-wash (Figure 13). As intended, plastic hinges formed
beyond the ends of the cast connectors within the free length
of the gusset plates during overall member buckling (Figure
14). At higher amplitude compressive excursions, a more
discrete plastic hinge formed at the mid-length of the brace
during inelastic buckling, however, the brace could not be
fractured within the displacement capabilities of the test
frame. White-wash applied to the cast connectors remained
intact throughout testing and no sign of yielding could be
8
Figure 12: Full-scale braced frame with brace equipped with cast connectors
prior to testing
Figure 13: Full-scale braced frame with brace equipped with cast connectors
during final tensile cycle; a loss of white-wash along the length of the HSS
segment is clearly observable, indicating significant yielding of the brace
Figure 14: Formation of plastic hinge in the free length of the gusset plate,
beyond the end of the connector, as induced during out-of-plane inelastic
buckling of the brace
Story Shear [kip]
400
300
Tremblay in relation to the laboratory testing conducted at
École Polytechnique, Montréal, and the support of Cast
Connex Corporation.
200
References
Adan, S. M., and Gibb, W., 2009, “Experimental evaluation
of Kaiser Bolted Bracket steel moment resisting
connections,” AISC Engineering Journal, 46(3): 181–196.
100
0
-100
-0.05 -0.04 -0.03 -0.02 -0.01 0.00
0.01
0.02
0.03
0.04
0.05
Story Drift [Rad]
Figure 15: Hysteretic response generated during full-scale braced frame test
– HSS 5.563 x 0.375 produced to ASTM A500 Grade C
observed on the connectors during or after the test. Figure 15
shows the hysteretic response generated during the frame test.
Conclusions
High-strength cast steel connectors for round hollow
structural section brace members have been developed at the
University of Toronto for use in Special and Ordinary
Concentrically Braced Frames (SCBF and OCBF). The
connectors have been capacity designed pursuant to the AISC
Seismic Provisions’ requirements for SCBF and OCBF brace
connections and brace assemblies fitted with the connectors
have been subjected to a number of structural tests to verify
the connector design philosophy. The full suite of tests
included monotonic tensile testing and quasi-static cyclic
inelastic testing of connector-brace assemblies having a
range of overall and cross-sectional slenderness and the
testing of a full-scale braced frame fitted with the connectors.
In all tests, the braces survived several inelastic cycles
beyond the test protocols that were applied and ultimately
failed during a tensile excursion at the center of the brace.
The cast steel connectors showed no sign of yielding in all
tests, validating the overall design rationale for, and
performance of, the connectors in seismic-resistant braced
frame applications. The connectors, now available from Cast
Connex Corporation, have been used in the construction of a
number of seismic-resistant braced frame structures in North
America.
Acknowledgments
The authors gratefully acknowledge the research funding
support by the Natural Sciences and Engineering Research
Council of Canada (NSERC), the research and
commercialization funding and support of the Ontario
Centres of Excellence (OCE), the work of Professor Robert
American Institute of Steel Construction (AISC), 2005a,
Seismic provisions for structural steel buildings, ANSI/AISC
341-05 and ANSI/AISC 341s1-05, American Institute of
Steel Construction, Chicago, Illinois.
American Institute of Steel Construction (AISC), 2005b,
Prequalified connections for special and intermediate steel
moment frames for seismic applications, ANSI/AISC 358-05,
American Institute of Steel Construction, Chicago, Illinois.
American Institute of Steel Construction (AISC), 2009,
Supplement No. 1 to Prequalified connections for special and
intermediate steel moment frames for seismic applications,
ANSI/AISC 358-05s1-09, American Institute of Steel
Construction, Chicago, Illinois.
American Institute of Steel Construction (AISC), 2010a,
Specification for structural steel buildings, ANSI/AISC 36010, American Institute of Steel Construction, Chicago,
Illinois.
American Institute of Steel Construction (AISC), 2010b,
AISC Steel design guide 24: Hollow structural section
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