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10NCEE
Tenth U.S. National Conference on Earthquake Engineering
Frontiers of Earthquake Engineering
July 21-25, 2014
Anchorage, Alaska
INELASTIC PERFORMANCE OF THE
“MODIFIED-HIDDEN-GAP” CONNECTION
FOR SQUARE HSS BRACE MEMBERS
R. Moreau1, C.A Rogers2 and R. Tremblay3
ABSTRACT
Square hollow structural section (HSS) brace members are typically connected to a steel braced
frame using a slotted tube-to-gusset plate (conventional) connection or knife-plate connection. A
net section exists in this connection which often requires reinforcement when seismic capacity
design methods are applied. The “Modified-Hidden-Gap (MHG)” connection represents an
attractive alternative to traditional connection reinforcement; however, no seismic design or
detailing rules exists for the MHG connection for square HSS members. Thus, this paper
describes a laboratory testing program aimed at determining the minimum overlap length
required to develop the yield resistance of a brace. Two brace sizes (152x152x9.5 &
203x203x13.0) were examined which led to a total of 15 specimens: 5 conventional connections
and 10 MHG connections. Strain rate effects were also examined through the monotonic tensile
testing of one MHG connection specimen and the reverse cyclic testing of a 4.86 m long brace
with MHG connections. The smallest overlap length of 5% of the weld length for each brace size
was sufficient to develop the yield resistance of the brace with fracture occurring on the gross
area away from the connection after extensive overall yielding along their gross area. The
maximum load recorded for all the MHG connection specimens corresponded to the ultimate
tensile resistance of the tube on its gross area with no noticeable material fractures at the
connections. The dynamically loaded MHG brace specimen attained greater ductility than its
statically loaded counterpart.
1
Graduate Research Student, Dept. of Civil Engineering, McGill University, Montreal, Canada
Associate Professor, Dept. of Civil Engineering, McGill University, Montreal, Canada
3
Professor, Dept. of Civil, Geological & Mining Engineering, Ecole Polytechnique of Montreal, Montreal, Canada
2
Moreau R, Rogers CA, Tremblay R. Inelastic performance of the “Modified-Hidden-Gap” connection for square
HSS brace members. Proceedings of the 10th National Conference on Earthquake Engineering, Earthquake
Engineering Research Institute, Anchorage, AK, 2014.
10NCEE
Tenth U.S. National Conference on Earthquake Engineering
Frontiers of Earthquake Engineering
July 21-25, 2014
Anchorage, Alaska
Inelastic Performance Of The “Modified-Hidden-Gap” Connection For
Square HSS Brace Members
R. Moreau1, C.A Rogers2 and R. Tremblay3
ABSTRACT
Square hollow structural section (HSS) brace members are typically connected to a steel braced
frame using a slotted tube-to-gusset plate (conventional) connection or knife-plate connection. A
net section exists in this connection which often requires reinforcement when seismic capacity
design methods are applied. The “Modified-Hidden-Gap (MHG)” connection represents an
attractive alternative to traditional connection reinforcement; however, no seismic design or
detailing rules exists for the MHG connection for square HSS members. Thus, this paper describes
a laboratory testing program aimed at determining the minimum overlap length required to
develop the yield resistance of a brace. Two brace sizes (152x152x9.5 & 203x203x13.0) were
examined which led to a total of 15 specimens: 5 conventional connections and 10 MHG
connections. Strain rate effects were also examined through the monotonic tensile testing of one
MHG connection specimen and the reverse cyclic testing of a 4.86 m long brace with MHG
connections. The smallest overlap length of 5% of the weld length for each brace size was
sufficient to develop the yield resistance of the brace with fracture occurring on the gross area
away from the connection after extensive overall yielding along their gross area. The maximum
load recorded for all the MHG connection specimens corresponded to the ultimate tensile
resistance of the tube on its gross area with no noticeable material fractures at the connections.
The dynamically loaded MHG brace specimen attained greater ductility than its statically loaded
counterpart.
Introduction
Square hollow structural sections (HSS) brace members are commonly connected to the beamto-column joint of a steel bracing bent frame using a slotted tube-to-gusset plate connection or a
knife-plate connection, herein referred to as a conventional connection (Fig. 1a). The slot in the
tube creates a net section; as well, the unconnected flanges of the tube create shear lag. As a
result, the tensile resistance of a brace is diminished such that challenges often arise when
attempting to satisfy CSA S16-09 [1] seismic capacity design requirements at the net section.
Consequently, an engineer has to specify some type of connection reinforcement at the net
section, such as side plates or wrapped-around welds. Side reinforcement plates are
1
Graduate Research Student, Dept. of Civil Engineering, McGill University, Montreal, Canada
Associate Professor, Dept. of Civil Engineering, McGill University, Montreal, Canada
3
Professor, Dept. of Civil, Geological & Mining Engineering, Ecole Polytechnique of Montreal, Montreal, Canada
2
Moreau R, Rogers CA, Tremblay R. Inelastic performance of the “Modified-Hidden-Gap” connection for square
HSS brace members. Proceedings of the 10th National Conference on Earthquake Engineering, Earthquake
Engineering Research Institute, Anchorage, AK, 2014.
uneconomical from a fabricator’s perspective, and wrapped-around or return welds are not
recommended for seismic application [2].
a)
b)
c)
Figure 1: a) Unreinforced conventional connection, b) MHG connection, c) transparent view of
MHG connection.
The “Modified-Hidden-Gap (MHG)” connection [3] was developed for circular tubes to
allow the brace to yield along its gross area without requiring connection reinforcement. This
connection is similar to the conventional connection with one geometrical modification; a slot is
created in the gusset plate such that a portion of the gusset plate extends onto the gross area of
the tube. (Figs. 1b,c). The fillet weld begins on the gross area which subsequently allows the
strain concentrations to be transferred away from the net section (conventional connection) to the
gross area of the brace, thereby allowing the brace to yield over its entire length. A tolerance for
fabrication and erection is left between the ends of the slot in the gusset plate and tube (Fig. 1c).
The MHG connection for square HSS braces may represent an attractive alternative to
traditional connection reinforcements; however, no physical test data or design guidelines exist.
Thus, a research program was initiated to develop seismic design and detailing guidelines. This
paper reports on the first part of this program which involved the laboratory testing of MHG
connections for square HSS braces with the goal of determining the minimum overlap length
required to develop the yield resistance of the brace.
Background
Conventional Connection
In seismic design, the connections of a brace are designed for the probable resistances of the
brace in tension and compression according to a capacity design process. The probable tensile
resistance of a brace, Tu, as defined in CSA S16-09, is given by AgRyFys, where Ag is the gross
cross-sectional area of the brace, Ry is the ratio of probable yield-to-nominal yield stress (taken
to be 1.1) and Fys is the nominal static yield stress of the brace. The probable yield stress, RyFys,
must be at least 460 MPa for HSS braces, or alternatively R yFys may be calculated using the
average of material properties extracted from the flat walls of the HSS in accordance with CSA
G40.20 [4]. On the other hand, the net-section tensile resistance of a brace, Tr,net, connected with
a conventional connection is given by (1.2/ϕu) ϕuAneFus, where 1.2 accounts for the increase in
the ultimate stress over the nominal stress, Ane is the effective net area which accounts for the
effects of shear lag and Fus is the nominal static ultimate stress of the brace. Ane is given by the
product of the shear lag factor, U, and the net area, An, of the brace. The shear lag factor for
square HSS braces can be calculated using the equations provided by CSA S16-09 [1] or AISC
360-10 [5].
An engineer is typically confronted with the challenge of satisfying Tr,net > Tu at the
reduced section, owing to the variability of HSS material properties [6], as well as the
application of well-known conservative shear lag design factors [7]. On the other hand, the AISC
341-10 Seismic Provisions [2] require an engineer to reinforce the net section of a conventional
connection such that the effective net-area (UAn) is at least equal to the gross area of the tube
(Ag). This requirement is stipulated to ensure that a brace can develop adequate ductility before
rupturing on its net area.
MHG Connection
Martinez-Saucedo et al. [3] conducted quasi-static reverse cyclic testing of three circular brace
specimens (one conventional, two MHG) with an outside diameter of 168 mm. One of the MHG
brace specimens exhibited classical brace behaviour under a symmetric displacement history and
ultimately fractured at its plastic hinge region. Conversely, the other MHG brace specimen
experienced extensive yielding along its length under a tension-dominated loading protocol and
fractured at its plastic hinge region. Packer et al. [6] performed quasi-static reverse cyclic testing
of four circular brace specimens with MHG connections under a symmetric loading protocol.
The MHG connections possessed similar geometric values (tube diameter, weld length, overlap
length and gap size) as those tested by Martinez-Saucedo et al. The research program evaluated
the seismic performance of the HSS braces fabricated from different steel grades. All the brace
specimens sustained their loading protocol with no noticeable material fracture at their
connections.
Testing Program
Connection Design
The brace connection specimens were fabricated from cold-formed HSS 152x152x9.5 and HSS
203x203x13.0 tubes conforming to CSA G40.20-21 350W Class C [4]. The laboratory program
was divided into two phases: Phase 1 consisted of the testing of conventional brace connections,
while Phase 2 consisted of the testing of MHG brace connections.
Phase 1 - Conventional Connections
The measured mechanical properties of the test braces, Fys,m and Fus,m, are shown in Table 1.
Two different heat stocks were used to fabricate the HSS 152x152x9.5 specimens. The majority
of these specimens (six) were fabricated from HSS 152x152x9.5-M1 (HSS 152-M1), while three
additional specimens were fabricated from HSS 152x152x9.5-M2 (HSS 152-M2) in order to
evaluate the influence of strain rates on the performance of the MHG connection. The HSS
203x203x13.0 (HSS 203) specimens (six) were fabricated from the same heat stock.
Table 1.
Measured properties of test braces [8].
Brace
Ag
Fys,ma
Fus,m
Size
(mm2)
(MPa)
(MPa)
HSS 152-M1
5123
389
445
1.14
HSS 203
8991
416
472
1.13
a
Avg. of three coupons taken from the flat walls of the tube and using the 0.2% offset method.
A detailed description of the design of the conventional connections can be found in
Moreau et al. [8], however a summary will be provided herein for completeness. A total of four
conventional brace connections (specimens 1-4) were fabricated: two identical brace specimens
for each tube size. For each tube size, one specimen was tested under monotonic static loading
(specimens 1 and 3), while the other was tested under monotonic dynamic loading (specimens 2
and 4). Using the measured static material properties, specimens 1 and 3 were designed using the
minimum possible weld length, Lw. This design represented a worst-case scenario for the
connection since shear lag effects were maximized. A fillet weld size, Dw, as well as a gussetplate width and thickness, tg, were subsequently selected. Table 2 summarizes the design of the
conventional brace specimens which were tested under static strain rates. It can be seen that the
brace specimens are expected to rupture on their net area before yielding on their gross area.
Table 2.
Specimen
#
Brace
Size
Summary of Phase 1 conventional connection designs [8].
Lw
(mm)
Tua = AgFys,m
UCSA UAISC
(kN)
Tr,net / Tu
1
HSS 152-M1 230
0.92
2027
0.69 0.75
0.72
0.79
3
HSS 203
310
0.92
3852
0.69 0.75
0.72
0.79
a
Nominal areas were used. Ag = 5210 mm2 and 9260 mm2 for specimens 1 and 3, respectively.
Phase 2 - Conventional Connections
A typical MHG connection for a square HSS brace specimen is shown in Fig. 2. The overlap
length, Lwg, is defined as the length of weld on the gross portion of the tube; it is believed to be
the most important geometric parameter affecting the response of the connection. This parameter
was identified through detailed finite element analyses (FEA) [9] and it was the main parameter
varied in Phase 2 of testing.
The conventional connections from Phase 1 (specimens 1 and 3) were used as a
benchmark against which the performance of the MHG connections could be measured; thus, the
gusset-plate width and thickness, weld length and tube length for the MHG connections were
identical to their corresponding conventional connections, with the overlap length being the only
parameter that was varied. The overlap length was normalized with respect to the weld length to
yield an overlap length ratio, Lwg/Lw, where Lwg/Lw = 0 represents a conventional connection.
Using FEA, overlap length ratios of 0.05, 0.15 and 0.30 were selected for each tube size giving
MHG specimens 5, 7 and 8 for HSS 152-M1 and specimens 9, 10 and 11 for HSS 203 (Table 3).
Also, specimen 6 was fabricated to evaluate the effects of having the same overlap length as
specimen 5, but using a weld length and size that would be more representative of field
conditions. A hidden gap varying between 23 mm and 27 mm was present in all MHG
connection specimens to allow for fabrication and assemblage tolerances (Fig. 2).
Figure 2: Typical MHG connection with a close-up view of the overlap length (detail 1).
Table 3.
Laboratory testing matrix for all HSS specimens summarizing varied parameters.
Loading Protocol
LB
Lw
Lwg
Dw
tg
(mm) (mm)
(mm) (mm) (mm)
Type
(mm/s)
Phase 1 - Conventional Connections
1
HSS 152-M1 1000 230
0
0
16
19.1
Static
0.004
2
HSS 152-M1 1000 230
0
0
16
19.1 Dynamic
50.0
3
HSS 203
1000 310
0
0
20
25.4
Static
0.004
4
HSS 203
1000 310
0
0
20
25.4 Dynamic
50.0
Phase 2 - MHG Connections
5
HSS 152-M1 1000 230
0.05
12
19
19.1
Static
0.025a
6
HSS 152-M1 1280 510
0.024
12
10
19.1
Static
0.025
7
HSS 152-M1 1000 230
0.15
35
19
19.1
Static
0.025
8
HSS 152-M1 1000 230
0.30
69
19
19.1
Static
0.025
9
HSS 203
1000 310
0.05
16
25
25.4
Static
0.025
10
HSS 203
1000 310
0.15
47
25
25.4
Static
0.025
11
HSS 203
1000 310
0.30
93
25
25.4
Static
0.025
12
HSS 203
4857 310
0.15
47
25
25.4 Dynamic Varied
13
HSS 152-M2 1000 230
0
0
18
19.1
Static
0.004
14
HSS 152-M2 1000 230
0.05
12
20
19.1
Static
0.025
15
HSS 152-M2 1000 230
0.05
12
20
19.1 Dynamic
50.0
a
The actuator piston speed was varied from 0.004 to 0.025 mm/s for the static MHG connections.
Specimen
#
Brace
Size
Laboratory Setup and Instrumentation
A typical brace specimen consisted of a 1000 mm long tube (Fig. 3a) with a conventional- or
MHG- connection at one end and a 76 mm thick base plate (455x495 mm) welded to the other
end. The base plate of the specimen was bolted to a 127 mm thick base plate of a reusable T-stub
assembly (Fig. 3a) using twelve 1-1/8” Ø ASTM A490 pre-tensioned bolts. The T-stub assembly
and gusset plate were clamped using two specially designed grips which were fastened to the
upper and lower platens of the loading frame using high-strength pre-tensioned bolts. The upper
platen was secured to a moveable cross-head of a double-acting, double-ended 12 MN actuator.
a)
b)
Figure 3: a) Typical setup for a brace specimen, b) location of displacement potentiometers.
Two string potentiometers were deployed over the length of the specimen to measure the
overall tube elongation (Fig. 3b). Also, string potentiometers were employed to measure the local
elongation over the gross portion of the tube (Fig. 3b). The local distance was kept constant for
the corresponding MHG connection. Additional test information is provided in Moreau [10].
Loading Protocol
The average of the two overall string potentiometers (Fig. 3b) was used to control the loading of
the specimens. All specimens with the exception of specimen 12 were tested under monotonic
tension using either static or dynamic strain rates (Table 3). In order to evaluate the effects of
strain rate on the performance of the MHG connection, three additional specimens (13,14,15)
(Table 3) were fabricated from HSS 152-M2. The conventional brace specimen 13 was used as a
benchmark to measure against the performance of two identically fabricated MHG brace
specimens 14 and 15, which were tested under different loading rates (Table 3). The material
properties were not known at the time of design, thus the probable brace material properties
given in CSA S16-09 were used to design these specimens.
One 4857 mm long brace, LB, specimen 12 (HSS 203), was tested under reverse cyclic
dynamic loading to evaluate the effects of compression loading and strain rate on the
performance of the MHG connection when the most efficient overlap length ratio of 0.15, as
determined by FEA, was used. The first part of the loading protocol (Fig. 4) consisted of a few
small elastic and inelastic cycles, followed by one large tensile displacement at the 9½ cycle to
an IDR of 3.14% in order to place the connection under a large tensile demand that may occur
during a near-fault ground motion. In the event the brace withstood the near-fault history, a
second loading protocol was appended to the first protocol which consisted of a far-field history
with increasing displacement amplitudes (Fig. 4).
Figure 4: Applied displacement history for MHG specimen 12 showing the number of cycles
sustained and the location of fracture.
The IDR was converted to brace axial deformations by assuming the brace was part of a
chevron frame with a storey height, hs, and inclined at an angle of θ = 35° to the horizontal (Fig.
4). All the axial deformations were assumed to occur over the length between brace hinge points,
LH, (LH = LB + 2tg) and a 1.3 factor was used to transform the deformations of the brace from its
centre-to-centre dimension to its LH dimension (Fig. 4). The applied displacement rates were
such that the brace experienced average strain rates of 9800 με/s in the first tension yielding
excursion beginning at t = 7 s and 4100 με/s in the second, larger tension yielding excursion in
the 12-16 s time period. Additional loading protocol information is provided in Moreau [10].
Laboratory Results
HSS 152x152x9.5-M1 Connection Specimens
The results from Phase 1 of testing are reported in Moreau et al. [8] and Moreau [10]; only the
results from the statically loaded conventional connection (specimen 1) are compared with its
corresponding MHG brace specimens in this Section. Figure 5 shows the normalized overall
load-displacement response for the HSS 152-M1 brace specimens. This graph can be used to
compare the influence of the overlap length ratio since the weld lengths were the same for all the
specimens, hence specimen 6 was omitted from this graph. All the specimens yielded on their
gross cross-sectional area (Texp/AgFys,m > 1); however, the conventional brace specimen
experienced the majority of yielding near its slotted region as indicated by the elongation of its
tube slot (Fig. 5b), while all the MHG connections experienced extensive yielding along their
tube length directly in front of the weld as evidenced by the lack of white wash (Figs. 5c,d,e).
The conventional brace specimen ruptured on its net area (Fig. 5b), while all the MHG
connections fractured on their gross area away from the connection (Fig. 5e).
a)
b)
c)
d)
e)
f)
Figure 5: HSS 152-M1 specimens at ultimate displacement: a) overall load-displacement
response, b) specimen 1, c,d) specimen 5, e) specimen 7, f) typical FEM.
All the MHG specimens attained a maximum load which corresponded to the ultimate
tensile resistance on the gross area (AgFus,m). Although the MHG connections attained an overall
displacement substantially higher than their corresponding conventional connection, the MHG
brace specimen with the smallest overlap length ratio of 0.05 reached the highest final overall
displacement compared to the other two MHG specimens which attained approximately the same
final overall displacement (Fig. 5a). Similar behaviour was observed for the HSS 203 specimens,
whereby the specimen with the smallest overlap length ratio of 0.05 (specimen 9) achieved the
highest ductility and fractured on its gross area away from its connection. Additionally, FEA
were performed in parallel with the physical tests [10] (Fig. 5f) to develop a better understanding
of the connection response; one such result is presented as an example in Fig. 5a.
HSS 152x152x9.5-M2 Connection Specimens
Figure 6a shows the normalized overall load-displacement response for the HSS 152-M2 brace
specimens. Similar to the HSS 152-M1 specimens, the statically loaded MHG brace specimen
with an overlap length ratio of 5% experienced extensive yielding along its length and fractured
on its gross area away from its connection (Fig. 6b). The dynamically loaded MHG brace
specimen achieved an ultimate load increase of approximately 9.8% over its statically loaded
counterpart (Fys,m = 398 MPa, Fus,m = 455 MPa). Moreover, the greatest ductility was observed
for the dynamically loaded MHG brace specimen which experienced extensive overall brace
yielding (Fig. 6c) with no noticeable material fractures at its connection.
a)
b)
c)
Figure 6: HSS 152-M2 specimens at ultimate displacement: a) overall load-displacement
response, b) specimen 14, c) specimen 15.
HSS 203x203x13.0 - MHG Specimen 12
Figure 7a shows the normalized load-IDR response curve for specimen 12. The specimen
attained a maximum IDR of 3.14% and a maximum load of 1.15AgFys,m (1.02AgFus,m) while
undergoing an average strain rate of 4100 με/s during the largest tension excursion. The brace
sustained approximately 44 cumulative cycles and fractured at its mid-length plastic hinge region
(Fig. 7b) after experiencing severe local buckling. Yielding was observed at the plastic hinge
region and at the fold lines of the gusset plates, with little distress detected at the connections.
a)
b)
Figure 7: MHG specimen 12: a) normalized load-IDR response, b) fracture at plastic hinge.
Conclusions
The MHG connection for square HSS brace specimens in this research project showed marked
structural improvement over their corresponding conventional connection by distributing
inelastic demand along the tube and away from the connection. All the MHG connections for
both tube sizes attained their ultimate tensile resistance and fractured on their gross area away
from the connection without having their connection reinforced. The maximum tube ductility for
both tube sizes was found to occur with an overlap length ratio of 5%. This ratio is applicable for
the weld lengths investigated in this research program. No noticeable material fracture was
observed at any of the MHG connections. The dynamically loaded MHG brace specimen
displayed greater ductility than its corresponding statically loaded MHG specimen.
Acknowledgements
This research project was sponsored by the Natural Sciences and Engineering Research Council
of Canada. The authors thank ADF Group Inc. and DPHV Structural Consultants for their
financial, technical and fabrication contributions. The inputs by Brad Fletcher and the donation
of the HSS tubes by Atlas Tube, as well as the technical assistance by Professor Jeffrey A.
Packer are appreciated. The support and dedication provided by the staff at the Hydro-Quebec
Structural Engineering Laboratory at Ecole Polytechnique of Montreal is acknowledged.
References
1.
CSA. Design of Steel Structures S16-09. Canadian Standards Association: Mississauga, ON, Canada, 2009.
2.
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Construction: Chicago, IL, USA, 2010.
3.
Martinez-Saucedo G, Packer JA, Christopoulos C. Gusset Plate Connections to Circular Hollow Section
Braces under Inelastic Cyclic Loading. Journal of Structural Engineering 2008; 134 (7): 1252-1258.
4.
CSA. General requirements for rolled or welded structural quality steel / structural quality steel
G40.20/G40.21. Canadian Standards Association: Mississauga, ON, Canada, 2009.
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AISC. Specification for Structural Steel Buildings. ANSI/AISC 360-10. American Institute of Steel
Construction: Chicago, IL, USA, 2010.
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Packer JA, Chiew SP, Tremblay R, Martinez-Saucedo G. Effect of material properties on hollow section
performance. Proceedings of the Institution of Civil Engineers Structures and Buildings 2010; 163 (SB6):
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Willibald S, Packer JA, Martinez-Saucedo G, Puthli RS. Shear lag in slotted gusset plate connections to
tubes. Connections in Steel Structures V, June 3-4: Amsterdam, 2004. pp 445-456.
8.
Moreau R, Rogers CA, Tremblay R. Experimental testing of conventional HSS slotted tube-to-plate
connections loaded under static and dynamic strain rates. 3rd Specialty Conference on Material
Engineering & Applied Mechanics. Canadian Society for Civil Engineering: Montreal, Canada, 2013.
9.
Moreau R, Rogers CA, Tremblay R, Packer JA. Finite element evaluation of the "modified-hidden-gap"
HSS slotted tube-to-plate connection. Connections VII, 7th International Workshop on Connections in Steel
Structures: Timisoara, Romania, 2012. pp 525-534.
10. Moreau R. Evaluation of the “Modified-Hidden-Gap” connection for square HSS brace members. Master’s
thesis: Department of Civil Engineering, McGill University, Montreal, Canada, 2014.

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