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Re-engineering Composite Connections for a Higher Construction and Cost
Effectiveness
Conference Paper · December 2015
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11th International Conference on
Advances in Steel and Concrete Composite Structures
Tsinghua University, Beijing, China, December 3-5, 2015
RE-ENGINEERING COMPOSITE CONNECTIONS
FOR A HGIHER CONSTRUCTION AND COST EFFECTIVENESS
Aaron J. Wanga
a
Director, Project Design and Development Centre, CapitaLand Management (China) Co., Ltd.
E-mails: aaron.wang@capitaland.com
ABSTRACT
Keywords:
Composite structures;
seismic
design; performance-base design;
high-rise building; design and
construction; value engineering.
This paper introduces two case studies on the alternate design and re-engineering of complex
composite connections in modern ultra-highrise buildings to achieve a higher efficiency of
construction, easier site quality control and better cost effectiveness. One of the case studies is
on the twin twisting composite towers of 250 m in Raffles City Hangzhou, the structural design
of the composite connection between CFT columns and SRC beams needed to safeguard the
overall structural stability through the fully rigid connections and avoid scarifying any tailored
interior space in the meantime. The conventional ring beam type composite connection was
regarded to be bulky and not suitable because of its inference with the façade erection and
interior decoration. An innovative and high performance corbel type composite connection was
proposed with a minimum intrusion into the interior space to achieve the fully rigid connection.
Physical tests under both monotonic and quasi-static cyclic loads were conducted to investigate
the load carrying capacities and deformation characteristics of this new type of composite
connection. In the second case study, the steel-concrete hybrid outrigger truss was developed in
the high-rise towers of 380 m in Raffles City Chongqing. Both the steel truss and concrete
outrigger wall works compositely to enhance the overall structural performance of the tower
structures under extreme loads. Through rigorous numerical and experimental investigations, the
hybrid outrigger system was proved to be safe and effective to withstand both wind and seismic
actions. The design allows the contractor to break through the critical path of the tedious
wedding on the steel outrigger truss in the outrigger floor, which shortens the overall
construction period and lowers the overall material cost in the meantime.
1 INTRODUCTION
According to Eurocode 4: Part 1.1 (BSI, 2004a), the
design of both rotational stiffnesses and moment
capacities of composite beam-column joints are based on
the relevant clauses in Eurocode 3: Part 1.8 (BSI, 2005)
for steel joints with the incorporation of the contribution
of tensile reinforcement. Other codes of practice with
similar design philosophy are also available (AISC, 2005;
Brockenbrough & Merritt, 2006; SCI & BCSA, 2002;
Lawson & Gibbon, 1995). According to these codes of
practice,
different
components
of
composite
beam-column joints are to be analyzed and designed
separately for different failure locations. By summing
up the load carrying capacities and the stiffnesses of these
components with the consideration of their associated
lever arms, the moment capacities and the rotational
stiffnesses of the composite beam-column joints can be
obtained. However, none of the design codes gives
guidance regarding the rotational capacities of composite
joints and they should be determined according to
physical tests. (BSI, 2004a).
This paper introduces the following two case studies
on the alternate design and re-engineering of complex
composite connections in modern ultra-highrise buildings
to achieve a higher efficiency of construction, easier site
quality control and better cost effectiveness.
1.1 Corbel types composite connection in Raffles City
Hangzhou
The structural design of the composite connection
between CFT columns and SRC beams needed to
safeguard the overall structural stability through the fully
rigid connections and avoid scarifying any tailored
interior space in the meantime. The conventional ring
beam type composite connection was regarded to be
bulky and not suitable because of its inference with the
façade erection and interior decoration. An innovative
and high performance corbel type composite connection
was proposed with a minimum intrusion into the interior
Wang
space to achieve the fully rigid connection. Physical tests
under both monotonic and quasi-static cyclic loads were
conducted to investigate the load carrying capacities and
deformation characteristics of this new type of composite
connection.
- Concrete encasement
All above mentioned components are encased with C35
concrete to ensure a composite action.
1.2 Hybrid outrigger truss in Raffles City Chongqing
The steel-concrete hybrid outrigger truss was
developed in the high-rise towers of 380 m in Raffles
City Chongqing. Both the steel truss and concrete
outrigger wall works compositely to enhance the overall
structural performance of the tower structures under
extreme loads. Through rigorous numerical and
experimental investigations, the hybrid outrigger system
was proved to be safe and effective to withstand both
wind and seismic actions. The design allows the
contractor to break through the critical path of the tedious
wedding on the steel outrigger truss in the outrigger floor,
which shortens the overall construction period and lowers
the overall material cost in the meantime.
In order to achieve a full strength connection between
the SRC beam and CFT column, the corbel together with
the ring stiffener is strengthened to the strength and
rigidity of an ordinary SRC beam. Thus, satisfactory
deformation and plastic energy absorbing capacities can
be achieved with a preferred failure mode and location of
the plastic hinge.
2 CORBEL TYPE COMPOSITE CONNECTION
The proposed corbel type composite joints include the
following key components as shown in Figures 1:
- The corbel and ring stiffener as butt welded to the CFT
column:
In order to ensure a full strength rigid connection, the
I-section corbel is enlarged and stiffened together with a
ring stiffener as welded inside the steel tube, so that the
overall rigidity and load carrying capacity of the
connection is not less than that of a typical SRC beam
section.
- The tapered section from the corbel to the steel beam:
In order to ensure a smooth loading and stress transfer
from the corbel in the joint region to the ordinary SRC
beam, a tapered steel section is proposed with a slope of
1:6.
- The steel section in the SRC beam:
The ordinary I-steel section in the composite SRC beam
is fully connected to the outer edge of the corbel through
full bolted joints on both flanges and webs.
- Lapped reinforcement bars:
All the longitudinal reinforcements are lapped around the
flanges of the steel corbel, so that both the loads and
stress can be transferred from the longitudinal main
reinforcements onto the corbel in the connection region.
The set-up of the physical test is shown in Figure 2.
The geometrical scale of the test specimen is 1:2 to
ensure a proper and quality erection of the test specimens,
and in the meantime, sufficient capacities of loading jacks
and rigs as well. The depth of the SRC beam of the
specimen is scaled down to 250 x 400 mm, and the
diameter of the CFT column in the specimen is 500 mm.
The thickness of all steel webs, flanges and stiffeners is
also scaled down accordingly with a thickness of 28, 10
and 10 mm respectively. Various instrumentations are
carefully arranged on the specimen to capture accurately
the structural response throughout the tests.
Wang
Figures 3a and 3b present the results of the monotonic
tests on Specimens SP1 and SP2, while Figure 4 presents
a typical failure mode. A close observation on the strain
development also shows that the direct tensile strain at
the top flange is 30 to 50% higher than the compressive
strain of the bottom flanges due to the contribution of the
concrete material. It is noted that the shear strain in the
web is significantly smaller than the strain in the flange,
which is just above the yield strain. This is preferred for a
high-rise building in a seismic sensitive region like
Hangzhou, where the Project located. The quasi-static
cyclic loading tests were conducted on both Specimens
SP3 and SP4. Figures 3c and 3d present the
load-deflection and moment-rotation curves of Specimens
SP3 and SP4. The cumulative plastic deformations of
both Specimens SP3 and SP4 are 0.3 and 0.24 rad
respectively, which are corresponding to 88 and 80 times
the first yield rotation of the composite connections. This,
again, demonstrates the high ductility and energy
absorbing capacities of the corbel type composite
connections.
To study the structural behaviour of the corbel type
composite connection, a generalized nonlinear
three-dimensional finite element model was set up using
the commercial finite element package ANSYS 12.1
(2011). The meshes of the finite element model are
shown in Figures 5a and 5b. In order to simplify the
problem and save computational time, only half of the
specimen was modelled. The finite element simulation
gives a quite close prediction of the load-deformation
characteristics in the connection regions as shown in
Figure 5c, which is demonstrated through the comparison
Wang
of the load-deformation curves at the end of the
connection corbel. As such, the corbel type composite
joint was verified to be of high strength, rigidity and
ductility and suitable for highrise buildings in seismic
sensitive regions.
3 HYBRID COMPOSITE OUTRIGGER
The design and construction of high-rise buildings in
China require a rigorous consideration on the impact of
winds and earthquakes. In the current national seismic
design
codes
(MHURD,
2010
and
2011),
performance-base design approaches were introduced,
which requires the structurally complex building to meet
the corresponding stringent requirements under
earthquakes with exceeding rates of 63%, 10% and 2-3%
respectively. ‘Dual system’ requirements also need to be
met for tall buildings in many circumstances. Wind is
another concern for many coastal cities, where the
typhoon is normally an issue. The structural engineer
normally faces the double challenges of extreme loads
from both wind and earthquakes, and needs to keep the
overall structural and spatial efficiency in the meantime.
Energy dispersing devices, like dampers and isolating
bearings, are getting popular in high-rise buildings to
enhance the overall structural performance under
disastrous loads, instead of putting in additional steel and
concrete material and making the overall structure trunky
and costly.
An innovative type of steel-concrete hybrid outrigger
truss is being developed in two mega high-rise towers of
370 m tall in RCCQ, in which the steel truss is embedded
into the reinforced concrete outrigger wall as shown in
Figures 6a and 6b. Both the steel truss and concrete
outrigger wall works compositely to enhance the overall
structural performance of the tower structures under
extreme loads. Meanwhile, metal dampers were also
adopted as a ‘fuse’ device between the hybrid outrigger
and the mega column. The dampers are designed to be
‘scarified’ and yielded first under moderate to severe
earthquakes in order to protect the structural integrity of
important structural components of the hybrid outrigger.
As such, not brittle failure happens in reinforced concrete
portion of the hybrid outrigger system.
Wang
a)
Physical Tests
Figure 6c shows the numerical simulation of the
hybrid outrigger system under earthquakes. The design
may allow the contractor to break through the critical
path of the tedious wedding on the steel outrigger truss in
the refugee floors, and shoot the core first by leaving the
construction joints between the core and the outrigger
walls. This helps to shorten the overall construction
period of the tower.
As per verification tests, the metal dampers work
effectively under Level 2 and Level 3 earthquakes and
enhance the overall structural performance. Both finite
element modelling and physical component tests were
conducted to verify the effectiveness of the hybrid
outrigger system. Figure 7a shows the overall test set up
and load deflection curves under cyclic actions. The scale
of the tests is taken to be 1:4.5. The overall depth of the
specimen is 1590 mm with a thickness of the outrigger
wall of 200 mm. C45 concrete is adopted in the concrete
part of the specimen. The steel section of the specimen is
typically box sections of 100 x 150 mm with a steel grade
of Q345B. The hybrid outrigger system exhibits
sufficient ductility under seismic actions with the
effective protection for the ‘fuse’ devise of low yield steel
metal dampers. Figure 7b is the results of the
three-dimensional finite element simulation. It also
demonstrated the sufficient ductility at the ‘fuse’ device
while the cracks in the concrete outrigger wall are well
controlled even under the action from the severe
earthquake. Figure 7a also shows the load-deflection
curves from the test and predicted by the finite element
modelling. It is demonstrated that the proposed finite
element model is able to provide a relatively conservative
while yet proper prediction towards the load-deflection
characteristics of the hybrid outrigger under cyclic loads.
b)
Finite element modelling
Figure 7: Study on hybrid outrigger system
Figure 8 is the shaking table test that has been done
on the overall tower structural system, which verified the
suitability of the hybrid outrigger system as fit into the
overall tower structure.
4 CONCLUSION
This paper introduces two case studies on the
alternate design and re-engineering of complex composite
connections in modern ultra-highrise buildings to achieve
a higher efficiency of construction, easier site quality
control and better cost effectiveness. Through
comprehensive experimental and numerical studies both
the corbel type composite joint and hybrid outrigger
Wang
system are verified to be effective from a performance
base point of view. In the meantime, both the cost
effectiveness and construction efficiency were achieved.
The re-engineering of composite joint system will add
value to the design and construction of modern highrise
hybrid buildings in wind and seismic sensitive regions.
This will benefit the new generation of highrise mega
buildings with a higher level of structural performance
and integrated building functions.
5 REFERENCES
British Standards Institution (BSI) (2004), Eurocode 4: Design
of Composite Steel and Concrete Structures, Part 1.1:
General Rules and Rules for Buildings, European
Committee for Standardization.
British Standards Institution (BSI) (2005), Eurocode 3: Design
of Steel Structures, Part 1.8: Design of Joints, European
Committee for Standardization.
American Institute of Steel Construction (2005), ANSI/AISC
360-05: Specification for Structural Steel Buildings, AISC.
Brockenbrough R.L. and Merritt F.S. (2006), Structural Steel
Designer's Handbook, American Institute of Steel
Construction.
The Steel Construction Institute (SCI) (2002), the British
Constructional Steelwork Association Limited (BCSA),
Joints in Steel Construction, the Steel Construction Institute.
Lawson R.M. and Gibbons C. (1995), Moment Connections in
Composite Construction: Interim Guidance for End-plate
Connections, the Steel Construction Institute.
The Ministry of Housing and Urban-Rural Development
(MHURD), Code for Seismic Design of Buildings:
GB50011-2010, 2010.
The Ministry of Housing and Urban-Rural Development
(MHURD), Technical Specification for Concrete Structures
of Tall Buildings: JGJ3-2010, 2011.
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