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NUMERICAL MODELLING OF CENTRICALLY BRACED
FRAMES EQUIPED WITH FRICTION DAMPERS IN THE
BRACINGSFILIP VACARESCU NORIN
Department for Steel Structures and Structural MechanicsPolitehnica University Timisoara
Str. Ioan Curea Nr. 1A
ROMANIA
Abstract: - The paper is concerned with the numerical simulation of the behavior of centrally braced frames
under seismic action using SAP2000 engine. A test frame was analyzed using performance based design under
the action of ground motion recording of Vrancea 77. In order to improve the behavior of the dual frame,
energy dissipation devices in the form of friction dampers were equipped in the bracing system. The aim of the
paper is to use numerical modeling to study the behavior of centrically braced structures equipped with a
specific type of friction dampers in the bracings.
Key-Words: -Numerical simulation, seismic, friction damper, performance based design.
1 IntroductionThe static configuration of a building represents in
all configurations a spatial system capable of
transmitting to the foundations the effect of vertical
loads , own weight, live load and also the effects of
the horizontal forces that act on the structure from
wind and seismic loading.
The effect of spatial interaction is assured by both
the type of connection between the componentelements, columns, beams, and bracings or by
reinforced concrete diaphragms as well as by the
floor slabs of each floor which form horizontal
diaphragms and give high rigidity in their own
plane.
Steel frame structures generally fall into 3 main
categories according to the way they resist to the
action of lateral forces:
- un-braced frames (moment resisting
frames) (MRF)
- centrically braced frames (CBF)
- eccentrically braced frames (EBF)
For an optimal design of these structures one must
find a compromise between the strength , rigidity,
ductility and architectural demands.
For checking at ultimate limit states, the
methodology of dimensioning of structures situated
in seismic areas can lead to the following types of
structural design concepts:
- dissipative structures
- structures isolated from seismic action
- structures with supplemental damping
For structures isolated from seismic action and thosewith supplemental damping the structure is
conceived either not to enter plastic domain or by
prolonging and improving its behavior in the plastic
range by implementing devices which can absorb
the seismic energy and can modify the own period
of vibration of the structure to more favorable values
for global behavior.
The first alternative that of ductile structures leads
to designing of dissipative structures .These
structures are calculated and designed so they permit
the plastic behavior of certain zones also calleddissipative zones. These zones have the role of
dissipating the kinetic energy induced by the seismic
motion by hysteretic behavior in the plastic domain.
The structural elements however conceived as being
non-dissipative must be design in such a manner that
they remain in elastic domain.
Dissipative frames can be classified according to the
nature of their dissipative zones. We can mention 3
categories here:
- centrically braced frames Fig.:a,b,d
- eccentrically braced frames Fig.:c
- unbraced frames Fig.: e
a. b. c. d. e.
For centrically braced frames the dissipative zones
are the braces subjected to tension. The braces under
compression buckle. The dissipative performance ofthis type of structure is limited due to repeated
Proceedings of the 11th WSEAS International Conference on Sustainability in Science Engineering
ISSN: 1790-2769 381 ISBN: 978-960-474-080-2
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buckling which leads to a degradation of the cyclic
behavior with the increase in the number of cycles.
There are several types of devices that have the
purpose of dissipating seismic energy as follows :
- Hysteretic devices : yield metal device ,friction
dampers ;
-
Fluid viscous dampers
- Magnetorheological fluid dampers
- Tuned mass dampers
-
Base isolators
-
Visco-elastic dampers
The type of damper used for this model is a friction
damper placed in the diagonal bracing. Friction
dampers achieve a high amount of energy
dissipation through friction developed between two
or more sliding surfaces under compression forces
normal to their plan. Friction devices can produce
large rectangular hysteretic loops but can beconfigured to produce non-rectangular hysteretic
loops with a pinching effect. [1]
The main goal of the research is to analyze the
performance of centrically braced frames and to
improve the performance of these frames by placing
friction dampers in the bracing system. The
numerical analysis is done using SAP2000 as
modeling tool.
2 Geometry and designThe structure studied is a 5 storey steel building with
3 by 3 spans conceived in a dual configuration ofCBF combined with MRF frames .
The model used in the analysis comes from a
structure with 3x3 openings as depicted in Figure 1.
Fig.1 Plan view
For a simplified analyses a 5 level dual type plane
frame MRF + CBF with the height of each floor of2.4 m Figure 2.
Fig.2 Transversal section
The dimensioning of the frame was done accordingto Romanian Design Code and for dimensioning in
special combination
P100/2006 was used as a reference. The response
spectra used was for (Figure 3).
For a first iteration a seismic reduction factor of q =
2.5 was used.
Fig.3. Code Spectra
The dissipative elements (bracings) were
dimensioned from the load combination G + E +
0.4Q and non-dissipative elements (beams and
columns of central frame ) were dimensioned from
the load combination G + E +0.4Q. For a
simplified analysis a value of were chosen from
Tables (according to P100-1/2006 ).
For the determination of the response of the
structure to seismic loading a nonlinear dynamic
analysis (time-history) was applied using theacceleration spectra of the Vrancea 1977
Proceedings of the 11th WSEAS International Conference on Sustainability in Science Engineering
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-800
-600
-400
-200
0
200
400
600
800
-8
-6
-4
-2 0 2 4 6 8
Kinematic
SERB
Fig.6.
Fig.7.
Fig.8
The first 2 loops Kinematic and Takeda were used totry an approximation of the hysteretic curve supplied
by the manufacturer as mentioned above. In reality
the devices are only placed at the base of the bracing
but for the ongoing analysis the whole bracing was
replaced using an equivalent rigidity Kech resulting
from connecting in series the 2 elements the bracing
and the damping with their rigidities accordingly.
Consecutive push over tests with displacement
control at +/- 2, +/- 4, +/- 6 were done. The results
are presented graphically in terms of Force-
Displacement as follows :
Fig.9 :Kinematic type behavior of link element
-800
-600
-400
-200
0
200
400
600
800
-8
-6
-4
-2 0 2 4 6 8
10
TAKEDA
SERB
Fig.10 : Takeda type behavior of link element (kN-
m)
The best approximation of the area enclosed by
SERB control curve is achieved by the use of
Takeda type behavior . This behavior was used
further on in the analysis of the plane frame
presented previously. The main feature which is
recorded was again the inter story drift presented
graphically in Figure 11 in comparison to the drift
obtained for the un-damped structure:
Proceedings of the 11th WSEAS International Conference on Sustainability in Science Engineering
ISSN: 1790-2769 384 ISBN: 978-960-474-080-2
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0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04
Drift +Drift -
Fig.11 Relative drift for Takeda type curve (up) vs.
relative drift without damping devices (down).
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04
Dift +
Drift -
For a further comparison between the two a
Kinematic type behavior was also used (Figure 12)
which leads to a curve which completely overlaps
the control curve:
-800
-600
-400
-200
0
200
400
600
800
-8 -6 -4 -2 0 2 4 6 8 10
Kinematic
SERB
Figure 12 Kinematic type behavior (kN-m)
Using this behavior of the link elements the drift
was once again compared to the initial valuesobtained for the classical solution.Fig13
0
0.5
1
1.5
2
2.5
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04
Dift +
Drift -
Figure 13. Drift values for kinematic model (m)
Note: All drift values are given in meters.
4 Conclusion
Comparing drift curves in the two structural
configurations a first observation is that they have
almost the same shape. This shows that the
modeling of the behavior of the structure using link
type elements can be used for studying the global
behavior of the structure. This type of analysis can
therefore be used later on to determine the
performance of the structures under seismic loading
in different structural configurations and different
acceleration types. Following the values of inter
story drift you can observe close values for both
Kinematic and Takeda type behaviors .
For small values of the acceleration multiplier there
are no significant differences between the two
models with and without dampers the ones with
dampers having a slightly larger displacement
values due to reduction of the rigidity. The
advantage of dampers appears at large values of the
period of vibration ( for a multiplier of over 1.4)
when the braced structure reaches failure . The
structure with dampers records smaller deformations
and collapse is prevented .Figure 15The use of dampers leads to improvement of ductile
characteristics of the structure avoiding a brittle
failure.
Proceedings of the 11th WSEAS International Conference on Sustainability in Science Engineering
ISSN: 1790-2769 385 ISBN: 978-960-474-080-2
7/21/2019 Www.wseas.us E-library Conferences 2009 Timisoara SSE2 SSE2-20
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0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 0.02 0.04 0.06 0.08 0.1
Disipatori
CBF
Figure 15.Comparison of drift values
These SERB devices were designed to work
at a period of vibration of over 1.5 sec. The main
advantage is that they can give a predetermined
reduction of displacement if calibrated properly. A
reduction of drift values can be seen here also in Fig
14 for values of over 1.2 of the acceleration
multiplier meaning above the period of 1.36 of the
Vrancea 77 recording used.
The problem remains in modeling as
accurately as possible the behavior of link elements
in order to depict the real behavior of the dampers
with strengthening of the rigidity. This remains to bethe object of future studies with the main goal of
calibrating the dampers by numerical modeling to be
most suited for the structure under analysis.
At present, two SERB devices are in testing
at the laboratory of CEMSIG Research Centre
(www.cemsig.ct.upt.ro),at the Politehnica University
Timisoara.
The description of such a friction damper is shown
in Figure 16.
Fig.16
The two dampers under test have the capacity of
800kN and of 1000kN.The purpose of the test is to
calibrate the hysteretic behavior modeled so that it
can be used in the global analysis of the frame.
Based on these results further time history analyses
will be performed for different types of structures
under different sets of ground motion recordings in
order to evaluate the effectiveness of damping
introduced in the structures by this type of friction
damper.
References:
FEMA 356, Pre-standard and Commentary of
Seismic Rehabilitation for Buildings
FEMA 273, Guidelines for Seismic Rehabilitation
of BuildingsP100-1/2006
[1] PROHITECH, WP6 Report, Set-up of Advanced
Reversible Mixed Technologies for Seismic
Protection, Faculty of civil and geodetic
engineering Slovenia
Trevor E. Kelly, Design Guidelines, Holmes
Consulting Group, Revision 0, July 2001
Dubina Dan, Dan Lungu et.all , Constructii
amplasate in zone cu miscari seismice puternice,
Editura Orizonturi Universitare,2003
Dinu Florea, Metode de Calcul Neliniar al
Structurilor in Cadre Metalice Solicitate laActiunea Seismica, Editura Orizonturi
Universitare, 2006
Proceedings of the 11th WSEAS International Conference on Sustainability in Science Engineering
ISSN: 1790-2769 386 ISBN: 978-960-474-080-2
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