Www.wseas.us E-library Conferences 2009 Timisoara SSE2 SSE2-20

7/21/2019 Www.wseas.us E-library Conferences 2009 Timisoara SSE2 SSE2-20

1/6

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

[email protected]

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


2/6

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


ISSN: 1790-2769 382 ISBN: 978-960-474-080-2


3/6


4/6

-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:


ISSN: 1790-2769 384 ISBN: 978-960-474-080-2


5/6

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.


ISSN: 1790-2769 385 ISBN: 978-960-474-080-2


6/6

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


ISSN: 1790-2769 386 ISBN: 978-960-474-080-2

Documents

Www.wseas.us E-library Conferences 2009 Timisoara SSE2 SSE2-20