bartera 2004 journal of constructional steel research

8/8/2019 Bartera 2004 Journal of Constructional Steel Research

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Journal of Constructional Steel Research 60 (2004) 751769

www.elsevier.com/locate/jcsr

Steel dissipating braces for upgrading existingbuilding frames

F. Bartera , R. GiacchettiUniversity of Ancona, Ancona 60122, Italy

Abstract

The aim of this paper is to investigate how the dynamic response of an existing r.c. singlestorey frame may be upgraded by using different types of steel bracing dissipating system.

The dissipating bracing system is composed of steel braces in series with an energy dissi-pation device that is either a high damping rubber pad (HDRD) or a shape memory alloywire assemblage (SMAD), both characterised by a hysteretic behaviour.

Many tests have been performed by the authors, both in free and in forced vibration, tocompletely understand the behaviour of the equipped systems by the use of simple testingtechniques in a wide range of frequency and displacement amplitude, in order to simulatethe frequency content of an earthquake excitation.# 2003 Published by Elsevier Ltd.

Keywords: Steel braces; Rehabilitation of existing structures; Passive energy dissipation; High dampingrubber; Shape memory alloy; Dynamic tests

1. Introduction

During the last 15 years, a lot of research work has been done to evaluate the ef-fectiveness of different added damping systems in reducing the seismic response of buildings [13]. On this subject, it is of great importance to face the issue of eitherupgrading existing buildings provided with little inherent earthquake resistance orrehabilitating buildings not designed at all to withstand quakes. As a matter of fact, recent seismic events have evidenced that even earthquake resistant reinforcedconcrete structures may give poor performance under minor shakings.

To date, many devices able to dissipate a remarkable amount of the seismicinput energy have been proposed, all of them based on the properties of theparticular materials which they are made of. At the moment, the available passive

Corresponding author. Tel.: +39-071-2201; fax: +39-071-2202324.

0143-974X/$ - see front matter # 2003 Published by Elsevier Ltd.doi:10.1016/S0143-974X(03)00141-X


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dissipating devices are based on friction, plastic hysteresis, and viscoelasticity; eachof them is characterised by different degrees of thermodynamic efficiency, stabilityduring the load reversals, and re-centring capability. Generally, these devices work

by taking advantage of the interstorey drift and, so, it is widely accepted to im-plement bracing systems equipped with the selected dampers as connecting ele-ments between two consecutive oors.

How to couple the damping devices to the bracing system and how to insert thedissipating braces into the existing structures have been the subject of many inno-vative suggestions; but steel braces still remain the more frequently used materialbecause of its laying easiness and adaptability to any existing frame. Moreover,steel braces may realise a considerable increase of stiffness with a little weightaddition of structural material, which is particularly benecial when the structuraldeciencies of the existing building are to be charged to an excessive lateral exi-bility.

Besides, an adequate stiffening is commonly needed to enhance the damping sys-tem efficiency. On the other hand, as is known, large added stiffness may stronglyinuence the dynamic response of the system to high frequency shakings. In ad-dition, it may sometimes be necessary to limit the brace stiffness in order to dimin-ish the state of stress, particularly at the joints between the braces and the existingstructural members. In these circumstances, high added damping accompanied bylimited increase of lateral stiffness may be the winning solution in the upgradingdesign.

2. Objective and scope

This paper reports the results of an experimental investigation regarding thedynamic response of a reinforced concrete structural frame with added stiffness anddamping provided by steel dissipating bracing systems.

The effect of two specic dissipating systems carrying different damping deviceson the original frame is illustrated by making a comparison between the dynamicproperties of the bare frame and those of the frame equipped with the dissipating

bracings and then quantifying the structural modications.Both systems dissipate energy through the hysteretic behaviour of the dampingdevices, given the light damping, the steel bracings may offer: the former device(HDRD) [46] depends on the non-linear constitutive law of high damping rubberpads subject to shear deformation, the latter (SMAD) [7,8] depends on the highlyinelastic behaviour of pre-tensioned shape memory alloy wires axially loaded.

In addition, two kinds of bracing congurations with quite different lateral stiff-nesses, both carrying the same HDRDs, have been investigated with the purposeof understanding how the type of bracing system may affect the overall structuralresponse.

The purpose of the experimental campaign is to assess the suitability of differentenergy dissipating bracing systems in improving the seismic behaviour of a rein-forced concrete frame. To full this objective, some physical entities regarding the

F. Bartera, R. Giacchetti / Journal of Constructional Steel Research 60 (2004) 751769752


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frame dynamic response have been monitored, i.e. the equivalent damping ratio,the energy dissipated per cycle during the sine sweep tests, and the input energyrequired to attain the same elastic displacement throughout the different congura-

tions. 2.1. Description of the mock-up components

2.1.1. Reinforced concrete space frameThe real scale structure is a 3D one storey r.c. frame built on a 4.20 m spanned

square grid with a total height equal to 3.10 m ( Fig. 1). The columns are0:20 0:30 m, the beams are 0 :20 0:35 m in the X direction and 0 :30 0:35 m inY direction and the oor slab is 0.1 m thick. At the vertexes of the deck, four con-crete cubes of approximately 20 kN, each is xed to the slab and to the beams bysteel plates and expansion plugs; the total mass of the bare frame is about 17155kg. Prior to the current investigation, the structure underwent a large number of dynamic tests both as a bare frame and equipped with different frictional dampingsystems, so that it was lightly damaged after the preceding seismic simulations. Thelateral displacement at yield was designed to be equal to 20 mm, that is 0.65 % of total height.

2.1.2. Steel bracing systemsAll kinds of damping devices, described below, were mounted atop braces manu-

factured with commercial steel pipes having a diameter of 133 mm and a thickness

of 4 mm assembled in the chevron fashion. As mentioned before, two bracing con-gurations were obtained by either adding cross-stiffening rods to or removingthem from the bracings.

In order to transform the lateral displacement of the frame oor into sheardeformation for the rubber pads or axial deformation for the shape memory alloywires, the upper ends of the two braces were bolted to a steel plate (different forthe two kinds of dampers) which, in turn, was bolted to one of the connecting

Fig. 1. The r.c. frame layout.

753F. Bartera, R. Giacchetti / Journal of Constructional Steel Research 60 (2004) 751769


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obtained from (a) by removing the inner device; (c) with 1 damper for each bracearranged in a horizontal plane ( Fig. 4b ).

Starting from conguration (a), a kinematic bracing system was obtained bysimply removing the cross-bars and loosening the bolts: the resulting kinematicquadrilateral (conguration (d)) was free to move following a certain path derivedfrom its geometry ( Fig. 5). Obviously, in such a way, the damper would no longerbe stressed by pure shear, but would undergo a combined shear-torque effect.

2.1.2.2 The SMAD-based dissipating bracing system. Each of the four SMADs wasan assemblage of 20 groups of 10 nitinol wires with a diameter of 1 mm ( Fig. 6)

Fig. 3. HDRDs constitutive law at 100% shear deformation.

Fig. 4. (a) Congurations (a) and (b); (b) conguration (c).



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and a length of 427 mm. The maximum force level was about 80 kN in thedisplacement range xed for the experimental campaign ( 13 mm). The wiregroups were assembled by means of two circular end plates, 130 mm in diameter,

so that the nal length of the device was 553 mm. The SMAD constitutive law inFig. 7 shows two characteristic plastic thresholds (plateau).

Fig. 5. (a) Congurations (a) and (b), (b) conguration (c).



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Upon a suggestion of the manufacturer, the optimisation of the dissipative be-haviour of the device was attained by imposing a pre-tensioning axial strain equalto 3.5%, obtained by loading up to the end of the upper plateau and then unload-ing down to the lower plateau.

The SMADs and the braces were linked by a 30 mm thick steel plate, while thedampers and the r.c. beam were connected by a plate of 1200 300 30 mm,glued and bolted to the beam, to whose ends two gusset plates were xed. TheSMADs were then linked to the gusset plates by giving a light pre-tension as muchas needed to have a full contact between the steel plates and avoid local stress con-centration ( Fig. 8).

It is evident that this kind of devices requires the bracing system to be as rigid aspossible which excludes using the kinematic conguration.

Fig. 6. Section of the shape memory alloy device.

Fig. 7. SMADs constitutive law.



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The most suitable layout of the connecting bracing system is the rigid one usedfor the HDRD-based conguration; in fact, other kinds of assemblages have manylimitations when it is necessary to apply a pre-tensioning load to a pair of devicesand simultaneously realise an initial balanced system able to leave the r.c. frameundeformed.

3. Experimental investigations

3.1. Quasi-static cyclic tests (LC)

The monotonic test was carried out by loading one of the Y -laid beam at theheight of the medium axis of the slab; a hydraulic servo-controlled actuator xedon a reaction element furnished the load required to reach the target sinusoidal dis-placement time history at an extremely low frequency level (0.008 Hz). The actu-ator was linked to the mock-ups by means of a ball joint connected to a gussetplate linked by the symmetric one on the opposite r.c. beam by four pre-stressedbars.

3.2. Free vibration tests: snap back (SB)

Free vibration tests were performed both on the r.c. frame and on the various

equipped systems by imposing increasing initial displacement levels and analysingon average the results of three tests each. The aim of this investigation is to keepan eye on some parameters variation: rst of all, the free vibration frequency mod-

Fig. 8. SMAD bracings on the r.c. frame.



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ications at different displacement levels due to stiffness changes, and then the cor-responding equivalent damping ratios. The test procedure followed the samescheme of the quasi-static test, except that the ball joint was replaced by a T-shaped gusset plate, pushing the system at the required displacement amplitudeand then instantaneously removing the load by resetting the actuator at this zeropoint ( Table 1 ).

3.3. Forced vibration tests: displacement control (FVDC)

Displacement controlled sinusoidal loading tests have been performed in orderto investigate the behavioural differences of the damping systems when subjected toharmonic load at non-fundamental frequencies, mostly lower. The load applicationsystem was exactly the same of the quasi-static tests ( Fig. 9).

3.4. Forced vibration tests: force control (FVFC)

Sinusoidal loading time histories were applied to the mock-ups covering a fairlywide frequency range in order to pick up the resonance and analyse the overall

Table 1SB on the r.c. frame

Interstorey drift (mm) f ave (Hz) Stiffness (kN/mm) nave (%)

2.5 2.50 4.233 2.225.0 2.45 4.065 2.697.5 2.34 3.708 2.5710.0 2.27 3.490 2.5512.5 2.20 3.277 2.57

Fig. 9. The actuator for FVDC tests.



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behaviour through the estimation of the maximum displacement, the modal dampingratio and the amount of the energy dissipated. A hydraulic shaker was anchored tothe structure in the Y direction in the same position of the actuator of the previous

tests and its opposite end was hang up at the bridge crane hook ( Fig. 10). For theHDR damping systems, only the critical cycles were monitored, whereas for theSMAD damping system, all the sweeps, both raising and descending, were investi-gated.

4. Results of the experimental campaign

4.1. Dynamic behaviour of the bare frame

By the quasi-static test, a forcedisplacement relationship was found for a oorlateral displacement equal to 12.5 mm (at about 4.5 % of columns height), atwhich the bare frame should have remained elastic. As a matter of fact, the curveplot (Fig. 11) shows a little hysteresis; nevertheless, looking only at the loadingbranch, the regression curve is almost linear and its slope gives a value of stiffnessequal to 3.27 kN/mm , that is half of the original stiffness of the undamaged frame.

The snap back test procedure was then applied to highlight the dynamic behav-iour of the frame at displacements large enough to open the pre-existing cracks.

As a consequence of these major displacements, the larger crack opening caused

the decay of the stiffness and of the oscillation period in association with a littledamping increment. The average damping ratio came out to be equal to 2.52%(Table 2 ).

Fig. 10. The shaker for FVFC tests.



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Displacement controlled sine sweeps originated cycles whose related stiffness anddamping values were greatly coherent with those obtained during the free vibrationtest ones.

4.2. Dynamic behaviour of the braced frames

4.2.1. The HDRD-based bracing congurationsThe indeformability of the rigid bracing with respect to the r.c. frame was asses-

sed by overlapping the lateral displacement time history to the HDRD shear defor-

mation time history ( Fig. 12). In such conditions, the lateral stiffness of the bracedframe is actually equal to the sum of the stiffness of the bare frame and that of theHDRD ( Fig. 13).

Snap backs were then performed. It can be observed from Fig. 14 that the struc-ture oscillates around a deformed position; however, this residual deformation onthe rubber was completely absorbed in less than 24 h conrming the good re-cen-tring capacity of the material ( Fig. 14). Only the kinematic bracing system did notpresent any residual displacement at the end of the motion. The damping averagevalues for each rigid conguration were the following:

na 39 :2 % nb 27 :4 % nc 26 :5 % nd 19 :3 %

Fig. 11. LC rst cycle on the r.c. frame.

Table 2Free vibration tests on SMAD-based bracing system

Interstorey drift (mm) f ave (Hz) Stiffness (kN/mm) nave (%)

1.0 6.53 29.80 5.732.5 5.99 25.07 7.63

5.0 5.65 22.31 10.067.5 4.83 16.30 11.6110.0 4.69 15.37 12.93



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The displacement controlled forced vibration tests were conducted at 2.55.0mm with a sweep in the frequency range from 0.1 up to 3.5 Hz. On varying the ex-citation frequency, poorly scattered values of the damping ratios (calculated in en-ergetic terms) and of the dissipated energy were detected, except for the kinematicbracing system which seemed to have an internal resonance problem at the fre-quency of 1.0 Hz. Furthermore, the damping ratios appeared to be unrelated toboth frequency and amplitude ( Figs. 15 and 16 ).

On the average, the equivalent damping ratios for the rigid conguration werethe following:

na 12 :2 % nb 10 % nc 14 %

The force controlled vibration tests substantially conrmed the results describedabove (Fig. 17). After these tests, it was possible to observe a perfect agreement

Fig. 12. Time histories comparison.

Fig. 13. LC forcedisplacement relationships.



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between the resonance frequency and the consequent stiffness at every displacementlevel and the corresponding value obtained by the constitutive law.

Many frequency sweeps at different constant force level were performed whichallowed detecting the existence of a critical frequency, about 40% smaller than theresonance, which is related to the maximum displacement amplication. Duringthis critical situation, the braced frame attained a lateral displacement up to 1.80

times that at resonance, while the energy dissipated per cycle was equal to that inthe resonance condition. In Fig. 18, a comparison between the forcedisplacementrelationships at the resonance and at the critical condition is shown. Taking intoaccount, the displacement amplication and the frequency reduction for the crucialfrequencies at every tested load level, the authors have pointed out that themaximum displacement amplication may be calculated by adopting a secant stiff-

Fig. 14. SB time-histories.

Fig. 15. SB damping ratios.



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ness 20% lower than that obtainable from the constitutive law at the same displace-ment.

4.2.2. The SMAD-based bracing congurationLateral displacement equal to 1.0, 2.5, 5.0, 7.5, 10.0 mm was applied to the

equipped conguration to carry out the free vibration tests.Even with this kind of hysteretic dampers the structure oscillated around a non-

zero position, except for the 1 mm test, when the dampers behaviour is almostelastic and very little hysteretic damping occurs. The residual deformation at theend of the free vibration is permanent and re-centring is inhibited unless an exter-nal mechanical action is applied ( Fig. 19).

Fig. 16. FVDC: damping ratio vs. frequency.

Fig. 17. FVFC: damping ratio vs. force level.



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Like the HDRD-based bracing systems, the most signicant parameters havebeen evaluated. The results, shown in Table 2 , have conrmed that the dampingratio increases with the initial displacement as the SMAD damping capacity isassociated to the strain level.

Displacement controlled sine sweeps were performed in the frequency range0.0011.0 Hz at displacements of 1.0, 2.5, 5.0, 7.5, 10.0 mm.

Like the HDRD-based congurations, the overall behaviour of the equipped sys-tem follows the dampers; in fact, the forcedisplacement relationship remainsmostly linear elastic with a stiffness equal to 2.73 kN/mm (the new stiffness of ther.c. frame) and almost all the dissipative capacity is exhibited by the SMA devices.

The cycles at constant amplitude are almost stable in the frequency sweep, but inorder to establish the overall stability of this damping system, the authors repeatedall the test history three times with the same procedure. Comparing the forcedis-placement curves of the rst and second series of cycles ( Fig. 20), it is evident thata signicant reduction of the energy dissipated was caused by the pinching effect

the SMADs have shown.The numerical and graphical comparisons show that the dissipated energy re-duction decreases as the number of cycles performed increases, that is, for example,between the rst and the second series of tests at 0.1 Hz, there is a reduction atabout 30%, while between the second and the third series, the reduction is set to10% of the initial value.

This behaviour is well known; as a matter of fact, this kind of wire assemblageneeds to be stabilised by applying a certain number of cycles before implemen-tation, even though that reduces the dampers efficiency. Stabilisation was actually

executed in the factory by cycling them 20 times at 3.5% axial strain; neverthe-less, the test results have evidenced that the devices require a larger number of pre-liminary cycles to exhibit quite a stable performance.

Fig. 18. Crucial forcedisplacement relationships for HDR-based braced frame.



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The shaker induced sine sweeps were performed at different force amplitudes.For each of them, both raising and descending frequency sweeps were carried outin order to appreciate the differences of the transfer functions originated by theSMA strongly non-linear behaviour ( Fig. 21).

The displacement amplitude vs. frequency plots ( Fig. 22) at different force levelsshow that the performance is almost regular, excepting an anomalous condition inwhich the damping ratio reaches the minimum value.

Those critical frequencies are characterised by the in-phase movement of boththe r.c. frame and the bracing system and they are different for each forcing level.Consequently, the dampers are not capable of dissipating energy.

5. Conclusions

The tests results are as follows:

. starting from an equivalent damping ratio of about 23% in the bare frame, it ispossible to provide additional damping up to as much as 10121% by equipping

Fig. 19. (a) SB time-histories: re-centring behaviour, (b) SB time-histories: non-re-centring behaviour.



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the frame with steel dissipating bracing systems. As a matter of fact, both theHDRDs and the SMADs are capable of adding such an amount of damping;

. supplemental stiffness and damping are benecial to a exible structure becausea dramatic reduction in the structural response of the braced frame can beobserved in terms of interstorey drifts, whereas for structures that cannot with-stand serious reactions at the beam-to-column joints, the use of a kinematic bra-cing system may add damping without an increase in the overall stiffnessexcessively;

Fig. 20. Pinching effect due to plateau modication.

Fig. 21. FVFC: forcedisplacements relationships during a rising sweep (5.0 kN).



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. the response of the braced frames equipped with the HDRDs, both rigid andkinematic, does not go through the most critical situation at the classical reson-ance frequency, but at a lower frequency level mathematically correlated to theresonance one at every displacement amplitude;

. the kinematic bracing system leaves the frame undeformed; on the contrary, therigid one has shown an acceptable re-centring capability of the HDRDs;

. the damping capacity of the HDRDs is widely stable only if the stress applied ispure shear; otherwise the composite shear-torque stress caused by the kinematic

motion gives rise to very scattered damping ratios;. the hysteretic behaviour of SMADs is unstable and shows a progressive decay of the energy dissipated;

Fig. 22. (a) FVFC: raising sine sweeps at different force level, (b) FVFC: descending sine sweeps at dif-ferent force level.



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. the SMAD-based bracing system may be affected by internal resonance, just likethe kinematic bracing system, so that there could be a lack of energy dissipationat certain frequencies;

. the great irregularity of the SMADs response is a hard limit to pass through if an analytical law able to predict the damping capacity is looked for;

. the SMADs have shown poor re-centring capability.

Finally, the dissipative capacities of both the damping systems are interesting,but the reliability of the HDRDs may be very advantageous to bring to fruitionsimple numerical models even for complex structural assemblages and so that canbe helpful for the preliminary design. On the contrary, the SMAD behaviour, evenif their dissipative resources appear very interesting, is not yet so predictable.

Acknowledgements

The high damping rubber pads are furnished by T.A.R.R.C, Tun Abdul RazakResearch Centre, Hertford, London, UK and come from many studies on civilengineering usage of natural rubber devices (REEDS European Project).

The shape memory alloy wire assemblages are furnished by FIP Industriale, Sel-vazzano, Padova, Italy and are the results of researches in SMA damping optimi-sation under high frequency excitation for the protection of cultural heritage(ISTECH European Project).

References

[1] Hanson RH. Supplemental damping for improved seismic performance. Earthquake Spectra1993;9(3).

[2] Soong TT, Dargush GF. Passive energy dissipation systems in structural engineering. John Wiley &son; 1997.

[3] Aiken ID, Nims DK, Whittaker AS, Kelly JM. Testing of passive energy dissipation systems. Earth-quake Spectra 1993;9(3).

[4] Dumoulin C, Magonette G, Taucer F, Fuller KNG, Goodchild IR, Ahmadi HR. Viscoelastic energydissipaters for earthquake protection of reinforced concrete buildings. Eleventh European Confer-

ence on Earthquake Engineering. Balkema; 1998.[5] Shen KL, Soong TT, Chang KC, Lai ML. Seismic behaviour of reinforced concrete frame withadded viscoelastic dampers. Engineering Structures 1995;17(5).

[6] Ungar EE, Kerwin EM. Loss factors of viscoelastic systems in terms of energy concepts. The Journalof the Acoustical Society of America 1962;34(7).

[7] Castellano MG. Development and experimental characterisation of shape memory alloy devices(SMADs), Proceeding of the Final Workshop of ISTECH Project, 2000.

[8] Tirelli D, Renda V, Bono F. Characterisation and t to seismic protection of shape memory alloys.Proceedings of the Fourth European Conference on Structural Dynamics, EURODYN 99, Prague,Czech Republic, 7 9 June. 1999.


bartera 2004 journal of constructional steel research

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