laura anania, antonio badala, sebastiano costa - wit press · analytical and experimental analysis...

Analytical and experimental analysis on a r.c.

braced frame coupled to a new hysteretic

dissipation device

Laura Anania, Antonio Badala, Sebastiano Costa

Istituto di Scienza delle Costruzioni - University of Catania -V.le A.Doria,6 - 95125 Catania (Italy)-Phone n.:+39.95.7382257- Fax n.:+39.95.738229 7- abadala(a),isc. ing, imict. it lanania@isc. ing. unict. it

AbstractThe seismic upgrading of r.c. framed structures can be carried out by means ofthe introduction of new structural elements capable of dissipating part of or allthe energy transmitted by the earthquake. The work concerns the development ofan innovative hysteretical device designed by the same authors, for using withina reinforced concrete frame building. Namely, in the paper both the numericaland experimental analysis are carried out on a coupled system as well as on no-coupled system. A comparison is then made between the data of the two system.

1. Introduction

In the conventional seismic design it is recognized that the structure canwithstand both a medium earthquake without any damage and a strong groundmotion which produces some inelastic deformations with a low hazard damageon the structure. However, the structural damaging is often very difficult torepair and, sometimes, very expensive, so, in order to avoid this, new techniquescapable of guaranteeing high performance of the structure are required. Amongthese, the adding of new structural elements called "hysteretical devices", seemsto be the most useful and the cheapest procedure actually used.The design of both dissipative braces and damper devices has been treated bymany authors following different approaches. But, in any case, the mainrequirement that they must fulfill, consists in guaranteeing the uniformplasticitsion of the same under cyclic loads. In some previous works [Anania&co. l]we treated a new hystereticical device capable of dissipating energy bymeans of plastic mechanisms (fig.l). This prototype has been developed in orderto insert it inside a mesh of braced frame structure.

Transactions on the Built Environment vol 38 © 1999 WIT Press, www.witpress.com, ISSN 1743-3509

154 Earthquake Resistant Engineering Structures

Hinge forconnection ,-

between deviceand frame

Figure 1: a) Statical scheme, b) Experimental scheme of I beam device

As reported in other papers, the dissipating part of the device is constituted by anI beam element, derived from the cutting of a commercial beam profile of the IEseries, with flanges much more rigid than the web so that the plasticdeformations occur only on the latter. This device fulfills both the requirementsof uniform plasticisation and ideal cyclic response throughout the peculiarstructural scheme. This is in contrast as regards the majority of the dissipatingdevices proposed so far, that, being subjected to multi-axial stress condition, theyrequire complex shapes in order to maximise the dissipation. In this paper,infact, the attention will be focused on both the behaviour of the whole systembraced frame-dissipating device and on the problem of the choice of the stiffnessas well as the yield threshold in order to achieve an optimum protection of theanalysed structure.

2. Insertion within the braced frame structures

In the theoretical model the braced frame has been considered as a shear typemodel (Figure 2a); this schematization seems to be very useful in order todetermine the parameters that play a very important role for an efficient frictionof the dissipation device. In the small displacement theory, both applying thekinematics chains' theorems and neglecting the compressed brace contribution itis possible to treat the scheme reported in figure 2a as reported in figure 2b. Theangular stiffness Kj is the term due to the device and it represents the bendingmoment to apply on the structure to obtain a unit rotation OT of the hinge. Thisbending moment is evaluated by either eqn (1) during the elastic phase or eqn (2)during the elastic-plastic phase:

2M6(1)

2M,

3M-2M 12EI.(2)


Earthquake Resistant Engineering Structures 155

Figure 2: a) Frame schematization, b) Connections schematization

In this scheme the rigid beam is considered as a roller bearing while the dash linerepresents the term to neglect due to the compressed diagonal of the brace. It ishypothesized that the brace leads to the compression state after a 8 displacementof the frame applied from left to right (positive sense).A suitable behavior of the dissipating device is obtained when it starts to workunder frame displacements lower than yielding frame one. In other words, thedevice must work before the frame starts with the damaging process.The horizontal frame displacement 5 is strictly connected to the OT rotation ofthe hysteretical device; thus, if 5 represents the yielding displacement of theframe, the corresponding OT rotation must be greater than the yielding rotation ofthe device because the rotation is in function of the curvature of the web bymeans of eqn (3):

(Or

*=i: ™

In small displacement theory the frame displacement is completely resisted bythe rotation COT according to the expression (eqn 4) reported above:

cor = arctan — (4)1%J

where a^ represents the distance between the load application hinge and theconnection hinge to the frame (Figure Ib, Figure 2b). Under this condition thediagonal of the brace placed on the right cannot undergo buckling phenomenaand so it can work immediately when the displacement 5 is inverted.Now, removing the small displacement hypothesis and considering both thebracing diagonal and the "c" portion as infinitely rigid, the new position of the"T" hinge, due to an imposed displacement 6 of the roller bearing, will be givenby the intersection between the circle arcs with Lp and a^ radius (Lp bracediagonal length) whose center lie at the column base of the frame and on theroller bearing respectively as described in figure 2b. The dimensioning of thehysteretical device to be inserted within a braced frame structure depends onsome energetic parameter as well as on the choice of both the device and thedistance a^ (as shown by eqn 4) which represents the arm of the couple.



Figure 3: Deformed scheme of the coupled system under a 8 displacement

The possibility of changing the position of the connection hinge between deviceand braces, i.e. aligning either to the connection hinge between frame and deviceor in eccentric position respect to the latter, permits us to obtain a different shapeof the hysteretical cycles. On the other hand, a very small a^ arm determines thetransferring of a very high load from braces to device, although it permits it tocome into play under very small 5 displacements (i.e. comparable with the r.c.frame displacements).

3. The r.c. frame: experimental model

The experimental analysis is carried out on a mesh of frame model scaled 1:2size. The two fundamental scales Si and Sf, respectively regarding the geometryand the applied load, have been fixed according to the following eqn. 5:

s, = * real

model= 2 , (5)

The stress ratio has been fixed equal to 1:1 scale so as not to scale the resistantcharacteristic of the transversal section. Thus, the load scale becomes equal to1:4 (eqn. 6)

Sf=S0-Sl=4 (6)

The design of the transversal section of the frame has been carried out byreferring to the ground floor of a four-storey building located in a high seismichazard zone according to Italian rules.The transversal section in the experimental model has the followingcharacteristics: B=H=25 cm ; N=l 13 kN ; M=1380 kNcm.The transversal section in the real model has the following characteristics:B=H=50 cm ; N=225 kN; M=5520 kNcm. Thus, two frame models areperformed and tested. The first one was a no braced frame; it has been tested inthe absence of the hysteretical device and to have a better understanding of thebehavior of the structure in the case of the presence of the device.



IPE 550

HydraulicactuatorINSTRON

Figure 4: Testing equipment for the model frame

The second one was a dissipating braced frame capable of dissipating the inputenergy by means of the I beam device described above.

4. Test on no-braced frame r.c. model

The r.c. no-braced frame withstands a constant vertical load of 100 kN appliedover each column by means of a hydraulic jack fixed to an IEB 240 steel profilelocated just under the frame foundation. The horizontal load was, instead,applied by means of a particular equipment, INSTRON, constituted by a digitalcontrolled hydraulic actuator with ±250 kN capacity and a ±125 mm stroke. Boththe actuator and the loading jack have been erected on a very rigid testing frame(Figure 4). The system has also been constrained out plane by a doublependulum system located on the frame top as shown in photo n.la.

Photo 1: a) Out-plane constrained system, b) Device-frame connection



Previously, a numerical investigation had also been carried out on a no-bracedframe in order to evaluate both the elastic and plastic behavior of the frame.The results obtained are shown in figure 5 where the plastic hinge succession isreported. The F.E. incremental analysis has given a yield strength equal to 17.5kN and a yielding displacement equal to 1,3 mm. The test has been carried outby imposing the nodal displacement at the frame top. The test program hasforeseen three cycles in elastic domain up to a controlled displacement equal±lmm, three cycles at imposed displacement of ±1.5 mm corresponding to yieldstrength. Furthermore three series of cycles have been carried out in elastic-plastic domain at controlled displacements equal to ±3 mm, ±6 mm, ±9 mm, ±12mm, ±15 mm, ±18 mm, ±21 mm. The experimental measured load at animposed displacement of ±25 mm was equal to 70 kN circa. At this loading statesome cracks have occurred in each node of the structure. Namely, some of theseoccurred at the top of the frame columns and others at the beginning of the span.The plastic hinges formation during the experimental test reflects the theoreticalinvestigation except that of the second hinge which occurred on node 9 ratherthan node 13 (the node numbering is reported in Figure 8b).The structural behavior in terms of load-displacements could be considered aslinear up to a horizontal load equal to 20 kN, although a very light decrement ofstiffness is visible around a horizontal load of 15 kN. Other stiffness variationsare found for both a load of 27 kN and of 40 kN (Figure 6), probably when thefractures occur on the frame. During this experimental investigation no fractureoccurs on the mid-span of the frame beam.

f F f

a) b) c) d)

Figure 5: Succession of the plastic hinge formation on no-braced frame

Load vs. displacement on the top of the frame

-1OO -• Displacement [mm]

Figure 6: Experimental hysteretical cycles of no-braced frame



5. The coupled system: braced frame-device

When we known the yielding displacement value as well as that of the yieldforce of the frame, we can dimension the hysteretical device to insert within it(Photo no.lb). The prototype, capable of working before the frame achieves theyielding displacement, has been obtained from a IEB 100 length 100 mm withvertical eccentricity equal to a =2l.92 mm (Figure 7a). Two twins prototypes areperformed. One of these has been tested alone in order to know the hystereticalcycles and each parameter whose knowledge was very important for the test tobe carried out on the coupled system. The hysteretical cycles obtained arereported in figure 7b when it is possible to note the presence of soft hardening,due mainly to internal friction effects. In fact, this hardening has not been foundduring numerical simulation of the device behavior, where it has shown perfectlyan elastic plastic cycles. Data is measured by means of displacement transducersand of strain gauges located along the web of the I device. The prototype no.2was inserted within the frame by means of "K" braces. In order to connect thebraces and the device together with the frame, some steel plates are made andfixed by bolting. To this aim, little holes 12 mm wide and 100 mm long are madealong the frame. Each hole has been cleaned and a particular binding, Hilti HITHY 50, has been injected. Then, a § 10 screw bar has been located for a length of130 mm. Every bar was capable of resisting to 6.3 kN traction solicitation and to6.2 kN of shear solicitation. Once hardened, the steel plate was bolted (Photono.3a). At this stage the device was located inside the frame mesh and the bracewas anchored by means of two steel tubular turnbuckles.A 5 kN pre-strength was applied on each brace. The coupled system has beeninstrumented and tested in the same way used for no-braced frame. The data wasmeasured by means of displacement transducers and strain gauges. Numericalinvestigation has also been carried out by using the discretization and the schemereported in figure no.Sb. This investigation has been stopped at the formation ofthe first plastic hinge because the application of the hysteretical device mustpreserve the frame which, in this way, remains in elastic domain.

(a)

F[Kg]

-8<)LS [mm]

(b)

Figure 7: a) 3D view of hysteretical device, b) Hysteretical cycles



Load (kN)

80706050403020100

D - No braced -frame

Bracedframe

_o or , > - O* * ' Node 5 Node 9O* " " Node 13

' /Node I

(a)

6 8.2 10.8 13.5Displacement (mm)

Figure 8: a) Plastic hinge formation, b) Adopted theoretically scheme

Controlled displacement +/- 15 mm

F[KN]

Figure 9: Hysteretical cycles of braced frame coupled with device

The testing program differs from the case of no-braced frame because the loadfound at the same controlled displacement was higher than the previous case.Namely, in this case the maximum value of the achieved displacement during thetest was equal to ±15 mm for a horizontal load of 80 kN circa. Namely, the firsthinge occurs at node no.5 at the extreme beam section for a load equal to 40 kNfor a displacement of+/- 9 mm as in the numerical forecasting (Photo 2a).The section located at the bottom of the column (node no.l) has shown somefractures for a load of 68 kN (Figure 8b). In this case some cracks also occur atthe beam mid-span (node no.7, Photo no.2) owing to the strength transmitted bythe device to the frame beam (Figure 9b). During the first cycle at +/- 15 mm thecracks appeared on the node no. 13. At the ultimate loading stage, three plastichinges occurred near the extreme sections of the beam, others appeared near theextreme sections of the columns. The hysteretical cycles drawn by the system aremuch wider in respect to the no-braced frame. In the figure no.9 the hystereticalcycles are reported in function of the imposed displacement both in the case ofelastic and elastic-plastic domain.



Photo 2: Distribution of the crack in the critical section of the frame

Photo 3: Crack at the base column: a) Braced frame, b) No-braced frame

Displacemet-lond on left brace

Ft[kN]60 n

-25 -15 -520 5-40-1

S [mm]

Displacemet-load on right braceFt [kN]

-25 -15 -520 J 5-40 !-60 -iS (mm)

Figure 10: Hysteretical cycles of the braces during the test

Figure 10 reports the hysteretical cycles described by the brace during the test. Itis possible to note that they work alternatively.

6. Analysis of the results and conclusions

The experimental investigation has shown that: in the case of braced frame thehysteretical cycles are much wider in respect to the case of no-braced frame.



Comparison between no-braced frame and braced frame

F[KN]

Figure 11: Comparison between the hysteretical cycles

In fact, at the same imposed displacement, we find a different value of load witha considerable increasing in the case of braced system (Figure 11). The base ofthe column has shown some cracks in both cases (Photo no.3). Namely in a no-braced frame system, the cracks appeared because of a low value ofdisplacement corresponding to the first hinge formation. In the case of bracedframe, the cracks which occurred at the base column appeared only at very highvalue of load approximately near to the collapse of the no-braced structure;besides they were placed at higher position than the previous case. This fact isdue to the presence of the steel plate for the connection with the brace rod. Thissteel plate works as a wrapping system, and so it strengthens the bottom sectionof the column. Another advantage to resistance of the bottom section of thecolumn is given by the use of special binding to collect the plate with the frame.The plastic hinges at the column-beam intersection node occur on the beaminstead of on the column. This shows the efficiency of the device whose usepermits us to avoid global mechanism formation for the examinated structure.

References:

[1]. Anania L., Badala' A., Cuomo M. "An elastic-plastic dissipation device forseismic protection of constructions: numerical and experimental analysis"proceedings of the international symposium "Plasticity '97" JeuneauAlaska, 12-18 lug. 1997

[2]. Anania L., Badala' A., Cuomo M. "Primi risultati teorici e sperimentalirelativi ad un nuovo dissipatore isteretico per la protezione sismica dellestrutture" proceedings of 8° convegno nazionale Anidis 1997 - Taormina,21-24 sett. 1997

[3]. A. Badala', M. Cuomo, L. Anania, S. Costa "A New elastic-palstic devicefor seismic protection of the structures: experimental analysis andanalytical modelling of the hysteretic response" Proceedings of EleventhEuropean Conference on Earthquake Engineering , Paris France, 6-11 sett.1998


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