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details of beam-to-column connections are investigated by means of non-linear dynamic analyses, carried out by IDARC 2D program, for increasing levels of the seismic intensity measure. As the work on this topic is still ongoing, only the preliminary results are presented and discussed in this paper.

In these preliminary results, record-to-record variability is accounted for performing dynamic non-linear analyses, assuming 3% of critical damping, considering

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three earthquake records selected from PEER database. The main data of the considered records (name of earthquake, date, component, spectral acceleration corresponding to the period of vibration of the considered Moment Resistant Steel Frames, PGA and length) are reported in Table 5.

Table 5. Main data of considered records

Earthquake Date Component Sa (T=1.6sec)

(g) Sa (T=1.7sec) (g) PGA (g) Length (sec) Northridge 17/01/1994 N-S 0.13 0.11 0.233 39.99 Imperial Valley 15/10/1979 N-S 0.27 0.30 0.370 28.35 Kobe 16/01/1995 N-S 0.22 0.29 0.251 40.95

Aiming to perform an incremental dynamic non linear analysis, all the records have been properly scaled to provide increasing values of the spectral acceleration Sa(T1) corresponding to the fundamental period of vibration of the structure equal to

T1=1.6 sec for connections EEP-CYC 02 and EEP-DB-CYC 03 and equal to T1=1.7

sec for connection TS-CYC 04. In particular, the analyses have been repeated increasing the Sa(T1)/g value until the attainment of the experimental ultimate value

of the plastic rotation supply of connections. Scaling the records at the same value of Sa gives the possibility to reduce the variability of structural seismic response.

In particular, in the top left corner of Figure 3, the elastic spectra of the analysed records are given; in addition, the same figure provides also the IDA curves giving the maximum spring rotation versus the spectral acceleration. In these figures, the ultimate plastic rotation for each considered connection is also represented. The IDA analyses have been stopped when the available ductility has been exceeded at least for one spring element. In particular, it can be observed that for Imperial Valley earthquake, the maximum value of spectral acceleration is equal to 0.64g, 1.17g and 1.44g for MR-frames with connections EEP-CYC 02, EEP-DB-CYC 03 and TS-CYC 04, respectively; for Kobe earthquake the maximum value of spectral acceleration is equal to 0.56g, 1.96g and 1.31g for MR-frames with connections EEP-CYC 02, EEP-DB-CYC 03 and TS-CYC 04, respectively; finally, for Northridge earthquake the maximum value of spectral acceleration is equal to 0.64g, 1.48g and 0.75g for MR-frames with connections EEP-CYC 02, EEP-DB-CYC 03 and TS-CYC 04, respectively.

It is evident the dependence of the results on the considered earthquake. In particular, the MR-Frame with TS-CYC 04 connections behaves better than the one with EEP-DB-CYC 03 connections for the Imperial Valley earthquake, while EEP- DB-CYC 03 connections lead to the best behaviour in case of Kobe and Northridge earthquakes. The MR-Frame with EEP-CYC 02 connections always exhibits the worst behaviour. However, as this is due to the limited plastic rotation supply provided by this connection, it is useful to remember that, during the experimental test, as the displacement amplitude increased, the plastic engagement of the end- plate at the welded flange-to-end plate connection zone increased, leading to the formation of a crack along the whole width of the end-plate starting from the heat affected zone which progressively propagated along the thickness up to the

complete fracture of the endplate (Iannone et al., 2011). Even though this failure mode is consistent with the design purposes of type-1 collapse mechanism for the end-plate in bending, it provides a reduction of the plastic rotation supply under cyclic loads.

Figure 3. Response spectra of considered ground motions and maximum spring rotation versus spectral acceleration

In Figure 4 the maximum interstorey drift ratio (MIDR) and the roof drift angle (RDA, ratio between top displacement and building height) versus spectral acceleration are reported for the Imperial Valley earthquake. The behaviour of the three structures are similar; the differences are due again to the different ultimate plastic rotation of the connections. In particular, the MR-Frames with TS-CYC 04 connections exhibits a more regular increase of considered parameters with spectral acceleration.

In Figure 5 and 6 the same parameters are reported for Kobe and Northridge earthquakes respectively. Also in these cases the main differences among the three structures are due to the different ultimate plastic rotations of the spring elements.

Connections in Steel Structures VII / Timisoara, Romania / May 30 - June 2, 2012 143 144 Connections in Steel Structures VII / Timisoara, Romania / May 30 - June 2, 2012

Figure 4. Maximum Interstorey Drift Ratio and Roof Drift Angle versus spectral acceleration for Imperial Valley earthquake

Figure 5. Maximum Interstorey Drift Ratio and Roof Drift Angle versus spectral acceleration for Kobe earthquake

Figure 6. Maximum Interstorey Drift Ratio and Roof Drift Angle versus spectral acceleration for Northridge earthquake

5. CONCLUSIONS

Nowadays semi-rigid partial-strength connections, if well designed, can be considered to have ductility and dissipation capacity in order to satisfy the seismic demand. Therefore, in this paper the influence of beam-to-column connections on the seismic response of MR-Frames has been studied.

Starting from the knowledge of the cyclic rotational behaviour of beam-to- column joints, three different MR-Frames have been considered. Each structure is characterized by a different structural detail of beam-to-column connections. The cyclic behaviour of each joint has been modelled by means of the spring element of IDARC 2D computer program with the polygonal hysteretic model, either vertex oriented or yielded oriented depending of the shape of the hysteresis loops, whose parameters have been calibrated on the base of available experimental tests obtaining a good agreement between experimental and modelled behaviour.

The observation of the results obtained from IDA analyses shows that the behaviour of the analysed MR-Frames with EEP-CYC 02 connections, i.e. bolted end- plate connections, and TS-CYC 04 connections, i.e. bolted double split tee connections, is very similar to the one of the MR-Frame with EEP-DB-CYC 03 connections, i.e. RBS connections. In particular, for the Imperial Valley earthquake record the behaviour of the structure with bolted double split tee connections (TS-CYC 04) is the best one. Conversely, RBS connections have led to the best behaviour in case of Northridge and Kobe earthquakes.

However, it has to be underlined that the results herein presented are just the preliminary results of an ongoing research activity, so that general conclusions cannot be still provided. Only three ground motions have been considered up-to-now, so that in next future further IDA analyses, considering many earthquakes and different MR-Frames, have to be performed. In addition, also different modelling options regarding the cyclic response of beam-to-column joints will be investigated by means of other software with the aim of analysing the influence of this kind of epistemic uncertainty.

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INFLUENCE OF STEEL-TO-CONCRETE CONNECTION