B. Gorenc/D. Beg · Curtain wall façade system under lateral actions with regard to limit states
Fig. 4. MRF configuration used to define test protocols for wind and earthquake
(MRF) with three bays each spanning 6 m (Fig. 4) according to Eurocode standards [4], [5], [10], [11] and then analysing it using non-linear dynamic analysis based on three selected ground motion records. HEB sections were chosen for the structural members, with elastic plastic material law and a kinematic isotropic hardening and characteristic minimal yield strength of 355 N/mm2. The characteristic used in all test protocols was the relative storey drift ratio 6 (Fig. 4). Rate of drift application was 0.2 mm/s for monotonic and 4 mm/s for cyclic tests. The monotonic rate was intended to capture response and detect possible failures of the specimen in in-plane shear through instruments and observation. The cyclic rate was considered to be similar to the influence of gust or earthquake in order to avoid the influence of the viscoelasticity of the materials within the specimen and to be executed within a reasonable amount of time. The rates were confirmed through the T1 and T2 tests. The stiffness of the sample through two different types of test remained the same until very high drift ratios.
3.1 Defining experimental test protocols for exposure to wind actions The raw data for the maximum average wind speed over 10 min and 3 s (gust) intervals were obtained from the national Agency of the Republic of Slovenia for Environment (ARSO). It confirmed that the base wind speed vb,0 in the region of Slovenia, defined in the national annex [10], was gener-
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Steel Construction 9 (2016), No. 1
ally higher than the statistical vb,0 from 15 years of data collected since 1997. Considering a characteristic combination, there is a 2 % probability of wind pressure at a level of SLS or higher each year, which would occur at least once in 50 years of the design life. This would cause a maximum structural relative storey drift 6SLS to be limited with 1/300 of the storey height according to Eq. 6.14b [11] and the values from Table A.1.4 [11]. However, from the frequent combination, the probability would be higher and would account for about 50 occurrences in the design life, but at a lower 6SLS. To be on the safe side, the benchmark amplitude taken was that of the characteristic 6SLS translated into deflection on the zaxis (Fig. 1), uSLS " t8.5 mm with 10 times the number of cycles (500) at the frequent combination. Since for ULS the characteristic combination is factored with 1.5 in unfavourable conditions, an additional 20 cycles were performed for uULS " t12.0 mm. The tests using these protocols were designated T2-1, T2-2 and T2-3 (see Table 1).
3.2 Defining experimental test protocols for exposure to earthquake actions Testing protocols for earthquake actions vary, depending on the materials used [12]. To construct protocols, the non-linear dynamic analysis procedure was performed on an MRF building model (Fig. 4) using SAP 2000 software [13]. The target spectrum used for peak ground acceleration was ag,475 " 0.25 g (g " gravitational acceleration), with a return period TR,ULS "
475 years [14]. A weaker ag,SLS " 0.13 g with approx. TR,SLS " 50 years was chosen for damage limitation, and a stronger ag,1.3ULS " 0.33 g with approx. TR " 1300 years for the near-collapse situation, designated by SLS and 1.3 ULS respectively. Combined average response spectra with 5 % damping ratio, see section 3.2.3.1.2.(1) [5], were developed from 22 recorded accelerations taken from the European Strong Motion Database [15]. Records were sized to comply with the requirements of section 3.2.3.1.2(4) [5] for three different ground accelerations using a procedure similar to [16]. Out of 22 records analysed, three were selected as representative, i.e. 333X, 333Y (Korinthos-OTE Building) and 1230Y (Iznik-Karayollari Sefigi Muracaati) (Fig. 5). The highest average drift ratio 6 was obtained in the 3rd storey. The spectrum that became the basis for the protocol, designated by MRF10-acc, was constructed (Fig. 6a) from these responses by counting events when the storey peaked at an interval of )6I " t0.002 starting from 61 " t0.001. Within the spectrum ground accelerations were grouped together based on peak ground acceleration ranged from weakest to strongest, with ag,SLS first and ag,1.3ULS last. Within one group, the least intense protocol was the first, followed by the most intense and the average last. Only one (1230Y) was used for ag,1.3ULS as it is less likely to occur more than once within the design life of the structure. After every group there was a short pause to check for possible damage. Two tests were carried out: the test on QAir1 was designated T3 and that on QAir2 as T4 (Table 1).