Zveza društev gradbenih inženirjev in tehnikov slovenije
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Objavljeno v: Gradbeni vestnik - oktober 2020

Avtorji

prof. dr. Tatjana Isaković, univ. dipl. inž. grad.
Antonio Janevski, mag. grad.
doc. dr. Matija Gams, univ. dipl. inž. grad.
prof. dr. Matej Fischinger, univ. dipl. inž. grad.

EKSPERIMENTALNE RAZISKAVE INTERAKCIJE MED ARMIRANOBETONSKIMI PLOŠČAMI IN STENAMI NA POTRESNI MIZI

UDK: 624.012.45+624.042.7(047.31)

POVZETEK

Na potresni mizi smo v okviru projekta HORIZONT 2020 SERA-TA na preizkušancu v velikem merilu testirali interakcijo med armiranobetonskimi ploščami in armiranobetonskimi stenami pri različnih nivojih potresne obtežbe. Preizkusili smo trietažno konstrukcijo v merilu 1 : 2. Bila je sestavljena iz štirih sten pravokotnega prečnega prereza, ki so bile v višini etaž povezane z gladkimi armiranobetonskimi ploščami. Preizkušanec smo obremenjevali s potresno obtežbo, katere intenziteto smo postopno povečevali. Pri zadnjih treh testih so se plošče in stene vidno poškodovale.

Testi so pokazali, da sta v obravnavnem primeru upogibna nosilnost in togost plošč pomembno vplivali na mehanizem odziva konstrukcije. Po končanih testih so bile vse plošče vidno poškodovane, še zlasti v prvih dveh etažah, kjer je prišlo do plastifikacije zgornje in spodnje armature. Razpoke so se razširile po celi širini plošč. Poškodbe plošč so bile pretežno upogibne. Sodelujoča širina plošč je bila približno enaka celotni širini plošč. Zato je bila upogibna kapaciteta plastičnega členka, ki se je ustvaril v plošči na stiku s stenami, dokaj velika. Posledično so bile prečne sile v plošči dokaj velike, kar je povzročilo tudi velike spremembe osnih sil v stenah.

Potresni odziv preizkušene konstrukcije bi lahko glede na določila standarda Evrokod 8 [SIST, 2005] računali kot odziv štirih konzolnih sten, saj običajno ne pričakujemo, da bodo plošče brez gred stene med sabo močno povezale v okvir. Vendar se je izkazalo, da sta bili zaradi razmeroma velike togosti plošč in razmeroma velike kapacitete plastičnega členka v ploščah leva in desna stena v posamezni osi razmeroma močno povezani. Posledično se je ustvaril dokaj močen okvir, sestavljen  iz sten in plošč, ki je prevzel velik del (približno 50 %) prevrnitvenega momenta, ki ga je v konstrukciji povzročil potresni vpliv. To je potrdilo posamezne indice iz literature, da lahko že samo plošče brez gred slope stene močno upogibno povežejo (ne le v horizontalni ravnini).

Glede na razmeroma močno povezanost sten lahko rečemo, da je bil preizkušanec sestavljen iz dveh povezanih sten, sestavljenih iz dveh slopov sten in ene vrste odprtin. Pri močnejšem potresnem vzbujanju je bil v enem časovnem koraku v obeh stenah en slop obremenjen z močnimi nateznimi osnimi silami. Istočasno je bil drugi slop na nasprotni strani odprtine močno tlačno obremenjen. Zibanje natezno obremenjenega slopa okoli notranjega vogala (na strani odprtine) je bilo vidno s prostim očesom. Na zunanjem vogalu tlačno obremenjenega slopa se je navpična armatura uklonila. Razpoke v posameznih slopih sten so bile nesimetrične in bistveno drugače razporejene od križno razporejenih razpok, ki so tipične za konzolne stene. Pri zadnjem testu je bil dosežen povprečni 1,1% zasuk sten. Nihajni čas osnovne nihajne oblike preizkušanca je od začetnih 0,14 s narasel na 0,32 s.

Odziv je bil drugačen, kot ga običajno ocenimo z elastičnimi linearnimi postopki analize, in kaže na potrebo po reviziji postopkov projektiranja.

Ključne besede: armiranobetonske stene, eksperiment v velikem merilu, potresna miza, interakcija med ploščo in stenami

Brezplačen elektronski izvod revije:
Celoten članek si preberite v elektronski različici revije Gradbeni vestnik v PDF obliki.

SHAKE TABLE EXPERIMENT OF RC SLAB-TO-WALL PIERS INTERACTION

SUMMARY

A shake table test of a half-scale three-story specimen subjected to different intensity levels was conducted to study the slab-to-wall piers interaction within HORIZON 2020 SERA-TA project. The specimen consisted of four rectangular walls linked by three slabs. It was subjected to a series of seismic excitations of increasing intensity. In the last three tests the non-linear response of the slabs and wall piers was observed.

Flexural strength and stiffness of slabs considerably influenced the response mechanism in wall piers. All slabs were fully activated. They were significantly damaged, particularly in the first two floors, where the yielding of the top and bottom reinforcements was observed. Cracks were observed at their top and bottom surfaces over the entire width of the slabs. Their damage was predominantly flexural. The effective width of the slabs was equal to their total width. Therefore, the flexural capacity of the plastic hinge formed in the slabs near the joints with walls was considerable. The shear forces in the slabs were consequently quite large, resulting in considerable variations of axial forces in walls.

According to Eurocode 8 [SIST, 2005], the seismic response of the tested structure can be estimated by means of the numerical model consisting of four cantilever walls under the assumption that slabs with no beams cannot link the walls into the frame. However, due to the considerable stiffness and considerable flexural capacity of plastic hinges in slabs, the left and right walls in the same axis were considerably coupled. Consequently, walls and slabs formed quite strong frame, which resisted considerable shear of the overturning moment (50%) due to the seismic excitation. This confirmed some indications from the literature that the slabs alone (without beams) can provide considerable coupling (flexural) of wall piers.

In fact, the specimen was made of two coupled walls, consisting of two wall piers and openings between them. When the specimen was subjected to stronger seismic excitations, in both walls one pier was subjected to large tensile axial forces. At the same time, the pier at the opposite side of the opening was subjected to strong compression forces. The rocking of the wall pier subjected to tension around inner corners (near the openings) was visible to the naked eye. In the wall pier subjected to compression the buckling of the longitudinal bars was observed at the outer edge of the wall. The crack pattern in the wall piers was asymmetric and different from the cross-shaped damage pattern typical for cantilever walls. The maximum drift of the wall piers of 1.1 % was observed within the last test. The fundamental period of vibration of the specimen was increased from the initial 0.14 sec to 0.32 sec.

The response of the specimen was considerably different from that which is typically obtained by linear elastic analyses of such structures.

Key words: Reinforced concrete walls, Large-scale experiments, Shaking table, Slab-to-wall interaction

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