Potresna odpornost varjenih spojev prečka-steber v hibridnih pomičnih in zavetrovanih okvirih : doktorska disertacija

Čermelj, Blaž (author), Beg, Darko (mentor)

V nalogi je bila, na osnovi eksperimentalnih cikličnih testov, opravljena študija potresne odpornosti varjenih polnonosilnih spojev prečka-steber ojačanih z rebroma in veznima pločevinama. Pri tem so bili uporabljeni različni protokoli obremenjevanja. Za dosego večje robustnosti koncepta močan steber/šibka prečka so bili uporabljeni sovprežni stebri. Upoštevana je hibridna zasnova momentnih okvirov, pri čemer je jeklo visoke trdnosti uporabljeno v stebrih, nedisipativnih elementih, mehko konstrukcijsko jeklo pa v disipativnih prečkah. Dodatni parameter študije je bil visok nivo osne sile v stebru. Vsi preizkušanci so izkazali obnašanje skladno s konceptom načrtovane nosilnosti: varovan neduktilen varjeni stik ob pasnici stebra je ostal nepoškodovan, vse plastične deformacije so bile skoncentrirane v prečki za ojačanim delom. Vsi spoji testirani s spremenljivo amplitudo rotacije so zadostili prekvalifikacijskim pogojem iz EN 1998-1 in ANSI/AISC 341-10. Rezultati testov skupaj z eksperimentalnimi podatki iz literature so služili za verifikacijo numeričnega modela, ki je bil uporabljen v nadaljni parametrični študiji obeh konfiguracij ojačanih spojev pri monotoni in ciklični obremenitvi. Pomemben del študije obsega razvoj krivulje poškodb za analizo nizkocikličnega utrujanja spojev. Namen numerične parametrične študije je podkrepitev predlaganih priporočil za detajliranje in dimenzioniranje obeh obravnavanih tipov ojačanih spojev; predvsem v smislu zajema morebitnih neugodnih vplivov iz naslova različnih velikosti in geometrije I in H prečnih prerezov prečk, ki se pogosto uporabljajo v Evropi. Vsi analizirani polnonosilni spoji izpostavljeni monotoni in ciklični obremenitvi so izkazali polno plastično kapaciteto priključene prečke. Parametrična študija je potrdila rezultate testov: spoj z veznima pločevinama se obnaša bolje kot spoj ojačan z rebroma; tlačna osna sila v stebru do vrednosti 60 % njegove tlačne nosilnosti ne vpliva na obnašanje varjenega stika prečkasteber; za analiziran nabor prerezov prečk predlagan koncept dimenzioniranja daje povsem sprejemljive dimenzije ojačilnih pločevin. Študija predstavlja pomemben doprinos tudi v smislu mejne upogibne nosilnosti jeklenih prečk in potrjuje potrebo po uporabi dvojne klasifikacije jeklenih elementov pri potresno odpornem projektiranju: glede na njihovo duktilnost in dodatno upogibno nosilnost. Pri tem je bila posebej obravnavana večja možna nosilnost prečke zaradi pričakovane vrednosti tečenja materiala γov in dodatna nosilnost s zaradi plastičnega utrjevanja jekla, kar še vedno ostaja nejasno obravnavano v EN 1998-1. Največja vrednost skupnega faktorja dodatne nosilnosti v skladu z EN 1998-1 znaša 1.1× γov = 1.375. V študiji je jasno pokazano, da ta vrednost v določenih primerih ne zagotavlja dovolj velike dodatne nosilnosti spoja za razvoj polnoplastične kapacitete priključene prečke. Predlagani vrednosti γov = 1.25 in s = 1.2 dajeta vrednost skupnega faktorja dodatne nosilnosti γov' = s·γov = 1.5, kar se je izkazalo zadovoljivo.

• Language: English
• Keywords: civil engineering, thesys, full strength beam-to-column joint, stiffened welded connection, composite column, low-cycle fatigue, plastic rotation, overstrength, ductility, EN 1998-1
• Work type: Dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FGG - Faculty of Civil and Geodetic Engineering
• Place of publishing: Ljubljana
• Publisher: [B. Čermelj]
• Year: 2015
• Number of pages: XXXIII, 296 str., pril.
• PID: 20.500.12556/RUL-32526
• UDC: 624.014.2: 624.21(043)
• COBISS.SI-ID: 6954081
• Publication date in RUL: 10.07.2015

Secondary language

• Language: Slovenian
• Title: Seismic resistance of welded beam-to-column connections in hybrid moment and braced frames

Abstract

Cyclic tests with different loading protocols were carried out on full-strength welded ribstiffened and cover-plate beam-to-column joints to study their seismic performance. Composite columns were used to increase the robustness of the applied strong column/weak beam design concept. Hybrid-steel approach for moment-resisting and braced frames was implemented with high strength steel used for columns as non-dissipative elastic members and mild carbon steel for beams acting as dissipative members. Additional parameter of the study was axial force of high level compression in the column. All rib-stiffened and coverplate beam-to-column joints fulfilled the objective of the applied capacity design concept: the whole inelastic action was transferred away from the face of the beam-to-column connection into the beam section behind the stiffened region. All the joints subjected to variable amplitude loading met prequalification requirements from EN 1998-1 and ANSI/AISC 341-10. The test results with additional experimental data from literature were used to verify numerical model, which was employed for further parametric studies on both types of stiffened connection subjected to monotonic and cyclic loading. An important part of the study was also dedicated to the development of damage curve for low-cycle fatigue resistance evaluation of joints. Numerical parametric study was performed to support the proposed joint detailing and design guidelines for the two stiffened joint types against possibly adverse beam member size effects within the selected range of practically applicable I and H beam profiles typically used in Europe. All designed stiffened beam-to-column joints developed full plastic capacity of the connected beam under monotonic and cyclic loading. Parametric study confirmed the outcomes from experimental tests: cover-plate is superior to rib-stiffened connection; column axial compressive load up to 60 % of its capacity should not undermine welded beam-to-column connection; the applied joint design concept results in reasonable stiffening plate dimensions for the considered range of beam profiles. The study presents an important contribution also in terms of flexural ultimate behaviour of steel beams and confirms the need for a twofold classification of steel members according to their ductility and overstrength as the most appropriate approach for seismic design applications. Clear distinction is made between overstrength due to random material variability γov and overstrength due to strain-hardening s, which is still not clearly addressed in the current European seismic design code. Maximum value of the complete overstrength factor 1.1× γov = 1.375 as exists in EN 1998-1 was shown to be too small to guarantee a sufficient overstrength of a connection in order to assure the development of the full plastic capacity of the connected beam. Proposed values for both γov = 1.25 and s = 1.2 result in overall overstrength factor γov' = s·γov = 1.5, which proved to be satisfactory.

• Keywords: gradbeništvo, disertacije, polnonosilni spoj prečka-steber, varjen ojačan spoj, sovprežen steber, nizkociklično utrujanje, plastična rotacija, dodatna nosilnost, duktilnost, EN 1998-1