The paper discusses the structural behaviour of concrete-filled steel tube columns (CFT) when applied to the top-down construction method as temporary internal supports for ceilings. Their ultimate capacity to take into account the actual boundary conditions of the column is essential for successful application in construction. The paper presents the full-scale in situ testing of four slender specimens with variable D/t ratios under concentric axial loading. The CFT columns were supported on the previously jacked concrete piles. In addition, detailed finite element numerical models in ABAQUS and PLAXIS computer programs were developed. The models include the nonlinear behaviour of materials and the nonlinear behaviour of soil. The soil–pile–column interaction and impact of the CFT column–pile connection stiffness on global column stability were considered. The numerical model was validated by comparison with the experimental results. In conclusion, the coefficient for the effective buckling length of the studied columns is proposed. Finally, the experimental results of the critical buckling forces were compared with widely used international design codes Eurocode 4-EC4, American standard-ACI and the Australian standard-AS.

Professional paperCFT columns are one of the basic structural elements in "top-down" construction method. Because of the large depth of excavation the question of stability of these columns raises. This paper presents a procedure for calculation of critical buckling force of CFT columns which are on the upper side elastically fixed in the concrete ceiling and on the bottom side elastically fixed in the drilled pile. Using computer programs ABAQUS 6.12 and LPILE, as well as appropriate constitutive models the boundary conditions on the upper and lower end of the CFT columns are determined. For a building called "Cvijetni prolaz" in Zagreb, using program created in Excel the critical buckling force of elastically fixed CFT columns is determined. The dependence of the critical buckling force decreasing with increasing depth of excavation is presented. The impact of concrete class, diameter and wall thickness of the steel section, length and diameter of the pile, the thickness of the ceiling on the critical buckling force of CFT columns is analysed. The results are shown in diagrams and tables. Keywords: CFT column; FEM; p-y curve; stability of bifurcation Nosivost i stabilnost elastično uklještenih CFT vitkih stupovaStručni članak CFT stupovi su jedan od osnovnih konstrukcijskih elemenata u "top-down" metodi gradnje ("odozgor prema dolje"). Kako su dubine iskopa velike postavlje se pitanje stabilnosti ovih stupova. U radu je dan prijedlog postupka proračuna kritične sile izvijanja CFT stupa koji je s gornje strane elastično uklješten u izvedeni strop, a s donje strane elastično uklješten u bušeni pilon. Primjenom programa LPILE i ABAQUS 06.12, kao i odgovarajućih konstitutivnih modela određeni su granični uvjeti na gornjem, odnosno donjem kraju CFT stupa. Za objekt Cvjetni prolaz u Zagrebu primjenom programa modeliranog u Excell-u izračunata je kritična sila izvijanja obostrano elastično uklještenog CFT stupa. Prikazana je ovisnost opadanja kritične sile izvijanja s porastom dubine iskopa. Analiziran je utjecaj klase betona, promjera i debljine zida čeličnog profila, dužina i promjer šipa, debljina stropa na vrijednost kritične sile izvijanja CFT stupa. Dobiveni rezultati prikazani su u dijagramima i tablicama.

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Stubovi od kružnih šupljih čeličnih profila ispunjenih betonom, u daljem tekstu CFT stubovi (eng. Concrete Filled Tubes), zbog svojih konstruktivnih prednosti, našli su široku primenu u savremenoj inženjerskoj praksi. Proračun nosivosti i stabilnosti CFT stubova zasniva se na proračunu prema graničnim stanjima. Određivanje granične nosivosti CFT stubova složeno je zbog nelinearnih karakteristika betona i čelika, imperfekcije vezane za geometriju i oblik nosača, zaostalih napona u čeličnom profilu, istorije opterećenja, ekscentričnosti opterećenja, uticaja drugog reda i tako dalje. Kod umereno vitkih i vitkih CFT stubova, gubitak nosivosti zasniva se na problemu stabilnosti. U tom slučaju moraju se uzeti u obzir uticaji drugog reda. Tačnost rešenja, pre svega, zavisi od izbora nelinearnog konstitutivnog modela za materijal, kao i od načina definisanja graničnih uslova oslanjanja CFT stuba. U radu je prikazan program sopstvenog eksperimentalnog ispitivanja nosivosti i stabilnosti umereno vitkih i vitkih CFT kružnih stubova određenih geometrijskih i materijalnih karakteristika. Primenom računarskog programa ABAQUS detaljno je prikazano numeričko modeliranje nosivosti CFT kružnih stubova, pri čemu je uzeta u obzir Marija Lazović, mast. inž. građ.

The paper presents retaining structure to stabilize the landslide in the middle section from 878+775 to km 878+825, and slope protection on both left and right sides in order to provide vertical cutting and excavation to the level of highway platform. Retaining structure was designed as micro-pile wall with three different types of walls depending on the pile position. Micro-pile is made from steel pipe dia. Ø139.6 mm and 8.8mm thick wall, which is filled with cement mortar of minimum strength 30 MPa. In a constructive view, the designed retaining structure is a plate that is resting on the continual tie beams, which are supported in discrete points by geotechnical anchors. Calculation of stability and bearing capacity of protective structure was carried out in several phases. Phases are modeled according to the technology of execution of works. The calculation was performed in FEM program called Plaxis 2D. This calculation is based on the finite element method, using an incremental, iterative process. Based on the performed analysis, calculation of stability and bearing capacity, it can beconcluded that designed support structure satisfies all the necessary conditions of stability and bearing capacity.