The present study explores the structural properties of regular convex polygonal cross section (RCPS). Such profiles are commonly used to construct masts and towers, e.g.  transmission towers, light poles etc. The aim is to provide a better understanding of how the RCPS profiles behave on the local level under meridional compression.

For that matter, the study is limited to short specimens to avoid interaction with flexural buckling. Depending on the geometric and material characteristics, local buckling of an RCPS can either be contained within the width of individual facets, with the polygon edges acting as simple supports, or overcome the edge barriers of a facet extending to its neighbouring ones (spillover), which is best described as a type of shell buckling behaviour. Distinguishing between these two cases is the main task of this study.

The investigation is performed by means of numerical simulations and laboratory experiments, in both cases paying due attention to the influence of initial geometric imperfections. Four different imperfection patterns are identified by observing the collapse mechanisms occurred on experiments.

Through Finite Element Modelling (FEM), the influence of the four imperfection types as well as of the yield strength are studied over a wide range of RCPS models with varying number of vertices and local slenderness. Based on the minimum FEM obtained resistances, the RCPSs are categorised into two groups, one where a plate-like (facet contained) buckling is more critical than the spillover and vice versa.

The suitability of the Eurocode parts corresponding to plated and shell structures (EN~1993-1-5 and EN~1993-1-6 respectively) is evaluated for the design of RCPS on the basis of the two identified regions. The comparison showed that the two codes in conjunction can cover the design of RCPS. A method expressed as a simple empirical formula is suggested for categorising RCPS and choosing the respective Eurocode part.

The results of the numerical simulations are verified through an experimental matrix consisting of nine High Strength Steel (HSS) specimens. The specimens are of 16, 20 and 24 vertices and range from class 1 to class 4. Precise measurements of the geometric initial imperfections are taken by 3D scanning and digital processing of the specimens.

Experimental results from the present study and from literature come in good agreement with the conclusions from the numerical simulations, validating the proposed model.

Profiler med slutna polygonala tvärsnitt är vanligt förekommande för konstruktioner som master och torn, t.ex. radiomaster och lyktstolpar. Denna avhandling fokuserar på hur profiler med polygonala tvärsnitt (i avhandlingen betecknade RCPS för regular convex polygonal sections) beter sig i brottgränstillstånd och vilka faktorer som påverkar detta beteende.

Det grundläggande syftet med arbetet har varit att få en bättre förståelse för hur RCPS-profiler påverkas av lokal buckling vid belastning i längdriktningen. För att kunna renodla fenomenet lokal buckling och undvika interaktion med global buckling har studien avgränsats till korta provkroppar. Lokal buckling av RCPS-profiler kan, beroende på geometri och materialegenskaper, begränsas till att uppstå inom bredden av tvärsnittets kanter (en kant är ett individuellt plåtfält) där de längsgående hörnen som avgränsar kanterna verkar som fria upplag (här kallad kantbegränsad buckling), eller så kan den lokala bucklingen spilla över till närliggande kanter (här kallad spillover), vilket närmast kan liknas vid ett skalbucklingsliknande beteende. En viktig del av syftet med denna studie har varit att kunna särskilja dessa två fall från varandra.

Undersökningen bygger på numeriska simuleringar och laboratorieförsök, där båda metoderna riktar extra fokus på inverkan av geometriska initialimperfektioner. Genom att observera kollapsmekanismer vid experimentella försök kunde de identifierade initialimperfektionerna grupperas i fyra olika elementarfall.

Finita Element Metoden (FEM) har använts för att studera hur initialimperfektioner enligt de fyra olika elementarfallen påverkar bärförmågan för lokal buckling för RCPS-profiler. Dessa FE-analyser har omfattat en stor mängd RPCS-profiler med varierande sträckgräns, tvärsnittsutformning (antal kanter) och slankhet. Resultatet av FE-analyserna visar att de varierade parametrarna påverkar vilken typ av lokal buckling, kantbegränsad buckling eller spillover, som är avgörande för en RCPS-profil.

RCPS-profilernas bärförmåga enligt FE-analyserna har jämförts med den bärförmåga som ges av dimensioneringsreglerna i EN 1993-1-5 (Plåtbalkar) och EN 1993-1-6 (Skal). Denna jämförelse visar på en god överensstämmelse mellan resultaten från FEM och den dimensionerande bärförmågan enligt de två eurokoderna, förutsatt att rätt typ av lokal buckling antas. Baserat på denna jämförelse föreslås en metod i form av ett enkelt empiriskt uttryck för att kategorisera ett specifikt RCPS-tvärsnitt med avseende på vilken av de två eurokoderna som bör tillämpas för beräkning av bärförmågan.

Resultaten av de numeriska beräkningarna verifierades genom en experimentell studie omfattande nio provkroppar av höghållfast stål (HSS). Provkropparna var utformade med tvärsnitt med 16, 20 respektive 24 enskilda fasetter och med slankhet varierande från tvärsnittsklass 1 till 4. Innan provkropparnas belastades gjordes precisa mätningar av de geometriska initialimperfektionerna med hjälp av 3D-skanning och digital bearbetning.

De experimentella resultaten från denna studie och från litteraturundersökningar överensstämmer väl med slutsatserna från de numeriska simuleringarna och validerar därmed den föreslagna metoden.

The design, laboratory investigation and main results of an experimental programme on the joints of an innovative 3D truss made of high strength steel (HSS) is presented. An entire truss of triangular cross-section was fabricated (two compression chords and a single tension chord) to provide realistic loading conditions. Both compression and tension chords were fabricated by cold forming hot-rolled HSS plates by means of press braking. The chords were as follows:

• — an open angle-type profile with 45° contained angle as tension chord (U-chord),
• — a semi-closed octagonal profile made of 3 sectors bolted along their length every 1.2 diameters as compression chords (P-chord).

The focus of this study is on the tension chords joints where a pair of tension and a pair of compression diagonals converge. Joints were tested to ultimate load by introducing shear to one bay of the truss at a time. Two design approaches for the joints were tested resulting in four individual tests.

Ultimate load levels, stiffness and failure modes varied between the considered joint configurations. Stiffening increased the ultimate load by as much as 170% of that of the unstiffened version achieving higher utilisation of the tension chord, thus making more efficient use of HSS. The stiffness loss at loads close to failure varied between 60% – 80%. Two distinctive failure modes were observed, one caused by cracking of the welds connecting the diagonal's gusset plates to the tension chord while the second by cross-section tensile fracture of the circular hollow section (CHS) diagonal.

The on-going RUOSTE project aims to improve understanding of HSS by means of tests and FEA, addressing issues of ductility and stability of structures made of HSS. Various material models used in FEA are verified by tests. This paper presents calibration and verifica-tion of ductile damage material model in Abaqus FE software package referring to series of tensile test experiments on coupons and plate specimens with a single circular hole. Nominal steel grade S700MC and S960Q are used. Damage initiation criterion and evolution law are derived analysing localization of plasticity by FEA. Quasi-static analysis using explicit dy-namic solver is chosen in order to create the most realistic FEA of the specimens.

A novel type of connection, referred to as the “crocodile nose” (CN) connection for circular hollow sections (CHS) is investigated in the framework High Strength Long Span Structures (HILONG) project. This connection provides an aesthetically improved alternative to the commonly used slotted-end CHS connection. The end of the CHS member is doubly bevelled and a pair of inflected plates, welded on the edges of the cuts, undertakes the load transfer. This investigation focuses on two parameters, the bevelling angle and the influence of a stiffener connecting the inflected plates. The program is completed through FE calcula-tions and laboratory tests. The design of the test specimens, preliminary FEM and test results are presented in this paper.

The work carried out at LTU as a part of preparation for the experimental program of the so called Crocodile Nose (CN) connection, is presented. This work is performed in the framework of the RFCS-funded project "High Strength Steel Long Span Structures" (HILONG) that focuses on the development of applications of higher-strength steel grades for the longer spans. The advantage of this connection is in avoiding the classical abrupt right-angle termination of the CHS which, along with the protruding gusset plate, results in what is regarded by many architects as aesthetically unappealing joint. In a CN connection, the CHS member end is obliquely bevelled along two planes symmetric to its axis. On the lips of the resulting semi-elliptic cut-offs, appropriately cut cover plates are fillet-welded; past the tip of the member these are inflected and extend in parallel to the member axis forming a gap wherein a gusset plate is inserted. The inflected plates are bolted to the gusset thus securing the connection of the CHS to the other side without any eccentricity. Preliminary FE results of the crocodile nose experiments are shown.