The effect of moment-rotation joint behaviour on the displacements of portal frames

Albertyn, Heindrich Louw (2011-12)

Thesis (MScEng)--Stellenbosch University, 2011.

Thesis

ENGLISH ABSTRACT: Higher grade steels are being rolled in South Africa by suppliers and results in structural members having an increased axial and bending moment capacity due to an increased yield stress. Structural elements used in designs are stronger and therefore lighter sections with sufficient axial and bending moment capacity are used. Displacements of structural elements are calculated using the stiffness and Young’s modulus of a profile. These values are not affected by the increased yield stress in higher steel grades and therefore have a negative effect on the displacements of the structure. The potential of these higher grade structural elements are not utilized through serviceability limit state criteria, since the displacement determination does not account for the increased capacities of higher grade steels, but only stiffness and elasticity of the members. Structural analysis of portal frames does not account for the real behaviour of steel connections and column bases. It is assumed that connections and bases are either fully rigid or perfectly pinned. This assumption is used in the analysis and design of the structure. Although it is assumed that connections and bases are either rigid or pinned, the real behaviour is in between these two extremes. Rigid connections exhibit a certain flexibility under loading whereas pinned bases provide a certain restraint under loading. The real behaviour of connections and bases are referred to as the moment-rotation behaviour of the connection. For a certain applied moment to the connection or base, the connection exhibits a certain rotation. The focus of this study is placed on the accuracy and feasibility of modelling the real behaviour of connections and bases in a structural analysis of a portal frame. A connection stiffness is determined from the connection’s moment-rotation behaviour, and is assigned to a rotational spring of zero length in a structural analysis. An experimental investigation was conducted to obtain the real displacement data of a portal frame subject to loads for two different support conditions, i.e. a perfect hinge and grouted-support. A perfect hinge support was used to isolate the moment-rotation response of the ridge and eaves connection. The experimental results were used to compared to the results obtained from a structural analysis to determine the accuracy of the numerical results. A real design case was investigated with load combinations imposed on the frame in accordance with SANS 10160:2011. Three methods of modelling connections and bases in an analysis were considered. Firstly modelling connections as rigid and bases as pinned, secondly modelling connections as linear rotational springs and bases as pinned. Lastly was to model connections as linear rotational springs and bases as non-linear rotational springs. The outcome of the research was that more accurate displacements of a portal frame could be obtained by modelling the real behaviour of rigid connections as rotational springs, but this is not the case with grouted column bases. It is thus not feasible to model the real behaviour of connections and bases in a structural analysis as the current method of modelling connections as rigid and bases as pinned provides reliable and accurate displacement results.

AFRIKAANSE OPSOMMING: Hoë graad staal word tans in Suid Afrika gerol deur verskaffers en lei daartoe dat strukturele elemente oor ’n groter aksiale- en buigmomentkapasiteit het as gevolg van ’n groter vloeispanning. Strukturele elemente in ontwerpe is sterker en gevolglik het ligter elemente die benodigde aksiale- en buigmoment-kapasiteit. Verplasings van strukturele elemente word bepaal vanaf die styfheid en Young modulus van die element. Hierdie waardes word nie beïnvloed deur die groter vloeispanning van hoë graad staal nie, en het dus ’n negatiewe uitwerking op die verplasings van die struktuur. Die potensiaal van die gebruik van hoë graad staal word nie benut in die geval van voldoening aan diensbaarheids kriterium nie, aangesien verplasings bepaal word vanaf die styfheid en elastisiteit van die elemente, en nie vloeispanning nie. Strukturele analise van portaalrame neem nie die ware gedrag van konneksies en kolomvoetstukke in ag nie. Die aanname word gemaak in analises en ontwerpe dat konneksies en voetstukke óf rigied óf geskarnierd is. Hierdie is slegs ’n aanname en in die werklikheid lê die ware gedrag van konneksies en voetstukke tussen hierdie grense. Rigiede konneksies toon ’n sekere buigbaarheid tydens belasting en geskarnierde voetstukke toon ’n sekere beperking teen rotasies. Die ware gedrag van konneksies en voetstukke word gedefinieer as moment-rotasie gedrag. Vir ’n spesifieke aangewende moment, ondergaan die konneksie of voetstuk ’n sekere rotasie. Hierdie studie fokus op die akkuraatheid en uitvoerbaarheid van die modellering van die ware gedrag van konneksies en voetstukke in ’n strukturele analise van portaalrame. Die styfheid van ’n konneksie word bepaal vanaf sy unieke moment-rotasie gedrag, en word ingevoer as ’n styfheid van ’n rotasieveer in ’n strukturele analise. ’n Eksperimentele ondersoek was gedoen om verplasingswaardes van ’n portaalraam onder belastings te bepaal. Twee ondersteunings is ondersoek in die eksperimentele program, naamlik ’n geskarnierde ondersteuning asook ’n breivul ondersteuning. Die gebruik van die geskarnierde ondersteuning isoleer die moment-rotasie gedrag van die nok en dakrand konneksies. Die eksperimentele resultate was gebruik om die akkuraatheid van die resultate vanaf die strukturele analise te ondersoek. Laastens was ’n ontwerpsprobleem ondersoek deur laskombinasies, soos bepaal volgens die riglyne van SANS 10160:2011, op ’n portaalraam aan te wend. Drie gevalle van modellering van konneksies in ’n strukturele analise is ondersoek. Eerstens om konneksies as rigied en voetstukke as geskarnierd te beskou. Tweedens was die konneksies as linieêre rotasievere gemodelleer en voetstukke as geskarnierd te beskou. Laastens was om konneksies as linieêre rotasievere te modeleer en voetstukke as nie-linieêre rotasievere. Die navorsing het getoon dat meer akkurate verplasings van portaalrame bepaal kan word deur rigiede konneksies te modelleer as rotasievere, maar dit is nie die geval met breivul ondersteunings nie. Die gevolg is dat die uitvoerbaarheid van die modellering van konneksies en voetstukke as rotasievere nie effektief is nie, aangesien die huidige metode van die modellering van konneksies as rigied en voetstukke as geskarnierd akkurate en betroubare resultate lewer.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/18046
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