An investigation into the structural behaviour of a novel cellular beam structure in fire

Kloos, Michael (2017-12)

Thesis (MEng)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: The Southern African Institute of Steel Construction (SAISC) has developed a novel cellular beam structure (CBS), which has been specifically designed for the construction of two to ten storey office blocks. The concept is based on the factory production of modules, which can be transported to site and quickly erected, thereby reducing construction time and cost. The modules consist of cellular steel beams and a ceiling board system, thus avoiding any wet trade, and remaining lightweight. The modularity allows for architectural freedom, as multiple configurations are possible. Furthermore, modules can later be added, or even re-used in another structure. However, the primary challenge hindering the commercialisation of this system is its unknown fire resistance. In South Africa, and worldwide, all structures require a fire rating, and thus an investigation into the fire resistance of the CBS has been carried out in this thesis to assist in bringing the system to production. As part of a larger project, this thesis focuses on the structural behaviour of the CBS under fire conditions, with an emphasis on numerical modelling. With the unconventional layout of the structure, standard fire design methods do not necessarily apply, or conversely could result in an over-conservative and costly specification of passive fire protection. Thus, a rational/performance-based approach has been developed, in which non-linear finite element (FE) models developed in Abaqus are used to characterise the behaviour of the CBS. Non-linear FE modelling procedures are required to model the CBS, as the material properties, geometry, structural loads and temperatures change over the course of a fire. The models developed have been validated based on three case studies in the literature. The CBS is then investigated using a total of 18 single element models consisting of isolated elements, and 9 global structure models. Three different time-temperature fire scenarios are considered: (a) a standard fire with the fire-rated ceiling remaining in place, (b) a standard fire with the ceiling failing, leading to significantly hotter beam temperatures, and (c) a parametric fire. This allowed the CBS to be tested under a variety of boundary conditions, thermal loads and possible fire scenarios. Overall, the models indicate that the CBS performs well under fire conditions. The structure is able to deffect and expand as the steel heats up, which reduces the internal forces. An ultimate failure mode, which could cause a collapse, is only detected under conservative conditions that are unlikely to occur. The models developed are used to predict the maximum vertical and lateral defections of the steel members. The vertical deflections were found to be relatively small in terms of typical fire deflections, with a maximum predicted value of 35 mm (span/229) under the standard fire with the ceiling in place (the primary design scenario). However, a maximum lateral deflection of 185 mm is anticipated, which will require careful detailing considerations. Under parametric fire conditions, negligible permanent deformation is predicted by the global structure models once the steel returns to ambient temperature. Ultimately, design recommendations are made to increase the fire resistance. Firstly, steel end connections should be designed to provide negligible moment restraint, and to allow free thermal expansion using slotted bolt holes. Secondly, if the ceiling system is designed to accommodate the predicted deflections, the integrity of the entire system is protected and the chance of a structural failure is significantly reduced.

AFRIKAANSE OPSOMMING: Die Suider-Afrikaanse Instituut vir Staalkonstruksie (SAISC) het 'n nuwe sellul êre balkstruktuur (SBS) ontwikkel, wat spesifiek ontwerp is vir die bou van kantoorgeboue met twee tot tien verdiepings. Die konsep is gebaseer op die fabrieksproduksie van modules, wat vinnig opgerig kan word op die perseel, waardeur konstruksietyd en -koste verminder kan word. Die modules bestaan uit sellulêre staalbalke en 'n plafonbordstelsel, waardeur enige beton vermy word, en die struktuur lig bly. Die modulariteit maak voorsiening vir argitekturele vryheid, aangesien verskeie konfigurasies moontlik is. Verder kan modules later bygevoeg word, of selfs weer in 'n ander struktuur gebruik word. Die primêre uitdaging wat die kommersialisering van hierdie stelsel belemmer, is egter die onbekende brandweerstand. In Suid-Afrika en wêreldwyd vereis alle strukture 'n brandgradering, en daarom is 'n ondersoek van die brandbestandheid van die SBS in hierdie tesis gedoen, om hierdie stelsel tot produksie te bring. As deel van 'n groter projek, fokus hierdie tesis op die strukturele gedrag van die SBS onder brandtoestande, met die klem op numeriese modellering. Uitgesien die onkonvensionele uitleg van die struktuur, is standaard brandontwerpmetodes nie noodwendig van toepassing nie, aangesien dit 'n oor-konserwatiewe en duur spesifikasie van passiewe brandbeskerming kan veroorsaak. Dus, is 'n rasionele/prestasiegebaseerde benadering ontwikkel, waarin nie-lineêre eindige element modelle wat in Abaqus ontwikkel is, gebruik word om die gedrag van die SBS te karakteriseer. Sulke modelleringsprosedures word benodig, aangesien die materiaal eienskappe, meetkunde, strukturele laste en temperature verander deur die loop van 'n brand. Die ontwikkelde modelle is bevestig op grond van drie gevallestudies in die literatuur. Die SBS word ondersoek deur gebruik te maak van 'n totaal van 18 enkel-element-modelle, wat bestaan uit geïsoleerde elemente, asook 9 globale-struktuurmodelle. Drie verskillende brandsituatsies word oorweeg: (a) 'n standaardvuur met die brand gegradeerde plafon in plek (b) 'n standaardvuur met die plafon wat misluk, wat lei tot aansienlik warmer temperature in die balke en (c) 'n parametriese vuur. Hierdeur is die SBS getoets onder 'n verskeidenheid van randvoorwaardes, termiese vragte en moontlike brandsituasies. Oor die algemeen, wys die modelle dat die SBS goed vaar onder brandtoestande. Die struktuur is in staat om te buig en uit te sit soos die staal verhit, wat die interne kragte verminder. 'n Strukturele vaaling, wat 'n ineenstorting kan veroorsaak, word slegs onder konserwatiewe toestande waargeneem wat onwaarskynlik sal voorkom. Die ontwikkelde modelle word gebruik om die maksimum vertikale en laterale defleksie van die staallede te voorspel. Die vertikale defleksies is relatief klein ten opsigte van tipiese branddefleksies, met 'n maksimum waarde van 35 mm (span/229) onder die standaardvuur met die plafon in plek (die primêre ontwerp situasie). 'n Maksimum laterale defleksie van 185 mm word voorspel, wat egter sorgvuldige detailering sal benodig. Onder die parametriese vuur, word weglaatbare permanente vervorming voorspel deur die globale struktuurmodelle nadat die staal terugkeer na omgewings temperatuur. Uiteindelik word ontwerpaanbevelings gemaak om die brandweerstand van die SBS te verhoog. Eerstens moet die staal-eindkonneksies ontwerp word om weglaatbare momentbeperking te bied, en om vrye termiese uitbreiding toe te laat deur middel van gleufboutgate. Tweedens, as die plafonstelsel ontwerp is om die voorspelde defleksies te akkommodeer, word die integriteit van die hele stelsel beskerm en die kans van 'n stukturele vaaling verminder.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/102683
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