Mechanical response of curved timber beams with variable height under fire conditions : doctoral thesis

Pečenko, Robert (author), Hozjan, Tomaž (mentor), Turk, Goran (comentor)

V doktorski disertaciji je obravnavano obnašanje ukrivljenega nosilca s spremenljivo višino izpostavljenega požaru. V ta namen je bil razvit nov tri-fazni numerični model za geometrijsko in materialno nelinearno požarno analizo. V prvi fazi je določen časovni razvoj temperatur znotraj požarnega sektorja. V drugi fazi, ki predstavlja glavno temo te disertacije, je bil razvit nov matematični in numerični model za določitev povezanega prehoda toplote in vlage po lesu med požarom. Model upošteva prenos vezane vode, vodne pare in zraka ob hkratnem prenosu toplote. Glavne novosti modela so naslednje: (i) prenos vezane vode je izpopolnjen in vključuje Soretov efekt, (ii) podan je modificiran zapis sorpcije, primeren tudi za temperature nad točko vrelišča, (iii) vpeljana je premikajoča se robna ploskev na stiku med nepoškodovanim lesom in zoglenelo plastjo, kjer sta predpisana tok vodne pare in pritisk. Sistem osnovnih enačb je rešen z metodo končnih elementov v programskem okolju Matlab®. Osnovne neznanke modela so: temperatura, tlak plinske mešanice ter koncentracija vodne pare in vezane vode v lesu. Z validacijo modela in ob upoštevanju parametričnih študij je ugotovljeno naslednje: (i) model omogoča natančno določitev temperaturnega polja in debeline zoglenele plasti, (ii) višja začetna vlažnost lesa vodi v počasnejši razvoj temperatur, (iii) odkrit je bil velik vpliv difuzije vezane vode glede na skupno vsebnost vlage lesa in (iv) vpliv konvekcijskega prenosa toplote na razvoj temperatur je zanemarljiv. V zadnji fazi je predstavljen mehanski model za določitev napetostno-deformacijskega stanja ukrivljenega nosilca in nosilca s spremenljivo višino med požarom. Model je formuliran na podlagi Reissnerjevega geometrijsko točnega modela ravninskega nosilca in zajema vpliv membranske, strižne in upogibne deformacije. Sistem osnovnih enačb je rešen z metodo končnih elementov. Z validacijo in verifikacijo modela je bilo ugotovljeno, da je model zelo natančen in kot tak primeren za požarno analizo omenjenih tipov nosilcev. Poleg tri-faznega numeričnega modela, disertacija prikazuje tudi metodologijo za določitev zanesljivosti lesenega nosilca med požarom, s čimer je prikazan performančni pristop za implementacijo negotovosti v požarno analizo.

• Language: English
• Keywords: building environment, civil engineering, thesis, timber beam, fire analysis, coupled heat and moisture transfer, sorption, char formation, reliability analysis
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FGG - Faculty of Civil and Geodetic Engineering
• Place of publishing: Ljubljana
• Publisher: [R. Pečenko]
• Year: 2016
• Number of pages: XXIV, 121 str.
• PID: 20.500.12556/RUL-81520
• COBISS.SI-ID: 7435105
• Publication date in RUL: 11.04.2016

PEČENKO, Robert, 2016, Mechanical response of curved timber beams with variable height under fire conditions : doctoral thesis [online]. Doctoral dissertation. Ljubljana : R. Pečenko. [Accessed 11 April 2025]. Retrieved from: https://repozitorij.uni-lj.si/IzpisGradiva.php?lang=eng&id=81520

Secondary language

• Language: Slovenian
• Title: Mehanski odziv ukrivljenih lesenih nosilcev s spremenljivo višino pri požarni obtežbi : doktorska disertacija

Abstract

The thesis discusses the behaviour of a curved and tapered timber beam exposed to fire. For this purpose a new three-phase numerical model for the geometrical and material non-linear fire analysis is developed. In the first phase, the development of the gas temperature with time, within the fire compartment is determined. New mathematical and numerical model to determine the coupled heat and moisture transfer in timber exposed to fire is introduced in the second phase. The model accounts for the transfer of bound water, water vapour and air, coupled with the heat transfer. The main novelties of the model are: (i) the bound water transfer is refined by including the Soret e�ect, (ii) a modified description of sorption applicable for the temperatures above the boiling point of water is given, (iii) a moving boundary surface to prescribe water vapour flux and pressure at the contact between the unburned timber and the char layer is introduced. The system of governing di�erential equations is solved by the finite element method developed in the Matlab® environment. The basic unknowns of the model are: the temperature, the gas pressure, the water vapour and the bound water concentration. After validating the model and conducting parametric studies, the following conclusions can be given: (i) the model accurately predicts both the temperature field and the charring depth, (ii) the higher initial moisture content results in a slower development of temperatures, (iii) a significant e�ect of bound water di�usion on the total moisture content is shown and (iv) the influence of the convective heat transfer on the temperature distribution is negligible. In the third phase, a mechanical model to determine the stress–strain state of a curved and tapered timber beam exposed to fire is presented. The Reissner geometrically exact planar beam model is employed. Membrane, shear and flexural deformations of the beam are accounted for. The finite element method is used to solve the system of the governing non-linear equations. By validating and verifying the model it can be concluded that the model is very accurate and therefore suitable for the fire analysis of a curved and tapered timber beam. Beside the three-phase numerical model, the thesis also extensively discusses the methodology for estimating reliability of a timber beam exposed to fire, thus giving a firm basis for the modern performance based approach aiming at introducing uncertainties in the fire analysis.

• Keywords: grajeno okolje, gradbeništvo, disertacije, lesen nosilec, požarna analiza, model povezanega prenosa toplote in vlage, sorpcija, razvoj zoglenele plasti, analiza zanesljivosti