Fire Resistance of Concrete Structures - The Global Behaviour

1 fire Resistance of Concrete Structures - The Global BehaviourTiago Filipe Domingos Gon thesis summary1 IntroductionAs it is known, Concrete presents a good Resistance when exposed at high temperatures. It is non-combustible and has a slow rate of heat transfer, which makes it a highly effective barrier to the spreadof International Codes and Standards allow performing member analysis, thus neglecting the effectsof the indirect fire actions. This method does not take into consideration the arising of members internalforces and can lead to non conservative results. The increase of shear force in the columns induced bythermal expansion of beams and slabs can cause the collapse of the main objective of this work is to understand the Global Behaviour of Concrete Structures whenexposed to fire .

2 Hence, it is performed a simplified analysis to a building that suffered a fire . This analysisconsists in the application of temperature variations (uniform and linear gradient) to the slabs and beamsof the building and comparing the results with those verified in-situ. It is used a commercial computerprogramme named SAP2000 for the modelling of the structure and for the application of Methods of Assessment of fire Resistance and General Pro-cedures for the Verification of fire ResistanceThe fire Resistance of Concrete Structures can be assessed by one of the five distinct methods, listedin order of increasing complexity [1]: standard fire tests; tabulated data (largely prescriptive but also increasingly based on calculations); simplified calculations, neglecting complex effects such as thermal stresses.

3 Advanced calculations, that can be used on simulation of Global Behaviour of the structure , parts ofthe structure (frames) and isolated elements (beams, columns or slabs), neglecting the interactionbetween them; full scale fire to Part 1-2 of Eurocode 1 [2], fire Resistance of Structures should take into account thefollowing steps: definition of the thermal action; definition of the mechanical actions in fire situation;1 calculation of the temperature evolution din structural elements; determination of the design value of the relevant effects of actions in the fire situation at exposuretimet,Efi,d,t; determination of the design value of the relevant Resistance in fire situation at exposure timet,Rfi,d,t; verification of the fire Resistance , that can be done in three different domains:1.

4 In time domain:tfi,d tfi,requ(1)2. in strength domain:Rfi,d,t Efi,d,t(2)3. in temperature domain: d cr,d(3)Where,tfi,d- is the design value of the fire Resistance ;tfi,requ- is the required fire Resistance time; d- is the design value of material temperature; cr,d- is the design value of the critical material The Global BehaviourWhen a member analysis is done, the indirect actions arising in the Structures due to thermal expansionare not taken into consideration which, in case of statically-indeterminate members, can lead to nonconservative Riva, referred in [4], performed an extensive study in order to analyse this problem in frames.

5 Inthis kind of Structures , the continuity of the beams with the columns may induce a non negligible axialforce in the beams, which in turn may generate high shear forces in the columns and cause a possibleshear failure, as often observed in real fires. This study is summarized in the Parametric study of framesIn this study, it was considered two different fire exposures to the columns of the frames: fire on threesides, with the fourth side at ambient temperature, and fire on one side, with the remaining three sidesat ambient temperature. The first case represents a column with the external side flush with the wallof compartment in fire , and the second case a column with the internal side flush with the wall of existence of the upper floors has been introduced by considering a portion of the columns above thefire compartment, assuming that the inflection points are located at mid-height between two continuousstoreys, and by applying to the columns an axial forceNsd= 1000kNrepresenting the effects of the upperfloors 2.

6 The columns belonging to the upper floor, being outside the fire compartment, are assumed tobe at ambient temperature. In the base, the columns are considering as fully - Time - Strength - Temperature 1:Verification of the fire Resistance [3].Figura 2:Parametric study of frames [4].3 The analysed frames are shown in figure 2. The geometry of the frames and the reinforcement of thecritical sections satisfy the tabulated values in [5] for R60. In the following, the results of the analysiscarried out on a frame with a 6 m span beam having rectangular section and with the column heated onone side are the obtained results, which are shown in figure 3, the following comments can be drawn: Both bending moment and shear force increase dramatically in the columns for the first 30 minutes,because of the heating of the beam.

7 However, no further increase is observed later on, because ofthe progressive damage of the 3:R/C frame with the columns exposed to fire on one side and with 6 m span beam (rectangularsection) [4]. The bending moments in the lower columns increase approximately seven times because of thethermal deformations of the beam (elongation and end rotation), while the bending moments inthe upper columns change sign, and their values increase more than twice of the value at ambienttemperature. The shear forces in the lower columns increase approximately four times with respect to ambientconditions. As a result, the columns which are lightly reinforced in shear and not confined, mayexhibit a shear failure, as often observed during real fires.

8 Neglecting the effects of beam in the design of R/C frames may lead to highly non conservativeresults because of the increasing bending moments and shear forces in the columns during the first30 minutes of fire practical applications, detailing rules for columns similar to those generally adopted in seismicdesign are recommended for fire design as well. The adoption of closely spaced hoops is importantin improving section strength and ductility in combined bending and axial force, and helps controllingconcrete spalling [4].The column exposure to fire on three sides do not shown relevant differences, except that the increaseof the bending moment and shear is less pronounced, due to the smaller temperature gradient in The buildingA simplified analysis to a building that suffered a fire was conducted.

9 The building has four floors,the ground floor and three elevated floors. The beams and columns of the building were made with insitu Concrete and they are structurally connected. The slabs of the ground floor are solid Concrete slabsand those of the upper floors are one way slabs with pretensioned beams and ceramic blocks. On theground floor there is a ramp that connects to the first floor, which is used as truck s access. This rampwas made with Concrete beams, a solid Concrete slab and is monolithic to some the real fire , the columns of the ramp were excessively damaged and so far, they did not sufferany repair (figure 4). The connection between the slab of the ramp and the columns lead to an imposeddisplacement of the columns, with the direction of the ramp, due to the expansion of the Concrete resultingfrom the increase of temperature during the 4:Shear failure in the ModellingThe structural modelling of the building was done with a commercial computer program namedSAP2000.

10 To simulate the beams and the columns, frame elements were used. The slabs were simulatedwith rectangular finite elements of Shell-Thick type and one meter side length. In the base, the columnsare considered as fully figure 5 is shown a tri-dimensional view of increase of temperature inside a section can be divided into two parts, one linear and one self-equilibrated (figure 6). In turn, the linear part is divided into the sum of a constant and a lineartemperature 5:3D model 6:Temperature linearisation the analysis it was study the influence of the two previous parts separately, with the objective ofassessing the Global Behaviour of the structure , namely in the columns of the ground floor.