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Mechanics of wide-flanged steel sections that develop thermal gradients due to fire exposure
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This paper examines the behavior of wide-flanged (WF) steel sections with axial load and a thermal gradient through the section depth due to uneven exposure to fire. Such conditions may produce a shift of the section’s effective centroid (i.e. the center of stiffness), which will move away from the section’s geometric centroid toward the cooler side. If the axial loads carried by the section are applied at its geometric centroid (as is typically assumed), the presence of a thermal gradient produces a bending moment because the axial loads are now acting eccentrically to the section’s effective centroid. These moments become large enough to reverse the direction of moment in the member. This paper describes the mechanics of this behavior in detail and evaluates the effects of different levels of axial load on the magnitude of the centroidal shift and on the moments that develop as a result. It is observed that larger axial loads produce larger shifts of the effective centroid, and yet the total moment in the section becomes smaller.
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... The temperature gradient within the cross-section may shift the effective centroid towards the colder region, thus increasing the bowing of the sample towards the heating source. These effects have been shown to result in different failure modes at elevated temperatures for other types of structural systems [22,23]. However, there have been no reported studies that properly consider these combined effects on PCWs. ...
... The shift of the failure plane is linked with the combined effect of the initial load eccentricity and thermal gradient within the cross-section of the sample. When subjected to a thermal gradient, the effective centroid tends to be shifted towards the colder region [23]. Thus, the eccentricity was naturally increased during heating. ...
... As a result, the steel sheet and concrete fibres on the heated side enter the inelastic region earlier because of the presence of compressive stress. Thus, the ratio of stress to the strain of the concrete and steel is much smaller than those in the elastic region [23]. The effective centroid of the samples is therefore no longer at the same location as the geometric centroid but moves toward the colder region. ...
Profiled composite walls (PCWs) are regularly used in construction because they provide enhanced ductility, shear resistance and damage tolerance when compared to traditional reinforced concrete walls. Although much research has been conducted to understand the structural performance of PCWs at ambient temperature, studies into their performance at high temperatures remain limited. In this work, a comprehensive set of experiments has been conducted to investigate the performance of PCWs at both ambient and elevated temperatures. A heat source comprising of radiant burners and 1MN MTS machine were employed to deliver known and actively controlled thermal and structural boundary conditions on the PCW samples. The experiments were conducted to understand the effects of an incident heat flux when combined with loads. The results from this study have shown that (i) the axial load capacity of PCWs decreases as the temperature increases; (ii) the PCWs tends to exhibit ductile failure modes when cold but brittle failure at high temperature; (iii) due to thermal bowing, the failure plane of the PCWs subjected to one-side heating shifts closer to the heating source; and (iv) applying a load in an eccentric manner can compensate for the effect of temperature gradient.
... In 1991, Hamerlinck [4] conducted a numerical and experimental study regarding the thermal and mechanical behaviour of reinforced composite slabs under fire conditions. Both numerical models were experimentally validated with loaded and unloaded tests. ...
... With the aim of validating the approach, the results of the numerical simulations are compared with experimental results published by Lim and Wade [7], Piloto et al. [11], Abdel-Halim, Hakmi, O'Leary [6], and Hamerlinck [4]. In addition, a comparison between the fire resistance obtained numerically, experimentally and analytically (using the Eurocode 4 calculation method [8]) is presented. ...
... Four different composite slabs with trapezoidal profile have been selected to perform the numerical validation. These slabs correspond to experimentally tested slabs: slab 1 was tested by Lim and Wade [7] (test number 4), slab 2 was tested by Piloto et al. [11] (test number 1), slab 3 was tested by Abdel-Halim, Hakmi and O'Leary [6] (test number 2), and slab 4 was tested by Hamerlinck [4] (test number 2). The slabs were exposed to the ISO 834 standard fire from below in controlled furnaces. ...
This book gathers selected, extended and revised papers presented at the 5th Iberian-Latin American Congress on Fire Safety, CILASCI 5, held on 15-17 July 2019, in Porto, Portugal. The respective chapters address experimental efforts and the computational and numerical modelling of materials (e.g. wood, concrete, and steel) and structures to assess their fire behavior and/or improve their fire resistance. In addition, they present simulation studies on fire events and findings from fire performance tests on walls. Given its scope, the book offers a valuable resource for researchers, graduate students, and practitioners whose work involves fire safety-related topics.
... Secondly, the current paper is based on the previous work of the authors in the field of the mechanics of thermal response of columns in a fire. This previous work is included in the following papers: Garlock and Quiel [22,23] and Quiel and Garlock [24]. The main contribution of this work is a simplified closed-form methodology used to predict the thermal and structural response of steel perimeter columns in high-rise building frames exposed to fire. ...
... This data is used to develop new temperature-fire curves which are describing real fire scenarios. Quiel [22,23] and Quiel and Garlock [24]) are used to make accurate predictions for the temperature of the different wideflange components of the structure as the fire scenario is developing. • Lastly, for the steel material behavior, the Eurocode 3 material laws are used (CEN [26]). ...
Infrastructure resilience is the ability of an infrastructure asset to limit the effect and duration of damaging extreme events. The four main components of the concept of resilience are robustness, resourcefulness, recovery and redundancy most of which are very difficult to be quantified. In addition to this difficulty, a careful study of extreme event cases can demonstrate that it is very common for an extreme event scenario to include cascading multi-hazard events such as blast, floods, earthquake, and fire. This paper studies the resilience of a multi-story steel frame with multi-hazard considerations which include a post-event fire scenario. The initiating extreme event is simulated through the threat-independent alternate load path method of analysis and a post-event fire is considered following the extreme event. The work in the paper combines previous work by the authors on stability-induced collapse of damaged steel structures and closed-form solutions for temperature predictions of wide-flange components during a fire scenario. For the purposes of the fire scenarios, new fire time-temperature curves are developed based on experimental data from the well-known Cardington fire tests. The results show that even when a structure can withstand an extreme event scenario, a post-event fire consideration is highly critical to evaluate the remaining time of survival of the structure before collapse. It is shown that the sequential method of multi-hazard analysis can lead to very short available time periods before the post-event fire leads to the complete collapse.
... Nonuniform heating introduces asymmetry through the cross section and thermal bowing along the column length, both of which adversely affect the load-bearing capacity of steel columns. For example, as discussed in Garlock and Quiel (2007), thermal gradients through the cross section move the location of the instantaneous neutral axis toward the cooler (stiffer) side. If the column is loaded concentrically, then the moment induced by the shift of the neutral axis makes the column deform toward the cooler side (i.e., concave part in the hotter side). ...
... As a consequence, short columns bend toward the cooler side but slender columns bend toward the hotter side. Both Agarwal et al. (2014) and Garlock and Quiel (2007) have described this phenomenon in detail. The fire resistance of gravity columns is more sensitive to the length effects, which can exacerbate thermal bowing. ...
This paper presents the behavior of axially loaded steel columns subjected to thermal gradients through their cross sections. Experimental tests were conducted on full-scale wide-flange steel columns with W8×35 and W14×53 sections made from standard grade 50 steel. The experimental investigations confirmed that variations in fire protection thickness had significant influence on the thermal gradient developing through the steel cross section. Thermal gradient along the flanges caused bowing of column specimens toward the hotter side. Thermal bowing introduced second-order moments and adversely affected stability, leading to failure of column specimens by inelastic flexural column buckling. The experimental behavior and results were compared with those obtained from detailed nonlinear finite-element analyses of the tested specimens. These nonlinear finite-element models utilized standard (temperature-dependent) material properties, and reasonably predicted the axial load-temperature-deformation behavior and failure temperature of column specimens subjected to nonuniform heating.
Read More: http://ascelibrary.org/doi/10.1061/%28ASCE%29ST.1943-541X.0001500
... Structural members that are not uniformly heated may experience a combination of thermally-induced forces and moments due to restraint and thermal bowing (Dwaikat 2011). A member with a thermal gradient through its cross section may experience a shift of the section's center of stiffness away from its geometric center, resulting in additional bending moments (Garlock 2007). A neutral axis shift will induce unanticipated moments for axially loaded members if the member's ends are rotationally restrained. ...
This chapter reviews how temperature affects the mechanical properties of steel, concrete, masonry, and wood. Many tests have characterized the mechanical properties of reinforcing steels at elevated temperatures. The chapter covers some fundamental concepts about the effects of temperature on materials’ mechanical properties and provides references to the most widely accepted material models found in design codes and the research literature. All mechanical properties used for timber design derive from extensive sampling and analysis procedures, and these properties are represented as either minimum strength or average stiffness. Because the strength and stiffness of the char layer are nearly zero, the properties of wood from room temperature to 300°C are of most interest to structural fire engineering design.
... The behaviour of composite elements is more complex due to the existence of non-linear thermal gradients in concrete. In 2007, Garlock and Quiel [4] presented the effect of thermal gradients in steel columns with wide flanges, which are responsible for modifying the position of the neutral axis to the coldest side, introducing extra bending moments. This shift effect also depends on the amount of axial load. ...
Partially encased columns (PEC) have better fire resistance when compared to bare steel columns, due to the existence of concrete between the flanges. The aim of this study is to develop a new proposal for the calculation of the buckling strength of partially encased columns, embedded on walls, under fire conditions. This proposal is based on the current calculation method proposed in Annex G of EN 1994-1-2. This study uses the finite element method to calculate the average temperature of seven components. The average temperature is then used to find the buckling resistance of composite columns when submitted to fire from one side. This solution method is carried out using 30 different cross sections. All cross sections are exposed to the standard fire curve ISO834 from one side, assuming the specific fire rating of 30, 60, 90 and 120 min.
... One of the reference works in the area was carried out, in 2007, by Garlock and Quiel [1]. The authors performed a numerical study in unevenly heated columns. ...
This paper presents the results of an experimental research on steel and composite partially encased steel columns embedded on brick walls and subjected to fire. The specimens were made of HEA 100 and HEA 220 steel profiles, with the web parallel and orthogonal to the wall's surface. The thickness of the tested walls was 7, 11, and 15 cm depending on the steel profile used in the column. Due to the generated thermal gradient, this type of columns experienced thermal bowing, bending first towards the exposed side and then to the opposite side of the fire. When the column's web is parallel to the wall's surface, a less pronounced thermal gradient is developed, and the column behaves more like to a uniform heated one. The axial restraining forces increasing and after reaching the peak value decreasing quite suddenly. When the column's web is orthogonal to the wall's surface, the steel flange is directly exposed to fire, resulting on its rapid thermal elongation accompanied by a rapid degradation of the mechanical properties of the materials, while the remaining cross-section heats slower. The restraining forces after reaching a peak value descended a little bit and then increasing again up to second peak value, thus part of the cross-section is slowly heating and elongating. The thickness of the brick wall influenced the stiffness of the tested columns, affecting the development of the restraining forces and displacements.
... Zjawiska zachodzące w elementach poddanych nierównomiernemu nagrzewaniu, takie jak zmiana położenia efektywnego środka przekroju, czy też osi obrotu plastycznego opisano m.in. w [6] oraz [7]. W pracy [7] analizowany jest przypadek, w którym temperatura jest niejednorodna zarówno w przekroju słupa, jak i względem jego wysokości. ...
Wprowadzenie: W artykule przedstawiono problemy związane z odpornością ogniową słupów stalowych i żelbetowych nagrzewanych z mniej niż czterech stron, z uwzględnieniem elementów wbudowanych w ściany oddzielenia przeciwpożarowego. Jednostronne nagrzewanie słupa sprawia, że na jego przeciwległych stronach występują różne temperatury. Prowadzi to do powstawania przemieszczeń poziomych, które wywołują dodatkowe siły wewnętrzne w samym słupie, jego zamocowaniu oraz w przylegającej konstrukcji.
Metody: W odniesieniu do słupów stalowych wskazano, że w normie PN-EN 1993-1-2 brakuje metod uproszczonych pozwalających na obliczenie nośności słupów stalowych poddanych oddziaływaniu wysokiej temperatury tylko z jednej strony. Opisano najważniejsze wnioski z prac badawczych przeprowadzanych w tym obszarze w ostatnich latach i wskazano możliwą do zastosowania metodę obliczania nośności tego typu elementów, uwzględniającą nierównomierny przebieg temperatury w elemencie. Tego typu nagrzewanie i związana z nim nierównomierna temperatura w obrębie przekroju sprawiają, że stal, w zależności od jej położenia w przekroju, wykazuje różne wartości parametrów takich jak granica plastyczności i moduł sprężystości. Uwzględnienie w obliczeniach rzeczywistej temperatury stalowego słupa pozwala na oszacowanie jego przemieszczeń, wywołanych wpływem nierównomiernej temperatury, od których zależą wielkości sił wewnętrznych i jego nośność. W odniesieniu do słupów żelbetowych opisano podstawowy podział metod obliczania nośności zawartych w PN-EN 1992-1-2, ze szczególnym zwróceniem uwagi na ich ograniczony zakres stosowania z uwagi na: typ konstrukcji, długość elementu, smukłość oraz wielkość mimośrodu obciążenia. Dokonano przeglądu metod pod kątem stosowania ich przy narażeniu na działanie ognia z mniej niż czterech stron. Przybliżono metodę zawartą w niemieckim załączniku krajowym do DIN-EN 1992-1-2, która pozwala na obliczenie nośności słupów wspornikowych poddanych działaniu ognia z jednej, trzech lub czterech stron. Przywołano pozycje literaturowe podające uproszczone metody obliczeń wraz z wnioskami.
Dyskusja i Podsumowanie: Spośród potencjalnych rozwiązań omawianego problemu można wyróżnić: umieszczenie słupów poza ścianą oddzielenia pożarowego, zaprojektowanie połączenia pozwalającego na zawalenie się konstrukcji dachu do wnętrza hali, bez wywoływania nadmiernych sił poziomych oddziałujących na głowicę słupa lub wykorzystanie usztywnienia przylegającej konstrukcji do przeniesienia sił poziomych. Z uwagi na poziom skomplikowania bardziej zaawansowanych metod obliczania nośności, wskazane jest opracowanie metod uwzględniających nierównomierne nagrzewanie w sposób uproszczony, na bazie faktycznych przypadków pożarów budynków.
Profiled composite walls (PCWs) are regularly used in construction because they provide enhanced ductility, shear resistance and damage tolerance when compared to traditional reinforced concrete walls. Although much research has been conducted to understand the structural performance of PCWs at ambient temperature, studies into their performance at high temperatures remain limited. In this work, a comprehensive set of experiments has been conducted to investigate the performance of PCWs at both ambient and elevated temperatures. A heat source comprising of radiant burners and 1MN MTS machine were employed to deliver known and actively controlled thermal and structural boundary conditions on the PCW samples. The experiments were conducted to understand the effects of an incident heat flux when combined with loads. The results from this study have shown that (i) the axial load capacity of PCWs decreases as the temperature increases; (ii) the PCWs tends to exhibit ductile failure modes when cold but brittle failure at high temperature; (iii) due to thermal bowing, the failure plane of the PCWs subjected to one-side heating shifts closer to the heating source; and (iv) applying a load in an eccentric manner can compensate for the effect of temperature gradient.
A secant stiffness method was previously developed for efficient analysis of isolated steel beams under the influence of thermal distributions caused by fire. This method has been developed considerably further, to include the thermally degrading characteristics of the connections between adjacent members and also the effects of axial force, the latter including both material and geometric nonlinearities. This enables full planar skeletal frames̀ to be analysed up to failure under local or overall fire conditions. In particular terms it allows the investigation of the real behaviour in fire of frames designed as ‘simple’ for ambient-temperature, strength and serviceability criteria. Although the normal design assumption for such frames is that beams transfer no moments to columns, the rotational stiffnesse of the connections can considerably affect fire survival, as can other restraint forces from the surrounding structure. In this paper the theoretical background to the analysis is developed, and validation studies are reported. These include comparisons with previous analytical work and with fire test results. An original example is presented of a three-bay x three-storey frame with a local compartment fire in the bottom storey.
In general the temperature profile within a steel beam subjected to fire is non-uniform. To predict the structural behaviour of such a beam the effect of the variation of the steel's characteristics with temperature across the beam must be considered. In addition stresses due to non-uniform thermal expansion need to be included. A procedure for the analysis of such cross-sections with a non-uniform temperature profile is presented. This has been developed specifically to be used on personal computers, rather than the mainframes normally employed for numerical analysis of this degree of complexity. The actual temperature profile is idealised by a finite number of constant temperature steps, and the thermal stresses induced are converted to a set of external loads. These are used in a secant stiffness approach to predict the deflection-temperature characteristics of beams. A comparison with test results for different types of beam illustrates the degree of accuracy that can be achieved using the techniques described.
This paper uses a finite-element model to investigate the stability of the Twin-Towers of the World Trade Center, New York for a number of different fire scenarios. This investigation does not take into account the structural damage caused by the terrorist attack. However, the fire scenarios included are based upon the likely fires that could have occurred as a result of the attack. A number of different explanations of how and why the Towers collapsed have appeared since the event. None of these however have adequately focused on the most important issue, namely ‘what structural mechanisms led to the state which triggered the collapse’. Also, quite predictably, there are significant and fundamental differences in the explanations of the WTC collapses on offer so far. A complete consensus on any detailed explanation of the definitive causes and mechanisms of the collapse of these structures is well nigh impossible given the enormous uncertainties in key data (nature of the fires, damage to fire protection, heat transfer to structural members and nature and extent of structural damage for instance). There is, however, a consensus of sorts that the fires that burned in the structures after the attack had a big part to play in this collapse. The question is how big? Taking this to the extreme, this paper poses the hypothetical question, “had there been no structural damage would the structure have survived fires of a similar magnitude”?