In high-seismic regions, design codes emphasize the critical role of RC beam-column joints; however, fire exposure can induce significant, unpredictable changes in the strength and ductility crucial for resisting future earthquakes. This paper proposes a detailed nonlinear finite element framework, incorporating three approaches of varying complexity, to evaluate the residual seismic behavior of RC beam-column joints through a sequential two-stage analysis involving heat transfer and subsequent mechanical loading. The first approach (A1) uses nodal temperatures at the end of the heating phase, assuming they represent peak fire temperatures. The second approach (A2) determines maximum nodal temperatures from the full heating-cooling history to capture delayed peaks caused by thermal inertia. The third and most comprehensive approach (A3) involves a sequential coupled analysis including the cooling phase, explicitly accounting for irreversible material damage, followed by lateral loading in the subsequent step. These methodologies were implemented via the integration of custom Python scripts and user-defined subroutines. The framework was validated against experimental data for both RC beams under fire and post-fire seismic behavior of joints, demonstrating good agreement with test results. Parametric studies revealed that extended heating duration (up to 120 min) severely degrades structural integrity, reducing initial stiffness by up to 62%. Crucially, the cooling rate significantly influences the post-fire behavior; by neglecting thermal lag, A1 yields unconservative results in slow-cooling scenarios, whereas A2 and A3 estimate the residual stiffness to be 38% and 21% lower, respectively. Furthermore, gravity loads were identified as a critical factor inducing seismic asymmetry; under a gravity load ratio of 55%, A3 predicted column instability during the cooling phase. These findings underscore that while simplified approaches suffice for strength estimation in mild scenarios, the comprehensive strategy is indispensable for assessing residual seismic characteristics and stability under high gravity loads and extended fire exposure.

This paper presents a numerical and experimental study on stability until burnout of glued-laminated timber columns in compartment fires. In a previously reported experimental study, six 3.7 m long, 280 x 280 mm2 timber columns designed for 60 min of standard fire resistance failed to survive various compartment fires, with four of the six columns failing due to thermal wave penetration into the cross section during the decay phase of the fire. Here, six additional compartment fire tests are conducted, supported by numerical simulations, to investigate the design of timber columns for burnout resistance. A finite element model is used to design the size of the columns to mitigate the effect of the thermal wave. Five tests on columns up to 400 x 400 mm2 show that increasing the section size at constant loading enhances fire endurance, but the columns later failed due to local smoldering. While failure in the smaller columns was driven by the thermal wave and occurred within two hours of ignition, failure in the larger columns was caused by smoldering and occurred more than 10 h after ignition. In a sixth test, on a 280 x 280 mm2 column, water-based fire suppression was used 35 min after ignition. The column remained stable until burnout, showing that early fire suppression may prevent failure of timber columns by addressing both the thermal wave and smoldering mechanisms.

The use of parametric/natural fires in the design of reinforced concrete structures in fire conditions requires an accurate definition of the temperature-induced evolution of the thermal and mechanical properties. Within this context, the characterization of four normal-strength concretes (f(c)(20) = 4200 to 6800 psi [29 to 47 MPa]), with siliceous and carbonate aggregates, are studied herein concerning the thermal diffusivity D (between 68 and 1652 degrees F [20 and 900 degrees C]) and under uniaxial compression after different thermal cycles, with reference maximum temperatures of 392, 752, and 1112 degrees F (200, 400, and 600 degrees C). The results show that thermal diffusivity exhibits mostly irreversible behavior after exposure to temperatures over 1382 degrees F (750 degrees C). Concerning the compressive strength, the hot and residual values (when Ttest = 68 degrees F [20 degrees C]) are, overall, in line with the most common standard provisions. Quite interestingly, the tests carried out at intermediate temperatures (with T-test not equal T-max and T-test > 68 degrees F [20 degrees C]) highlighted a strength decay, which is not simply an interpolation between hot and residual values.

Timber framing is increasingly used as a load-bearing structural system in mid-rise and high-rise buildings. While extensive data exists from standard furnace tests, there is a lack of data on the fire performance of loaded timber members in full-scale compartment fires. Compartment fire tests are crucial to investigate a structure's performance under conditions where the fire is let to grow naturally with the fuel and oxygen present in the room and then let to cool down as the fuel burns out. This paper presents the findings from six fire tests performed on glue laminated timber columns in a custom-built compartment. Wood cribs are used as fuel. The columns, 3680 mm long with a 280 x 280 mm2 section, are subjected to constant axial loading during the whole fire duration. Column design and loading are based on a 60-min fire resistance. The tests vary in opening factor, fire load, and wood crib size. Column failure was observed in the six tests, with failure times ranging from 35 to 71 min. In four of the tests, failure occurred after the gas temperature had started to cool down. Comparisons with tests on similar columns made in a fire resistance furnace allow comparing charring rates observed in standard conditions and in natural fires.

Modeling bond behavior in either ordinary or high-temperature conditions requires the knowledge of bond shear modulus-called also slip modulus or simply bond stiffness-that has received so far scanty attention because of the greater interest for bond as a guarantee of equilibrium at the Ultimate Limit State (and in fire conditions) than as a means to guarantee both equilibrium and compatibility at the Serviceability Limit State (and in fire/post-fire conditions). The limited knowledge of bond shear modulus makes it difficult to numerically model such phenomena as tension stiffening, that controls the structural behavior in both ordinary and fire conditions. The general trends identified by examining eleven experimental campaigns with anchored bars covering 27 different cases and temperatures ranging between 20 and 800 degrees C are the starting point of the design-oriented laws proposed in this study about bond shear modulus as a function of concrete residual strength and temperature. A simple shear-lag model is introduced for bond shear modulus at room temperature, as its evaluation from test data is no simple matter due to initial chemical adhesion and different test procedures. Bond shear modulus is shown to be a decreasing function of concrete residual compressive strength and of the maximum temperature reached by the bar-concrete system. Design charts are proposed to allow the designer to identify the value of the bond stiffness on the basis of the max. temperature, of concrete residual strength and of bar diameter, making it possible to realistically model tension stiffening in fire-damaged RC structures.

This paper presents the data and the results of seven fire tests performed on glue laminated timber columns in a compartment built especially for the tests and in which timber wood cribs created a so-called natural fire.These tests are part of a research programme titled "burnout resistance" to establish a new methodology to better describe performance of structural elements during the whole duration of a fire.Comparisons with similar tests made in a fire resistance furnace allow comparing charring rates observed in standard conditions and in natural fires.

EN 1992-1-2 generally limits the redistribution of bending moments from the intermediate supports to the span for continuous reinforced concrete slabs and beams in fire conditions to 15%. While higher redistributions are allowed if sufficient rotation capacity is provided, EN 1992-1-2 does not indicate how to assess the rotation capacity. However, plastic hinges caused by the rotation demand under fire conditions are highly relevant when predicting the global response and structural safety of a structure (partially) exposed to fire. Rotation capacity is specifically necessary at support regions subjected to negative bending and fire, where concrete in compression undergoes thermal degradation while the tension chord remains close to ambient temperature. This article presents a comprehensive model for the behavior of statically indeterminate members in fire conditions, enabling to estimate whether sufficient rotation capacity is provided. Material properties specified by EN 1992-1-2 are applied combined with complementary considerations concerning (i) the biaxial compressive strength of concrete, (ii) strain hardening and limitations of the ultimate strain of reinforcement, as well as (iii) tension stiffening. Tension stiffening detrimentally influences the ductility of the tension chord, limiting the rotation capacity. When comparing predictions obtained by the model to experimental results given in the literature, the correlation is good for the investigated one-way continuous slabs and beams. However, considerable uncertainty exists regarding the type of concrete aggregate used. Moreover, uncertainties concerning the behavior of concrete under compression and fire conditions are highly relevant for modeling the region of supports with rotational restraint.

Fire performance of tunnels can represent a critical issue in the design phase, even more than for other structures and infrastructures, due to some inherent features such as (a) the fire compartment geometry often leading to very high temperature (also making difficult the intervention of fire brigades), (b) the structural redundancy caused by soil restraint fostering the development of relevant indirect actions, and (c) the high compression state in the lining (all the more during the fire exposure) that increases the spalling propensity and severity. Within this context, the role played by key parameters such as lining thickness and stiffness are investigated by comparing the fire performance of two different technological solutions for the lining: (1) traditional cast-in-situ lining and (2) pre-cast segmental tunnel lining. The fire scenario also considers the cooling phase, in order to discuss the main critical points to be solved when facing the final stage of the fire. 3D finite element analyses have been performed, proving that (I) higher thickness and stiffness does not necessarily correspond to a higher safety factor due to the indirect actions, and (II) fire cooling phase (if any) can be even more critical than the heating phase.

Fire exposed structures may collapse during or after the fire decay phase, with risks for building occupants and firefighters; yet, understanding of the effects of the fire decay phase on structural loadbearing capacity remains limited. This paper describes a numerical investigation on the behavior of reinforced concrete columns, beams, and walls under natural fires including cooling down phases. Finite element models are benchmarked against experiments capturing the behavior during heating. The models are then used to simulate the structural response of the concrete members under fires with various cooling rates and load ratios. The analyses capture the irreversibility of material properties through tracing of the temperature history in the structure. The results show that temperatures and deformations continue increasing after the end of the fire heating phase. As a result, concrete columns, beams, and walls may fail during the cooling phase. Faster cooling rates reduce the likelihood of failure in cooling. For beams, failure can be inferred from the maximum reinforcement temperature reached throughout the fire, but for columns and walls a thermal-mechanical analysis of the member throughout the fire history is needed. A relationship is proposed to evaluate the burnout resistance from the fire resistance rating and cooling rate. The presented numerical method allows assessing the structural stability throughout a fire event, an important requirement for designing a fire resilient built environment.

This paper describes fire tests on loaded glued laminated timber columns in which the structural response was measured during the heating and cooling phases. Identical columns with 280 × 280 mm 2 cross-section and 3.7 m length were tested under various heating durations in a standard furnace to investigate integrity to full burnout. Two of the columns were subjected to ISO 834 heating until failure and their measured fire resistance was 55 and 58 min, respectively. Two columns were subjected to 15 min of ISO 834 heating followed by controlled cooling; these columns failed during the cooling phase, respectively after 98 and 153 min. Flame self-extinction occurred after approximately 40 min while smoldering continued locally. Two columns tested under 10 min of ISO 834 heating both survived the defined heating–cooling exposure. Thermocouples inside the columns show sustained temperature increases for hours after the end of the heating phase. These full-scale furnace experiments show that timber columns may fail during the cooling phase after exposure to standard heating for about 25% of the standard fire resistance duration. These results, in line with previous numerical predictions, highlight the need for further investigation into fire safety until full burnout for timber structures.

This paper describes the results of a numerical investigation carried out on the dynamic behavior of a footbridge recently built in Milan, Italy. Following excessive vibrations under crowd loading, experimental tests under ambient-induced vibrations were carried out on the footbridge, and the structure was strengthened through the addition of diagonal braces in the piers. The objective of the different analyses illustrated in the paper is to assess the dynamic characteristics of the bridge, as well as the expected vibration level induced by crowd loading, in order to evaluate the risk of discomfort problems, and to evaluate the effectiveness of the strengthening strategy adopted. The structural behavior was studied by carrying out modal and linear dynamic analyses, as well as through the application of a simple and straightforward criterion. The modal analysis shows good agreement with the measurements, and the adopted strengthening solution is shown to greatly reduce the likelihood of vibration problems under crowd loading.

Structural ConcreteVolume 24, Issue 3 p. 3119-3123 ISSUE INFORMATIONFree Access Issue Information: Structural Concrete 3/2023 First published: 25 June 2023 https://doi.org/10.1002/suco.202370311AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat Volume24, Issue3June 2023Pages 3119-3123 RelatedInformation

In flat-slab frames, which are typically designed as secondary seismic structures, the shear failure of the slab around the column (punching failure) is typically the governing failure mode which limits the deformation capacity and can potentially lead to a progressive collapse of the structure. Existing rules to predict the capacity of flat slab frames to resist imposed lateral displacements without losing the capability to bear gravity loads have been derived empirically from the results of cyclic tests on thin members. These rules account explicitly only for the ratio between acting gravity loads and resistance against concentric punching shear (so-called Gravity Shear Ratio). Recent rational models to estimate the deformation capacity of flat slabs show that other parameters can play a major role and predict a significant size effect (reduced deformation for thick slabs). In this paper, a closed-form expression to predict the deformation capacity of internal slab-column connections as a function of the main parameters is derived from the same model that has been used to develop the punching shear formulae for the second generation of Eurocode 2 for concrete structures. This expression is compared to an existing database of isolated internal slab-column connections showing fine accuracy and allowing to resolve the shortcomings of existing rules. In addition, the results of a testing programme on a full-scale flat-slab frame with two stories and 12 columns are described. The differences between measured interstorey drifts and local slab rotations influencing their capacity to resist shear forces are presented and discussed. With respect to the observed deformation capacities, similar values are obtained as in the isolated specimens and the predictions are confirmed for the internal columns, but significant differences are observed between internal, edge and corner slab-column connections. The effects of punching shear reinforcement and of integrity reinforcement (required according to Eurocode 2 to prevent progressive collapse after punching) are also discussed.

Structures may collapse during the cooling phase of a fire, yet standard furnace tests only measure the response under heating. There lacks experimental test protocols and design methods to assess resistance until burnout. This paper describes a new experimental approach for burnout resistance evaluation, reports experimental data on loaded reinforced concrete columns in furnace tests with cooling down phases, and presents numerical models of the tests. The test results show that columns designed for a standard fire resistance of 60 min exhibited a fire resistance of 83 min in the furnace but failed during the cooling phase when the burners were shut off after 72 min while the load was maintained. Two other specimens survived exposure to heating of 45 and 55 min, respectively, and their residual capacity was measured. Finite element analyses show agreement with the tests, showing applicability of numerical methods for evaluating burnout resistance of concrete columns. These findings demonstrate experimentally that delayed thermal-mechanical effects can jeopardize structural stability in real fires, and provide a framework to measure these effects. Moving beyond fire resistance to quantify the response until burnout will support designs for safety of occupants and firefighters throughout the fire and promote repairability and resilience.

Ultra-High Performance Concrete (UHPC) proved to be very durable in harsh environments, primarily because of the extremely low porosity of the matrix (uncracked). However, the limited availability of design standards is still a barrier to widespread applications of UHPC, together with still limited knowledge of its durability in the real (cracked) service conditions. In this paper, the use of tailored UHPC is introduced, whose composition has been specifically designed to achieve enhanced durability in the cracked state combined with extremely aggressive environments. Validation of the material and structural design concept on a full scale structure is presented, with reference to a tank intended to contain geothermal water from the cooling tower at a geothermal power plant. This pilot structure was designed and constructed using both ordinary reinforced concrete and durability enhanced UHPC, which is called hereafter Ultra High Durability Concrete (UHDC), for comparative assessment purposes. Upon the completion of the pilot construction and entering its service life, periodic assessment and validation tests have been carried out to validate the structural design assumptions and to check the serviceability requirements. Results of these tests, performed over the span of two years are reported in detail in this paper to validate the material and structural concepts. The study highlights the most important parameters that could affect the performance of UHPC structures during casting and service life. The overall project framework presented in this paper has to be intended as a pioneer study in moving towards a performance based durability-design approach for UHPC structures.