M.B. Wong

The effects of heat treatment on the recovery stresses of shape memory alloys

The influence of annealing temperatures on the thermo-mechanical behavior of NiTi alloy in terms of transformation temperatures, mechanical properties at ambient temperature and the recovery stress under constrained end conditions were investigated experimentally. An attempt is made to correlate results obtained from the DSC test and tensile test at room temperature with results from recovery stresses at elevated temperatures. It is found that annealing the alloy above the recrystallization temperature (600 °C) reduces the maximum recovery stress significantly, even though the alloy still exhibits thermal transformations and a stress-induced martensite plateau at an annealing temperature above 600 °C. A pre-strained amount above approximately 2.4% strain is sufficient to achieve the maximum recovery stress at constrained end conditions. It is recommended that the alloy used in this study be annealed at temperatures below 450 °C in order to produce the desired thermo-mechanical properties in the alloy for applications that exploit the shape memory effect.

Sensitivity analysis of heat transfer formulations for insulated structural steel components

The aim of using fire protection in a building is to reduce the rate of temperature rise of its structural components in case of fire. For protected structural steel, the thermal properties of the insulation materials affect the rate of temperature rise and are crucial in determining the minimum requirements for fire safety for both the steel and the insulation materials. The determination of the required thickness of the insulation materials can be performed by means of test results, analytical solutions or numerical methods. The current Eurocode 3 provides simple analytical solutions for estimating the temperature rise of both protected and unprotected structural steel in a fire. This paper presents a sensitivity analysis to examine the appropriateness of using these analytical solutions for structural steel components protected with insulation materials of contrasting properties including thermal conductivity and density. Results of the analysis show that, for certain types of insulation materials, the temperatures predicted by the Eurocode may differ substantially from those by exact analytical solution. An alternative formulation is presented when these types of insulation materials are used for fire protection of structural steel.

Elastic and plastic methods for numerical modelling of steel structures subject to fire

Abstract An approach based on the elastic and plastic methods for fire resistance analysis is presented. It is intended to fill the gap between the simple calculation method and the finite element method currently being used by engineers. This approach can be applied to simple framed structures for which the failure temperature of the structure can be calculated. The elastic method includes the effect of interaction between the thermal loading and static loading. The load ratio in each member is calculated, resulting in a formulation whereby the limiting temperature of each member can be obtained. The plastic method is based on the plastic hinge concept for the calculation of critical temperature of frames at collapse. The method makes use of the fact that the collapse load factor of a frame is a linear function of the collapse temperature for the same collapse mode. For situation where the collapse mode changes as the temperature in the frame increases, a demarcation collapse temperature multiplier is introduced to monitor the change. The main advantage of this approach is that the frame as a whole can be considered without using complex analytical technique.

Determination of steel emissivity for the temperature prediction of structural steel members in fire

AbstractA numerical method, with which experimental results were processed, was adopted to measure the total emissivity of mild steel specimens at high temperatures. This method is derived from considering the transient thermal energy equilibrium between a steel specimen and its surrounding heating environment. As a first step to validate this method, the results obtained at a low temperature were compared with those using infrared thermographic techniques, and a good correlation of 87% was achieved. The numerical model was then extended to high temperatures to investigate the variation of emissivity of steel with temperatures. The convective heat transfer coefficient used in the numerical model was examined in great detail using results obtained from transient high temperature tests. The emissivity of steel obtained from this study shows that steel emissivity varies over a range of temperatures and the variation becomes more abrupt between 400 and 500°C. Formulation for the emissivity of steel at rising ...

Durability of CFRP-strengthened Concrete Members under Extreme Temperature and Humidity

Abstract Fibre reinforced polymer (FRP) systems have emerged as one of the most promising and affordable solutions for strengthening civil engineering infrastructure. Nowadays, this application is widely used in outdoor structures such as bridges, silos and towers. The major challenge in composites applications for outdoor structures is the long-term durability of bond between FRP and substrate under different environmental conditions. Elevated temperature and humidity are common issues that may have adverse effects on the polymer matrix, especially on thermosetting adhesives such as epoxies. This paper presents the results of long-term bond performance of carbon fibre reinforced polymer (CFRP) strengthened concrete members. CFRP- plated concrete specimens were exposed to combination of cyclic temperatures (20–50 °C) and 90% constant humidity. In addition to environmental exposure, some of the specimens were subjected to sustained loading. At the end of the exposure period, the residual strength was measured using single lap shear test method.

Unit load factor method for limiting temperature analysis of steel frames with elastic buckling failure mode

Temperature increase in a steel frame may lead to strength degradation of the frame due to deterioration of the strength of individual members at elevated temperatures. In this paper, strength degradation of steel frames at elevated temperatures is reflected by the corresponding decrease in the elastic buckling strength of the structure. In obtaining the elastic buckling load of the structure, the mechanical properties and the internal forces of the members first have to be evaluated. Moreover, the internal forces induced by the external loads and by the temperature effect need to be separated in order to obtain the elastic buckling load factor, pertaining to the external loads. Using a proposed unit load factor method, the limiting temperature at which the frame fails by elastic buckling can be obtained. It is shown that a temperature gradient existing across the cross-section of the members has little effect on the elastic buckling strength of the frame.

A novel active fire protection approach for structural steel members using NiTi shape memory alloy

A novel active fire protection approach, based on integrating a shape memory alloy, NiTi, with a steel structure, was proposed to satisfy the fire resistance requirements in structural design. To demonstrate the principles of this approach, a simple structure in the form of a simply supported steel beam was used. The internal action of the beam due to a transverse applied load was reduced by utilizing the shape memory effect in the NiTi alloy at rising temperatures. As a result, the net internal action from the load design was kept below the deteriorated load capacity of the beam during the fire scenario for period of time that was longer than that of the original beam without the NiTi alloy. By integrating the NiTi alloy into the beam system, the structure remained stable even though the steel temperature exceeded the critical temperature which may have caused the original beam structure to collapse. Prior to testing the composite NiTi–steel beam under simulated fire conditions, the NiTi alloy specimens were characterized at high temperatures. At 300 °C, the stiffness of the specimens increased by three times and its strength by four times over that at room temperature. The results obtained from the high-temperature characterization highlighted the great potential of the alloy being used in fire engineering applications.

Modeling of Heat Transfer in Gas-Filled Furnace for Steel Members

AbstractA heat transfer model for unprotected steel structural members was proposed in this study. The model accounts for both the variation of the steel emissivity during the fire growth and the view factor effect. The participation of the gases in the heat exchange by radiation also was discussed. The calculation of the view factor of concave steel sections was derived and justified using the heat transfer theory in a realistic situation. In addition, a new formulation incorporating the transmissivity, absorptivity, and emissivity of the participating medium into the view factor was presented. A numerical verification was conducted to validate the predicted temperatures with experimental data and a good correlation was achieved. Further, the model was simplified, while retaining its accuracy to facilitate the numerical computations carried out by the structural engineers in structural fire design.

Use of kinetic model for thermal properties of steel at high temperatures

Abstract Steel thermal properties determination is essential in steel structure temperature prediction for structural fire design or metallurgical evaluation. Thermal properties include specific heat, thermal expansion and heat transfer coefficients. Since the steel thermal properties variation at high temperature may be considered as a chemical process, kinetic theory could be used to explain the variation. To examine this concept, steel specific heat is first considered in this paper. The validity of the kinetic model that successfully applied to steel specific heat can be used to estimate the variation of other steel thermal properties at high temperatures. Complex non-linear equations for steel specific heat computation are provided in design codes. In this paper, it is found that the way steel specific heat varies with temperature during solid state phase transformation is not unique. Instead of using high-order equations, models using simple linear equations and average specific heat for phase transformation are proposed. These models are based on the latent heat principle whereby the latent heat consumed by steel during phase transformation is constant. Furthermore, the proposed simple models can effectively simplify the steel specific heat computation and provide good steel temperature development simulation shown in the case study.

WORLD TREND FOR THE DEVELOPMENT OF PERFORMANCE-BASED FIRE CODES FOR STEEL STRUCTURES

The paper provides a description of major developments of fire engineering requirements for steel structures in steel design codes in several countries and regions. This development not only reflects the trend for the integration of both the structural design and fire engineering design processes, it also gives indication to the research effort that individual organizations and countries have contributed to the development of performance-based fire codes. The paper has made an effort to include countries involving major fire code developments while recognizing that there is a large amount of work in this area still being done in many countries which may not have been mentioned.