Optimal design of long-span steel portal frames using fabricated beams

This paper considers the optimal design of fabricated steel beams for long-span portal frames. The design optimisation takes into account ultimate as well as serviceability limit states, adopting deflection limits recommended by the Steel Construction Institute (SCI). Results for three benchmark frames demonstrate the efficiency of the optimisation methodology. A genetic algorithm (GA) was used to optimise the dimensions of the plates used for the columns, rafters and haunches. Discrete decision variables were adopted for the thickness of the steel plates and continuous variables for the breadth and depth of the plates. Strategies were developed to enhance the performance of the GA including solution space reduction and a hybrid initial population half of which is derived using Latin hypercube sampling. The results show that the proposed GA-based optimisation model generates optimal and near-optimal solutions consistently. A parametric study is then conducted on frames of different spans. A significant variation in weight between fabricated and conventional hot-rolled steel portal frames is shown; for a 50 m span frame, a 14–19% saving in weight was achieved. Furthermore, since Universal Beam sections in the UK come from a discrete section library, the results could also provide overall dimensions of other beams that could be more efficient for portal frames. Eurocode 3 was used for illustrative purposes; any alternative code of practice may be used.

Evolution of microstructure and changes of mechanical properties of CLAM steel after long-term aging

The China Low Activation Martensitic (CLAM) steel has been developed as a candidate structural material for future fusion reactors. It is essential to investigate the evolution of microstructure and changes of mechanical properties of CLAM steel during thermal exposure. In this study, the long-term thermal aging of the CLAM steel has been carried out in air at 600 °C and 650 °C for 1100 h, 3000 h and 5000 h. The microstructural evolution with aging time was studied, including characteristics of the growth of M23C6 carbides and the formation of Laves-phase precipitates as well as the evolved subgrains. The microstructural evolution leads to the changes of mechanical properties of the CLAM steel. The Ductile–Brittle Transition Temperature (DBTT) increases significantly during the thermal aging, which is related to the formation of Laves-phase in the steel matrix. The possible mechanism of stabilizing microstructure during the thermal exposure has been analyzed based on the interaction between M23C6 carbides and subgrain boundaries.

Experimental investigation of thermal inertia properties in hemp-lime concrete walls

Hemp-lime concrete is a sustainable alternative to standard building wall materials, with low associated embodied energy. It exhibits good hygric, acoustic and thermal properties, making it an exciting, sustainable building envelope material. When cast in temporary shuttering around a timber frame, it exhibits lower thermal conductivity than concrete, and consequently achieves low U-values in a primarily mono-material wall construction. Although cast relatively thick hemp-lime walls do not generally achieve the low U-values stipulated in building regulations. However assessment of its thermal performance through evaluation of its resistance to thermal transfer alone, underestimates its true thermal quality. The thermal inertia, or reluctance of the wall to change its temperature when exposed to changing environmental temperatures, also has a significant impact on the thermal quality of the wall, the thermal comfort of the interior space and energy consumption due to space heating. With a focus on energy reduction in buildings, regulations emphasise thermal resistance to heat transfer with only less focus on thermal inertia or storage benefits due to thermal mass. This paper investigates dynamic thermal responsiveness in hemp-lime concrete walls. It reports the influence of thermal conductivity, density and specific heat through analysis of steady state and transient heat transfer, in the walls. A novel hot-box design which isolates the conductive heat flow is used, and compared with tests in standard hot-boxes. Thermal diffusivity and effusivity are evaluated, using experimentally measured conductivity, based on analytical relationships. Experimental results evident that hemp-lime exhibits high thermal inertia. They show the thermal inertia characteristics compensate for any limitations in the thermal resistance of the construction material. When viewed together the thermal resistance and mass characteristics of hemp-lime are appropriate to maintain comfortable thermal indoor conditions and low energy operation.

Finite-element investigation of cold-formed steel portal frames in fire

This paper presents the results of a full-scale site fire test performed on a cold-formed steel portal frame building with semi-rigid joints. The purpose of the study is to establish a performance-based approach for the design of such structures in fire boundary conditions. In the full-scale site fire test, the building collapsed asymmetrically at a temperature of 714°C. A non-linear elasto-plastic finite-element shell model is described and is validated against the results of the full-scale test. A parametric study is presented that highlights the importance of in-plane restraint from the side rails in preventing an outwards sway failure for both a single portal and full building geometry model. The study also demonstrates that the semi-rigidity of the joints should be taken into account in the design. The single portal and full building geometry models display a close match to site test results with failure at 682°C and 704°C, respectively. A design case is described in accordance with Steel Construction Institute design recommendations. The validated single portal model is tested with pinned bases, columns protected, realistic loading and rafters subject to symmetric uniform heating in accordance with the ISO 834 standard fire curve; failure occurs at 703°C.

Effects of temperature and strain rate on the tensile behaviors of SIMP steel in static lead bismuth eutectic

In order to assess the susceptibility of candidate structural materials to liquid metal embrittlement, this work investigated the tensile behaviors of ferritic-martensitic steel in static lead bismuth eutectic (LBE). The tensile tests were carried out in static lead bismuth eutectic under different temperatures and strain rates. Pronounced liquid metal embrittlement phenomenon is observed between 200 °C and 450 °C. Total elongation is reduced greatly due to the liquid metal embrittlement in LBE environment. The range of ductility trough is larger under slow strain rate tensile (SSRT) test.