Structural fire design is exceedingly adopting the performance based approach. There are evidentadvantages of this approach compared to the prescriptive methods from codes. An analytical procedure, based on thereal performance, must accurately predict the beam behaviour in fire. The study presented here proposes one suchsimplified analytical procedure aim to predict the real behaviour of a restrained steel beam. The proposed analyticalprocedure is validated through FE Analysis using FE models validated through test results. The study also attempts toestablish the importance of using semi-rigid connection strength with respect to accurately predicting the behaviorof the restrained beam at catenary stage.

The methods proposed by the design codes for single member design in fire situation assume that these members are isolated in their response. The real response of structural members such as beams is, however, more complex due to thermal expansion and the presence of restraints against this expansion by the surrounding structure. It is therefore imperative to study the response of structure at high temperature in a way which includes its interaction with its surroundings such as in a full-scale fire test and in numerical analysis. This paper focuses on the numerical investigation of steel beams, with a concrete slab and connected to concrete filled tubular (CFT) columns through reverse channel connections. The finite element software ABAQUS has been used in this study. The aim of the investigation is to study the behaviour of the composite steel-concrete beam exposed to increasing temperature in fire.

The so-called Reverse Channel connection has been conceived for the purpose of accommodating the thermal expansion of beams so that premature failure due to thermal buckling is avoided. The connection is made of a channel-shaped element, welded along the tips of its flanges onto the face of a hollow section column; an endplate welded on the beam is bolted onto the web of the channel. In a fire situation, the thermal expansion of a reverse-channel supported beam causes extensive bending deformation of the connection, therefore preventing the development of significant axial stress in the beam. Furthermore, this connection offers a high rotational capacity, if designed properly, which is beneficial in a fire situation where excessive deflections of beams can be expected. This paper aims to provide analytical stiffness assessment tools for reverse channel connections in compression and tension under uniform temperatures. The proposed analytical models are compared to results of Finite Element simulations, which in turn have been benchmarked with experiments. In addition, a comprehensive parametric study is conducted in order to identify all influencing factors on the initial stiffness response: reverse channel geometry and thickness, plate thickness, bolt position, and bolt diameter. Correction factors that account for 3D effects and bolt size are presented and discussed. The obtained expressions for the reverse channel stiffness are found to provide an accuracy that is acceptable for structural applications and can, therefore, be used as a design tool.

Sweden has a strong demand on the construction of student accommodations and therefore significant efforts have been taken towards an affordable and easy solution of the problem. A concept combining these requirements may be based on the use of structural steel frames in combination with prefabricated 3D modules fully equipped and suitable for student accommodations. Therefore, the need to investigate and develop a system suitable for an effective assembly of student residences is considered in this paper, as part of an international project, Optimization of the frames for effective assembling - FRAMEUP. The Fig. 1 reveals an overview of the system within the execution process.The Frameup system introduces a new approach in terms of execution technique which consists of the execution of a building starting from the roof to the 1st floor. The existence of a lifting system constituted of a horizontal rigid frame - grid - in combination with lifting towers - pylons - permits the erection of the building, promoting each time the building is lifted, a clearance of one-floor-height plus tolerances at the ground level. This creates room enough for the assembly of the lower floor from below the previously assembled floor. The procedure is repeated several times according to the number of floors until the 1st floor of the building, the last floor of the execution sequence, is assembled. Moreover the Frameup system introduces an innovation, the Frameup conveyor system, which streamlines the assembly process so to move/slide the elements, as they come, directly from the lorry to their final position in the building.The development of the Frameup system benefits from a stepwise detailed 3D modeling and structural analysis and design tools. However, when it comes to attest the reliability and efficiency of the system, a full scale feasibility test is essential and it is performed on the majority of the sequences of construction.

The design methods currently proposed by the codes prescribe the strength assessment of structures to be based on their strength limit state. These design methods can be applied to isolated steel members to determine their design strengthin fire. The real response of a structural member is, however, more complex due to the thermal expansion and the presence of restraints against this expansion by the surrounding structure. It is therefore imperative to study the response of a structural member at high temperature in a way which includes its interaction with its surroundings. This paper focus on the numerical investigation of steel beams in structural frames connected to concrete filled tubular (CFT) columns through reverse channel connections and comparison to hand calculation procedures. Finite element models (FEM) of the sub-frames were validated against fire tests conducted on sub-frames and then their results were compared to the proposed simplified hand calculation procedures (HCM).

The novel joint presented in this paper is a friction connection used for column-splice connections of modular buildings as part of the innovative construction method introduced in the research project Optimization of frames for effective assembling - FRAMEUP. This type of joint provides a quick assembly and can deal with misalignments by introducing a connection gap. A filler and finger plate are welded to the upper part of the column to this end.The gap between finger plates and lower column faces is closed during tightening of the bolts and, thus, establishes a slip-resistant connection. The efficiency of the joint resistance based on different connection gaps subjected to uniform compression is assessed.The column-splice is composed of four slip-resistant connections, one at each side of the tube. Each finger plate consists of three long slotted holes and is welded to the upper column face. Long slotted holes are used to accommodate vertical misalignments and, therefore, allow fitting the bolts which are pre-installed in the lower column. Filler plates with different thicknesses (4, 6 and 8 mm) welded between the finger plate and upper column face are used to create a connection gap which allows balancing horizontal misalignments. The lower column faces consist of each nine holes with no clearance in order to pre-fit the bolts in a workshop. Thus, the assembling process on the construction site can be speeded up as once the lowercolumns are in place all bolts can be tightened immediately.

The robustness of a structure in a fire situation greatly depends on the rotational capacity of the connection region. High rotational capacity is required at elevated temperatures since the steel beams lose their bending stiffness and exhibit increasingly large deflections under constant load. Beam deflections result in increasing rotations at the supports and may lead to collapse due to connection failure. The reverse channel has been proposed as a practical alternative to assemble beams to tubular columns. In a simple implementation, the bending moment generated in the joint due to rotation of the beam may be neglected; however, research efforts are being attempted to quantify the level of constraint. The typical arrangement of the connection type consists of a reverse channel with its flanges welded onto the face of concrete-filled tubular columns and the web bolted to the endplate of a beam. Thicknesses and depths of the reverse channel determine the level of rotational restraint at high temperature. The reverse channel has the ability to undergo catenary deformation in the tensile zone due to the applied rotation at the support and similarly it is relatively ductile in the compression zone. Overall, the reverse channel connection response is rather ductile in terms of its ability to undergo large rotational deformation as long as bolt failure is avoided through proper design.Various tests have been performed to study the behaviour of this type of connection such as full scale buildings, sub-frames, isolated joints and individual sections. The aim of these tests was to capture the connection behaviour in relation to other structural components in fire. This paper, however, focuses on the derivation and verification of analytical models to assess the initial stiffness of reverse channel/partial-depth endplate connections. The results from finite element analyses have been utilized to validate analytical models that describe the behaviour of this type of connection at ambient and elevated temperature. Insight into the analytical models provides proper background to a structural designer to estimate the initial stiffness and understand the behaviour of the reverse channel in the connection.