Serdar Selamet

Modeling and Behavior of Steel Plate Connections Subject to Various Fire Scenarios

Shear connections are common connection types and they are designed to resist only shear loads. In a fire event, the axial restraint provided by adjacent structure creates unanticipated compressive and tensile forces in the beam and thus the connection. Using finite-element models, this study examines single-plate shear connections that are bolted to the beam and welded to the supporting girder. A floor subassembly, which includes the beam, girder, slab, and connection, is modeled so that appropriate forces are applied to the connection. The model is validated with the experiments of bolted lap splice plates at elevated temperatures, as well as full-scale experiments. This paper (1) illustrates efficient modeling methods for these floor subassemblies; (2) evaluates the importance of the slab in the connection response; and (3) examines the effects of the rate of heating and cooling on the connection. The results show that care needs to be taken as to how the concrete slab is represented in the model. The heating and cooling rates affect the beam stress distribution, peak temperatures, and peak displacements, but not the peak beam axial force. Also, the cooling phase creates large tensile forces in the connection which can lead to failure.

Plate Buckling Strength of Steel Wide-Flange Sections at Elevated Temperatures

AbstractAt ambient temperature, estimations of the postbuckling strength of steel plates (web and flanges) in wide-flange beams are based on the assumption that the stress at the edge of the plate equals the yield stress of the material. However, at elevated temperatures material behaves in a nonlinear manner beginning at very small strains. The work presented in this paper has shown that at elevated temperatures the ultimate buckling load occurs when stresses at the plate edge are smaller than the yield stress, which are typically defined at large strains such as at 2%. Hence, the current expressions for plate buckling strength at ambient temperature cannot be directly applied at elevated temperature. By taking into account the nonlinear behavior of steel at elevated temperatures, a new postbuckling strength equation for webs and flanges in wide-flange beams that correlates well with finite-element studies at elevated temperatures is proposed.

LOCAL BUCKLING STUDY OF FLANGES AND WEBS IN I-SHAPES AT ELEVATED TEMPERATURES

Local buckling in floor beams has been one of the important observations in several fire events in steel buildings such as World Trade Center Tower 7 and large-scale fire experiments such as Cardington in UK. Utilizing three dimensional finite element methods for complex geometry and nonlinear behavior of such connections, local buckling of the web followed by the buckling of the lower flange is observed to occur in early stages in fire, which causes instability to the floor system, and a reduction in the connection strength. To fully capture the behavior of floor systems, one needs to be able to predict such buckling behavior of the beam. This paper contributes to such knowledge by investigating the local buckling of floor beams at elevated temperatures using nonlinear finite element models. The results are compared to AISC provisions of plate buckling under ambient and elevated temperatures. I - Introduction Recent experimental and finite element (FE) observations [Moore 2003; Garlock and Selamet 2010] show that local buckling of a floor beam in the vicinity of the connection greatly reduces the axial capacity of the beam during both the heating and the cooling period in a natural fire. During the heating phase, the beam is under compression and the axial forces increase until the lower flange buckles at which point the compressive forces decrease. The deformations caused by the buckling near the connection reduce the tensile capacity of the connection [Selamet and Garlock 2010]. Therefore local buckling controls the maximum compressive and tensile force that a beam experiences in a fire. The aim of this paper is to investigate the strength of wide flange beams considering local buckling under fire conditions. Previous research on local buckling of steel members focused on isolated plate buckling studies without consideration of the flange (an “unstiffened plate”) and the web (a “stiffened plate”) interacting with each

Symmetric and Asymmetric Collapse Mechanisms of a Multi-Story Steel Structure subjected to Gravity and Fire

Fire risks pose significant threats to the integrity and stability of the multi-story steel structures. The robustness of these structures against a fire hazard requires further attention. The progressive collapse of a high-rise structure is detrimental not only to the inhabitants in the building but also to the surrounding infrastructure. The collapse of World Trade Center Twin Towers showed that the impact location and the fire distribution could cause symmetrical as well as asymmetrical types of total collapse. This study investigates the uncoupled structural-thermal response of a 49 story steel high-rise structure. The structural load carrying system of the high-rise structure is assumed to be a moment resisting frame. The results show that the structural response and the progressive collapse differ depending on the fire spread and it does not significantly change due to the fire location as long as the fire is contained on a single floor. This study intends to provide a better understanding of the effect of fire loading leading to the collapse mechanism of a multi-story steel structure.

A Comparison between the Single Plate and Angle Shear Connection Performance under Fire

The strength and stability of connections in a floor system is an integral part of a building structure. A connection is subjected to large compressive and tensile forces during heating and cooling phase of a fire, respectively. Since shear connections are only designed for gravity loads that produce shear, their behavior in a floor assembly at elevated temperatures needs to be investigated. This paper compares the behavior of three types of shear connections (single plate, single angle and double angle) under fire conditions using the finite element software ABAQUS. The single plate shear connection was validated by a full-scale building fire tested in Cardington. Adopting Eurocode and AISC provisions on the shear connection design, the Cardington connection was redesigned using the single and double angles. While the single plate connections can provide substantial rotational ductility and tensile strength, it could fail during cooling phase of a fire by bolt-hole bearing or bolt shear. The bolted double angle connections are generally more ductile in tension which is advantageous during cooling phase; however they are prone to develop prying forces which could cause the failure of the bolts. In all of the connection models, the beam near the connection experiences local buckling at elevated temperatures.

Thermal Gradient Estimation due to Surface Heat Exchange in Steel I-Sections

AbstractFire poses a great threat to steel structures. An accurate estimation of the temperatures in structural member cross sections is the first and most crucial step to calculate fire-induced fo...

Fire performance of single plate shear connections in a composite floor

Purpose This paper aims to present a numerical investigation on the fire performance of a single plate shear connection in a steel-framed composite floor. Large-scale fire experiments show that the tensile membrane action of the concrete slab enhances the fire performance of composite floors. The enhancement in the performance is contributed to large slab deflections. However, these deflections cause significant rotations and tensile force in the single plate connection. Design/methodology/approach A finite element model is constructed, which consists of a secondary steel beam, concrete slab and shear connection components. The interaction between the connection components such as bolts and single plate is defined by contact surfaces. The analysis is conducted in two uncoupled phases: thermal analysis by creating fire boundaries on the composite floor model with convective and radiative heat transfer, and mechanical analysis by considering thermal expansion and changes in the material stiffness and strength due to temperature. Findings The thermo-mechanical analysis of the composite floor finite element model shows that the structure survives the 2-h Standard fire, but the connection fails by bolt shear and buckling of the connection plate. Originality/value This paper investigates the fire performance of a shear connection in a steel-framed concrete slab. Previous work generally focused on the concrete slab behavior only. The originality of the research is that the connection is considered as part of a sub-assembly and is subjected to forces due to concrete and steel beam interaction.

Modified Connection Details for Single Plate Steel Connections under Fire

Single plate (shear tab) connections connect a beam to a girder or to a column, and they are designed to resist only shear loads. In a fire event, the axial restraint provided by adjacent structure creates unanticipated compressive and tensile forces in the beam and thus the connection. Using finite element (FE) models with contact elements, this study examines shear tab connections under fire. The model, validated by experimental data, represents the connection as well as a portion of the surrounding structure in a composite floor framing system. By extending the model beyond the connection region, the combination of shear, axial force and moment are properly represented in the connection during the fire. Previous studies have shown that large beam rotations and tensile forces result in bolt tear out (beam bearing) or bolt shear failure. This study shows how some simple modifications in the connection model affects the response (if at all).