Maria Garlock

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.

Review and Assessment of Fire Hazard in Bridges

Bridge fires are low-probability but high-consequence incidents. Generally, bridge design codes and standards, in contrast to building codes, do not take into account the concept of fire safety. However, recent high-profile fire incidents on bridges and in other infrastructure have opened a debate on the need for fire resistance requirements on bridges. An overview of fire hazard in bridges is presented. A state-of-the-art review related to the bridge fire hazard was carried out. Different conditions and complexities associated with characterizing fire hazards in bridges are discussed, and a design strategy to integrate performance-based fire safety into bridge design is suggested. Further, a strategy to assess and repair fire-damaged bridges is proposed. A case study is presented to evaluate the fire performance of a composite steel bridge girder. Finally, needed research that can lead to improved performance of bridges during fire incidents is highlighted.

Damage Assessment of the 2010 Chile Earthquake and Tsunami Using Terrestrial Laser Scanning

In the wake of the 2010 Chile earthquake and tsunami, a reconnaissance survey recorded earthquake and tsunami damage using terrestrial laser scanning (TLS), which is capable of detecting details that most traditional reconnaissance methods cannot. TLS enables precise measurements of structural deformations and damage (including shear cracking of concrete walls, concrete spalling, and damage of rebars), as well as soil deformations and damage (including erosion, scour, liquefaction, lateral spread, slope failure, and ground displacement). Advanced measurements such as minute structural rotations, spatial distribution of cracks, volumetric and positional change calculations can also be obtained. Herein, we present various types of detailed measurements and analyses using TLS data obtained at several sites that were damaged by the earthquake and/or tsunami in Concepción, Constitución, Dichato, and Talcahuano. Moreover, this high-resolution data has enabled a unique avenue for virtual, post-visit analysis, providing additional insights that were not readily observable during the field visit.

Closed-Form Procedure for Predicting the Capacity and Demand of Steel Beam-Columns under Fire

During a fire, columns on the perimeter of a building will be subject to moments induced by both a thermal gradient and the restraint of axial expansion by adjacent heated beams, which themselves develop axial load. These members thus act as beam-columns because they are then subject to a combination of axial load plus moment caused by a combination of gravity plus thermal loading. This paper presents a two-pronged procedure to predict the behavior of the perimeter column as a beam-column, considering both the individual member response (including thermal gradients) and the global response (including the interactions of adjacent members). All methods discussed in the paper are closed-form (i.e., they require no iteration) and can therefore be solved by using a spreadsheet or simple mathematical algorithm. The framework is sufficiently simple for use in codified structural-fire design and could be included in a reference of performance-based analysis methods for steel structures. Although this paper specif...

Closed-Form Prediction of the Thermal and Structural Response of a Perimeter Column in a Fire

This paper proposes a simplified closed-form methodology with which to predict the thermal and structural re- sponse of steel perimeter columns in high-rise building frames exposed to fire. Due to their orientation in the building compartment, perimeter columns are heated on three sides and will develop a thermal gradient through their cross- sectional depth. Restraint of the thermal expansion associated with this gradient will cause these members to experience a combination of axial load (P) and bending moment (M), thus acting as beam-columns. At high temperatures, the thru- depth gradient will alter the plastic capacity and mechanical behavior of the perimeter column, leading to plastic P-M be- havior that is not captured under the assumption of uniform cross-sectional temperature. Simplified methodologies are proposed to calculate the following: (1) the thru-depth temperature distribution that develops due to three-sided heating, (2) the gradient-induced changes in plastic capacity, and (3) the gradient-induced changes in demand (i.e. P and M). These methodologies are sufficiently simple for use in code-based design and can be implemented via a spreadsheet because they are closed-form. The individual results of each simple methodology as well as their combination are validated against the results of computational thermal and structural analysis, showing good agreement.

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.

Parameters for Modeling a High-Rise Steel Building Frame Subject to Fire

This paper examines the level of detail and complexity that one needs to incorporate in a computational finite element (FE) model to predict the thermal and structural response of steel high-rise building frames to fire. Comparisons are made between these models in terms of accuracy and efficiency. Performance related to three parameters was examined: (1) the representation of the structural system as a 3-D full frame model versus a 2-D plane-frame model, both of which include the steel frame and the floor slab; (2) the representation of the slab in the 2-D plane frame model; and (3) the effects of modeling the temperature profile of each steel member cross-section as non-uniform (i.e. allowing a thermal gradient to develop) versus uniform. Results indicate that the 2-D plane frame model can be reasonably used in some cases to predict the performance of the perimeter column and floor beams framing into them in a fire-exposed high-rise moment-resisting frame (MRF) with a significant savings in analysis run...

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

Probabilistic performance-based evaluation of a tall steel moment resisting frame under post-earthquake fires

Purpose This paper aims to develop a framework to assess the reliability of structures subject to a fire following an earthquake (FFE) event. The proposed framework is implemented in one seamless programming environment and is used to analyze an example nine-story steel moment-resisting frame (MRF) under an FFE. The framework includes uncertainties in load and material properties at elevated temperatures and evaluates the MRF performance based on various limit states. Design/methodology/approach Specifically, this work models the uncertainties in fire load density, yield strength and modulus of elasticity of steel. The location of fire compartment is also varied to investigate the effect of story level (lower vs higher) and bay location (interior vs exterior) of the fire on the post-earthquake performance of the frame. The frame is modeled in OpenSees to perform non-linear dynamic, thermal and reliability analyses of the structure. Findings Results show that interior bays are more susceptible than exterior bays to connection failure because of the development of larger tension forces during the cooling phase of the fire. Also, upper floors in general are more probable to reach specified damage states than lower floors because of the smaller beam sizes. Overall, results suggest that modern MRFs with a design that is governed by inter-story drifts have enough residual strength after an earthquake so that a subsequent fire typically does not lead to results significantly different compared to those of an event where the fire occurs without previous seismic damage. However, the seismic damage could lead to larger fire spread, increased danger to the building as a whole and larger associated economic losses. Originality/value Although the paper focuses on FFE, the proposed framework is general and can be extended to other multi-hazard scenarios.

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.