Reduced-Order Modeling of Composite Floor Slabs in Fire. I: Heat-Transfer Analysis.

This paper presents a reduced-order numerical modeling approach for the analysis of heat transfer in composite floor slabs with profiled steel decking exposed to fire effects. This approach represents the thick and thin portions of a composite slab with alternating strips of shell elements, using a layered thick-shell formulation that accounts for both in-plane and through-thickness heat transfer. To account for the tapered profile of the ribs, layered shell elements representing the thick portion of the slab adopt a linear reduction in the density of concrete within the depth in the rib. The specific heat of concrete in the rib is also proportionally reduced to indirectly consider the heat input through the web of the decking, because the reduced-order model considers thermal loading only on the upper and lower flanges of the decking. The optimal ratio of modified and actual specific heat of concrete in the rib is determined, depending on the ratio of the height of the upper continuous portion to the height of the rib. The reduced-order modeling approach is validated against experimental results.

Reduced-order modeling of composite slabs in fire. II: Thermal-structural analysis.

This paper describes a reduced-order modeling approach for thermal and structural analysis of fire effects on composite slabs with profiled steel decking. The reduced-order modeling approach, which uses alternating strips of layered shell elements to represent the thick and thin portions of the slab, allows both thermal and structural analyses to be performed using a single model. The modeling approach accounts for the trapezoidal profile of the concrete in the ribs; the structural resistance provided by the steel decking, including the webs of the decking; and the orthotropic behavior of the decking, which provides greater resistance along the ribs than transverse to the ribs. The modeling approach is validated against experimental data from one-way composite slabs tested under ambient-temperature, a one-way composite slab tested under fire conditions, and a two-way composite slab tested under fire conditions. Both implicit and explicit solution schemes are evaluated for the structural analysis, and the results show that it is feasible to scale down the hours-long fire duration to a simulation time of seconds in an explicit dynamic analysis, without adversely affecting the accuracy of the results. The steel decking contributes significantly to the structural resistance at ambient temperature, but as expected, its contribution is found to decrease rapidly under fire exposure. The modeling approach can account for the location of reinforcing bars (i.e., at a specified depth in either the thick or thin portion of the slab), and it is found that reinforcement location can have a significant effect on the structural response, because heat transfer in the composite slab results in higher temperatures in the thin portions of the slab between the ribs.

An Empirical Component-Based Model for High-Strength Bolts at Elevated Temperatures.

High-strength structural bolts are used in nearly every steel beam-to-column connection in typical steel building construction practice. Thus, accurately modeling the behavior of high-strength bolts at elevated temperatures is crucial for properly evaluating the connection capacity, and is also important in evaluating the strength and stability of steel buildings subjected to fires. This paper uses a component-based modeling approach to empirically derive the ultimate tensile strength and modulus of elasticity for grade A325 and A490 bolt materials based on data from double-shear testing of high-strength 25 mm (1 in) diameter bolts at elevated temperatures. Using these derived mechanical properties, the component-based model is then shown to accurately account for the temperature-dependent degradation of shear strength and stiffness for bolts of other diameters, while also providing the capability to model load reversal.

Thermal performance of composite slabs with profiled steel decking exposed to fire effects.

This paper presents a systematic investigation of the influence of various parameters on the thermal performance of composite floor slabs with profiled steel decking exposed to fire effects. The investigation uses a detailed finite-element modeling approach that represents the concrete slab with solid elements and the steel decking with shell elements. After validating the modeling approach against experimental data, a parametric study is conducted to investigate the influence of thermal boundary conditions, thermal properties of concrete, and slab geometry on the temperature distribution within composite slabs. The results show that the fire resistance of composite slabs, according to the thermal insulation criterion, is generally governed by the maximum temperature occurring at the unexposed surface of the slab, rather than the average temperature. The emissivity of steel has a significant influence on the temperature distribution in composite slabs. A new temperature-dependent emissivity is proposed for the steel decking to give a better prediction of temperatures in the slab. The moisture content of the concrete has a significant influence on the temperature distribution, with an increment of 1 % in moisture content leading to an increase in the fire resistance of about 5 minutes. The height of the upper continuous portion of the slab is found to be the key geometrical factor influencing heat transfer through the slab, particularly for the thin portion of the slab. Heat transfer through the thick portion of the slab is also significantly affected by the height of the rib and the width at the top of the rib.

Component-Based Model for Single-Plate Shear Connections with Pretension and Pinched Hysteresis.

Component-based connection models provide a natural framework for modeling the complex behaviors of connections under extreme loads by capturing both the individual behaviors of the connection components, such as the bolt, shear plate, and beam web, and the complex interactions between those components. Component-based models also provide automatic coupling between the in-plane flexural and axial connection behaviors, a feature that is essential for modeling the behavior of connections under column removal. This paper presents a new component-based model for single-plate shear connections that includes the effects of pre-tension in the bolts and provides the capability to model standard and slotted holes. The component-based models are exercised under component-level deformations calculated from the connection demands via a practical rigid-body displacement model, so that the results of the presented modeling approach remains hand-calculable. Validation cases are presented for connections subjected to both seismic and column removal loading. These validation cases show that the component-based model is capable of predicting the response of single-plate shear connections for both seismic and column removal loads.

Integrity of Bolted Angle Connections Subjected to Simulated Column Removal.

Large-scale tests of steel gravity framing systems (SGFSs) have shown that the connections are critical to the system integrity, when a column suffers damage that compromises its ability to carry gravity loads. When supporting columns were removed, the SGFSs redistributed gravity loads through the development of an alternate load path in a sustained tensile configuration resulting from large vertical deflections. The ability of the system to sustain such an alternate load path depends on the capacity of the gravity connections to remain intact after undergoing large rotation and axial extension demands, for which they were not designed. This study experimentally evaluates the performance of steel bolted angle connections subjected to loading consistent with an interior column removal. The characteristic connection behaviors are described and the performance of multiple connection configurations are compared in terms of their peak resistances and deformation capacities.

New Component-Based Model for Single-Plate Shear Connections with Pre-tension.

Although simple shear connections are typically idealized as perfectly pinned, the actual resistance of the gravity framing system to flexural and axial loads can be critical in evaluating the robustness and stability of steel buildings subjected to extreme loads such as earthquakes, fire, and column loss. There are several key reasons for including more realistic connection behaviors in the design and analysis of steel buildings for extreme loads: (i) the gravity connections may develop large localized deformations under combined flexural and axial loading, potentially precipitating their failure (e.g. due to local buckling, fracture of the bolts, etc.), (ii) the gravity connections provide critical lateral bracing to the columns, and failure of connections could lead to global instability (potentially resulting in disproportionate collapse), and (iii) accurately accounting for contributions from the gravity system in design could effectively reduce the demands on the lateral load-resisting system, thus reducing costs. In order to include contributions from the steel gravity frames in structural analysis and design, validated and computationally efficient analysis tools are needed. This paper describes a component-based model for single-plate shear connections that includes the effects of pre-tension and accommodates both standard and slotted holes, accounting for deformations associated with bolt slip, bolt bearing, and bolt shear. The model also accounts for load reversals and pinching effects associated with hysteresis, thus providing the capability to model the connections under arbitrary in-plane load histories. Validation cases show that the model is capable of simulating connection response under both earthquake and column removal loading.