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.

Analysis of Wind Pressure Data on Components and Cladding of Low-Rise Buildings.

This paper presents a methodology for analyzing wind pressure data on cladding and components of low-rise buildings. The aerodynamic force acting on a specified area is obtained by summing up pressure time series measured at that area's pressure taps times their respective tributary areas. This operation is carried out for all sums of tributary areas that make up rectangles with aspect ratios not exceeding four. The peak of the resulting area-averaged time series is extrapolated to a realistic storm duration by the translation method. The envelope of peaks over all wind directions is compared with current specifications. Results for one low-rise building for one terrain condition indicate that these specifications can seriously underestimate pressures on gable roofs and walls. Comparison of the proposed methodology with an alternative method for assignment of tributary areas and area averaging is shown as well.

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.

Evaluation of Structural Robustness against Column Loss: Methodology and Application to RC Frame Buildings.

A computational methodology is presented for evaluating structural robustness against column loss. The methodology is illustrated through application to reinforced concrete (RC) frame buildings, using a reduced-order modeling approach for three-dimensional RC framing systems that includes the floor slabs. Comparisons with high-fidelity finite-element model results are presented to verify the approach. Pushdown analyses of prototype buildings under column loss scenarios are performed using the reduced-order modeling approach, and an energy-based procedure is employed to account for the dynamic effects associated with sudden column loss. Results obtained using the energy-based approach are found to be in good agreement with results from direct dynamic analysis of sudden column loss. A metric for structural robustness is proposed, calculated by normalizing the ultimate capacities of the structural system under sudden column loss by the applicable service-level gravity loading and by evaluating the minimum value of this normalized ultimate capacity over all column removal scenarios. The procedure is applied to two prototype 10-story RC buildings, one employing intermediate moment frames (IMFs) and the other employing special moment frames (SMFs). The SMF building, with its more stringent seismic design and detailing, is found to have greater robustness.