Fire Performance of a Steel Substructure
tall steel buildings, progressive collapse, fire, substructure, thermal elongation, World Trade Center Towers
This research aims to investigate structural response of tall buildings to high temperatures during an intense fire. Designing structures resistant to high temperature fire has become an important area in engineering field after 9/11 attacks. The main cause of catastrophe was not the impact of airplanes into the buildings, but the geometric instability of column/beam structural system due to wide-spread intense fire after impact. Most of the high-rise buildings such as WTC towers are not designed to resist long duration of such high temperatures and thus, a new method should be developed to make buildings resistant to intense fire. The progressive collapse is initiated by the floor beams, which expand and push the columns outwards. Once the beams lose their axial stiffness, columns are pulled inwards, thus the overall stability of the building is weakened. Excessive thermal expansion over time is the cause of the instability of columns. Thermal expansion could be reduced using floor systems composed of steel and carbon epoxy sheets, which have a lower thermal expansion coefficient. A 3-floor building model proves that overall lateral outward deflection of columns is reduced significantly. Reduction of 35% in lateral deflection is obtained using 0.5” thick carbon/epoxy (C/EP) sheets.
Selamet S, Hueckel T (2006). Design of Intrinsic Safety Mechanisms in Tall Buildings under a Thermal Attack. Pratt Research Fellowship Article, Duke University, NC.
Fire Performance of Steel Shear Connections
lap-joints, shear connections, fire, substructure, finite element, contact mechanics, Cardington building test
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.
A robust design for single plate connections is needed to overcome fire induced forces and moments during both fire growth and fire decay in order to improve the structural integrity of floor systems. Using finite element studies validated by a full-scale frame Cardington building test, we investigated simple and cost-effective modifications of single plate connections to resist the forces and deformations induced by thermal loads during a real fire scenario. Results show that significant improvements in the behavior of such connections could be achieved by any of the following: adding a doubler plate to the beam web, matching the single plate thickness to the beam web thickness, using a larger distance from the bolt-hole centerline to the beam end and increasing the gap distance between the end of the beam to the connected member. Using larger bolt holes can also improve the fire performance through less axial restraint and thus more freedom for the beam to move (expand, contract, rotate) with the fire-imposed thermal loads.
Glassman JD, Garlock M, Selamet S (2012). Shear buckling behavior of steel plate girders at elevated temperatures. Structures. Accepted for Proceedings of the 7th International Conference on Structures in Fire (SIF), Zurich, Switzerland.
Selamet S, Garlock ME, Papadopoulos P (2012). Steel connection finite element modeling under ambient and fire temperatures. Journal of Constructional Steel Research, under review.
Selamet S, Garlock ME (2010). Robust fire design of single plate shear connections. Engineering Structures, 32(8): 2367 – 2378.
Garlock ME, Selamet S (2010). Modeling and behavior of steel plate connections subject to various fire scenarios. Journal of Structural Engineering, 136(7): 897–906.
Selamet S, Garlock M (2010). Improved details for fire-induced steel single plate shear connections. In Proceedings of the 6th International Conference on Structures in Fire (SIF), pages 621–628, East Lansing, MI.
Selamet S, Garlock M (2010). Guidelines for modeling three dimensional structural connection models using finite element methods. In International Symposium Steel Structures: Culture and Sustainability 2010, pages 351–360, Istanbul, Turkey. ECCS.
Selamet S, Garlock ME (2009). Modified connection details for single plate steel connections under fire. In Structures Congress: Don’t Mess with Structural Engineers, pages 642–649, Austin, TX. ASCE.
Selamet S, Garlock M (2008). Behavior of steel plate connections subject to various fire scenarios. In Proceedings of the 5th International Conference on Structures in Fire (SIF), pages 139–149, Singapore.
Local Buckling during Fire
local buckling, AISC buckling curves, fire, connection, axial force, plate buckling
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 building test in U.K. 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 of fire, which causes instability to the floor system, and a significant reduction in the connection strength. The observations also suggest that the maximum compression in the floor beam is limited to the buckling capacity of the web and flanges near the connection. This research contributes to such knowledge by investigating the local buckling of floor beams for different connection types at elevated temperatures using nonlinear finite element models. Moment connections are found to be more resistant to local buckling when compared to the shear connections. The results are also compared to the AISC design equation for plate buckling under ambient and elevated temperatures. Compared to the finite element analyses of this study, it is observed that at ambient temperature the AISC curve conservatively captures the buckling capacity of webs and flanges; at higher temperatures, AISC overestimates the capacity.
At 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 research 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 post-buckling strength equation for webs and flanges in wide-flange beams that correlates well with finite-element studies at elevated temperatures is proposed.
Related Publications:
Selamet S, Garlock ME (2013). Plate buckling in wide-flanged beams considering nonlinear steel behavior at elevated temperatures. Journal of Structural Engineering, ASCE 139 (11): 1853-1865.
Selamet S, Garlock ME (2012). Predicting the maximum compressive beam axial force during fire considering local buckling. Journal of Constructional Steel Research 71: 189-201.
Selamet S Garlock ME. (2010). Local buckling study of flanges and webs in I-shapes at elevated temperatures. In Structures Congress, pages 1592–1603, Orlando, FL. ASCE.
Fire Experiment of a Compartment with Angle Shear Connections
fire experiment, floor compartment, angle connections, fire
Experimental results of double angle connections with thermally-induced loading are presented in this paper. Two beam assemblies were tested inside a furnace by simulating typical building fire conditions. These beams were connected to a restrained steel frame outside of the furnace with double angle connections. The test variables included fire scenario, load level and composite action arising from beam-slab effect. Data generated from the fire tests indicate that double angle connection exhibit inherent rotational rigidity in spite of being designed as simple shear connections and are capable of transferring thermally-induced moments. Despite undergoing permanent deformations, no failure occurred in the tested assembly, thus illustrating the ductile behavior of the double angle connections.
Related Publications: Pakala P, Kodur V, Selamet S, Garlock M (2012). Fire behavior of shear angle connections in a restrained steel frame. Journal of Constructional Steel Research 77: 119-30.
Fire Performance Comparison of Single Plate, Single Angle and Double Angle Connections
shear connections, angle connections, single plate connections, fire, finite element modeling
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 research 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 re-designed 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.
Related Publications:
Selamet S, Garlock ME (2013). Fire Resistance of Shear Connections. Fire Safety Journal, under review.
Selamet S, Garlock ME (2012). A comparison between the single plate and angle shear connection performance under fire. In Structures Congress: Don’t Gamble on your Future, Las Vegas, NV. ASCE.