Effect of Location of Axial Restraint on Beam-Columns behavior in Fire Mahmud M.S. Dwaikat Civil & Env. Engineering Michigan State University

Outline Background: Beam-Columns Modeling Location of Axial Restraint Finite Element Model Idealization Validation Parametric Studies Effect of Fire Scenario + Beam Length Effect of Beam Slenderness Ratio Conclusions

Background Steel Beam-Columns L At room temperature steel beams are designed for flexure Axial Restraint Under fire, steel expands non-uniformly due to thermal expansion The restrained member will develop axial force and bending moment because of the restraints Expansion = Δℓ The member will no longer behave like a beam, but like a beam-column Axial force Bending moment Beam-column

Modeling Location of Restraint In real situations, location of axial restraint is variable

Finite Element Model Idealization ANSYS software – SUR151 and PLANE55 elements for thermal analysis – SHELL93 element for structural analysis Steel temperature is independent of its stress state. Non-uniform temperature over the cross-section, and uniform along the length of the member. ANSYS Creep Model 11: Generalized high-temperature creep - Time hardening, including primary and secondary creep strains L/3 P ks 1D mesh using SURF151 element 2D mesh using PLANE55 element 1 I J x y K SURF151 1D-thermal Surface-effect element 4 1 2 3 I J K L SHELL93 element PLANE55 2D-thermal solid element Thermal Discretization Structural Model

Model Validation Fire Test Data (Li and Guo 2007) Restrained beams tested under design fires No local buckling, no lateral torsional buckling Unprotected steel beams H250X250X8X12 Fire exposure H300X300X12X20 H300X300X16X25 Ceramic fibre blanket 3 mm thickness 4.5 m 150 300 450 600 Temperature ˚C 750 Time min 4 12 20 28 36 42 900 H250X250X8X12

Model Validation Thermal Response H250X250X8X12 Ceramic fibre blanket 3 mm thickness Test data 900 Fire Bottom flange 750 Web middle Top flange 600 450 Temperature ˚C 300 150 Model predictions 4 12 20 28 36 42 Time min

Model Validation Structural Response

Parametric Studies Fire Scenarios – Beam Length ks W24x76

Parametric Studies Fire Scenarios – Beam Length W24x76 L/3 P ks

Parametric Studies Beam Slenderness Ratio Exposed L/3 P ks

Conclusions The location of the axial restraint in a beam exposed to fire has a significant influence on the fire performance of the beam. Shifting the axial restraint in the beam to the bottom flange greatly enhances the ultimate fire resistance for beams with low to moderate slenderness ratio (L/r ≈ 20). For beams with high slenderness ratio, the location of the axial restraint has little influence on the ultimate fire resistance of the beam. Fire scenario does not affect the influence of shifting the axial restraint on the fire resistance of the beams. This because fire scenarios affect the heating rate, and has little effect on the thermal gradient developed in the section

Acknowledgment Thank you!