Critical Factors Governing Crack Propagation at the Interface of Fire Insulation and Slender Steel Trusses

This article presents a numerical approach in which the implicit finite element method and fracture mechanics concepts are applied to simulate crack propagation at the interface of fire insulation and truss members in steel framed buildings. An intrinsic cohesive zone model (CZM) in conjunction with contact interaction analysis is applied to model the progression of fracture at the interface of fire insulation and slender steel truss members. Experimentally determined cohesive zone properties are utilized to simulate the progressive delamination in three types of commercially available spray-applied fire-resistive material (SFRM) applied on a truss chord. The numerical model, which is initially validated against the previously conducted fracture experiments, is employed to perform a sensitivity analysis with respect to CZM parameters, SFRM elastic modulus, and thickness of SFRM. Results obtained from a sensitivity study are subsequently utilized to define a delamination characteristic parameter ($d_{ch}$) that could represent the interdependency among the influencing factors, namely elastic modulus, thickness, fracture energy, and displacement ductility over the cohesive zone. Further, the strain ductility demand in steel, at which delamination of SFRM is initiated and subsequently gets completely detached, is quantified and related to $d_{ch}$. Results from analysis show that there is a power-law relationship between $d_{ch}$ and strain ductility demand of steel at the onset of delamination and complete detachment. For instance, by increasing the $d_{ch}$ value from 0.2 to 2, the strain ductility demand of steel at the onset of delamination dramatically reduces from 18 to 6; however, beyond a $d_{ch}$ value of 2, a steady trend is noticed.