Fire resistance simulation using LS-DYNA Mikael Schill
Background The aim of the simulations is to find the collapse of a structure due to fire The increase in temperature reduces the properties of the material and subsequently the critical load is reached Further, it is of interest to determine the position and amount of Passive Fire Protection (PFP) through FE-simulation Franssen, J.M. and Real, P.V. Fire design of steel structures, ECCS Eurocode Design Manuals, Ernst & Sohn, Berlin Gemany, 2010
Plastic Work Displacement Solver options Can be solved using either a coupled or un-coupled approach Thermal Solver Implicit Double precision Thermal Solver Implicit Double precision MPP/SMP Temperature ONLY SMP Temperature Mechanical Solver Implicit / Explicit Double precision / Single precision Mechanical Solver Implicit / Explicit Double precision / Single precision
Solver options Coupled solution: Staggered approach Mechanical t mech t mech Thermal t thermal t thermal t thermal Uncoupled approach Thermal Temperatures Mechanical
Mechanical and thermal properties Both a mechanical and a thermal material is needed Mechanical Young s Modulus Poisson s ratio Heat expansion Thermal Specific heat Thermal conductivity Yield stress Material hardening All input is temperaturedependent
Thermal Boundary conditions The heat flux is applied as a convective and/or a radiation boundary condition. q Total = q conv + q rad The convection/radiation coefficients can be prescribed as a function of time or as a function of the surrounding temperature.
Example: I beam Flexural-Torsional Collapse Paik, J., Czujko, J., Kim, J. Park, S., Islam, S. and Lee D. A New Procedure for the Nonlinear Strucural Response Analysis of Offshore Installations in Fires, SNAME Annual Meeting, Bellevue, Washington, 2013
Example: Simulation of topside Paik, J., Czujko, J., Kim, J. Park, S., Islam, S. and Lee D. A New Procedure for the Nonlinear Strucural Response Analysis of Offshore Installations in Fires, SNAME Annual Meeting, Bellevue, Washington, 2013
Passive Fire Protection (PFP) modeling In fire resistance simulation software, PFP (passive fire protection) is modeled using a convection function. Thus, the heat transfer is expressed as: q = h i (T s T e ) where T e is the temperature of the structure,t s is the temperature on the insulation surface and h i is the film coefficient (heat transfer coefficient). By this, it is assumed that the PFP has no thermal mass.
PFP modeling Solid elements The PFP can be modeled using one layer of solid elements. The thickness of the solid PFP element could be equal to the actual thickness of the PFP layer. The heat conduction, hc, of the PFP material is then calculated through: hc = h i t i where t i is the thickness of the PFP. If the solid elements are of different thickness than the PFP material, e.g. for pre-processing reasons, the heat conduction is calculated through: hc = h i t solid t i where t solid is the thickness of the solid element.
PFP modeling Shell elements The shell can be built up by several different materials through the thickness using *PART_COMPOSITE Mechanical material Thickness Thermal material Easy to vary the thickness of the PFP Reducing the number of elements Reducing the pre-postprocessing
PFP modeling PFP BEAM
I-Beam with PFP example All solids (coupled) Shells + Solid PFP(coupled) Composite PFP + Shells (uncoupled)
I-Beam with PFP example
I-Beam with PFP example
Conclusion Fire resistance simulations in LS-DYNA are performed using the thermal and implicit mechanical solvers. The solvers can be either coupled or un-coupled. LS-DYNA has a vast number of mechanical and thermal material models necessary to handle the temperature dependencies Passive Fire Protection (PFP) can be modeled using either solid elements or stacked shell elements for simple pre-processing.
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