Simulating the Inferno: Understanding ISO 834 and Performing Fire Analysis in CalculiX

Fire. It’s a powerful and destructive force, and understanding how structures behave under its intense heat is crucial for ensuring safety and minimizing damage. This is where fire resistance testing and simulation come into play. One of the most widely recognized standards for fire resistance testing is ISO 834, and thankfully, even with free and open-source tools like CalculiX, you can perform realistic thermal analysis simulations to predict structural behavior in fire.

In this blog post, we’ll delve into the fundamentals of ISO 834 and explore how to leverage CalculiX to simulate the thermal impact of this standard fire curve on a structure.

What is ISO 834?

ISO 834 is an international standard that defines a time-temperature curve, often referred to as the “standard fire curve” or “nominal fire curve”. This curve prescribes a specific rate at which the temperature in a test furnace increases over time during a fire resistance test. It’s designed to provide a standardized and reproducible fire exposure for comparing the performance of different structural elements.

The ISO 834 curve is defined by the following equation:

T = T₀ + 345 * log10(8 * t + 1)

Where:

• T is the furnace temperature in degrees Celsius (°C) at time t.
• T₀ is the initial furnace temperature in degrees Celsius (°C), typically assumed to be room temperature (around 20°C).
• t is the time in minutes.

This equation dictates a rapid initial temperature rise, followed by a more gradual increase over time. Understanding this curve is crucial for engineers designing fire-resistant structures.

Why Simulate Fire Exposure?

While physical testing is invaluable, it can be expensive, time-consuming, and destructive. Simulation, particularly using Finite Element Analysis (FEA) tools like CalculiX, offers a cost-effective and efficient way to:

• Predict structural behavior under fire conditions: Determine how temperature distribution affects material properties, stress distribution, and ultimately, structural integrity.
• Identify critical areas: Pinpoint areas most susceptible to failure due to fire exposure.
• Optimize designs: Explore different materials, geometries, and fire protection strategies to enhance fire resistance.
• Reduce physical testing: Use simulations to narrow down design options before committing to expensive physical tests.

Performing Thermal Analysis in CalculiX Using the ISO 834 Curve

Here’s a step-by-step guide to performing thermal analysis in CalculiX using the ISO 834 curve:

1. Pre-processing (CAD Model and Mesh):

• Create a CAD model: Develop a 3D CAD model of the structure you want to analyze using a CAD software like FreeCAD or SolidWorks. Ensure the model accurately represents the geometry and relevant features.
• Mesh generation: Import the CAD model into a meshing software like Gmsh or Netgen. Generate a suitable mesh, using elements appropriate for thermal analysis (e.g., heat transfer elements). Refine the mesh in areas of high temperature gradients or stress concentrations.

2. CalculiX Input File (.inp):

This is where you define the simulation parameters and apply the ISO 834 fire curve. Here’s a breakdown of the key sections:

• *HEADING: Define a descriptive title for your simulation.
• *NODE: Define the coordinates of each node in your mesh.
• *ELEMENT: Define the element connectivity, specifying which nodes form each element. Choose the appropriate element type for heat transfer analysis (e.g., DC3D8 for 8-node brick elements).
• *MATERIAL: Define the material properties of the structural elements. This is critical for accurate fire analysis. You’ll need to define:
  • *DENSITY: Material density.
  • *CONDUCTIVITY: Thermal conductivity as a function of temperature. This is often the most temperature-sensitive property.
  • *SPECIFIC HEAT: Specific heat capacity as a function of temperature.
  • *EMISSIVITY: Surface emissivity for radiation heat transfer.
• *HEAT TRANSFER: This section defines the analysis type as a transient heat transfer analysis.
  • *STEADY STATE: Set to NO for transient analysis.
  • *DELTM: Initial time increment.
  • *TOTAL TIME: Total simulation time (e.g., 3600 seconds for 1 hour).
• *BOUNDARY: Define boundary conditions. You’ll likely need to define the initial temperature of the structure:
  • *INITIAL CONDITIONS, TYPE=TEMPERATURE: Set the initial temperature for all nodes.
• *DLOAD: Apply the ISO 834 fire curve as a surface heat flux. This is the heart of the simulation. Here’s how you can do it:
  • *DLOAD, OP=NEW: Start a new distributed load definition.
  • Surface Selection: You’ll need to define a surface (using *SURFACE) that represents the exposed faces of the structure to the fire. Then reference this surface in the *DLOAD command.
  • BF: Indicates that you’re applying a body flux.
  • ISO834: A user-defined name for the distribution (we’ll define it next).
• *DISTRIBUTION, NAME=ISO834: This is where you define the ISO 834 temperature curve. There are a few ways to do this:
  • Option 1: Tabular Data: Create a table with time and corresponding temperature values calculated from the ISO 834 equation. This is generally the simplest approach. *DISTRIBUTION, NAME=ISO834 0, 20.0 ; Initial temperature (T0) 60, 575.0 ; Temperature at 1 minute 120, 679.0 ; Temperature at 2 minutes 180, 738.0 ; Temperature at 3 minutes ... ; Continue with calculated time-temperature pairs 3600, 945.0 ; Temperature at 60 minutes
  • Option 2: User Subroutine (FORTRAN or Python): You can write a user subroutine to calculate the temperature dynamically based on the ISO 834 equation. This offers more flexibility but requires more programming expertise. You’ll need to compile the subroutine and link it to CalculiX. This is a more advanced technique and outside the scope of this introductory post.
• *RADIATION: Account for radiative heat transfer from the fire to the structure. This is essential for accurate results.
  • *EMISSIVITY: Define the emissivity of the surfaces.
  • *AMBIENT TEMPERATURE: Set to DISTRIBUTION=ISO834, using the same distribution you defined for the DLOAD.
  • *VIEWFACTOR: For complex geometries, you might need to calculate view factors between surfaces. CalculiX doesn’t directly calculate view factors, so you may need to use external tools or make simplifying assumptions.
• *OUTPUT, FIELD, VARIABLE=NT: Request temperature (NT) output. You can also request other variables like heat flux. Adjust the frequency of output as needed.

Example .inp Snippet (Simplified):

*HEADING
ISO 834 Fire Analysis

*NODE
1, 0.0, 0.0, 0.0
2, 1.0, 0.0, 0.0
3, 1.0, 1.0, 0.0
4, 0.0, 1.0, 0.0

*ELEMENT, TYPE=DC3D8
1, 1, 2, 3, 4, 5, 6, 7, 8

*MATERIAL, NAME=Steel
*DENSITY
7850
*CONDUCTIVITY, DEPENDENT=TEMPERATURE
20.0, 50.0
200.0, 40.0
600.0, 30.0
*SPECIFIC HEAT, DEPENDENT=TEMPERATURE
20.0, 450.0
200.0, 500.0
600.0, 600.0
*EMISSIVITY
0.8

*SURFACE, NAME=FireExposed
1, S1

*HEAT TRANSFER, STEADY STATE=NO, DELTM=10, TOTAL TIME=3600

*BOUNDARY
*INITIAL CONDITIONS, TYPE=TEMPERATURE
ALL, 20.0

*DLOAD, OP=NEW
FireExposed, BF, ISO834

*DISTRIBUTION, NAME=ISO834
0, 20.0
60, 575.0
120, 679.0
3600, 945.0

*RADIATION, AMBIENT TEMPERATURE=ISO834
FireExposed, 5.67e-8, 0.8, 20.0

*OUTPUT, FIELD, VARIABLE=NT
*END STEP

3. Running the Simulation:

• Save the input file as a .inp file.
• Open your command line or terminal.
• Navigate to the directory where you saved the .inp file.
• Run the simulation using the command: calculix -i your_file_name (replace your_file_name with the actual name of your .inp file).

4. Post-processing:

• Once the simulation is complete, use CalculiX GraphiX (cgx) or another post-processing tool like ParaView to visualize the results.
• Examine the temperature distribution within the structure over time.
• Analyze the temperature evolution at specific points of interest.
• If you’ve coupled the thermal analysis with a structural analysis (using *COUPLED TEMPERATURE-DISPLACEMENT), you can also visualize stress, strain, and displacement.

Important Considerations and Challenges:

• Material Properties at Elevated Temperatures: Accurate material properties at high temperatures are critical. These properties can significantly degrade in a fire. Refer to material standards and research data for reliable values.
• Meshing: A fine mesh is essential for capturing accurate temperature gradients, particularly near the fire-exposed surfaces.
• Radiation: Radiation heat transfer is a significant factor in fire scenarios. Accounting for view factors and surface emissivities is crucial.
• Computational Cost: Fire simulations can be computationally intensive, especially for complex geometries and long simulation times. Consider using efficient meshing techniques and parallel processing to speed up the simulation.
• Validation: Whenever possible, validate your simulation results against experimental data or established fire resistance tests.

Conclusion:

Simulating fire exposure using the ISO 834 standard and CalculiX can be a valuable tool for understanding and predicting structural behavior in fire. By carefully defining the material properties, boundary conditions, and heat transfer mechanisms, engineers can gain valuable insights into the performance of their designs and optimize them for enhanced fire resistance. While there are challenges to overcome, the benefits of simulation, including cost savings and faster design iterations, make it a worthwhile endeavor for anyone involved in structural fire safety.

---

CloudHPC is a HPC provider to run engineering simulations on the cloud. CloudHPC provides from 1 to 224 vCPUs for each process in several configuration of HPC infrastructure - both multi-thread and multi-core. Current software ranges includes several CAE, CFD, FEA, FEM software among which OpenFOAM, FDS, Blender and several others.

New users benefit of a FREE trial of 300 vCPU/Hours to be used on the platform in order to test the platform, all each features and verify if it is suitable for their needs

---

Contact Name/Surname *

Company Name *

Company address *

City *

State/Province *

Country *CountryArubaAfghanistanAngolaAnguillaÅland IslandsAlbaniaAndorraUnited Arab EmiratesArgentinaArmeniaAmerican SamoaAntarcticaFrench Southern TerritoriesAntigua and BarbudaAustraliaAustriaAzerbaijanBurundiBelgiumBeninBonaire, Sint Eustatius and SabaBurkina FasoBangladeshBulgariaBahrainBahamasBosnia and HerzegovinaSaint BarthélemyBelarusBelizeBermudaBolivia, Plurinational State ofBrazilBarbadosBrunei DarussalamBhutanBouvet IslandBotswanaCentral African RepublicCanadaCocos (Keeling) IslandsSwitzerlandChileChinaCôte d'IvoireCameroonCongo, The Democratic Republic of theCongoCook IslandsColombiaComorosCabo VerdeCosta RicaCubaCuraçaoChristmas IslandCayman IslandsCyprusCzechiaGermanyDjiboutiDominicaDenmarkDominican RepublicAlgeriaEcuadorEgyptEritreaWestern SaharaSpainEstoniaEthiopiaFinlandFijiFalkland Islands (Malvinas)FranceFaroe IslandsMicronesia, Federated States ofGabonUnited KingdomGeorgiaGuernseyGhanaGibraltarGuineaGuadeloupeGambiaGuinea-BissauEquatorial GuineaGreeceGrenadaGreenlandGuatemalaFrench GuianaGuamGuyanaHong KongHeard Island and McDonald IslandsHondurasCroatiaHaitiHungaryIndonesiaIsle of ManIndiaBritish Indian Ocean TerritoryIrelandIran, Islamic Republic ofIraqIcelandIsraelItalyJamaicaJerseyJordanJapanKazakhstanKenyaKyrgyzstanCambodiaKiribatiSaint Kitts and NevisKorea, Republic ofKuwaitLao People's Democratic RepublicLebanonLiberiaLibyaSaint LuciaLiechtensteinSri LankaLesothoLithuaniaLuxembourgLatviaMacaoSaint Martin (French part)MoroccoMonacoMoldova, Republic ofMadagascarMaldivesMexicoMarshall IslandsNorth MacedoniaMaliMaltaMyanmarMontenegroMongoliaNorthern Mariana IslandsMozambiqueMauritaniaMontserratMartiniqueMauritiusMalawiMalaysiaMayotteNamibiaNew CaledoniaNigerNorfolk IslandNigeriaNicaraguaNiueNetherlandsNorwayNepalNauruNew ZealandOmanPakistanPanamaPitcairnPeruPhilippinesPalauPapua New GuineaPolandPuerto RicoKorea, Democratic People's Republic ofPortugalParaguayPalestine, State ofFrench PolynesiaQatarRéunionRomaniaRussian FederationRwandaSaudi ArabiaSudanSenegalSingaporeSouth Georgia and the South Sandwich IslandsSaint Helena, Ascension and Tristan da CunhaSvalbard and Jan MayenSolomon IslandsSierra LeoneEl SalvadorSan MarinoSomaliaSaint Pierre and MiquelonSerbiaSouth SudanSao Tome and PrincipeSurinameSlovakiaSloveniaSwedenEswatiniSint Maarten (Dutch part)SeychellesSyrian Arab RepublicTurks and Caicos IslandsChadTogoThailandTajikistanTokelauTurkmenistanTimor-LesteTongaTrinidad and TobagoTunisiaTürkiyeTuvaluTaiwan, Province of ChinaTanzania, United Republic ofUgandaUkraineUnited States Minor Outlying IslandsUruguayUnited StatesUzbekistanHoly See (Vatican City State)Saint Vincent and the GrenadinesVenezuela, Bolivarian Republic ofVirgin Islands, BritishVirgin Islands, U.S.Viet NamVanuatuWallis and FutunaSamoaYemenSouth AfricaZambiaZimbabwe

Email *

Email

Confirm Email

Privacy *

• I accept the privacy conditions

Privacy Policy

Comment

Send registration