Did you know that you could use Consteel to perform structural analysis at room and elevated temperatures as part of design process for fire resistance?

In structural fire engineering, the mechanical response of steel structures must be evaluated under both room and elevated temperature conditions. Consteel permits this by incorporating temperature-dependent material behavior directly into the finite element analysis, allowing engineers to assess not only resistance but also changes in global structural response.

During fire analysis, Consteel determines the steel temperature and applies the corresponding reduction in material properties, most notably the modulus of elasticity and yield strength. These reductions are defined according to Eurocode 3 (EN 1993-1-2). As a result, the calculated internal forces and deformations reflect both the applied loads and the effects of thermal expansion and stiffness degradation. The analysis is performed on the global structural model, so compatibility effects and force redistribution are inherently captured.

Fire exposure is defined using nominal fire curves (Standard, External, Hydrocarbon), together with a specified fire resistance time. In addition, the model allows assignment of fire protection conditions, including unprotected members, hot-dip galvanized surfaces, and protected elements with either passive insulation or reactive (intumescent) coatings. These definitions influence the temperature development in the structural members and, consequently, their mechanical response.

For design verification, Consteel applies the resistance models of EN 1993-1-2. Cross-section resistance is calculated using temperature reduction factors​, depending on the type of internal force and cross-section class. Checks are performed for tension, compression, bending, and shear, as well as their interaction. For global stability, the software uses the Eurocode general method with modified buckling curves and reduction factors adapted for elevated temperatures.

In addition to elevated-temperature analysis, Consteel supports a complementary approach based on room-temperature analysis for critical temperature determination. In this case, the structural analysis is carried out with ambient material properties, and the objective is to find the temperature at which the reduced resistance equals the internal forces from the governing load combination. This method is particularly relevant for members with intumescent coatings, where the coating performance depends on the critical steel temperature. The calculated critical temperature can then be used to determine the required coating thickness based on product-specific data.

The difference between these two approaches can be illustrated using a two-storey frame model.

In the first case, the analysis is performed at room temperature. The beams develop a bending moment of approximately –59.55 kNm, while the columns carry primarily axial forces and show no significant bending moment along their length. This is consistent with the expected behavior based on the initial stiffness distribution of the structure.

In the second case, the analysis is performed at elevated temperature, where reduced stiffness and thermal expansion are taken into account. The beam moment remains –59.99 kNm, but the internal force distribution in the structure changes. Bending moments appear in the columns, reaching approximately –26.91 kNm and –42.21 kNm at midspan.

This difference is a direct consequence of two coupled effects. First, the reduction in modulus of elasticity decreases the stiffness of heated members, modifying the relative stiffness distribution within the frame. Second, thermal expansion introduces additional deformations, which are partially restrained by the structural system. In statically indeterminate structures, such restraint generates additional internal forces, leading to redistribution of moments and the appearance of bending in members that were previously dominated by axial force.

From an engineering perspective, this comparison highlights that the internal force system under fire conditions is not a simple scaled version of the ambient-temperature state. Instead, it is the result of a different equilibrium condition, influenced by temperature-dependent material behavior and compatibility effects.

By allowing both types of analysis within the same model, Consteel provides a consistent framework to evaluate these phenomena. This supports more accurate assessment of structural performance in fire and enables informed decisions regarding fire protection and member design.

Download the example model and try it!

If you haven’t tried Consteel yet, request a trial for free!

Did you know that you could use Consteel to determine the critical temperature of a steel beam protected against fire with intumescent painting?

To perform this calculation, you first need to create a Fire load group and include at least one fire load case.

For critical temperature determination, the analysis must be set to room temperature material properties, because the objective is to find the exact temperature at which the steel section can no longer resist the applied loads while the intumescent coating reacts.

The key step is to assign reactive fire protection to the beam. This activates Consteel’s intumescent coating model, which calculates the temperature at which the reduced cross-section resistance equals the internal forces from the applied load combination. The software accounts for tension, compression, bending, and combined loading using Eurocode reduction factors, including interaction checks for complex load scenarios.

Results are available in the Predesign Parameters, where the critical temperature is visualized per member. You can also inspect detailed information in the Section Module, including the applied fire curve, the steel temperature profile, and the achieved versus required fire resistance.

Additionally, Consteel can optionally determine the required thickness of the intumescent coating from product tables based on the calculated critical temperature, considering the environmental exposure and structural element type.

After completing the fire design, you can also include an intumescent paint design in the documentation. This report can list each cross-section selected for painting, along with the section name, fire protection surface to be painted, A/V ratio, and the critical temperature for each member.

Download the example model and try it!

If you haven’t tried Consteel yet, request a trial for free!

Example model for calculate critical temperature in Consteel

Watch our user guide about How to use the critical temperature feature to learn more.

Critical temperature calculation in Consteel

The calculation of the critical temperature is available in Consteel since the release of version 14. As an introduction of this feature, we prepared a video that gives some theoretical background on the topic, and demonstrates its usage in Consteel. It is shown how to prepare the model, how to execute the analysis and design, and how to create documentation about the critical temperature results.

Check out our user guide to learn more!

The case study exposes a practical evaluation of fire resistance of an old structure of a Spanish industrial building composed of steel built-up members; the truss members are angles connected through packing plates and the columns are battened chords. A simple calculation model was used element by element. First, heat is transferred to individual steel elements by convection and radiation in thermal study. The contributions of these two modes of heat transfer were treated by a practical approach. In mechanical study, the second order analysis was used with global imperfections. Finally, the fire resistance was evaluated R15 after some proposals.

Click the button bellow to download and read the full article.

In Hungary only since 2008 is allowed to use the structural fire engineering methods for fire design. The Eurocode provides methods and standards for these calculations. The practising engineers have no experience to apply these methods; therefore studies are made about them, like this, which is based on a bachelor thesis work. Our intention is also dual. First we study the fire design process of the frequently used steel structures and present it to the practising engineers. On the other hand we analyse the maximum fire resistance of these unprotected steel structures, and looking for different methods to fulfil the requirements of R30 fire resistance class.

Click the button bellow to download and read the full article at page 74-85.