Methods and regulations - Fire resistance checks

Time of fire resistance

The fire resistance period is defined as the period of time during which the structure must resist the effects of fire or fulfill the required limit state (or more limit states). The fire resistance period is determined in minutes. The basic classification periods are set at 15, 30, 45, 60, 90, 120, and 180 minutes.

Load during a fire resistance

According to the Eurocode, you can simplistically consider the extreme load during a fire as  70% of the load combination for ULS.

Fire resistance methods

The design methods vary according to the approach, accuracy, and complexity of the calculation. When analyzing a critical element, we can use the Tabulated method, which is the most conservative one.

If the code-check is not satisfactory, there is the possibility to proceed to more precise methods such as the Isotherm 500 or the so-called Zone Method. In the case of an analysis of a part of the structure or even the whole structure, numerical simulations such as conduction, convection or radiation can be used. 

Basic assumptions of the Tabulated method

• Primarily all values are for siliceous aggregates (most susceptible to the effects of fire)
• For limestone aggregate, the smallest required stone dimension can be reduced by 10%
• The humidity of concrete must be less than 3%, otherwise, there is a risk of explosive spalling
• The critical temperature of concrete reinforcement is 500 °C
• The critical temperature for prestressing reinforcement is 400 °C (bars) or 350 °C (tendons and wires) 
• The axial distance of the reinforcement from the surface must be less than 70 mm, otherwise, surface reinforcement must be considered
• In the case of multiple layers of reinforcement, the average axial distance from the edge shall be calculated, with the smallest axial distance of any layer fulfill at least criterion R30
• It is possible to interpolate linearly between the values in the tables
• The values given in the tables are minimal values and the design principles according to Eurocode must also be observed

Columns 

Columns are generally code-checked using method A or B. The decision of which method to use is solely dependent on engineering choice. The code-check is valid for columns that are mainly subjected to the normal pressure and for stiffened structures.

Method A

• The maximum effective length reaches 3 m.
• Effective length is equal to 0.7*L for the top floor and 0.5*L for each additional floor.
• Initial imperfections are less than 15% of the cross-section height.
• The maximum area of reinforcement is less than 4% of the concrete cross-sectional area.

Method B

• The method is valid only for stiffened structures. 
• The effective length of the column can be more than 3 meters.

Beams

The code-check of beam elements is only possible for rectangular, T-section, or I-section cross-sections. In the next step, it is necessary to distinguish between single span or continuous beams.

Plates

As with beam elements, you must choose between a simply supported plate or a continuous plate. Another aspect distinguishes between supports created by walls or columns.

a) One-way slab, b) Two-way slab, c) Continues plate - walls, d) Continues plate - columns

Walls

• The ratio between height to thickness must be lower than 40
• Fire resistance EI – integrity and insulating function, bearingless
• Fire resistance REI – integrity and insulating function, bearing function

Summary

For code-checking the fire resistance of structures, the Tabulated method is mostly preferred, according to which the engineer gets a pass/fail status very quickly. More advanced methods such as the Isotherm 500 or Zone Method are also frequently used methods. The advanced methods are used when critical elements such as a column, beam, wall or slab fail in the tabulated method. Numerical simulation methods like conduction, convection or radiation are mainly applied in academic settings. IDEA StatiCa RCS contains the aforementioned Tabuled methods. The development at IDEA StatiCa moves the analysis from Tabuled methods to the level of numerical methods. Thus, you can look forward to non-stationary heat conduction analysis in IDEA StatiCa Member in the near future.