Řešení požárně nebezpečného prostoru v rizikových místech objektu

Recently, there has been a growing interest in ecology and the environment, which is why wooden buildings are becoming increasingly popular. In the Czech Republic, there are opinions that the fire safety design of timber buildings is too strict, mainly because of the restrictive fire height (≤ 12 m). There are more requirements that seem to be too high comparing to noncombustible buildings, for example type of construction system, fire load, degree of fire safety, fire resistance of building structures, fire separation or falling out of burning parts. These requirements are briefly described in this paper. Another part of the paper is devoted to a case study. This case study consists of a building with offices and shops. Requirements of Czech regulations for fire safety design of timber buildings are implemented in this case study together with regulations of selected foreign countries (Great Britain, Canada, USA and Austria) to see if Czech design is strict or not. Graphical results (height of building and fire separation) are given at the end.

Detail

With increasingly dense urban structure and wider use of energy efficient (and very often combustible) materials both in and on walls, the requirements on fire separation distances become more actual than at any time in the past. Main parameter that affects separation distance its magnitude is radiation, critical heat flux respectively. The first approach was developed about 60 years ago. Critical heat flux used for the separation distance determination in the Czech Republic and Slovakia, highest in comparison with other countries, predicts the autoignition of exposed wooden materials in approximately 10 minutes. In the article a set of medium-scale fire experiments were conducted to verify this assumption. Wooden-based boards, namely oriented strand boards (OSB), were exposed to the critical heat flux of radiant panel of given power and time to ignition was measured. In this preliminary study, four scenarios were conducted using both insulated and uninsulated board with and without piloted ignition.

Detail

This paper presents a numerical study on the effect of the heating and cooling phases on the reduction of the effective cross-section of timber elements, in particular on the evolution of the char depth (300°C isotherm) and zerostrength layer. An advanced calculation method based a finite-difference heat transfer model is compared to the simplified approach suggested by Eurocode 5. For the heating phase, defined as the standard fire curve (ISO 834), the simplified Eurocode 5 method generally provides more conservative char depths, while the zero-strength layer is under-predicted. Nevertheless, the values of effective char depth are comparable. Including the cooling phase evidences that, during this phase, the heat wave penetration leads to a significant increase in the char depth and zero-strength layer. Particularly, this increase directly depends on the fire cooling rate: a slower cooling phase further reduces the effective cross-section of timber members. As a result, this research highlights how the heat wave penetration during the fire cooling phase can significantly reduce the load-bearing capacity of timber elements.

Detail

This research study investigates the effect of different heating and cooling regimes on the effective cross-section of timber elements exposed to natural fires. An advanced calculation method based on a 1D finite-difference heat transfer model and effective thermo-physical properties is adopted to analyse the heat penetration and the consequent reduction in mechanical properties. In particular, the research focuses on the evolution and penetration speed of the char depth (300 °C isotherm) and zero-strength layer (determined through in-depth temperatures and reduced mechanical properties). Results reveal how the char depth mainly develops during the heating phase, with non-negligible contributions from the cooling phase. In contrast, the zero-strength layer increases throughout the whole fire exposure, particularly during cooling and, possibly, after the end of the cooling phase. In general, the heating phase contributes about 2/3 to the total effective char depth, while the cooling phase about 1/3. The most challenging conditions were found for the fires of the longest durations (heating and overall), corresponding to low ventilation and high fuel load density conditions. The study emphasises the necessity of incorporating the cooling phase in performance-based methodologies for fire-safe timber structures to avoid under-estimating heat penetration effects.

Detail

This study investigates fire separation distances as essential means of passive fire protection in building design. The focus is on the inner corner configuration of building exterior walls, which represents the worst-case scenario for façade fire spread outside of a building. The inner-corner configuration appears to increase the intensity of the radiative heat flux due to reflection and reradiation of heat. Comprehensive approaches for determining fire separation distances around the various façade geometries can be found, but none of them is focused on detailed descriptions of the unprotected area in an inner corner. A medium-scale scenario was chosen and was experimentally validated with a radiant panel for a better understanding of heat flux spread. This paper compares the experiment with analytical and numerical models. The analytical model is based on the Stefan–Boltzmann law and the calculated configuration factor as per Eurocode 1. The numerical model combines radiative and convective components of the heat flux because convection is non-negligible near the heat source. Experimental data confirm the prediction based on the numerical and analytical model and show agreement. The final increase in heat flux due to the corner configuration investigated at the medium scale reaches up to 29%.

Detail