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Fire safety of staircases in multi-storey buildings The results of measurements in Buildings and Simulations
Grzegorz Kubicki, Ph.D.
Department of Environmental Engineering, Warsaw University of Technology
Izabela Tekielak – Skałka, M.Sc. Eng. Research and CFD Simulations Department, SMAY LLC
KEYWORDS Pressure differential systems, evacuation, high-rise buildings, fire safety INTRODUCTION The interior staircases are the only available escape routes for all occupants of the high-rise buildings in the case of fire. Protection of this area carried out by means of pressure differential systems must be as efficient as possible. THE ORIGIN OF THE RESEARCH PROGRAM The origin of the research program were numerous information and internal measurements of pressure distribution in staircases of high-rise buildings in different countries of European Union. These measurements indicated the unsuccessful operation of various types of smoke prevention systems. The obtained results were fully confirmed during the later tests carried out also with the participation of authors. As already observed, the stack effect may occur in the internal staircases before the start of fire ventilation system. Level of pressure distribution is strongly correlated with the difference between inside and outside temperature. The pressure difference is particularly evident in winter when the outdoor temperature falls well below 0 °C, but also occurs with varying intensity in other conditions of temperature. The observations also showed that air supply raises the initial difference of pressure to a higher pressure level. Pressure relief damper allows to reduce the problem of distribution of pressure, but does not solve it. GOAL AND STAGES OF RESEARCH The observations were the reason to start an program of research. The goal of the research was to design the pressure differential system. This system was intended for buildings higher than 50 m. The new system was intended to operate with greater efficiency than the systems consisting of a multi-point air injection system and the pressure relief damper. It should also provide effective solution to counteract the stack effects within a building in the temperate climatic zone (with significant fluctuations of the outside temperature during the year).
Tests were carried out in the following stages: Stage I: Research on the test rig in 2008-2010. The test rig was built in the staircase of reinforced-concrete frame building in Krakow (highest 92 m). Stage II: Research based on numerical analysis (CFD). Analysis were carried out in parallel with the Stage I. Stage III: Research carried out in the real building in the years 2012-2016. Studies have been conducted to verify the results of research and to verify the effectiveness of the technical solutions developed during the Stage I. THE TEST RIG The test rig was built to the long-term observation of physical phenomena. Research required continuous measurement of different systems. For this reason, the building operated every day were excluded as a location for research. Studies conducted in unfinished building at Lubomirska street in Cracow. One staircase was in the building of reinforced concrete. The staircase included 23-floor with a total height of 92 m. Two independent systems has been investigated on the test rig:
1. pressure differential system constructed in accordance with the recommendations of the European standard [5] (vertical duct, supply air every 3 storeys)
2. system consisting of two reversible fans located at the top and bottom of the building.
Figure 1. Real scale test stand 1 - control panel description – multipoint air-supply system
Figure 2. Real scale test stand 2 - control panel description – forced airflow system The test rig was equipped with a comprehensive system of monitoring physical parameters and registration data (Figure 1., Figure 2., Figure 3). The remote control panel allowed for supply of the air with varying intensity and various ways (concentrated air supply, multi-point air injection).
Figure 3. Schematic diagram of measurement and data acquisition system
The staircase was equipped with the necessary elements of the carpentry and joinery and adjusted to the level of the tightness corresponding to tight class in accordance to Annex A of European standard 12101-6 [5]. RESEARCH During two years research program performed hundreds of tests to check the operation of the system of pressurization in various configurations air supply and air flow. Research carried out at different periods of the year at different air temperatures. The research was conducted in three steps. Step I: Goals of the first step was defined as follows: Define the factors influencing the initial pressure distribution in an unheated staircase, and the correlation between weather conditions and internal temperature. Long-term observation of pressure in staircase confirmed that the phenomenon of the stack effect is closely related to temperature conditions. Significant changes in the vertical distribution of pressure appear in both in the daily cycle and or during a temporary worsening of the weather. Figure 4 show the change in the pressure caused by change of daily temperature observed on the test stand. Figure 5 show the change in the pressure caused by rapid decrease in air temperature during the storm.
Figure 4. Measurement data analysis – temperature difference -> stack effect
Figure 5. Measurement data analysis – temperature difference -> stack effect The measurements confirmed the influence of temperature on the pressure distribution in the staircase. The size of the pressure difference between the highest and lowest floors is proportional to the difference of external and internal air temperature. However, also in the space of a building is some of pressure, so the value of stack effect measured between usable space and the staircase is less than would appear from a typical formula describing the stack effect. Distribution of pressure in the building shown in Figure 6.
Figure 6. Distribution of pressure in the building in winter. Above relationship was taken into account in the equation describing the stack effect by adding a new coefficient “c”. Additional coefficient allows to determine the pressure difference between the staircase and the usable area.
∆𝑝𝑝𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠_𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑠𝑠𝑠𝑠 = (𝜌𝜌𝑒𝑒 − 𝜌𝜌𝑖𝑖) ∙ 𝑔𝑔 ∙ ℎ ∙ 𝑐𝑐 [𝑃𝑃𝑃𝑃]
Where: 𝜌𝜌𝑒𝑒 - the density of the outside air [kg/m3], 𝜌𝜌𝑖𝑖 - the density of the indoor air [kg/m3], 𝑔𝑔 - gravitational acceleration [m/s2], ℎ - height of staircase [m], 𝑐𝑐 - additional coefficient (𝑐𝑐0T ϵ 0,5 ÷ 0,9) [-]. Coefficient depends on the ratio tightness of the staircase to tightness of the building - building on the tight staircase compared to full of holes elevation will have a high rate of c-factor. Coefficient depends also on the height of the building. The value of c-factor is higher for high-rise buildings and lower for low-rise buildings. Step II: Goals of the second step was defined as follows: Determination of the practical effectiveness of various forms of air supply and active control of supply air. During the tests were examined the effectiveness of different methods of pressurization: concentrated on the lowest and highest levels (single point air supply, double point air supply) and air flow distributed over the entire height of the staircase. In all cases achieved a similar result: the pressure increases but line of pressure kept their initial deviation. This means that the air supply into the staircase does not change the initial pressure difference between the lower and the high part of this space.
Figure 7. Pressure distribution in the staircase for different method of supply air. Step III
Goals of the third step was defined as follows: Study of the effectiveness of the system utilizing the airflow within the staircase to forming the pressure distribution. The system consists of two reversible fans located on the lowest and the highest floor of the staircase. One of the fan supply air into the staircase in the direction of the natural flow of air. The second fan operates in the extract function with less air flow. The air flow direction is calculated based on the initial pressure distribution, determined by measuring the inside and outside temperature. The fans are equipped with frequency inverters. Suitably selected airflow allows to stabilize the pressure on the entire height of the staircase. An integral part of the system are sensors of pressure, which signal is the basis for determining the quantity of air supply and exhaust air. Change the scenario (opening doors) changes the amount of supply air in a very short time. Supply of the air is increases. The airflow on the exhaust fan is restricted. The system returns to the previously air flows after closing the door. The solution provides the effectiveness of the system, reducing the time necessary to stabilize the pressure and automatically adapts to the changes in the work environment such as e.g. change in the level of tightness of the staircase.
Figure 8. Pressure distribution stabilization inside staircase during summer (a) and winter (b) periods. Verification of assumptions and full confirmation of the effectiveness of the system was carried out on the test rig. Flow system can be used for forming any pressure in the staircase. For example, below were shown the results of measurements for a test create a pressure difference between the extreme floors of the building. The purpose of test was to obtain the pressure difference between the highest and the lowest floors equal to - 60 Pa. The initial distribution of the pressure was 10 Pa. During the test were set appropriate settings of the pressure controllers (100 Pa at the 1 floor and 40 Pa at the 23 floor). The system obtained set values without any problem. The results demonstrated high efficiency flow system to control the pressure in the staircase.
Figure 9. Example of possibility of pressure changes in the staircase using system based on flow. The test showed that it is possible achieve the desired pressure by the generation an air flow within the staircase. The analysis of the test results allowed to develop an advanced system using air flow resistance through the staircase. The method allows for the active prevention of the stack effect. USE OF CFD TOOLS TO VERIFY THE TEST RESULTS Research were supplemented by a program of computer simulations performed using the ANSYS FLUENT. Simulations were conducted in parallel with the measurements on the test rig, so that it was possible to validate the developed numerical model. Simulation results were compared to real all time. In order to carry CFD simulations developed a three-dimensional computational model. Numerical model takes into consideration all important architectural details as well as air supply, evacuation doors dimensions and exact location of air supply and air exhaust points. Air leakages were simulated as slots with located at the bottom of the door frame. In the model applied tetrahedral computational mesh composed of 1 500 000 elements. the maximum dimension of the mesh at the boundaries is limited to 0.20 m. the size of model elements have been reduced in places characterized by small dimensions. In the model were used the following settings:
• CFD simulations were carried out in Double Precision Solver, parallel computing; • two-equation k-epsilon turbulence model (RNG with full buoyancy effect); • force of gravity has been taken into consideration; • heat transfer inside the stairs was taken into consideration; • Convergence criterions were taken into consideration;
CFD simulations were conducted for the theoretical winter conditions (outside temperature -20°C, internal temperature +20°C). Criteria for the correct functioning of the pressure differential system was determined based on the European standard EN 12101-6:
• minimum pressure was 50 Pa +/- 10% (45 Pa), • maximum pressure was 60 Pa.
The examples of results of the simulation are shown below.
Figure 10. Pressure distribution in the staircase for different pressure differentiation systems. In all analyzed cases the system based on flow have the highest efficiency to stabilize the pressure in the staircase. Compare the effectiveness of the presented solution with other types of installations a clear indication of the objective of the research project - creating a system based on flow capable of producing of a stable pressure even in the stack effect. VERIFICATION OF THE RESULTS OF RESEARCH IN PRACTICE The results of experiments and numerical analyzes have been confirmed many times in tests and measurements on real objects. In the years 2010-2015 studies carried out in several buildings with a height exceeding 50 m in Poland. These buildings were equipped with different solutions of pressure differential systems. Below presents selected results of the two examples. Sky Tower in Wroclaw (the tallest building in Poland equipped with a system that uses air flow to stabilize the pressure) At the time of construction it was the tallest building in Poland, with a height exceeding 200 m. Inside the building there were two staircases. Stairways were equipped with a system composed of two main reversible fans located at the extreme floors of staircase and three additional fans. The distance between the points of air supply was between 10 and 23 storeys (37 and 87 m). Measurements during the winter (outside temperature +3°C, internal temperature +18°C). The examples of results of the measurements are shown below.
storey ∆p staircase - corridor (volume flow)
+50 +59 Pa
(out 23 200 m3/h) (supply 7 700 m3/h)
+27 +48 Pa (supply 8 000 m3/h)
+17 +54 Pa (supply 8 000 m3/h)
+3 +56 Pa (supply 60 000 m3/h)
Table 1. The results of the measurements of pressure in the staircase. Measurements carried out in the building confirmed the ability of the system to stabilize the pressure of the flow in a stairwell, regardless of external conditions. Student House Riviera in Warsaw The staircase consisted of 21 floors above ground and one underground, with a total height of 66 m. Installation went was submitted with the fan connected to a vertical duct. The points of air supply arranged every 3 storeys. In the wall under the ceiling of the staircase was placed pressure relief damper, which, however, was closed during testing. Tests carried out during the winter (outside temperature ranged from -14 to -20 °C). For such conditions in the staircase initial dissection of pressure was characterized by the value of 80-90Pa. The effect persisted after the start of the pressure differential system (Figure 11. Test 1 and Test 2). The normative value of the pressure (50 Pa) obtained only at the height of the middle of the staircase. Pressure in the bottom of the staircase achieved value -10 Pa and on the top floor achieved value + 60Pa.
the main fan
additional fan
Figure 11. The test results of pressure and temperature changes in the stairwell SUMMARY The conducted measurements show that the effectiveness of the pressure differential system is heavily dependent on external conditions, which is especially visible in the high-rise buildings. Systems composed of multipoint air-supply and pressure relief damper does not guarantee the correct distribution of pressure. The high-rise buildings require an individual approach. Studies have shown that an alternative way to protect vertical evacuation routes is a forced airflow system, which uses flow resistance inside the staircase to form the suitable pressure difference. .
LITERATURE: 1. Tamura G.T., “Assessment of stair pressurization systems for smoke control”, ASHRE Transactions Vol. 98 part 1, 1992, 2. Tamura G.T. and Shaw C.Y., “Experimental studies of mechanical venting for smoke control in tall office buildings”, ASHRE Transactions Vol. 84 part 1, 1978, 3. Konrath B. “Pressure differentia systems - The German experience with the standard EN 12101-6- considerations about the influence of the static pressure differential in the high-rise buildings”, 2007 4. Horst A. Ermer “Smoke Control by Pressurization in High-Rise Buildings - Empirical findings” EuroFire 2009 5. EN 12101-6 “Smoke and heat control systems - Part 6: Specification for pressure differential systems – Kits”, 2007 6. prEN 12101-13 “Smoke and heat control systems — Part 13: Pressure differential systems (PDS) design and calculation methods, acceptance testing, maintenance and routine testing of installation”
