On the steel building fire scene recognition technology

On the steel building fire scene recognition technology

     Steel system with high strength, light weight, low cost, short construction period and other advantages, has been widely used in industrial plants, airports garages, stadiums, shopping centers and other buildings, but the frequency of occurrence of such buildings fire upward trend, especially when there is a major fire occurred. Such as the 1998 Beijing Yuquanying Furniture City Fire Island; August 2003 Taizhou leap AG fire. In the fire scene investigation process, after the building collapsed due to the complexity of the scene, to identify cause of the fire has brought great difficulties.

     Steel at high temperature exhibit unique properties, steel building fire scene is also showing its unique traces of evidence. Through the study of the fire scene steel building components collapse marks, deformation marks, color marks and signs of melting, etc., summarized the effective steel building fire scene recognition technology, for the steel construction of a large fire prone area collapsed, site excavation inquest time-consuming, difficult to determine the fire point the fire site and other issues to provide solutions and technical support.

     Steel is a very important form of civil engineering structures, has been widely used in industrial and civil construction civil engineering. Steel construction including single or low-rise buildings, multi-storey or high-rise buildings, is steel skeleton, with a fire, wall panels and floor waterproof, sound insulation, heat insulation and other features assembled together. Steel is characterized by high strength, low mass; toughness, ductility, high seismic performance; create a simple, short construction period; steel components heat deformation, resulting in the collapse of buildings.

     After the increasing popularity of the use of modern steel, steel building fire and brought the frequency gradually increased, in 1990 the British multi-storey steel building fires occurred during the construction phase, causing steel columns, beams and floor steel severely damaged trusses; September 11, 2001 New York World Trade Center building north and south, respectively, after the hijacked airliner hit the fire, after more than an hour north and south of the burning building have collapsed.

     Studies have shown that the strength of steel decreased with increasing temperature, when the temperature reaches 350 ℃, 500 ℃, 600 ℃, its intensity decreased 1 / 3,1 / 2 and 2/3, respectively; when the temperature exceeds 700 ℃, the component strength To reduce the 80% point easily deformed steel building collapse and so on. In practice, most of the steel building fires often occur in warehouses, barns, shopping malls, and other simple shed, spatial span, doors and windows, good ventilation, combustible materials and more, in case of fire, the fire will be burned more prosperous, in short within the time the fire will develop into a violent stage; this time due to the fire resistance of the steel itself is low, thermal speed again quickly, very short time will be lost in the steel strength, coupled with a lot of steel construction steel components fire Protection is not used, even with a fire retardant coating, because of different quality and price level, some units of investment options to save the poor quality of fire-resistant coating; and a steel body design life is generally 50 years, while the life far fireproof coatings less than 50 years, companies are generally less likely to repaint itself after fire coatings fail, thus resulting in the use late in steel, coated in fire-resistant coating will not achieve the role of fire on it. Also in some steel factories or warehouses, sheds and the like, filling its walls and roof material is often used in polyurethane and polystyrene fire resistance of these two materials is very low, when a fire, often accelerate the temperature dependence of steel components, resulting in a faster component is destroyed, even the collapse of the building and therefore deformed.

     Because executives and super-tall steel building fires small, limited cases, and fires high-rise steel buildings collapse after deformation, live trace feature is not obvious, therefore this paper daily life often occur Strut and large-span steel building fire scene focuses on identifying marks.

     As the inquest collapse marks an important traces of the fire scene evidence, the steel building fires should not be ignored. Formation of collapsed steel building components, a large reason is because the mechanical properties of high-temperature steel external manifestations: Steel stiffness refers to the ability of steel to resist deformation deformation size, stiffness K = EA / L, where E is the elastic modulus amount, A is the cross sectional area, L is the length perpendicular to the cross section. When the fire scene, the cross-sectional area A of its thermal expansion and contraction due to changes in the size of negligible elastic modulus with increasing temperature drastically reduced, resulting in a sharp decrease stiffness K, steel just too small, unable to resist deformation maintain the status quo. Steel fire collapsed and so deformed by instability.

     A basic rule of steel building collapsed traces of fire as a building fire traces, which follows the construction of fire: the fire toward the fire site, or facing the direction of the fire spread to the collapse, but the collapse of the steel structure analysis and certification marks another role different from other fire scene collapse marks, it has its own characteristics. Steel building collapsed into partial collapse and the overall collapse, but no matter what kind of collapse, are part of a part of the collapse, is generally not collapse all at once. After the steel building collapsed in the fire because of their overall connectivity is good, less affected by the fire affect the position change. Xu is now widely used as a building roof trusses are made of non-combustible material made of color steel or galvanized iron, can still save more complete scene after the fire, but unlike timber roof after building fire, by fire and other factors, roof collapse litter larger position changes. Steel roof collapsed building in the shape of the roof of the fire are common U-shaped, beveled shape, and trapezoidal collapse marks.

     Steel roof building fire, if the high fire load within the building, the roof on both sides often appear to form a U-shape facing the collapse of the roof collapsed, which is characterized by both sides of the beams and columns are still together. Bevel-shaped roof collapsed shape is due to a fire in a building close to the side of the formation. After the steel roof building on fire, burned to the site as a result of relying on one side of the building, leading to the connection point of the beams and columns to lose strength, deformation off the formation of the bevel-shaped roof collapsed shape. In multi-span building, if a large area of the roof steel roof collapsed, both parts of the roof collapsed shape bevel shape, another U-shape collapsed, the fire occurred in the U-shaped roof collapsed certainly marks at, because if the first generation of the bevel shaped collapsed shape, indicating that the process of fire spread, the fire burned before the fire across the face of the column cross-share, the share of post and crossbar junction adjacent shedding the first to lose strength under high temperature, and in this case the shape of the roof collapsed under the scarf is certainly not in line with the collapse of the U-shaped features, it should be developed into a U-shaped beveled shape. Trapezoidal shape is usually caused by the collapse of the roof beams or steel roof off the building roof at the side of the seat of the column after the collapse of the scarf did not fall off the side of the fire continued to spread formation.

     Steel building components deform because the strength of steel with the fire temperature increases, the steel in the initial stages of heating, elastic, plastic little change, but at about 250 ℃, the steel tensile strength and impact toughness of the decline this phenomenon is called blue brittle phenomenon (surface oxide film appear blue). When the temperature exceeds 300 ℃ after the yield point and ultimate strength decreased significantly, reaching 500 ℃, which dropped by about half of the original intensity contrast, reach 600 ℃, its strength had declined by two-thirds of contrast. When the steel construction in general the designer, the considered room temperature by a cross-sectional stress under load is half the yield point, and therefore, the temperature rose to 500 ℃, the yield strength decreased to half of the normal temperature, the plastic deformation and damage member to 600 ℃, the strength is almost exhausted. At this point in the dual role of building vertical pressure and horizontal stresses, steel distortion occurs. Since the same coefficient of thermal expansion of steel is not small, the thermal expansion of the fire after the more serious, the steel members at both ends of each surface is restricted or fixed steel member, may also have traces of swelling deformation, the degree of expansion by the same level of influence of temperature, The higher the temperature, the greater the degree of thermal expansion, the greater the deformation.

     Steel building components in fire due to the high temperature effect, its intensity decreases, usually from the fire site closer, its strength relative to other parts of the other parts of the steel components and steel structures, reduced itself more, in the absence of applied load under the circumstances, the performance for the fire deformed downward direction soften, when in the presence of external load situations, the deformed toward the direction of the force. At the same time due to thermal expansion of the role of the degree of thermal expansion of the steel surface by the fire side is larger than the non-face by the fire, the steel members under fire towards the non-plane bending deformation. However, in general the degree of change is less than the degree of thermal expansion due to weakening of the deformation caused.

     In the fire, backfires steel surface under fire first thermal effect, strength decreased, under the premise of no external loads, generally toward the fire point deformation under applied load in the case, toward the direction of the force deformation, which is deformed steel building components acting on the basis of proof marks. By various types of steel buildings and other steel components connected by high strength bolts as a whole, when deformed steel components in a certain part of the scene of the fire by the flame and hot flue gas temperature and other factors, will inevitably bring to the axis of the member local deformation range of building components. With a light steel building basic load-bearing units - portal frame, for example, at the scene of the fire due to the high temperature, the beams and columns are different degrees of deformation, deformation There are two main forms: one-way deformation and deformation tilted toward the middle on both sides. If the fire site near the portal frame columns, steel columns are directly affected by the initial fire flame heat radiation, plastic increased, while the beams by high temperature flue gas rises above 700 ℃, the intensity decreased rapidly; due to their stress, steel door frame beams and columns toward the first heat of the fire point deformation, while the other end of the beam and column far away from the fire site, the role is not affected by temperature, smaller than the decline in the strength of steel, but by pushing the other end of the deformation stress deformation, steel column bent outward to form a one-way tilt deformed steel members; if the fire site is located in the central portal frame, the portal frame this combination, due to temperature effects of fire on both sides of the column is basically the same, and because the suspension hot smoke the role of gas, the first column after losing in the heat carrying capacity of steel beams and tilted toward the fire site, while pulling the sides of the steel beam tilt inward tilt column, the entire steel frame on both sides toward the middle to form a sloping steel components deformation traces its final similar traces shown in Figure 3.5, steel sloping from both sides toward the middle. Throughout the scene of the fire, the fire temperature effect gradually extended over time, initially subjected to high temperature of one or both of the deformed steel frame, steel frame composed of the remaining part of the building at this time of deformation has not occurred, and therefore, deformation of steel and is not deformed steel are drawn by connecting members, generate action and reaction, this form of action, would leave visible marks on the connecting member in the initial fire. The main parts of the two-door is a fire occurred opposite twisted steel steel columns, or near the site of the fire steel connecting member is elongated, pulled off the connection point.

     Steel building components will react with oxygen in the air, the surface oxide layer is formed steel members. Oxidation of the steel greatly affected by temperature, the higher the temperature, the faster the oxidation. Depending on the steel oxidation products under different conditions, the color change of steel, and therefore, there is a correspondence between the traces of steel and oxidation discoloration heating temperature. Steel oxidation reaction occurs much faster than the normal temperature at its surface under fire conditions, produce iron oxide rust layer. If we at high temperatures and in the presence of water or steam to generate part of the role will hydroxide, carbon dioxide in the atmosphere also generates a small amount of basic iron carbonate. When the fire temperature continues to rise by the decomposition of various steel components hydroxide and carbonate material for iron oxide rust. Therefore, the scene of the fire with a fire time and temperature changes, the color of steel components also undergone a series of changes.

     Steel components of different heating temperature and time, the oxide layer formed by different colors. In the scene, in different parts of the steel components, and even the temperature difference is also great on the same components in different parts. Therefore, the color formed on the surface of a significant level, especially the thin-type ferrous metal. Color In general, ferrous metal heat temperature is high, long duration of action of the various parts of the formation showed a red or pale color, color change levels significantly, especially temperature exceeds 800 ℃ portions on its surface also appeared shiny "iron scale "thin, hard and brittle texture. When uncoated steel covered in fire oxidation, the surface becomes dull first blue-gray. Oxidation can be formed thick oxide spalling. After the fire, if the metal has been wet, it can appear usually rusty oxide. On the stainless steel surface, moderate oxidized to form the color pattern, severe oxidation will be formed not shiny gray. Oxidation can form a boundary oxide thickness can show how the size of the fire, heat and processing. The higher the heating temperature, the longer the heating time, the more severe oxidation.

     In the role of fire, steel heating surface will mark oxidation discoloration, mainly iron at high temperature to produce a fire Fe2O3, Fe3O4, FeO, etc., are shown in different colors.

     In different locations and even different steel components due to the different members of the same scene and thus its heat radiation at different temperatures, the surface color is also showing a corresponding different steel building fire can be determined by observing the color change of the steel member hierarchy the fire site. In the actual scene of the fire, the steel components of the color of the surface will show a clear hierarchy, generally fire point as the center outward showing a pale yellow - black and red - blue; some black metal components coated with oil paints, or surface with paint , spray, easy to identify changes in color, but can be burned by the metal surface layer of paint discoloration, cracking, blistering and other changes in level, to identify the temperature change order. And in general we believe that the closer the fire started the fire site, by the time the longer the fire, the temperature will be higher.

     Steel building components melted to form traces in the scene, can be divided into two reasons, first because of the different types of metal components melting point, when the fire temperature reaches the melting point of the metal or melting temperature range, the metal absorbs heat, overcome intermolecular forces, the liquid-solid transition metal melts due to form a melt marks; the second is to produce instant high temperature under the action of the arc, making the metal produce melting traces. Different types of metal, and the relative melting point alloy, at a high temperature under the action of the fire, the metal member is generally difficult to be melted, only a relatively low melting point metal can be melted, such as aluminum and its alloys; However, due to other causes in a short circuit, an arc can leaving traces melt most metals.

     Under fire temperature, steel building components begin to melt the metal is heated to the melting point, the melting temperature of different metals is different, so the scene is formed in accordance with varying degrees of melting traces.

     Temperature continues to rise, reaction time increases, the melting area expanded, the length of the smaller, heavier degree of melting; and for the fire or the fire from spreading in one direction is first heated to melt, the degree of contrast heavier non-heated surface to form a clear the heating surface.

     On the other hand, if the electric arc causes molten metal conductor short-circuited due to the formation of scar, and generally marks the arc formed by molten metal bead and craters on the morphology and melting traces of fire have significantly different: the general arc weld mark for fine columnar crystals or cellular crystal composition, there are holes, one of which was a short circuit inside the pores less melt marks and small, secondary short circuit melt marks are many and large internal pores; and fire places melt marks coarse equiaxed based, almost nonporous.

     Steel building components at the extent of the fire temperature metal melting and temperature effects are intrinsically linked, different metals have different melting points, different degrees of similar metal melting, the site of a large degree of melting temperature is high, and the low melting point and high melting point unmelted molten metal has a high melting point metal explain the higher parts of the temperature. General metal traces by fire after the formation of the temperature effect of light - heavy order: metal surface coating color - blistering - metal surface discoloration - distortion - melted. This marks just the fire and the order corresponding to the temperature changes from low to high order, indicating the direction of fire spread. When the scene of the fire melted the metal structures, the side of the heating temperature for the spread of the direction of the high melting serious fire back surface melting and relatively light. Fire point generally is melted parts, namely, the highest temperature area. This judgment fire all parts of the temperature range and temperature of the fire investigators to provide a scientific basis. If it is found traces the arc melting, melting marks should be further analyzed before the formation of a short circuit or fire fire shorted form, namely a short circuit or shorted secondary. If it is a melting marks, also known as the primary short-circuiting the position is likely to fire point, it is necessary to find the cause of the fire to help the fire point.

     Through research and discussion of this topic, initially found a steel building fire scene traces common feature: the collapse of steel building components, deformation, discoloration, melting and other traces; summarizes the formation mechanism of these traces, through these types of trace evidence steel has been the role of the fire scene is simple, easy to prove the point of the fire or fire spread direction of the fire site approach. When looking for these signs and take advantage of its proven role, always grasp the basic principles of various types of high-temperature fire scene led to the formation of traces of these traces at the mechanical, chemical analysis, and to find the temperature distribution and the steel structure collapsed, deformed correspondence and then indicate the direction for the fire scene investigation.