An Assessment of the Use of Structural Deformation as a Method of Determining Area of Fire Origin

Current methodologies of origin investigation have yet to include the structural deformations seen in steel buildings as a viable indicator of the area of origin of a given fire. As many steel structures are of relatively large size, it is often difficult to determine the area of origin using the typical dig and sift methods advocated in NFPA 921, especially if the extent of the fire was large and there were no witnesses as to the origin of the fire. As has been investigated for years, the performance of steel is highly affected by the application of heat. The science of predicting the deformations of steel members is such that an investigator may be able to “reverse engineer” the fire to get an idea of its relative growth rate and length of combustion even if it is not possible to compute a heat release rate curve. The information derived from careful analysis of the deformations may also yield valuable input for use in computer fire modeling. Using several example cases, this paper explores the methodology that can be applied in order to use the structural deformations as a viable tool to determine the point of origin of large, single story steel framed structures. INTRODUCTION NFPA 921 states that the basic methodology of fire investigation should rely on the use of a systematic approach and attention to all relevant details. In order for a fire investigator to pay attention to all relevant details, it is imperative that they be able to recognize, record, and interpret all of the useful details of a scene. Typically, however, the scene is here today and may be gone tomorrow. Thus on scene documentation at the time of initial investigation is a necessity in order to properly determine the origin and cause of a fire. There are many indicators of a fire’s growth and travel left behind at most scenes. These indicators, when interpreted correctly, will almost always lead the investigator to the true origin of a fire. The real question is: Are these traditional “dig and sift” methods practical or advisable in large loss fires? Imagine a large warehouse fire, say 150 m by 150 m (500’ by 500’). The warehouse is a total loss. After the initial investigation, it is evident that the fire had spread to over half of the warehouse prior to the first engine company’s arrival. After a defensive fire attack was established the fire burned for over five hours and the fire became fuel controlled and basically self-extinguished. Prior to control of the fire, one of the sides of the building partially collapsed inward. There is 1 meter (3-4 feet) of debris on the floor of the warehouse and no witnesses as to the origin of the fire. Where would the investigation begin? Would it be possible or practical to “dig up” the entire half of the warehouse involved at the beginning of the fire? An investigator’s first guess on where to start should be the immediate area around the collapsed portion of the building. Why should the location of partial collapse be the first guess as to the origin of the fire? The method presented herein explains in detail the underlying principles and how they can be applied to the investigation of fires as described above along with almost any other fire encountered. FIRE EFFECTS ON STEEL While the effects of fire on steel have been long researched and documented, a more in-depth understanding of the exact engineering properties will be necessary to understand exactly what happens when a steel structure deforms to the point of collapse. An understanding of plastic theory and the method in which design engineers perform their analysis on buildings is also a necessity in order to understand this concept. Steel is an elastoplastic material used in all types of construction. Steel can have varying properties which are dependent upon the amount of carbon and alloying agents used in the production process. Structural steel is generally produced to be ductile as opposed to having a high yield capacity. In steel production, ductility must be traded for strength and vice versa. Steel’s behavior at room temperature as well as elevated temperatures is discussed in detail in the following sections. Modulus of Elasticity and Poisson’s Ratio The elastic modulus, E, is the ratio of stress to the strain it produces. It represents the stress required to produce unit strain. 2 The elastic modulus of steel is typically assumed to be 200 GPa (29,000 ksi) at room temperature and is subject to variation with temperature increases. The typical point of degradation of strength of steel is approximately 150°C. At this point, the strain increases under the action of a given stress, and the well defined yield point begins to vanish. It then becomes necessary to use the “proof stress” as the yield stress. The variation of the modulus of elasticity and the yield stress of steel can be found in Figure 1 below. Figure 1: Reduction Factors Poisson’s ratio (ν) is defined as the ratio of lateral strain to longitudinal strain. Poisson’s ratio basically quantifies the reduction in cross sectional area as the member is placed into tension or the increase in cross sectional area when the member is placed into compression. According to Cooke, it has been well documented that the Poisson’s ratio of steel is negligibly affected by the temperature of the member. Poisson’s ratio for steel will be taken as 0.3 for all calculations in this paper and is the recommended value to use in the field. Strain in Steel Deformation of steel members is a function of the strain generated by the three components of strain. The three components of strain that are of concern for analysis of deformations induced by fires are thermally induced strain, stress induced strain, and creep induced strain. In a typical structural analysis of a steel member, only stress induced strain is considered since the other two are generally small enough to be neglected. Strain, regardless of type, is defined as the change in length (δ) divided by the original length of the undeformed member (L). The typical maximum strain that structural steel can endure before failure is 0.0 0.2 0.4 0.6 0.8 1.0 0 200 400 600 80