火灾的发生和发展具有随机性和确定性的双重特点。随机性是指火灾发生的起火原因及时间、地点等因素是不定的，受到各种因素的影响，遵循一定的统计；确定性是指在某一特定场合下发生的火灾会按基本确定的规律发展蔓延。燃烧过程与烟气流动过程皆遵循燃烧学、流体力学等物理和化学规律。火灾的确定性规律可采用定量化工程研究，一般分成四个主要阶段，即：初起阶段、发展阶段、猛烈阶段和熄灭阶段。

钢结构厂房的火灾危险性主要体现在着火源荷载、厂房几何尺寸及火羽流的形态等方面。一般钢结构厂房的特点是空间较大，空气充足，四周无附属围合建筑，有的大跨度大空间厂房内部甚至形成空气对流，属于初期发展最快的轴对称火羽流。在初期增长阶段，热量迅速累积，在可燃物上方形成温度较高、不断上升的火羽流。当羽流受到顶棚的阻挡后，便在顶棚下方向以平均0.5m/s的水平速度四面扩散开来，形成沿顶棚表面平行流动的较薄的热烟气层，此后，烟气受到建筑围护的阻挡和冷却，达到了一定厚度时又会慢慢向内部火源处扩展，形成逐渐增厚的热烟气层，从而缩短进入发展猛烈阶段的时间。当着火源发展至多处时，在火源间形成漩涡，影响空气对流，造成热量加速积聚，达到充分发展阶段，热烟气层的温度与中心温度相差无几。

The occurrence and development of fire has dual characteristics of randomness and uncertainty. Randomness is refers to the cause of the fire and is uncertain factors such as time, place, influenced by various factors, follow certain statistics; Certainty that occurred in a particular situation of the fire will spread according to basic certain rule. Combustion process and gas flow process follows the combustion, fluid mechanics and other physical and chemical laws. Fire the certainty of law can be used quantitative engineering research, generally divided into four main stages, namely: at the beginning stage, developing stage, hard phase and phase out.

Steel structure factory building fire risk is mainly manifested in the fire load and fire plume and the factory building geometry size of shape, etc. Ordinary steel structure workshop is the feature of the space is larger, the air is enough, not affiliated to surround close around buildings, some large span large space buildings and even form the air convection, belongs to the early of the fastest growing axisymmetric fire plume. In the early growth stage, the heat accumulating rapidly, higher formation temperature, rising above the fuel of the fire plume. When plume after exposure to the roof of block, and under the ceiling direction at average level of 0.5 m/s speed spread in all directions, forming along the roof surface parallel to the flow of hot smoke layer with thin, after that, the flue gas block and cooling by building palisade, reached a certain thickness and will gradually expand to the internal fire place, form gradually thickening of hot smoke layer, so as to shorten the time of the stepped into the stage of development of violent. Development up to place in front of the fire, the fire between the vortex formation, affect air convection, cause heat accumulation, to achieve full development stage, the temperature of the hot smoke layer and the core temperature.

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