| Large space steel structure is an important form of building structure.Portal frame is the representative of light steel structure in China at this stage,which is widely used in factories,supermarkets,gymnasiums,airports and so on.Like other steel structures,portal frame structures also have some shortcomings,such as low strength at high temperature and poor fire resistance.Such buildings are usually crowded and exist large fire loads.Once a fire occurs,it is easily damaged or even collapsed,causing casualties.What’s more,the existing researches show that the large space building fire is different from the general indoor fire,and the fire temperature field is a non-uniform temperature field varying with time and space.Therefore,it is of great practical significance to study the fire resistance of large space steel structures.At present,scholars at home and abroad have carried out many studies on the response characteristics of large space steel structures under fire.Based on previous studies,considering the effects of direct flame radiation,flue gas radiation and heat convection on the temperature rise of steel components,this paper studies the influences of relative locations and start-up time of multiple fire sources with different power combinations under the same total fire power conditions on the displacement and critical temperature of plane portal frame,based on theory and numerical simulation.At the same time,the influences of span-to-height ratio and horizontal load on the displacement and critical temperature of portal frame are considered under the same stress ratio.Specific analytic works include the following aspects:(1)The ignition flame model,cone flame model and cylinder flame model are three commonly used flame models at present.Based on the theoretical formula of temperature rise of unprotected steel members verified by experiments,the local fire scenarios of steel beams under three kinds of flame models were established by finite element software ANSYS.The temperatures of steel beam were extracted and compared with the theoretical calculation results.It is found that the cylinder flame model is more suitable for simulating the temperature rise of steel members in fire.(2)The influence of the relative location of two fires on the displacement and critical temperature of portal frame was studied under the combination of 5MW-25MW,10MW-20MW and 15MW-15MW,keeping the total fire power of 30MW in space unchanged.The change of relative location of fires was based on the symmetry and asymmetry of fire location.When the fire position was asymmetric,a fixed fire was located at the critical position,the distance between the fire edge and the column is 3/40 of span.It is found that the 15MW-15MW fire combination has the most significant effect on the vertical displacement of the mid-span,and the asymmetric fire arrangement has a greater impact than the symmetrical one.The location of the fire with smaller power at the critical position has greater influence on the vertical displacement of the mid-span than that of the fire with larger power at the critical position.When the total power of the fire is constant in the portal frame space,the critical temperature of the portal frame under multiple fires combination condition is lower than that under single fire condition,and the critical temperature decreases with the decrease of the power difference between the two fires.When the combined working conditions of fire power are the same,the position of fire has little effect on the critical temperature of portal frame,which can be neglected basically,but decreases with the increase of the relative distance of fire.(3)Under the symmetrical position of 15MW-15MW fire,the effects of different initial combustion time of two fires at different relative distances on the fire resistance of portal frames were studied.The results show that the final temperature distribution and critical temperature of the portal frame are not affected by the combustion of two different fires,but the path of temperature rise and the time of failure of the portal frame are affected.(4)The effects of the span to height ratio and horizontal load on the displacement and critical temperature of portal frames with the same stress ratio and section height were studied.It is found that the influence of span-height ratio on the vertical displacement of the portal frame under fire is irregular,and the influence on the critical temperature of the portal frame is negligible.The influence of peripheral accumulated load on the fire portal frame depends on the stacking height,load size and whether both sides are loaded by accumulated load. |
