تاثیر انبساط حرارتی تیر بر روی رفتار سازه ستونها در سازه های قاب فولادی در یک آتش سوزی

In the UK, new design guidance is currently being developed for the behaviour of steel-framed buildings when subjected to fire. This is primarily based on recent research that considers the structural behaviour of all horizontal members, without applied fire protection, acting as a complete entity within the building. This guidance assumes that columns designed to current design procedures will always be adequate when used within this new design philosophy. For bare steel columns these existing design methods usually consist of applying some form of passive fire protection. Presented in this paper is an analytical investigation of the structural behaviour of columns when subjected to various structural and fire scenarios. The results from this study do not endorse the view that current fire design methods for columns are adequate. These design methods will require revision if instability of columns during a fire is to be avoided. In most cases this will result in the need for additional passive fire protection to be applied to the steel columns.

Recent research 1, 2, 3 and 4into the behaviour of steel-framed buildings during a fire has begun to consider the structural response of the building in its entirety instead of considering structural members in isolation. In the UK the nucleus of this research [1]is the recently completed fire tests, conducted on a full-scale eight-storey steel-framed building constructed at the Building Research Establishment (BRE) Laboratory in Cardington, Bedford. A total of six compartment fire tests were carried out [3], two by BRE and four by British Steel. The major aim of the tests was to provide quality data to validate and develop computer models, which will enable different structural and fire scenarios to be investigated. Before the Cardington tests, it was felt that existing design guidance on horizontal spanning structural members was too conservative, since it is based on the behaviour of isolated members. Results from the tests supported this theory, and together with supplementary computer modelling design guidance is now being developed based on whole building performance during a fire. The major aims of fire design, once ignition occurs, is to contain the fire within the compartment of origin and to limit structural damage so that overall collapse is avoided. Before the Cardington fire tests were conducted it was generally agreed, by the Research Organisations [1]involved, that to meet these criteria the bare steel columns would need to be designed to current practice 5, 6, 7 and 8(i.e., treated as isolated members). This involved applying passive fire protection to the columns. This effectively resulted in only the horizontal members being tested. Since the limit in terms of load-carrying capacity of these members in fire was unknown, it was decided to protect the columns to a higher standard than that given by current design methods, to ensure that the columns did not fail first. This is based on the assumption that columns designed to current UK practice will perform adequately in a fire, and therefore do not need to be tested to their limit in the full-scale fire tests. The design guidance, which is currently being developed, based on the Cardington test results, is effectively a hybrid design philosophy. The design of the beams (which could possibly be unprotected) and slabs is based on the behaviour of the structure as a whole, whereas the columns are based on current design practice by treating them as a series of isolated members. This paper investigates the validity of this approach and raises questions about its safety in relation to the overall stability of the structure.

Results from the Cardington full-scale fire tests indicated that high moments occurred in the columns during the test. At present these moments are not considered in existing design methods. Investigation of the test results, supplemented with computer models [10], concluded that these column moments were caused by the thermal expansion of the connecting beams. To carry out a detailed investigation into the consequence of these column moments, on the overall stability of the column, a simple structural model was developed (Fig. 8). This model represented a continuous column in a multi-storey frame, with beams framing into the minor and major axes of the column from one side only. It was shown that the column was forced into double curvature with high localised stresses occurring at the top and bottom of the heated column. It was assumed that the column was restrained laterally at floor level. In the simple model this was achieved by restraining the heated beams. In the Cardington frame the composite floor, due to its good inherent fire resistance, will provide the restraint. The analyses presented showed that instability could occur in the column, even though it was forced into double curvature and restrained at floor level. This instability was caused by the P−δ effect in the column, which was enhanced due to the enforced deflected shape of the column caused by the expansion of the connecting beams. The analyses showed that column instability was significantly affected by: 1. Beam to column heating rates 2. Beam cross-section size 3. Span of the beams 4. End rigidity of the heated column 5. Column axial load In addition the analyses showed that the following parameters had nominal effect on the behaviour of the column: 1. Column cross-section size 2. Beam-to-column connection rigidity 3. Horizontal restraint to the heated beams (provided realistic values are chosen) At present, only a limited parametric study has been produced. The results from this study have shown possible scenarios where column instability could occur. The analyses indicate that in certain situations the column Limiting Temperature (failure temperature) used in current design methods should be reduced. The required reduction will range from approximately 35°C to 195°C depending on the parameters explained in (1)–(5) above.