土木工程英文翻译资料.docx

1、土木工程英文翻译资料The Fire Resistance of Steel StructuresE. GEHRIFederal Institute of Technology, Zurich, SwitzerlandINTRODUCTIONSTEEL IS a noncombustible material, but the mechanical properties of structural steel are affected by heat. The fire resistance or more exactly, the fire endurance of a steel elem

2、ent varies greatly and, therefore, we need a better understanding of the conditions which affect fire resistance.Fire resistance is understood as the time during which the structural element can withstand the standard provisions of a fire test. For structural elements the only criterion to be consid

3、ered is the structural collapse of the element. Other criteria like temperature transmission may be disregarded.For a steel column the standard provisions for a fire test are given in Figure 1.We must note that the standard test prescribes the use of a constant load corresponding to the maximum desi

4、gn value. The thermal longitudinal or axial expansion of the column will be compensated during the test, i.e. there is no restraint, the column can move freely in the axial direction.By tests we can find the ultimate strength of the column for different fire endurances. Using the same type of column

5、 and same dimensions we obtain an interaction diagram (Figure 2).The reduction in strength with the passing of time depends on the temperature increase in the specimen tested. The temperature increase is a function of the so-called section factor and of the insulation applied. The scatter of test re

6、sults is quite normal, since the columns tested are not identical due to the dimensional tolerances and the variation in strength of the steel. More or less the same scatter is found when testing steel columns at room temperature.Now introducing the design value into Figure 2, we can directly find t

7、he fire resistance. Generally, the mean value of the test results will be higher than the fire resistance based on the nominal guaranteed strength of the steel.From Figure 2 it can also easily be seen, that a column which is subjected to a lower stress level - i.e., to an effective load lower than t

8、he design load - will have higher fire resistance.Finally, we must note that the conditions during the fire test do not correspond to those in practice. Besides different heating rates in a natural fire, we have to consider the real boundary conditions. Support conditions and the longitudinal restra

9、int may result in substantially different fire resistances.FIRE RESISTANCE OF CONCRETEThe fire resistance of concrete members is much higher than that of no insulated steel members. However the tendency to use higher strength concrete and steel reinforcements leads to smaller sections and therefore

10、to lower fire resistance. Furthermore, many steel constructions include concrete elements to a large extent or have so-called composite steel-concrete elements. We therefore need a general procedure to analyze the fire resistance of different structural materials and of composite elements. We could

11、also treat the reinforced concrete as a composite material, which it is.FIRE RESISTANCE - A SPECIAL ASPECT OF FIRE PROTECTIONThe fire resistance of steel structures is only a small part of the fire protection problem. We have to start by reducing the fire load, by using small compartments, by trying

12、 to remove the heat as quickly as possible, and finally, by preventing heat transfer to the structural members.The calculation of fire resistance is the last resort, which does not mean that fire resistance has no importance. But before calculating we should determine if the fire resistance problem

13、cannot be minimized by a combination of different subdivisions of the volume, or by placing the main structural parts in less exposed areas or by removing the heat through openings.Furthermore, we have to acknowledge that the actual fire conditions are difficult to know, which means that we have, on

14、 the so-called load side, great uncertainties.STRUCTURAL PERFORMANCE REQUIREDFIRE RESISTANCE - COLLAPSE LOADIn the introduction, fire resistance is defined as the time after which a collapse of the structural element occurs, assuming a rise of temperature in the furnace according to ISO 834.We know

15、from experience that test specimens may also collapse after the test during the cooling phase. This may be due to continuing loss of strength and due to redistribution of temperature in the specimen. For steel, in general, we do not need to consider that case, but it shows the difficulty of a proper

16、 definition of the fire endurance based on the collapse load.FIRE RESISTANCE - DAMAGE LEVELThe assumption of collapse as a criterion does not mean that we expect- even in a severe fire - the total collapse of the structure. It seems furthermore, useful to consider different criteria for main structu

17、ral and for secondary structural elements.Whether a structural element is part of the main structure or secondary structure is not always obvious. A guideline to follow can be: if the total collapse of an element does not seriously affect the structural behavior as a whole, it can be considered seco

18、ndary. Therefore lower requirements even none - may be laid down for such structural elements.We should, therefore, discuss main structural or secondary structural elements, total collapse or partial or local collapse, degrees of damage and possibilities of repair.Structural materials like steel, ti

19、mber or concrete, show different reactions to fire. After a fire, concrete and timber show a loss of resistance whereas protected steel structures may have the original strength after cooling.Repair possibilities and repair costs may therefore be very different. In extreme cases a replacement of a c

20、oncrete structure will be required, whereas only a simple reapplication of fire protection surfaces to the steel elements will be necessary.Depending on the structural performance required during and after a fire, we must establish reasonable and realistic criteria. The present orientation based onl

21、y on the collapse of an individual isolated element cannot be the entire basis of fire resistance classification.FIRE RESISTANCE CLASSIFICATIONSTANDARD FURNACE TESTSThe well-known procedure to establish fire-resistance time needs no explanation. I can refer here to ISO 834 where acceptance condition

22、s are specified. There is a certain criticism about those conditions, namely about the requirement to test the specimen, as far as possible, under supporting and restraining conditions that are similar to those in service.These conditions are seldom fulfilled in standard furnace tests, but I see no

23、problem in it as long as the actual boundary conditions are given in connection with the observed fire resistance. The same condition holds for testing insulated steel members, where, in addition to the insulation, the section factor or the steel element has to be introduced (see Figures 3 and 4).If

24、 appropriate design loads are applied (use of the same safety factor at room temperature the fire resistance of the three columns in Figure 3 will be nearly the same, if there is no axial restraint.With full knowledge of testing conditions and test limitations only simple elements like columns and s

25、imple girders; dimensional limitations in section and in length tests made in accordance with ISO 834 and fire resistance classification remain a valuable tool.ANALYTICAL PROCEDURESApart from the limitations mentioned, the furnace tests have the major disadvantages of being laborious and costly. Thi

26、s was the reason for developing different analytical procedures in the last 10 years.For steel elements the analytical procedure consists of (see also Figure 5) A calculation of the temperature level in the steel element, assuming a heating process corresponding to standard fire tests. A calculation

27、 of the effect of the temperature on the load bearing capacity of the element.With such an analytical procedure it should be possible to reproduce the results of fire tests in furnaces. If the analytical procedure is correct, the calculated time of failure should be identical with the fire-resistanc

28、e time obtained in a test, assuming structural elements of the same mechanical characteristics and the same load level. However, some dissimilarity must be expected, since also at room temperature, predictions of ultimate strength show a certain scatter.Of the existing analytical procedures it is pr

29、eferable to use those which are easy to handle but still have an appropriate level of accuracy. This leads to simplifications and as a result, the range of validity may be narrower.An idea of the appropriate level of accuracy may be seen from the following chart.Most analytical procedures are too go

30、od. They are also too laborious and prone to errors. We therefore need a simple tool to estimate with reasonable. Precision the fire resistance of a steel element. Such a tool was recently published by the European Convention for Constructional Steelwork under the title Calculation of the Fire Resis

31、tance of Load Bearing Elements and Structural Assemblies Exposed to the Standard Fire.The following calculation procedure is based on the above publication, but introduces some small modifications which allow a very simple and quick determination of the fire resistance of no insulated and insulated

32、steel elements. The method allows us also to take advantage of a lower stress level than the allowable design value. We have seen before that a lower stress level leads to higher fire resistance of the member.PRACTICAL APPROACHASSUMPTIONSThe following assumptions are made: Temperature in furnace according to ISO 834 The heating of the steel element is uniform (large and compact steel sections do not however Show a uniform temperature) The reduction of all strength and stiffness characteristics is proportional to the reduction of the yield point (valid for st