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FIRE RESISTANCE !.INGS e exposure or the "Vith columns, beams and girders the critical feature is usually the ability to carry the load under fire exposure, which in turn is usually dependent on the heat insulating value of the protective covering and its ability to stay in place during the fire exposure. \Vith noncombustible floor constructions the critical feature has usually been the temperature rise Oll the top (unexposed) surface in view of adequate protection provided on the under side, but ceiling protection is important and if deficient the critical feature becomes the load carrying ability under fire exposure which is dependent on the ceiling protection. V\'here the ceiling protection consists chiefly of plaster on some form of lath the method of attachment of the lath and the security with which it and the plaster are held in place during the fire exposure are often critical features. Falling of any considerable portion of the plaster and lath during the fire exposure period makes the possible collapse of such floors under load (whether of combustible or noncombustible construction) a cr,itical feature in the test results. This has been the case in a number of floor tests. ~ II il ij .~' .' ~ e e fire the critical feature 'may be the load carrying ability under resistance to hose stream penetration. .1 . It should be noted that ratings of plaster facings' on walls and partitions are not applicable to similar finishes on ceilings, for under fire exposure plaster may fall from a ceiling considerably before it will fall from a vertical surface, even though it be applied to the same plaster base in both cases. Fire Walls. Fire resistance ratings as determined by the standard fire test do not provide an adequate basis for specifying minimum thickness of fire walls of buildings, In brief general terms, the standard fire test is made on a sample wall about 10 feet square and to be rated as a 4-hour wall, for example. the sample must stand up under the standard exposure fire and bear its rated load, if of load bearing construction, for a period of 4 hours without attaining a temperature on its unexposed face higher than 250 F. above the starting tempera- ture. The hose stream test which is also specified is made on a like sample exposed to only a I-hour fire so that this test is not a critical factor in such walls, It is obvious that a fire test as described will not measure the ability of a wall many times the size of the test panel to stand up against fire exposure ex- tending over more or less its entire area. Walls expand when heated and in severe building fires it is common to see them curl and fall as the result of such expan- sion on the heated side. Ability to withstand the impact from collapsing floors and falling objects is also important. ,.~ . When a wall is called upon to stop a spreading fire that is reaching or has attained conflagration proportions it must have stability against collapse or over- turning far in excess of that presented by certain types of construction which develop a 4-hour resistance; in the standard fire test, if it is to serve its purpose. Therefore until tests have been developed that will measure the kind of per- formance recluired in fire walls of buildings, it is necessary to a considerable extent, as in the National Building Code, to specify thickness and required lateral support of walls on the basis of experience and their performance in actual fires, GENERAL NOTES Construction All ratings are based on use of materials and forms of construction in full conformity with requirements of the National Building Code, and in compliance with any applicable material specifications of the American Society for Testing Materials. Materials and Forms of I I~ ![' r I The thicknesses of walls, partitions and floor slabs specified in the tables are based on fire resistance only and are not to be construed as permitting such thicknesses where other considerations require greater thicknesses. See for ex- ample the Code provisions regarding height and thickness of masonry walls in section 909 of the National Building Code. 3 e e