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CRITICAL FEATURES IN FIRE RESISTANCE RATINGS. With masonry walls and partitions the critical feature in determining the fire resistance rating under the standard fire test is usually the temperature rise on the unexposed surface or in other words its resistance to heat transmission. However, with some constructions, 2 .' e HOLLOW UNITS. It should be noted that with hollow constructions and with constructions composed of hollow units the thickness of the hollow space is a relatively minor factor in the, fire resistance. Comparative tests have indicated that it is the total thickness of solid material in the unit rather than the total thickness of the unit which is the principal factor. Therefore, with constructions involving hollow block or tile, use of a thicker unit of the same material will increase the rating only if and as the thickness of solid m~terial in the unit is increased. An increase in rating may be obtained by filling the cores, WALLBOARD. Special gypsum wallboard or coreboard with a specially formu- lated core affords greater fire resistance than does the conventional or ordinary type gypsum wallboard or coreboard of the same thickness. Constructions in- corporating a, special wallboard or coreboard are so noted under details of con- struction. ... The increases The increase in fire resistance obtained by addition of a plaster coating is shown in the tables for certain constructions. In general, a facing of ~ -inch portland cement or gypsum plaster will usually increase the fire resistance of a I-hour assembly by ~-hour. On 2-, 3- and 4-hour assemblies the increase for the same kind and thickness of plaster will be progressively greater, amounting usually to about I hour on a 4-hour assembly. If applied to two sides of a masonry par- tition the effectiveness is double that of plaster on one side. Plaster coatings, to be effective in adding to the fire resistance periods when combustible structural members are framed in, must be on the exposed faces of the walls opposite to the faces in which the combustible members are inserted. aggregate in use its of perlite or vermiculite resistance to fire. place of sand in plaster, '. 'I. The richness of the plaster mix has a considerable eliect on the fire resistance, particularly with gypsum plaster, and if the listed ratings are to be obtained the mix must be as specified. I,'" '. . PLASTER. In constructions where plaster coatings provide an important part of the fire resistance, it is important to note the kind of plaster, its thickness, the type and proportions of the ingredients, and the type and method of fastening the lath. Gypsum plaster is superior to portland cement or lime plaster in resisting heat transmission. Unless asbestos or other fiber is added to portland cement plaster, its fire resistance is further limited by cracking and spalling. Applying Test Results to Modified Forms of Construction. Because small differences in quality of materials, forms of construction and dimensions of parts, in some cases, make large differences in the fire resistance, care must be exercised in applying test ratings to constructions which differ from those actually tested. Some of the factors which need to be considered are the following. CONCRETE. The fire resistance of concrete depends to a considerable extent on the kind and quality of coarse aggregate used. Siliceous gravel aggregates which contain a large percentage of chert or flint give lower ratings than limestone, trap rock, slag and a number of other aggregates. The relation between kind of aggregate and fire resistance well lustrated in the ratings given for concrete protections for steel rating IS columns. Many of the tests on columns were made prior to 1925 with column loads somewhat lower than permitted today. In certain cases this necessitates considera- tion of the effect of greater allowable loads on the fire resistance rating. Many tests have been made on samples smaller than the minimum size speci- fied in the standard fire test. Such tests cannot be considered the equivalent of tests made on samples of standard size. In some cases they may be satisfactory in determining heat transmission characteristics, from which fairly reliable esti- mates of the fire resistance rating may be made, provided other characteristics affecting performance of the assembly in the standard fire test have been estab- lished, but such estimates must be made with considerable caution. The above factors have been taken into account in arriving at the ratings shown in the following tables. \. .~ .: e FIRE RESISTANCE' RATINGS e e .