USEFUL DATA 



FLOORS AND ROOFS— WITH EXPLANATION 

 OF TABLES 



Types. The selection of the best type of floor and roof construction depends upon 

 the spans, loads to be carried, character of the building and local conditions. Build- 

 ings readily divide themselves into two general groups, — those primarily for the 

 housing of people, and those for warehousing and industrial purposes. 



In structures of the first group, comprising office and public buildings, schools, 

 hospitals, hotels, apartments, dwellings and garages the majority of the spans are 

 long and the loads light (40 to 125 pounds per square foot). Such conditions require a 

 greater depth of slab, to avoid undue deflection, than would be demanded for strength 

 alone, and the problem is usually solved through the use of a concrete ribbed floor 

 employing clay or composition tile, metal or wood forms of whatever depth may be 

 necessary. In the case of clay or composition tile they always remain a part of the 

 permanent floor and may be said to represent the best type of form or filler for con- 

 crete ribbed slabs; they add stiffness and strength to the construction and in no way 

 detract from its fireproofness. To reduce the cost, however, metal forms are frequently 

 used and may be of either the removable or permanent type. Permanent metal 

 forms are of light gauge steel sheets, stiffened transversely by corrugations, to enable 

 them to withstand the loads and impacts of service; they add nothing to the struc- 

 tural efficiency of the floor and, in fact, may damage the concrete through expansion 

 of the exposed metal in case of fire. For this reason, if metal forms are used, those of 

 a removable type would seem the proper selection. 



For buildings in the warehouse and industrial group the loads usually vary from 

 125 to 500 pounds per square foot. Where the panels are square, or approximately so, 

 the flat slab type of floor presents the utmost advantages structurally and economically. 

 If, however, the ratio of length of short to long side of panel exceeds 1:1^, a beam 

 and girder floor with solid concrete slabs should generally be used. 



Treatment. All beams, including the ribs in concrete ribbed slabs, may be classed 

 under one of two types — rectangular or T-section. In rectangular beams, the con- 

 crete above the neutral axis within the limits of the width of the beam must resist 

 the total compressive stress (assisted in special cases by additional steel in the com- 

 pression area) ; whereas, in the T-section beam the flange, when built monolithically 

 with the web, materially increases the compressive resistance of the beam and con- 

 sequently its carrying capacity. 



The tables of safe loads for ribbed slabs, continuous or partially continuous, are 

 based on a length of span equal to the distance center to center of supports, with the 

 condition that the tile or form, as the case may be, shall extend to withm not less 

 than twelve inches of the center of support as indicated by the illustrations at the 

 head of the tables. The critical section for bending is usually at this point and as the 

 ratio between the bending moment here and at the center of span varies with each 

 change of span, it would be uneconomical to maintain a constant steel area for each 

 fixed depth of slab on all spans. In these tables the proper steel area for each depth 

 and span length is given. 



Load and Moment Conditions. The carrying capacity of beams and slabs is 

 dependent upon the condition of fixity at the supports and the stresses allowed in the 



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