trSEFtTL DATA 



(a) Column Capital. — It is usual in flat slab construction to enlarge the supporting 

 columns at their top, thus forming column capitals. The size and shape of the column 

 capital affect the strength of the structure in several ways. The moment of the external 

 forces which the slab is called upon to resist is dependent upon the size of the capital; 

 the section of the slab immediately above the upper periphery of the capital carries 

 the highest amount of punching shear; and the bending moment developed in the 

 column by an eccentric or unbalanced loading of the slab is greatest at the under 

 surface of the slab. Generally the horizontal section of the column capital should be 

 round or square with rounded corners. In oblong panels the section may be oval or 

 oblong, with dimensions proportional to the panel dimensions. For computation 

 purposes, the diameter of the column capital will be considered to be measured where 

 its vertical thickness is at least 13^ inches, provided the slope of the capital below 

 this point nowhere makes an angle with the vertical of more than 45 degrees. In case 

 a cap is placed above the column capital, the part of this cap within a cone made by 

 extending the lines of the column capital upward at the slope of 45 degrees to the bot- 

 tom of the slab or dropped panel may be considered as part of the column capital in 

 determining the diameter for design piu-poses. Without attempting to limit the size 

 of the column capital for special cases, it is recommended that the diameter of the 

 column capital (or its dimension parallel to the edge of the panel) generally be made 

 not less than one-fifth of the dimension of the panel from center to center of adjacent 

 columns. A diameter equal to 0.225 of the panel length has been used quite widely 

 and acceptably. For heavy loads or large panels especial attention should be given to 

 designing and reinforcing the column capital with respect to compressive stresses and 

 bending moments. In the case of heavy loads or large panels, and where the condi- 

 tions of the panel loading or variations in panel length or other conditions cause high 

 bending stresses in the column, and also for column capitals smaller than the size 

 herein recommended, especial attention should be given to designing and reinforcing 

 the column capital with respect to compression and to rigidity of connection to floor 

 slab. 



(6) Dropped Panel. — In one type of construction the slab is thickened throughout 

 an area surrounding the column capital. The square or oblong of thickened slab thus 

 formed is called a dropped panel or a drop. The thickness and the width of the dropped 

 panel may be governed by the amount of resisting moment to be provided (the com- 

 pressive stress in the concrete being dependent upon both thickness and width), or 

 its thickness may be governed by the resistance to shear required at the edge of the 

 column capital and its width by the allowable compressive stresses and shearing 

 stresses in the thinner portion of the slab adjacent to the dropped panel. Generally 

 however, it is recommended that the width of the dropped panel be at least four- 

 tenths of the corresponding side of the panel as measured from center to center of 

 columns, and that the offset in thickness be not more than five-tenths of the thickness 

 of the slab outside the dropped panel. 



(c) Slab Thickness. — In the design of a slab, the resistance to bending and to shear- 

 ing forces will largely govern the thickness, and, in the case of large panels with light 

 loads, resistance to deflection may be a controlling factor. The following formulas 

 for minimum thicknesses are recommended as general rules of design when the diameter 

 of the column capital is not less than one-fifth of the dimension of the panel from 



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