COLUMNS AND STRUCTURES 271 



Cast steel bases are better than cast iron bases and built-up 

 steel bases are a great deal better than cast steel bases. Cast 

 steel buses an- designed in the same way as cast iron buses. Built- 

 up steel bases are illustrated in the steel handbooks. 



When the projection P of a cast iron, or cast steel, base is not 

 greater than six inches a plate may often be used to advantage. 

 The formula to use in the design of such a plate is given on page 

 152, where it is used to design a washer under the head of a bolt. 

 Mention is there made of ribs acting as cantilevers and if the 

 student wishes to attempt to design bearing plates and bases 

 of cast iron according to formulas, he is advised to procure from 

 the Engineering Experiment Station, University of Illinois, Ur- 

 bana, 111., a copy of the bulletin on the design of cast iron column 

 bases and bearing plates. He is advised also to procure a copy 

 of Bulletin No. 67 on the design of reinforced concrete footings. 



The size of a column is fixed by the compressive strength of 

 the material. The column rests on concrete, stone or brick foot- 

 ings, which have a lower strength in compression, so it becomes 

 necessary to enlarge the lower end in order not to overstress the 

 masonry. It is most convenient, in the majority of cases, to set 

 the spread base before erecting the column, so bases are made 

 to which the columns are bolted. The area of the bottom of the 

 base is obtained by dividing the load by the bearing strength of 

 the masonry. 



Eccentric Loads on Footings 



In Fig. 175 is illustrated a common case of eccentric loading 

 on the footing of a wall. The formula to use is given on pages 

 100 and 101. The direct load divided by the area of the footing 

 gives the pressure per square foot on the soil. A vertical line is 

 drawn through the center of gravity of the footing and a vertical 

 line is drawn through the center of gravity of the wall. The 

 horizontal distance e is the moment arm. Multiply the load in 

 pounds by the moment arm, e, in feet and use the bending mo- 

 ment in the formula. In the formula h is used as the depth of 

 the member. In the case of the footing h is the width, / shown 

 in the figure. The resulting fiber stress cannot exceed the safe 

 allowable bearing pressure on the soil. A positive (+) result 

 indicates compression and a negative ( ) sign indicates uplift. 

 The two shaded diagrams illustrate the action. The upper one 



