COLUMNS AND STRUCTURES 269 



thin that the aggregates have a tendency to separate. Get in 

 touch with the Portland Cement Association, Chicago, 111., when 

 reliable information on concrete is required. The Association 

 has a well equipped laboratory where all questions affecting the 

 use of portland cement and the manufacture and use of concrete 

 are investigated. 



Column Brackets and Bases 



Brackets on steel columns for carrying beams and girders are 

 rivetted to the columns. For light loads they are simple shelf 

 angles. For heavy loads they consist of plates stiffened with 

 angles. Details are given in the steel handbooks. 



Post caps for wooden columns are illustrated in a book entitled 

 "Heavy Timber Mill Construction Buildings" distributed free 

 of cost by the National Lumber Manufacturers Bureau, Chicago, 

 111. Mill construction has its place and every architectural de- 

 signer should have a copy of the book. Some of the statements 

 therein should be modified and the student is advised to obtain 

 from the Portland Cement Association, Chicago, 111., a bulletin 

 entitled "Why Build Fireproof," written by the author, and 

 thus obtain a glance at both sides of the question. The book on 

 mill construction illustrates cast iron and steel post caps for 

 carrying beams and girders. The girders do not rest on top of 

 the posts, for the carrying power of the posts would be reduced 

 thereby. It was formerly the custom to use wooden bolsters, 

 on which to rest the girders and beams, in order to shorten the 

 span. The side bearing strength of wood is much lower than the 

 strength with the fibers. When the direct load comes down 

 the post with a bearing stress, say, of 1100 Ibs. per square inch, 

 and rests on the side of a bolster the bearing stress is reduced at . 

 once to 235 pounds per square inch, or more, depending upon 

 the wood. Bolsters to-day are used only under roof girders. They 

 are better in case of fire than cast iron or steel but so greatly 

 reduce the carrying power of the posts that they are not eco- 

 nomical. Post caps come in a number of different shapes. To in- 

 vestigate the strength of a post cap obtain the moment of inertia 

 of the cross section of the carrying portion. Then find the fiber 

 stress by the method shown on page 67. The bending moment is 

 the moment caused by the reaction from the beam acting at half 

 the length of the projection from the face of the column. 



