39 



embodying this principle, and at the present time more 

 Thacher arches are being built in America than any other 

 one kind. 



The advantages gained by reinforcing concrete arches 

 with steel are: 



(1) In long spans the thickness of the ring can be 

 made very much less, thus reducing the dead load and 

 effecting a saving of 50 per cent or more in the cost of 

 the ring. 



(2) The load being reduced, the thrust is correspond- 

 ingly less, and the abutments may be very much smaller. 



(3) Reinforced with steel, the working unit stresses in 

 concrete can be considerably increased with perfect 

 safety. 



(4) The danger of cracks occurring from any cause 

 can be prevented by the proper distribution of the re- 

 inforcing metal. 



(5) Arch rings of such forms and proportions that the 

 bending moments would render them impossible with any 

 ordinary masonry, can be, and have been, constructed of 

 reinforced concrete both easily and successfully. 



(6) Much longer spans are practical for any given set 

 of conditions than could be seriously considered with any 

 other class of masonry." 



(21) That it is cheaper to build a J_ shaped retaining 

 wall properly reinforced than a solid concrete or counter- 

 fort wall. 



(22) Lay sheeting horizontal: use from % to 2 in. 

 stock and studding placed 2 ft. o. c. for % in. and 5 o. c. 

 for 2 in. sheeting. 



(23) Keep cement from getting wet or being in a moist 

 atmosphere. 



(24) Four men ought to mix and wheel 10H batches of 

 1:2V&:5 mix consisting of 4 bags Portland cement, 9.5 cu. 

 ft. sand and 19 cu. ft. of stone 50 ft. in 10 hours. 



Safe Strength of Portland Cement Concrete in Direct 

 Compression. (Taylor & Thompson.) 



Pounds per Tons per 

 Proportions sq. iu sq. ft. 



1:2:4 410 29 



860 25 



1:3:6 325 23 



1:4:8 260 18 



Wit h a larjfe mass foundation take values onr-eiphth greater. 

 \\ ith a vibrating or pounding load take one-half these value*. 



