278 FOUNDATIONS 



This gives tension on the windward side which will not do, and 

 so we will reinforce the footing with beams and make / = 10 ft., and 

 increasing weight so that W 40806 Ibs. 



P = 680 559 



= 1239 or 121 Ibs. per square foot, which 

 is safe for ordinary soils. 



If it had been necessary to drive piles for this pier, a small amount 

 of tension might have been allowed on the windward side if the tops of 

 the piles had been enclosed in concrete. 



Design of Footings. The thickness and length of the offsets in 

 a concrete or masonry footing are commonly calculated as for a beam 

 fixed at one end and loaded with a uniform load over the projecting 

 part equal to the maximum pressure on the footing. If p projection 

 of the footing in inches ; t = the thickness of the footing in inches ; P = 

 pressure on foundation in pounds per sq. ft.; and S = safe working 

 load of the material of which the footing is made in pounds per square 

 inch, by substituting in the fundamental formula for flexure and 

 solving for p f 



p = 4 ' V (88) 



The values of 5* in common use are : firct class Portland cement 

 concrete 50 Ibs.; ordinary concrete 30 Ibs.; limestone 150 Ibs.; granite 

 1 80 Ibs. ; brickwork in cement 50 Ibs. 



The projection and thickness of the footing course is sometimes 

 calculated on the assumption that the footing course is a beam fixed at 

 the center, in place of as above. This solution hardly appears to be 

 justified. 



Pressure of Column on Masonry. The following pressures in 

 pounds per square inch are allowed by the building laws of New 

 York. Portland cement concrete 230 Ibs. ; Rosendale cement concrete 

 125 Ibs.; Rubble stonework laid in Portland cement mortar 140 Ibs.; 

 brickwork laid in Portland cement mortar 250 Ibs. ; brickwork laid in 

 lime mortar no Ibs. ; granite 1000 Ibs. ; limestone 700 Ibs. It is very com- 

 mon to specify 250 Ibs. per square inch for bearing on good Portland 

 cement pedestals, and 300 Ibs. per square inch is not uncommon. 



