FOUNDATIONS 311 



As an example of a complete problem, the following case 

 is suggested: Design a reinforced-concrete arch for a 65-ft. 

 span. This arch is to carry a live load of 200 Ib. per sq. ft. 

 and a dead load of 300 Ib. per sq. ft. at the crown. 



First, the rise of the arch must be determined. This may 

 be taken as one-tenth the span, or 6.5 ft. The curve of the 

 arch may now be laid out. If the curve to be followed 

 is a parabola, the method of constructing it is given on 

 page 307. This curve may be used either as the center line 

 or as the intrados of the arch, preferably the former. 



The thickness of the arch ring at the crown may now be 

 determined by the formula on page 309. In this case, 5 = 65, 

 L = 200, and F = 300. Therefore, 



r 65 200 300 



Calling this thickness 18 in., the thickness of the arch ring 

 at the quarter points may be taken as 1^X18 = 27 in., and 

 its thickness at the haunches may be taken as 2 X 18 = 36 in. 



The amount of steel reinforcement required for each layer 

 must now be determined. For each foot of width of the arch, 

 this will be 18 X 12 X y&ff X 1% = .864 sq. in. A f-in. round 

 rod has an area of .4418 sq. in.; therefore, one J-in. rod every 

 6 in. near the extrados, and one 1 J-in. rod every 6 in. near the 

 intrados will be sufficient. These rods should be embedded 

 2 or 3 in. in the concrete. The two layers of reinforcement 

 should be either securely tied together or bonded into the 

 concrete with shear or diagonal members. 



FOUNDATIONS 



BEARING VALUE OF FOUNDATION SOILS 



There is some difference of opinion regarding the safe 

 bearing value of foundation soils, due probably to the diffi- 

 culty of arriving at any experimental results that will have 

 a general application. Conservative engineering practice, 

 however, dictates that the greatest unit pressure on the 



