in an Earthquake Country. 215 



for building material. Consequently the deflection of the centre 

 of gravity is equal to 



PA f Jl_ 31 

 EAl24K 2 + 2j ; 



so that if, for simplicity, we suppose the prism to vibrate as if 

 its mass were gathered at its centre of gravity, if T is the period 

 of a complete natural vibration, and if iv is the weight of the 

 material per unit volume, 



T=2 



PA f J^_ 31 



EA\24K 2 + 2j 



/yP. 



hAiu 



or 



T = 2 VS{A + S}- 



For a solid rectangular structure with a square horizontal 

 section of breadth b, 



12 

 or 



K 2 =^, 



T = 2 VS{fi+S} 



For a hollow square section, the sides of the outer and inner 

 square being respectively a and c f 



K 2 



12(d 2 -c 2 ) 



a 2 + c 2 

 12 ' 



V 2p3 \ a 2 + c 2 J 



Now for brick 



wr=ll21bs. 1 



and v about; 



E = 144,000,000 lbs. per square footj 



, T=0-001534^1(_^ +3 ). 



Q2 



