212 Professors Perry and Ayrton on Structures 



The amount of momentum which has to be transmitted 

 through the foundations of a building to the superstructure 

 depends on the nature of the earthquake (that is, its sudden- 

 ness and the amount of earth-motion), as well as on the mass of 

 the building, while the velocity of the foundations, if these are 

 rigidly connected with the earth, is independent of the mass 

 of the building — an important fact to which we have already 

 drawn attention. The earthquake energy gets destroyed by 

 the interior portions of the earth, as well as the mountains and 

 buildings at its surface not having exceedingly small periodic 

 times of vibration, in consequence of which interference takes 

 place at every surface of contact of the different portions. Of 

 course, however, any one particular building will destroy only 

 a very small portion of the whole energy of the earthquake- 

 vibration; so that its mass cannot in any perceptible way affect 

 the motions of its foundations. 



In the same way as we have shown that the more quickly 

 a house is capable of vibrating the less is its motion rela- 

 tive to the foundation, we might arrive at the result that 

 the smaller the natural period of vibration of the several 

 portions of a body subjected to shocks, the less internal fric- 

 tion must there be ; and this conclusion is consistent with the 

 well-known fact that there is more internal friction in non- 

 homogeneous bodies, or rather, we should say, in bodies which, 

 being non-homogeneous, have some of their materials only 

 capable of very slow natural vibrations compared with the 

 remainder. 



We have no doubt that with any given material what- 

 ever there is a best method of constructing buildings in an 

 earthquake country. Thus, with small stones set in bad mor- 

 tar, or in no mortar, as in the buildings destroyed by the Nea- 

 politan earthquake of 1857, the momentum which must pass 

 through any level joint depends (1) on the short time £ during 

 which the foundations are acquiring a great velocity v, (2) on 

 the mass of the building M above the joint, and (3) on the 

 natural time of vibration of the portion of the structure be- 

 tween the given joint and the foundations. If this time of 

 vibration is very short, then the momentum Mv must be trans- 

 mitted by the joint in the short time t ; that is, the joint must 



Mv 

 transmit the great force — ; whereas if the time of vibration 



of the building below the joint is considerable, the time of 



transmission of momentum is increased in a calculable way, 



say to the time nt, and hence the force transmitted by the 



Mv 

 joint becomes reduced to — - . It is for this reason that, if we 



nt 



