280 STRENGTH OF MATERIALS 



230. Transverse tests of brick. Bricks are tested transversely 

 by supporting them edgewise or flatwise upon two knife-edges and 

 applying the load centrally by means of an ordinary testing machine. 

 Care must be taken to provide suitable bearing surfaces for the knife- 

 edges, in order to prevent local failure. In this test the upper fibers 

 are in compression and the lower fibers in tension, and since brick is 

 stronger in compression than in tension, failure is caused by rupture 

 of the tension face. The fiber stress is computed from the formula 



Pie 

 P = J' 



where P is the breaking load in pounds, / is the length of span in 

 inches, e is half the bright, and / is the moment of inertia of a 

 section. The fiber stress on the outer fiber at failure is usually called 

 the modulus of rupture. 



For paving brick the modulus of rupture varies from 1000 lb./in.* 

 to 3000 lb. in. J . For pressed brick, common brick, and medium brick 

 the modulus of rupture varies from 300 lb./in.* to 1200 lb./in.-. 



The shearing strength of various grades of brick varies from 

 300 lb./in. a to 2000 lb./in. a . 



Problem 186. A brick having a depth ami a breadth of o.'.."> in. was 



loaded centrally on a span of ; iii. Tin- ultimate load was 164.~> lb. Kind tin- 

 modulus of rupture. 



231. Rattler test of brick. Paving bricks were formerly tested in 

 abrasion in order to determine their ability to withstand wear. This 

 test, however, does not approach the conditions of actual service, 

 which consist of the impact of horses' feet as well as the abrasive 

 action of traffic. To meet these conditions the rattler test was devised 

 The testing machine consists of a cast-iron barrel mounted hnri/on- 

 tally, and the test is made by placing the brick, together with some 

 harder material, such as cast iron, in the machine and revolving 

 it at a certain speed for a certain length of time. The rnti<> of the 

 amount of material broken or worn off in this way to the original 

 weight of the brick put into the machine indicates the value of the 

 brick in withstanding the conditions of service. 



The charge usually consists of nine paving bricks or twelve other 

 bricks, together with 300 lb. of cast-iron blocks, the volume of the 



