COMPRESSIVE AND TRANSVERSE STRENGTHS OF BRICKWORK. LXXI1I. 



and is:- ^ 



12 



but W the total weight of the brickwork over the girder is : — 



whtl m Wl whtl 2 



''' ~\2 7" 12 

 The moment of resistance of the brick beam is: — J th? (144 f) 



whtl* th* (144/) 

 '"'12 6 



h= wl2 



288/ 



It is sometimes assumed, for convenience, that w equals 144 lbs. 

 per cubic foot (although more frequently about 125) as then : — 



v 



i.e., the height equals the square of the span divided by twice the 

 modulus of rupture. 



If the wall, for example, is built over an opening 20 feet wide 

 and the modulus of rupture is 10 lbs. per square inch, (see Table 

 X.) then in order that it should be self-supporting we have: — 



i 20 x 20 OA , , 



h = = 20 feet 



20 



A height greater than this would be more than self-supporting, 

 and it is seen the height diminishes as the modulus of rupture 

 decreases. For any height less than that at which the brickwork 

 becomes self-supporting the wall would require to be supported 

 and would bring pressure upon the beam. When the bricks are 

 first laid the transverse strength will be much smaller, and the 

 load upon the beam correspondingly greater, but as time goes on 

 the strength will gradually increase, and the actual load upon the 

 beam becomes gradually less and less, and very frequently dis- 

 appears altogether. 



Since the net resistance of the wall increases simply as the 

 height, let h' equal the height which would produce the maximnm 

 load upon the beam, then : — 



