STRENGTH ANL> TESTING OF TIMBER 315 



timber, Lanza's tests on thirty-seven beams gave an 

 average modulus of rupture of 4,451 Ibs. per square incb ; 

 the minimum was 2,456, and he recommends 3,000 Ibs. 

 per square inch as the figure to be used. 



The results of tests made on superimposed beams by 

 Major Moritz Bock on the Continent and by Mr. H. D. 

 Smith in Liverpool are somewhat different to those which 

 would be anticipated and generally assumed by designers. 

 In both experiments it was found that the bolting together 

 of the beams gave no appreciable increase of strength over 

 beams simply laid one upon another ; in one of those tested 

 by Mr. Smith " the bolts bent at each joint of the baulks and 

 pressed sideways into the timber in some cases to the extent 

 of inch," showing that the beams slid upon one another. 

 The strongest of Major Bock's combined beams had only 

 50 per cent, the strength of a solid beam of the same size. 

 In the case of superimposed beams secured with oak keys 

 and wedges at the joinings, as well as being bolted with 

 inclined bolts, the strength was found to be 60 per cent, of 

 that of the solid beam. 1 



In cases, therefore, where it is found necessary, because 

 of heavy loads, to secure a greater depth of girder by having 

 two or three beams superimposed, although it is advisable 

 to have them secured with a few bolts, to ensure their not 

 being shifted or knocked out of position sideways, the 

 additional strength of the cumulative beams should only be 

 assumed as about 50 per cent, extra per beam. Beams 

 composed of three baulks are proportionally stronger than 

 those composed of four, and those composed of two baulks 

 are proportionally stronger than those composed of three ; 

 but under the same unit stresses three beams are much 

 stifTer than two, and, though in a less degree, two are 



1 Min. of Proc. lust. C.E., Yols. CVI. and CXXVIII. 



