1842.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



293 



4. The deflections within the limit of elasticity, were (very nearly) 

 equally increased by the addition of equal weights. 



5. The place of the neutral axis of fracture, was most distinctly 

 marked in every case, and the section of fracture, in all the models, 

 very closely resembled that shown in Plate III of Barlow's Essay on 

 Timber, 3rd ed : London, 1S2G. 



The elaslic strength of Model No. 1, did not exceed 392 lb. 



do. No. 2, do. 560 



do. No. 3, do. 560 



do. No. 4, do. 1120 



do. No. 5, do. 840 



As indicated by the breaking weights, excepting only No. 4, which 

 would have borne a few more pounds ; and No. 5, which would have 

 broken with less, if time bad been allowed. 



The ultimate strength of Model No. 1, was 

 do. No. 2, „ 



do. No. 3, „ 



do. No. 4, „ 



do. No. 5, „ 



868 lb. 



SG8 



868 

 1288+ 

 ]176- 



K comparison of the tabular residts will show 



1. That, contrary to the experiments of Duhamel, wedging the stick upon 

 the upper side to one-third of the depth, with slips of hard wood— at least 

 in the extent to which we carried it — neither augmented nor impaired the 

 ultimate strength of the timber, while it diminished its stiffness. 



2. That the suspension tniss, though it added to t)iiistiffness,ha.i\ noeff"ect 

 whatever, upon the ultimate strength, when only two bearing plates, (as b and 

 c, Fig. 1,) were used, and the weight applied midway between them. 



The reason of this appeared to be, that the deflection between the bearing 

 plates, when added to the compression of the timber, at these, and at the 

 abutments, allowed the beams to bend enough to exceed the deflection due 

 to their elastic strength, before the suspension truss came fairly into action. 



3. That the suspension truss, with a centre plate apphed at a. Fig. 1, 

 doubled the elastic strength of a plain girder of the stated dimensions, and 

 added near fiftg per cent, to its ultimate strength. 



4. That trussing a girder with hard wood, in the usual manner, increased 

 its elastic strength fifty per cent, and its ultimate strength about thirty-five 

 per cent. 



This last conclusion is nearly the same as that developed by Professor 

 Barlow, in experimenting upon a similar truss. (See Treatise on the Strength 

 of Materials : London, 1837, page 165.) 



The experiments referred to, and of which we subjoin a synopsis, showed 

 that though there was no eflicacy at all, in a common girder truss, of three 

 pieces of hard wood, with queen bolts, — indeed, that it was weaker than an 

 untrussed stick, — still, the truss of tioo pieces, with a ii>ig bolt, sindlar to 

 that of Fig. 3, did add considerably to the strength.^ 



Synopsis of Bartow's Experiments on the Strength of Girders of Wood. 



No. of Experiments. 



Distance between the Props 



Depth of Girder 



Breadth of Girder 



Ultimate deflection 



Ultimate weight in lb 



Trussed or untrussed 



Broken or not 



Inches. 



68 



2 



n 



225 

 500 



Trussed. 

 Not hrok. 



Inches. Inches. 



68 

 2 



11 

 L55 

 500 



50 

 2 



n 



L50-H 

 953 



Untrussed Trussed. 

 Not hrok. I Broke. 



4. 



Inches. 



50 



2 



n 



L00-> 

 717 I 

 Untrussed 

 Broke. 



2 S 



