974 THE STRENGTH AND ELASTICITY OF IRONBARK TIMBER 
for Existing Lines, for their kindness in allowing him to have the : 
remains of the large beam tested by the Railway Bridges Inqui: yo 
‘Commission, and for sending to the University the specimens cut 
for testing. 4 
Discussion. a 
Mr. J. A. M‘Donatp, A.M.1.C.E., said:—Hearing that Professor” 
Warren in his paper on the strength of Colonial timbers intended — 
touching on the question of compound beams, he thought it might 
be of interest to many members of this Society if he introduced a— 
practical and to some extent graphical method of calculating th 
strength of these beams. 
The beam shown on plate 15 is 29 ft. centres of bearings, 
and is composed of three logs, each 12 in. x 12in, bolted and 
= 
ction. 
The breaking strength of a solid beam 3 feet deep, 12 inches 
wide, and 29 feet centres, would be 170 tons with a centre load, 
or 340 tons with a distributed load, taking the modulus ¢ 
rupture of ironbark at 12,7 s. ; 4 
_ The total breaking strength of three beams cach 12 inches x 
12 inches and 29 feet centres, laid on top of one another witho ub 
bolts or keys, would be 57 tons with a centre load, or 114 tol 
with a distributed load, or one-third the strength of the 80 
beam. It will be seen, therefore, that the strength of the com 
pound beam can vary between one and three depending entirely 
the provision made to take the horizontal shear. 
e amount of horizontal shearing stress on the keys 
m 
curves 4 BC and D # F; the sum of these two is, of co 
the total mean vertical shear which we will call S. hen 4° 
= maximum horizontal shear at the neutral axis of the be 
Lay off G H = 4 S, and construct a parabola with height = G 
and base = total depth of beam. The horizontal shearing § 
at any point on the vertical section is then given by scaling 
iength of the ordinate at that point, as K L for the compe 
shown. On plate 15 these parabolas have been plotted # 
tical shear, the safe stress bein n 
Pid g taken at 4:5 tons per square 
By the above arrangement of keys and bolts it will be 
