166 



HORTICULTURE 



February 9, 1907 



FIG. IV. 



r^-rnTTr^rfT-T^T^n-frTrTTTr^TTl 



r?T=rTnrrf 



This roof covers 28 benches each 4 ft. In width. 



by truss work of the form shown in 

 figure IV. The load that each truss 

 is to carry in order to be strong 

 enough to support the glass under its 

 breaking load is found by taking the 

 area of roof supported by this truss 

 and multiplying this by the weight per 



TRUSS SECTION OF 150 FT. HOUSE 



PD5T 



pa'iT 



FIG. V. 



square foot that the glass will bear, 

 that is 30 pounds. Now this is the 

 breaking load of the glass that the 

 truss would be called on to bear, but 

 it would be safe to take the working 

 load as one-fourth of this, or 7 1-2 

 pounds per foot. This even would be 

 a high working weight, as ordinarily 

 the truss has only to carry the weight 

 of the glass, which is less than two 

 pounds per foot. Adding 2 1-2 pounds 

 per square foot to cover the weight of 

 the glass and bars, we have ten pounds 

 per square foot as the total working 

 load for the truss. Supposing the 

 posts are spaced 8 feet 4 inches apart 

 lengthwise of the house, then each 

 truss has to carry a total working load 

 of 8 ft. 4 in. X 28x10 pounds, or 2333 

 pounds. This loading may be consid- 

 ered as being distributed at the joints 

 and the ends of the truss as shown by 

 figure V. The stress or strains pro- 

 duced in the members of the truss can 

 now be determined by means of the 

 device known as the stress or strain 

 diagram. The stress diagram for the 

 truss we are considering is shown by 

 figure VI. It will be noticed that the 

 lines in figure VI are drawn to a scale 

 of 333 pounds to 1 inch, and that they 

 are all parallelled to one or other of the 

 lines in the truss shown in figure V. 

 Without attempting further explana- 

 tion it may be stated that the length 

 of each line in figure VI represents the 

 number of pounds' strain or stress in 

 the corresponding member of the truss. 

 For instance, the line k b in figure 

 VI is parallel to the lines between b k 

 on figure V, that is, the upper member 

 of the truss or the sash bar between 

 the two lower purlins. The lines b k 

 is 9.6 inches long and as 1 inch on this 



diagram represents 333 pounds, the 



stress or strain in the sash bar is 3200 

 pounds. In the same way the stress 

 in the lower truss rod is found to be 

 3000 pounds. After finding in this 

 way the strain in each member of the 

 truss, calculations must be made to ob- 

 tain the correct size for each. For in- 

 stance, if the sash bar is of cypress, 

 then it must be heavy enough to stand 

 a working end strain of the 3200 

 pounds. In the case under discussion 

 it has been calculated that this bar 

 must be 17-8x2 1-2 inches; this size 

 giving a working end strain of 3200 

 pounds and a breaking strain of 12,800 

 pounds. The lower member, which we 

 found must carry 3000 pounds, is in 

 tension, so should be of iron or steel, 

 and by a separate calculation it is 

 found that 5-8 inch diameter is suit- 

 able for this strain. The other mem- 

 bers of the truss are proportioned in 

 the same way. Having now designed 

 the truss to carry the roof, it is neces- 

 sary to design the posts or columns to 

 carry the truss. The strength of a 

 column depends greatly on its length, 



so by properly bracing a long, slendor 

 column it is practically divided by the 

 bracing into a corresponding number 

 of short columns. In the wide house 

 under consideration it is found that 

 2-inch pipe properly braced in every 

 direction will carry the roof. The de- 

 sign is now complete, leaving the glass 

 the weakest member as it should be, 

 for it is seen that although the break- 

 ing strength of the glass is 30 lbs. per 

 square foot, and the breaking strength 

 of the truss work is the same, yet the 

 glass is not uniform in strength where- 

 as the trusses are. and as 30 pounds is 

 an average, it follows that half the 

 glass would be broken by the time this 

 load would be reached; and if half the 

 glass is broken, half the weight has 

 gone with it, so our framework stands 

 to be subjected in the limit to only 

 one-half of its breaking load. There 

 is a good deal of theory about all of 

 this, but many of these here now will 

 likely be at the S. A. F. at Philadel- 

 phia in August, and there will be an 

 opportunity to see how it has all 

 worked out. 



STRESS DIAGRAIVl FOR TRUSS IN FIG. V. 



33 3 IBS -/ 



