162 PRACTICAL STRUCTURAL DESIGN 



splices will come in panels in which the stress is low, the full 

 area of the planks being available for carrying the tension in the 

 middle of the span where it is greatest. In any event the splicing 

 of the bottom chord of a truss, when said chord is composed of 

 plank, is a matter requiring a great deal of careful study. It is 

 an easy matter to make a poor splice. The writer has seen some 

 in which the designer calculated for one-half the stress going 

 through a certain splice when actually all the stress went through it. 

 In Fig. 96 is illustrated a detail of a bottom chord in which all 

 the tension is carried by a steel plate and underneath are two 

 pieces of timber large enough to carry the plate and take out the 

 sag. This is from a plan for a standard wooden highway bridge 



< 12, Bolts 



>USS- ------ /f'-g" -------- -*, 



TopViewSf Plate. * ; 



If*' L W er Chord 

 Bottom View of Chord. Cross Section. 



Fig. % 



designed by Hugh C. Lewis, Bridge Engineer in the State Highway 

 Department of Utah, E. R. Morton, State Road Engineer. The 

 figures were copied from Engineering News, Sept. 21, 1916. Note 

 the spliced joint between the two timber pieces in the chord. 

 This consists of a long splice plate, large enough to carry all the 

 stress, between the two outer chord pieces. The use of such a 

 splice leaves a wide ah* space between the two chord pieces for 

 ventilation. When a bottom chord consists of three planks 

 and the center plank between two vertical rods is long enough 

 to serve as a splice piece, the two outside pieces may break 

 joints at a section passing through the chord, as shown in this 

 detail. 



In Fig. 97 several methods are shown for making joints in a 

 piece under compression. These joints are used in the top chords 

 of trusses and also in columns, for the top chord of a truss is a 

 column. The detail shown at (a) is the best of the lot. The ends 

 should be carefully dressed to insure an even bearing. The detail 

 at (6) is in common use and is not so good as (a). It has two bear- 

 ing surfaces and this makes it very difficult to get an absolutely 

 true bearing. It may fit tight at one end and not fit evenly at the 

 other end. When the load is brought on one-half the member 



