222 



BIvIDGE TRUSSES NOW OUT OF DATE. Art. 122. 



The Whipple truss (Fig. 159) is called a double intersection 

 truss because it has two systems of web bracing. It is a combina- 

 tion of two Pratt trusses (Fig. 161) with common chords. The 

 object was to avoid long panels and the consequent expensive 

 floor system, and at the same time, to have an economic inclina- 

 tion for the diagonal web members. It was used for long spans 

 and many examples of it are still to be seen, particularly in high- 

 way bridges. It was displaced by the Baltimore type of truss. 

 See Figs. 163 and 164. 



The stresses are statically indeterminate, but it is the practice 

 to assume that the two systems act independently, the chord 

 stresses being added together (134). The division into two sys- 

 tems is not the same for partial loading as for full loading, and 

 certain ambiguities arise. Since about 1890 very few Whipple 

 trusses have been built. The simpler trusses, whose stresses are 

 statically determinate, are preferred by most engineers. 



All of the preceding trusses were of the pin-connected type 

 although, usually, there were some joints with special details. 

 The eyebar was not introduced until 1861. 



123. Modern Bridge Trusses. The Howe truss was pat- 

 ented by William Howe in 1840. This truss has been used exten- 

 sively, particularly in the first construction of railways. It is 

 still being used in sections of the country where timber is cheap 

 and economy in first cost is important. Many Howe truss bridges 

 have been replaced by steel bridges. 



In the Howe truss the diagonal members are in compression 

 and the vertical members in tension. (See Fig. 160.) All mem- 

 bers except the vertical tension members are made of timber. The 



HOWE TRUSS 



Fig. 160. 



vertical tension members are wrought iron rods with screw ends ; 

 their use, with cast iron angle blocks at the joints, against which 

 the diagonal members abut, perraits simple and satisfactory de- 

 tails with the minimum amount of metal. The main diagonals 



