216 



CONSTRUCTION. 



Art. 120. 



load are of opposite kind and may neutralize each other ; in which 

 case there will be no shear in the panel, and no diagonal member 

 will be required. If the live load is great enough, compared with 

 the dead load, the distortion will become such as to produce ten- 

 sion in the counter tie. As the live load moves to the right, the 

 stress in the tie decreases and becomes zero; then the stress in 

 the counter tie increases, reaches a maximum and decreases to 

 zero ; then the stress in the tie increases until the left end of the 

 live load stands somewhere in the panel in question. 



No counter ties are required in those panels where there is 

 no possibility of a reversal of stress, or where the live load nega- 

 tive shear is always less than the dead load positive shear. 



A counter brace is a diagonal web member capable of taking 

 compression and subject to alternating stresses of tension and 

 compression (as cD Fig. 167), or subject only to compression 

 under partial loading (as the dotted diagonals in Fig. 160). If 

 in Fig. 154 the counter tie were omitted and Cd were made a 

 compression member, it would be a counter brace, and its stress 

 would reverse during the passage of the live load. If the counter 

 brace happens to be so attached that it can take no tension, as is 

 the case in a Howe Truss (Fig. 160), it gets stress only under 

 certain partial loads just as a counter tie does, but the stress is 

 of opposite kind. 



It is the custom to use counter ties in some types of trusses, 

 and counter braces in others. In some types, counter ties can 

 not be used. 



Counter ties are made adjustable in length so that an initial 

 stress may be put into them. The initial stress in the counter tie 

 produces the same initial stress in the main tie, as is evident if 

 the distortion of the panel is considered. Stresses will also be 

 induced in the posts and chords of the panel, but no other mem- 

 bers of the truss will be effected. These stresses, whose magnetude 

 are usually much in doubt, may usually be neglected as the fol- 

 lowing consideration will show. Their purpose is to prevent 

 shock when the live load passes over the structure and the stresses 

 change from one diagonal to the other in a panel. 



Fig. 154 (b) represents the panel CDdc. When the counter 

 tie cD is shortened, the tendency is to deform the panel as shown 

 by the dotted lines. This deformation is resisted by the main tie 

 and consequently a tensile stress is set up in it. Take a section mn 



