SPECIFICATIOIsrS FOR STEEL HIGHWAY BRIDGES. 29 
Compression members. — The ratio of unsupported j J- ^^^^i^yj^^^e 
length to the least radius of gyration shall not exceed bers. 
120 for main compression and stiffening members nor 
140 for laterals and sway bracing. In proportioning the 
top chords of low trusses the unsupported length shall 
be assumed as the length between the rigid verticals. 
Tension members. — For main riveted tension members 
the ratio of length to least radius of gyration shall not 
exceed 200. 
The effective bearing area of a pin, bolt, or rivet shall ^^^^^Effective 
be its nominal diameter multiplied by the thickness of 
the metal on which it bears. 
RIVETS. 
In proportioning rivets, the nominal diameter of the lo. Effective 
rivet shall be used. et.""'''' '' "" 
Rivets shall be of the size specified but generally shall n. size of nv- 
be % inch or J^ inch in diameter. Five-eighth-inch ^*^' 
rivets shall not be used in members carrying calculated 
stress except in 23^-inch legs of angles and in flanges of 
6-inch and 7-inch beam and channels. 
The diameter of rivets in angles carrying calculated 
stress shall not exceed one-fourth of the width of the leg 
in which they are driven. In angles whose size is not 
so determined ^-inch rivets may be used in 2-inch legs, 
^-inch rivets in 23^-inch legs and J^-inch rivets in 3-inch 
legs. 
In no case, except in handrails, shall structural shapes 
be used which do not admit the use of 5^-inch rivets. 
The mininnum allowable distance between centers of gj^- ^^*''^°^"^' 
rivets shall be three times the diameter of the rivet but 
preferably shaU be not less than the following: 
For J^-inch rivets, 3 inches. 
For ^-inch rivets, 23^ inches. 
For ^-inch rivets, 234 inches. 
The maximum allowable pitch in the hne of stress 
shall not exceed 6 inches or 16 times the thickness of the 
thinnest outside plate or angle connected, except in angles 
having two gauge lines with rivets staggered, where the 
pitch in each line may be twice the above with a maximum 
of 10 inches. 
In webs of members composed of two or more plates 
in contact the rivets shall be spaced not more than 10 
inches between centers in gauge and pitch, provided such 
rivets serve no other purpose than to hold the plates in 
close contact. Tension members composed of two 
angles in contact shall be stitch riveted, using a pitch 
not greater than 12 inches. 
In the ends of built compression members the pitch 
^f rivets connecting the component parts of the member ^^l^ of^comprL" 
shall not exceed four times the diameter of the rivet for a sion members. 
length equal to one and one-half times the maximum 
width of member. Beyond this point the rivet pitch 
