30 BULLETIN 1259, U. S. DEPARTMEXT OF AGRICULTURE. 
shall be gradually increased for a length equal to one 
and one-half tmies the jnaximum width of the member 
until the maximum spacing is reached. In angles hav- 
ing two hues of staggered rivets in one leg, the pitch on 
each hne may be t^\'ice that specified above, but not 
greater than that allowed for the body of the member. 
14. Edge dis- The minimmn distance from the center of anv rivet to 
tance of rivets. i i j i n a, 
a sheared edge shall be: 
For J^-inch rivets, 13^ inches. 
For ^-inch rivets, 13kC inches. 
For ^-inch rivets, 13^ inches. 
The minimum, distance from rolled or planed edges, 
except flanges of beams and channels, shall be: 
For J^-inch rivets, 13^ inches. 
For %-inch rivets, 13^ inches. 
For ^-inch rivets, 1 inch. 
The maximum distance from any edge shall be eight 
times the thickness of the thinnest outside plate, but 
shall not exceed 5 inches. 
15. Long rivets. Loug rivcts subject to Calculated stress and having a 
grip in excess of four and one-half diameters shall be 
mcreased in number at least 1 per cent for each additional 
Yt inch of grip. If the grip exceeds six times the diame- 
ter of the rivet, specially designed rivets shall be used. 
16. Rivets in Rivets in direct tension shall, in general, not be used, 
ension. However, where so used their value shall be one-haK that 
permitted for rivets in shear. Countersunk rivets shall 
not be used in tension. 
17. Depth ra- Trusscs preferably shall have a depth not less than 
one-tenth of the span, plate girders a depth not less than 
one-twelfth of the span, and rolled beams a depth not 
less than one-twentieth of the span. If less deptns than 
these are used, the sections shall be increased so that the 
maximum deflection will not be greater than if these 
limiting ratios had not been exceeded. 
sibfe ^"^^^ ^^^^^' '^^® accessibility of all parts of a structm'e for inspec- 
tion, cleaning and painting shall be insured by the proper 
proportioning of members and the design of their details. 
19. Open sec- Closed sections shaU, in general, be avoided. 
^lons an poc - pQ(>]j^^g oj. dcpressious which will retain water shall be 
avoided as far as possible and those which are unavoid- 
able shall be provided with effective drain holes or shall 
be effectively filled with waterproof material. 
Detafls shall be arranged so that the retention of dirt, 
leaves, or other foreign matter will be reduced to a 
minimum. Wherever angles are used, either singly or in 
pairs, they preferably shall be placed with the vertical 
legs extending do\\'nwiird. 
20. symmetri- Main members shall be proportioned so that their 
sec lonb. neutral axes shall be as nearly as practicable in the center 
of the section. # 
In general, the gravity axes of main truss and other 
important members, meeting to form a joint, shall inter- 
sect in a common point so as to avoid eccentricity of stress. 
tios. 
