General Design Provisions

The general rules that follow are based on the AREMA Manual. They should be used where applicable but may be modified to reflect specific owners requirements.

Thickness of Material
Steel should generally not be less than 0.335 in thick, but fillers may be thinner. Where components are subject to corrosive conditions, they should be made thicker than otherwise required or should be protected. Some owners have adopted a more conservative minimum thickness of 0.50 in, except for fillers. Gusset plates used to connect chord and web members in trusses should be proportioned for the force transferred and should not be less than 0.50 in thick.

Slenderness Ratios
Slenderness ratios, expressed as the length divided by the least radius of gyration, should not exceed the following:
Main compression members 100
Wind and sway bracing in compression 120
Single lacing 140
Double lacing 200
Tension members 200

Fasteners and Net Section
The nominal diameter of fasteners should be used as the effective diameter. The effective bearing area of rivets and pins should be taken as the diameter multiplied by the length in bearing. For countersunk rivets, the bearing length should be reduced by one-half the depth of the countersink.
Fasteners should be arranged symmetrically about the axis of the member. The net section of a part should be taken as the thickness multiplied by the least net width of the part. Thenet section of a riveted or bolted tension member is the sum of the net sections of its parts, computed as the net width times the thickness.
The net width for a chain of holes extending across a part should be taken as the gross width, less the sum of the diameters of all holes in the chain, plus a quantity for each space in the chain computed as:
S^2 /4 g

where S  pitch of two successive holes in the chain in the direction of tensile stress and g  gage of same two holes, in the transverse direction.
The net section of the part is determined by using the chain of holes that gives the least width. The net width should not be considered as more than 85% of the gross width. The diameter of the holes should be taken as 1⁄8 in more than the nominal size of the fastener.
Bolted or riveted connections should have not less than three fasteners per plane of
connection, or the equivalent strength in welding. Fillet welds are preferred and should be parallel and symmetrical to the direction of the force.
Field connections should be made using rivets or high strength bolts. Field welds may be used for minor connections which are not subject to live load forces and for joining sections of deck plate or other items that do not function as part of the load carrying structure.
Otherwise, field welding should not be used for connections.
Welds acting in the same connection with rivets and/or bolts should be proportioned and aligned to carry the entire force. Rivets and high strength bolts working in the same connection plane should be considered as sharing the force. When the connection is subjected to fatigue conditions, the stress category and allowable stress for rivets should be used for both types of fasteners.

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