CHAPTER 3.

BOLTED CONNECTIONS FOR TENSION

MEMBERS 3.1 Types of Bolts

Bolting and welding have been the methods used for making structural steel connections for the past few decades and riveting is almost obsolete because they no longer provide the most economical connections. Rivets are still occasionally used for fasteners, but their use has declined to such a degree that most steel fabricators have discontinued riveting altogether.

Types of bolts for connecting members

* Unfinished bolts = sometimes called ordinary or common bolts. They are classified by the ASTM as A 302 bolts and are made from carbon steels with stress-strain characteristics similar to those of A-36 steel.

* High Strength Bolts = they are made from medium-carbon heat treated steel and from alloy steel and have tensile strengths greater than those of ordinary bolts. They are designated as A 325 and A 490 bolts.

High - strength hexagonal head bolt

D=1/2”

7/8" 5/16” Bolt tenght

Bolt

7/8” 31/64” Nut

Thread length l' '

3.2 Types of bolted connection

· Slip-critical or Friction type connection = bolted connections where high slip resistance is desired. When high-strength bolts are fully tensioned, they clamp parts being connected tightly together, this results on a considerable resistance to slipping on the surface equal to the clamping force times the coefficient of friction. So if the shearing load is less than the permissble frictional resistance, the connection is referred to a slip-critical or friction type. If the load exceeds the frictional resistance, the members will slip on each other and will tend to shear off the bolts and at the same time the connected parts will push or bear against the bolts. In this type of connection, the specification assumes the bolts are in shear and no bearing.

· Bearing type connection = bolting connections where high slip resistance is not necessary.

3.3 Sizes and types of holes for bolts

Standard-size bolt and rivet holes are 1.6 mm larger in diameter than the bolt or rivets.

· Oversized Holes = they are used in all plies of connection as long as the applied load does not exceed the allowable slip resistance. They should not be used in bearing type connection.

· Shord Slotted Holes = they are used regardless of the direction of the applied load if the permissible slip resistance is larger than the applied force. If the load be applied in a direction normal to the slot, these holes maybe used in any bearing type connections.

· Long Slotted Holes = they are used in only one of the connected parts of friction type or bearing type connections. For friction type these holes maybe used in any direction but for bearing-type connections the load must be normal to the axis of the slotted holes. If long-slotted holes are used in an outer ply, they will need to be covered by plate washers.