Structural Bolting and Types

The Research Council on Structural Connections (RCSC) prepares specifications and documents related to structural connections
RCSC’s Specification for Structural Joints Using ASTM A325 or A490 Bolts (2000) is a widely used specification which discusses joints, fasteners, limit states, installation, and inspections

Structural Bolting

During hoisting, connectors will install a minimum of two bolts per connection
• The rest of the bolts are installed and tightened after the structure is plumbed
• A systematic pattern must be followed when tightening bolts so that a joint is drawn together and all fasteners are properly installed

Per the Occupational Safety & Health Administration Standard 1926.754(b)(2), “At
no time shall there be more than four floors or 48 feet (14.6 m), whichever is less,
of unfinished bolting or welding above the foundation or uppermost permanently
secured floor, except where the structural integrity is maintained as a result of the

Structural Bolting

• There are many bolt types, installation methods, and joint types used in structural steel construction
• When left exposed, bolts may be used to make an architectural expression
(Green, Sputo, and Veltri)

ASTM Bolt Types

• A307 – Low carbon steel
 Not commonly used
 Only used for secondary members
• A325 – High-strength medium carbon steel (above left)
 Most common bolts used in building construction
• A490 – High-strength heat treated steel (above right)
 Cost more than A325’s, but are stronger so fewer bolts may be necessary
• Note that the ASTM designation is indicated on the head of the bolts above

Common Bolt Sizes

• A325 and A490 bolts are available in diameters ranging from 1/2” to 1-1/2”
• The most common sizes are 3/4”, 7/8”, and 1”
• High-strength bolts are commonly available in incremental lengths up to 8”



• Hardened steel washers are used in many structural connections to spread pressure from the bolt tightening process over a larger area
• Washers may also be used to cover an oversized or slotted hole (RCSC 2000)
• Flat washers are most commonly used
• Tapered washers (above left) are used when the surface being bolted has a sloped surface, such as the flange of a channel or an S shape
• A325 bolts require a washer under the element (head or nut) being turned to tighten the bolt (shown under the nut, above right)
• A490 bolts require a washer under both the head and nut (AISC & NISD 2000)

Parts of the Bolt Assembly

• The grip is the distance from behind the bolt head to the back of the nut or washer
 It is the sum of the thicknesses of all the parts being joined exclusive of washers
• Thread length is the threaded portion of the bolt
• Bolt length is the distance from behind the bolt head to the end of the bolt
(AISC & NISD 2000)

Bolted Joint Types

• There two basic bolted joint types:
o The load is transferred between members by bearing on the bolts
o The load is transferred between members by friction in the joint

Bolted Joint Failure Modes

• Bolts in bearing joints are designed to meet two limit states:
i. Yielding, which is an inelastic deformation (above left)
ii. Fracture, which is a failure of the joint (above left)
• The material the bolt bears against is also subject to yielding or fracture if it is undersized for the load (above right)
• Tension connections act similarly to bearing connections
 Many times, connections in direct tension are reconfigured so that the bolts act in shear (AISC)