Bolted Connections

Learning Objectives

  • Distinguish bolt specification/grade from joint type and installation condition.
  • Evaluate bolt shear and tension using nominal stresses from the exact adopted AISC/RCSC edition.
  • Calculate plate bearing/tear-out using actual hole type, clear distance, thickness, and material strength.
  • Separate bolt strength from connected-part net-section and block-shear strength.
  • Explain pretensioned and slip-critical joints without using a friction equation outside its RCSC scope.
  • Recognize combined shear/tension, eccentric bolt-group behavior, prying, and detailing requirements that prevent a simple per-bolt calculation from proving whole-connection adequacy.

Bolting code-basis rule

Current AISC manuals use ASTM F3125 terminology for high-strength structural bolts and coordinate with the applicable RCSC Specification. Legacy “A325” and “A490” names are still common in teaching and drawings, but exact nominal stresses, pretensions, slip coefficients, hole factors, and installation requirements must be taken from the adopted AISC/RCSC edition, not memorized as timeless constants.

Bearing-Type Joint

A joint whose strength design permits load transfer through bolt shear and bearing of the connected material against the bolt/hole after any initial slip that is not prohibited by the design requirements.

Pretensioned Joint

A joint in which high-strength bolts are installed to the required pretension using a permitted installation method. Pretension may be required by the governing connection category even when slip resistance is not the strength mechanism being used.

Slip-Critical Joint

A pretensioned joint designed so the required slip resistance at the faying surfaces satisfies the governing slip limit state under the load level specified by the adopted RCSC/AISC provisions.

Bolt Material and Joint Type Are Different Decisions

ASTM F3125 groups/grades

High-strength structural bolting is commonly specified under ASTM F3125 using grade/group designations that encompass legacy A325/A490 terminology. The bolt material determines tabulated nominal tensile/shear properties and required pretension. The joint type determines installation, slip, and detailing requirements. Do not use “A325” as if it means “bearing connection” or “A490” as if it means “slip-critical connection.”

Bolt Shear

Nominal bolt shear strength

Per shear plane for the applicable tabulated nominal shear stress.

Rn=FnvAbR_n=F_{nv}A_b

Threads in or out of the shear plane

The adopted AISC table distinguishes nominal shear stress according to bolt group/grade and whether threads are included in the shear plane. Use the nominal bolt area AbA_b specified by the provision; do not reduce the area again for threads when the table already accounts for thread condition through FnvF_{nv}.

Bolt Tension and Combined Loading

Nominal bolt tensile strength

Rn=FntAbR_n=F_{nt}A_b

Combined shear and tension

When a bolt simultaneously carries shear and tension, the available tensile/shear strength can be modified by the interaction procedure in the adopted AISC provision. Use the correct LRFD or ASD form and verify the separate shear limit as required. Do not apply a memorized interaction equation from another edition without checking the table definitions and factors.

Connected-Material Bearing and Tear-Out

Clear Distance (LcL_c)

The clear distance, in the direction of force, from the edge of a hole to the adjacent hole or material edge as defined by the governing connection provision.

Common standard-hole bearing/tear-out form

Representative nominal expression where hole deformation is a design consideration; exact scope/factors follow the adopted edition.

Rn=min⁡(1.2LctFu,  2.4dtFu)R_n=\min(1.2L_ctF_u,\;2.4dtF_u)

End bolts and interior bolts do not share the same Lc

For a straight bolt line, the end bolt uses the clear edge distance; interior bolts use clear spacing between adjacent holes. A calculator that applies one generic edge distance to every bolt can overstate or understate connected-plate strength.

Interactive straight bolt-line model

The following calculator uses explicit end edge distance, bolt spacing, hole diameter, shear planes, and connected-plate properties. Its adequacy badge applies only to the modeled bolt-shear and plate-bearing/tear-out limit states.

Bolted Connection — One Loaded Bolt Line

Concept and model scope

Bolt shear and plate bearing/tear-out use explicit hole, edge-distance and spacing geometry

End clear distance Lc1.094 in
Interior clear distance Lc2.188 in
Bolt shear design strength71.6 kips
Plate bearing/tear-out design strength146.0 kips
Governing modeled strength71.6 kips
Within modeled limit
This is a straight bolt line under concentric shear. It does not claim whole-connection adequacy: block shear, net-section rupture, combined bolt tension/shear, slip resistance, eccentric bolt-group action, prying, minimum spacing/edge detailing and connected-member limit states remain separate checks.

Net Section and Block Shear

The connection can fail outside the bolt

Even if every bolt passes shear/tension, the connected plate, gusset, angle, or beam web/flange can fail by gross yielding, net-section rupture, block shear, local bearing/tear-out, or other member-specific limit states. Calculate actual hole deductions and fracture paths rather than multiplying a per-bolt strength by the bolt count and stopping.

Slip-Critical Behavior

Slip resistance depends on the actual joint

Slip resistance depends on required bolt pretension, faying-surface class/slip coefficient, number of slip planes, hole type/direction, fillers, tension effects, and other RCSC factors. Those quantities and resistance/safety factors are edition-specific. A slip-critical design also requires the additional strength checks prescribed for the joint after/independent of slip.

Do not use one remembered slip formula as a universal design rule

A compact classroom equation is useful for understanding clamping/friction mechanics, but project design must use the exact RCSC/AISC slip-resistance equation, definitions, surfaces, hole factors, load level, and pretension table adopted by the project.

Spacing, Edge Distance, and Holes

Geometry is a strength and fabrication input

Minimum spacing and edge distance depend on bolt diameter, hole type, edge condition, load direction, fabrication method, and the adopted standard. Preferred detailing spacing can be larger than the code minimum. Oversized and slotted holes can change both bearing/slip behavior and detailing requirements.

Eccentric Bolt Groups

Direct shear plus moment

If the applied load does not pass through the bolt-group centroid, the group carries direct shear and an eccentric moment. An elastic vector method can provide a conservative/educational solution for suitable cases, while ultimate/instantaneous-center procedures account for nonlinear bolt-group load deformation where permitted. Do not divide the load equally among bolts when eccentricity is significant.

Prying and Flexible Connection Elements

Prying Action

Additional bolt tension caused by flexure/contact of a connection element such as a tee flange or end plate. Bolt demand depends on connection-element stiffness and geometry rather than only the externally applied tensile force divided by the bolt count.

Connection Design Workflow

Bolted-connection workflow

Key Takeaways
  • Bolt grade, joint category, and hole type are separate design inputs.
  • Exact bolt stresses, pretension, and slip factors are edition-specific table values.
  • End and interior bolts have different clear-distance geometry.
  • Whole-connection adequacy requires connected-part limit states in addition to bolt strength.
  • Slip-critical, eccentric, prying, fatigue, and seismic behavior cannot be reduced to a simple equal-load-per-bolt model.