Bolted Connections — Engineering Reference

Bolted connection design per AISC 360: bolt grades, shear and bearing capacity, slip-critical design, edge distance rules, and bolt shear/bearing calculator.

Overview

Bolted connections transfer forces between structural steel members through mechanical fasteners loaded in shear, tension, or combined shear-and-tension. They are the most common connection type in steel construction because they can be installed quickly in the field, inspected visually, and designed for a wide range of load magnitudes and directions.

The two primary categories are bearing-type connections (where bolt shanks bear against hole surfaces) and slip-critical connections (where clamping force from pretensioned bolts prevents slip at the faying surfaces).

Bolt grades and mechanical properties

Bolt shear capacity per AISC Table J3.2

phi = 0.75 for all values. Ab = pi/4 x d^2.

Bearing and tearout capacity

Bearing capacity per AISC J3.10

For standard holes with deformation at service load considered:

Rn = 1.2 x Lc x t x Fu (per bolt, limited to 1.5 x d x t x Fu)

Edge distance requirements per AISC Table J3.4

Bearing capacity by bolt size and plate thickness (Fu = 58 ksi, A36)

Bearing values assume Lc >= 2d. Shorter edge distances reduce capacity linearly.

Slip-critical design (AISC J3.8)

Slip resistance per bolt: Rn = mu x Du x hf x Tb x ns

Slip coefficient by surface class

Minimum bolt pretension (AISC Table J3.1)

Slip-critical capacity per bolt (Class A, Du=1.13, single slip plane)

phi = 1.0 for slip-critical at service level. Slip-critical capacity is much lower than bearing-type capacity.

Combined shear and tension (AISC J3.7)

Bolts loaded in combined shear and tension must satisfy:

F'nt = 1.3 Fnt - (Fnt / phi x Fnv) x Fnv,req <= Fnt

Interaction table for 3/4 in. A325-N

When shear reaches 100% of capacity, no tension can be resisted. The interaction is approximately linear.

Bolt spacing requirements

Minimum spacing by bolt diameter

Block shear rupture (AISC J4.3)

Block shear checks a tear-out failure path through the bolt group:

Rn = 0.6 x Fu x Anv + Ubs x Fu x Ant <= 0.6 x Fy x Agv + Ubs x Fu x Ant

where Anv = net shear area, Ant = net tension area, Agv = gross shear area, Ubs = 1.0 for uniform tension stress.

Block shear capacity for common patterns (3/4 in. bolts, A36, t = 3/8 in.)

Gross paths use Fy, net paths use Fu. Block shear often governs for coped beams and gusset plates.

Worked example — 3/4 in. A325-N bolt in single shear

Given: 3/4 in. A325-N bolt, single shear plane through the threads, connected plates are A36 (Fu = 58 ksi), plate thickness t = 3/8 in., edge distance = 1.25 in.

1. Bolt shear: Ab = pi/4 x (0.75)^2 = 0.4418 in^2. Fnv = 54 ksi. Rn = 54 x 0.4418 = 23.9 kip. phiRn = 0.75 x 23.9 = 17.9 kip.
2. Bearing: Standard hole = 13/16 in. Lc = 1.25 - 13/32 = 0.844 in. Rn = 1.2 x 0.844 x 0.375 x 58 = 22.0 kip. Upper limit = 1.5 x 0.75 x 0.375 x 58 = 24.5 kip. Bearing does not govern. phiRn = 0.75 x 22.0 = 16.5 kip.
3. Controlling capacity = min(17.9, 16.5) = 16.5 kip per bolt (bearing controls).

Code comparison — bolt shear capacity

Common mistakes to avoid

1. Ignoring prying action — when bolts are loaded in tension through a flexible fitting (angles, tee-stubs), prying forces can increase bolt tension by 20-40%. Use AISC Design Guide 16 or the T-stub model in EN 1993-1-8.
2. Using snug-tight for slip-critical joints — slip-critical design requires pretensioned bolts. Specifying snug-tight invalidates the slip resistance calculation entirely.
3. Neglecting bearing at short edge distances — when Lc is small (e.g., at beam copes or gusset plate edges), bearing/tearout often governs over bolt shear. Always check both.
4. Thread exclusion assumed but not verified — if the design relies on threads excluded from the shear plane, the bolt length must be specified precisely and verified during erection.
5. Mixing A325 and A490 in the same connection — while not explicitly prohibited, mixed bolt grades create inspection confusion. If different grades are on the same project, use different diameters to make them visually distinguishable.
6. Not checking block shear — block shear rupture often governs for coped beams and gusset plates. The failure tears out a block of material through the bolt group.

Frequently asked questions

What is the difference between bearing-type and slip-critical? Bearing-type connections allow the bolt to bear against the hole edge under shear. Slip-critical connections use pretensioned bolts to clamp the faying surfaces together, preventing slip at service loads. Slip-critical is required for oversized holes, fatigue loading, and load reversal.

When do I need slip-critical bolts? Per AISC J3.8: when oversized or slotted holes are used (except short slots perpendicular to load), when the connection is subject to fatigue, when bolts share load with welds, or when slip would compromise serviceability.

What is prying action? When a T-stub or angle leg bends under tension load, the flexibility of the connected part creates additional prying forces in the bolts. The total bolt tension = direct tension + prying force. This can increase bolt demand by 20-40%.

How many bolts do I need for a simple shear connection? It depends on the reaction. For a W16x26 beam with a 40 kip reaction and 3/4 in. A325-N bolts: 40 / 16.5 = 2.4 -> use 3 bolts minimum (bearing controls per bolt at 16.5 kip).

What is block shear? A failure mode where a block of material tears out through the bolt group along a path combining shear and tension planes. Per AISC J4.3, it combines shear yielding/fracture and tension fracture. Common in coped beams and gusset plates.

Should I specify threads included or excluded? Threads excluded (X) gives 25% more shear capacity but requires precise bolt length control. For most field connections, specify threads included (N) for simplicity. Use X only when the bolt length can be precisely controlled.

AISC Chapter J bolt design procedure

AISC 360-22 Chapter J governs the design of bolted connections. The following procedure ensures all applicable limit states are checked.

Design procedure for bolted shear connections

1. Determine factored loads (shear, tension, or combined) from LRFD combinations.
2. Select bolt grade and diameter (A325 or A490 for structural; specify N or X condition).
3. Determine bolt shear capacity per Table J3.2 (Fnv x Ab, with phi = 0.75).
4. Determine number of bolts required based on shear demand per bolt.
5. Lay out bolt pattern satisfying minimum spacing (2.67d, prefer 3d) and minimum edge distance (Table J3.4).
6. Check bearing and tearout at each bolt (J3.10): Rn = min(1.2LctFu, 1.5dtFu).
7. Check block shear rupture (J4.3) for the bolt group.
8. Check slip-critical capacity (J3.8) if applicable (oversized holes, fatigue, load reversal).
9. Check combined shear and tension (J3.7) if bolts see both.

Bearing-type vs slip-critical — comparison

Limit states checklist for bolted connections

Every bolted connection must be checked against all applicable limit states. The governing limit state is the one with the lowest capacity.

Worked example — simple shear connection design

Given: W16x36 beam (A992) framing into W14x82 column web. Beam reaction Ru = 42 kips. Design a single-plate shear connection using 3/4 in. A325-N bolts.

Step 1 -- Select plate: Single plate, 3/8 in. thick, A36 (Fy = 36 ksi, Fu = 58 ksi). Plate length to accommodate 4 bolts at 3 in. spacing = 12 in. Plate depth at cope = beam web depth minus top flange cope.

Step 2 -- Bolt shear capacity: 4 bolts, single shear (single plate), threads included:

phiRn = 4 x phi x Fnv x Ab = 4 x 0.75 x 54 x 0.4418 = 71.4 kips > 42 kips  (OK)

Step 3 -- Bearing on plate (Lc = 1.5 in. edge distance, 3 in. spacing):

Lc_edge = 1.5 - (13/16)/2 = 1.094 in.  (at end bolt)
Lc_inner = 3 - 13/16 = 2.188 in.  (at interior bolts)

Edge bolt: phiRn = 0.75 x 1.2 x 1.094 x 0.375 x 58 = 21.4 kip
Inner bolt: phiRn = 0.75 x 1.2 x 2.188 x 0.375 x 58 = 42.7 kip
Upper limit: 0.75 x 1.5 x 0.75 x 0.375 x 58 = 18.3 kip (governs for all)

Total phiRn = 4 x 18.3 = 73.3 kips > 42 kips  (OK)

Step 4 -- Block shear (plate, 4-bolt line):

Agv = (3+3+3+1.5) x 0.375 = 4.125 in^2
Anv = Agv - 3.5 x (13/16) x 0.375 = 4.125 - 1.069 = 3.056 in^2
Ant = (2 - 13/32) x 0.375 = 0.597 in^2 (assuming 2 in. width to plate edge)

phiRn = 0.75 x [0.6 x 58 x 3.056 + 1.0 x 58 x 0.597]
      = 0.75 x [106.3 + 34.6] = 0.75 x 140.9 = 105.7 kips > 42 kips  (OK)

Step 5 -- Plate shear yielding:

phiRn = 0.90 x Fy x Ag = 0.90 x 36 x (12 x 0.375) = 145.8 kips > 42 kips  (OK)

Summary:

Bolt shear governs at 59% demand-to-capacity. The connection has adequate margin.

Run this calculation

• Bolted Connections Calculator
• Section Properties Database

Related references

• Bolt Spacing and Edge Distance
• Bolt Hole Sizes
• Bolt Capacity Table
• Bolt Grades
• Steel Connection Design
• Column Base Plate
• Eccentric Connections
• Steel Fasteners
• How to Verify Calculations

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable standard and project specification before use. The site operator disclaims liability for any loss arising from the use of this information.

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