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). Bearing-type connections are more economical and suit most static applications. Slip-critical connections are required when slip would cause serviceability problems, when bolts share load with welds, or when the joint is subject to fatigue or load reversal.

Bolt shear and bearing capacity

The nominal shear strength of a single bolt per AISC 360-22 Section J3.6 is:

R_n = F_nv x A_b

where F_nv is the nominal shear stress from AISC Table J3.2 (e.g., 54 ksi for A325-N, 68 ksi for A490-N) and A_b is the nominal bolt area. The design strength is phi x R_n with phi = 0.75.

For bearing on the connected material (AISC J3.10):

R_n = 1.2 x L_c x t x F_u (deformation at service load considered) R_n = 1.5 x d x t x F_u (upper limit per bolt)

where L_c is the clear distance between holes or to the edge, t is the plate thickness, d is the bolt diameter, and F_u is the plate ultimate tensile strength.

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 (F_u = 58 ksi), plate thickness t = 3/8 in., edge distance = 1.25 in.

Bolt shear: A_b = pi/4 x (0.75)^2 = 0.4418 in^2. F_nv = 54 ksi. R_n = 54 x 0.4418 = 23.9 kip. phi x R_n = 0.75 x 23.9 = 17.9 kip.
Bearing: Standard hole = 13/16 in. L_c = 1.25 - 13/32 = 0.844 in. R_n = 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. phi x R_n = 0.75 x 22.0 = 16.5 kip.
Controlling capacity = min(17.9, 16.5) = 16.5 kip per bolt (bearing controls).

Code comparison — bolt shear capacity

Parameter	AISC 360-22	AS 4100:2020	EN 1993-1-8	CSA S16:19
Resistance factor	phi = 0.75	phi = 0.80	gamma_M2 = 1.25	phi = 0.80
A325/8.8 shear stress	54 ksi (threads included)	0.62 x f_uf (Category 8.8)	0.6 x f_ub / gamma_M2	0.60 x F_u
Slip-critical factor	mu = 0.30 Class A	mu = 0.35 (bare steel)	mu = 0.50 Class A	k_s = 0.33 Class A
Hole deduction	Threads in/out shear plane	Threads in/out (A_c or A_o)	Tensile stress area A_s	Core area or gross area
Bolt pretension (3/4 in.)	28 kip (A325)	~95 kN (M20 8.8)	0.7 x f_ub x A_s	28 kip (A325M)

Key design considerations

When designing bolted connections, verify the following:

Bolt grade and installation — A325 (F_ub = 120 ksi) and A490 (F_ub = 150 ksi) are the standard ASTM high-strength bolt grades. Snug-tight installation is acceptable for bearing-type connections in static applications. Turn-of-nut, calibrated wrench, tension-control (TC), or direct-tension-indicator (DTI) methods are required for pretensioned and slip-critical installations.
Thread condition — specify whether threads are included (N) or excluded (X) from the shear plane. Thread exclusion increases shear capacity by roughly 25% but requires careful detailing and inspection to confirm thread position during erection.
Hole types — standard, oversized, short-slotted, and long-slotted holes each have different bearing and slip-critical provisions. Oversized and slotted holes reduce bearing capacity and require slip-critical design in many cases.
Combined shear and tension — bolts loaded in combined shear and tension must satisfy the elliptical interaction equation per AISC J3.7. The available tensile stress is reduced based on the required shear stress.
Block shear — check the block shear rupture limit state (AISC J4.3) on the connected elements. This often governs in coped beams and gusset plates with small edge distances.
Bolt group eccentricity — when the line of force does not pass through the centroid of the bolt group, use the instantaneous center of rotation method (AISC Table 7-7 through 7-14) or the elastic method for the resulting bolt forces.

Slip-critical vs. bearing-type design

Slip-critical connections (AISC J3.8) prevent faying-surface slip at service loads. They are mandatory when:

Oversized or slotted holes are used (except short slots perpendicular to load)
The connection is subject to fatigue loading
Bolts share load with welds in a common faying surface
Slip would compromise serviceability (e.g., connections in bracing with oversized holes)

The slip resistance per bolt is: R_n = mu x D_u x h_f x T_b x n_s, where mu is the mean slip coefficient (0.30 for Class A surfaces), D_u = 1.13, h_f is the filler factor, T_b is the minimum bolt pretension, and n_s is the number of slip planes.

Common mistakes to avoid

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 equivalent T-stub model in EN 1993-1-8 to quantify prying.
Using snug-tight for slip-critical joints — slip-critical design requires pretensioned bolts. Specifying snug-tight installation invalidates the slip resistance calculation entirely.
Neglecting bearing at short edge distances — when L_c is small (e.g., at beam copes or gusset plate edges), bearing/tearout often governs over bolt shear. Always check both.
Thread exclusion assumed but not verified — if the design relies on threads being excluded from the shear plane, the bolt length must be specified precisely and verified during erection. A one-washer difference can shift threads into the shear plane.
Mixing A325 and A490 in the same connection — while not explicitly prohibited, mixed bolt grades create inspection confusion and should be avoided. If different bolt grades are on the same project, use different diameters to make them visually distinguishable.

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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.