Bolted Connections
Analysis and design of bolted connections, including shear, bearing, slip-critical resistance, prying action, and block shear.
Connections are critical for transferring loads between structural members. Bolted connections are widely used due to their ease of installation, quality control, and relative independence from weather conditions (unlike welding). While they reduce the net area of tension members, they are indispensable for field erection.
Bolt Types
Materials and grades of structural fasteners.
Common structural bolts are classified by ASTM standards:
- A307: Low-carbon steel bolts (similar to regular machine bolts). Used for light structures, secondary members (purlins, girts), and temporary connections. ksi.
- A325 (Group A / F3125 Grade A325): High-strength carbon-manganese steel bolts. The standard for most structural work. ksi (for ) or ksi (for ).
- A490 (Group B / F3125 Grade A490): Higher-strength alloy steel bolts. Used when A325 capacity is insufficient. ksi. Because they are harder and less ductile, they must not be galvanized.
Connection Types
The fundamental mechanisms of load transfer in bolted joints.
Bearing-Type (Snug-Tight) Connections
- Load transfer relies on the bolt shank bearing physically against the connected material (the plate hole).
- Some minor slip (about 1/16") is expected and acceptable as the bolts come into bearing under load.
- Bolts only need to be tightened to the "snug-tight" condition (the tightness attained by a few impacts of an impact wrench or the full effort of an ironworker with an ordinary spud wrench).
- Design checks: Bolt Shear, Bolt Bearing (on plate), Bolt Tension.
Slip-Critical (Pretensioned) Connections
- Load transfer relies entirely on friction between the faying surfaces (the contacting plates) clamped together by massive bolt tension.
- Used where any slip is detrimental to the structure (e.g., fatigue loading, vibration, oversized holes, or structures where large deformations would cause problems).
- Bolts MUST be fully pretensioned to at least 70% of their minimum tensile strength.
- Design checks: Slip Resistance + all Bearing-Type checks (as a backup in case of catastrophic overload that overcomes friction).
Spacing and Edge Distance
Geometric requirements to ensure proper installation and prevent material tearing.
To allow physical space for tightening tools (impact wrenches) and to prevent the bolts from tearing out through the base material, structural codes mandate minimum spacing between bolts and minimum distances from bolts to the edge of the plate.
- Minimum Spacing (): The distance between centers of standard, oversized, or slotted holes must not be less than , but is preferred ( = nominal bolt diameter).
- Minimum Edge Distance (): The distance from the center of a standard hole to any edge of a connected part depends on the bolt diameter and the edge manufacturing process (sheared vs. rolled/flame-cut). Common minimums range from to times the bolt diameter. AISC Table J3.4 provides the exact values.
Prying Action in Tension Connections
Additional tensile forces induced in bolts due to the deformation of connection fittings.
When bolts are used to transfer tension forces through flexible connection elements (such as the flanges of a WT hanger or the outstanding legs of an angle), the flexibility of the fitting fundamentally alters the force distribution.
As the applied tension load () pulls on the fitting, the fitting bends. The tips of the fitting's flanges bear against the rigid support, acting as a fulcrum. This bending lever action induces an additional tensile force in the bolts, known as prying force ().
Effects of Prying Action
The total tension force required to be resisted by the bolt () is the sum of the direct applied tension per bolt () plus the prying force ().
Prying action can significantly increase the required bolt size. To minimize prying forces, engineers must either:
- Thicken the fitting: A thicker, stiffer flange (e.g., a heavier WT shape) will not bend as much, reducing the lever action and the resulting prying force.
- Decrease the gage: Moving the bolts closer to the web of the WT reduces the moment arm causing the bending.
AISC Part 9 provides complex, iterative procedures to calculate the required thickness of a fitting to either eliminate prying action entirely or safely account for the amplified bolt forces.
Design Strength
The limit states that govern connection capacity.
- Bolt Shear Strength
The shear strength of a single bolt depends on its grade and whether the unthreaded shank or the threaded portion intersects the shear plane (the interface between the plates sliding past each other).
Bolt Shear Strength
Calculates the nominal capacity of a bolt in single shear.
$$
R_n = F_{nv} A_b
$$- N (Threads Included): Threads are included in the shear plane. The actual cross-sectional area resisting shear is reduced by the thread roots, so the assumed capacity is lower.
- X (Threads Excluded): Threads are deliberately excluded from the shear plane. The full, smooth shank resists the shear, providing a higher capacity.
- (LRFD).
Resistance Factor (): 0.75
- Bolt Tensile Strength
When bolts are subjected to direct tension (e.g., hanger connections, end-plate moment connections):
Bolt Tensile Strength
Calculates the nominal capacity of a bolt in pure tension.
$$
R_n = F_{nt} A_b
$$Resistance Factor (): 0.75
Prying Action in Tension Connections
When a flexible connecting element (like a tee flange or angle leg) is subjected to tension, it deforms outward and bears against its rigid support at the toes. This creates a lever-like "prying" effect that introduces an additional, unaccounted tensile force () into the bolts.
The total tension force on the bolt () is significantly greater than the applied external load alone.
Solution:
To safely resist the combined forces, the designer must either use thicker connection plates (to increase stiffness and eliminate the prying lever action) or use stronger/more bolts to carry the extra prying load. AISC Manual Part 9 provides complex formulas to calculate the required plate thickness () to prevent prying action entirely.
- Bearing Strength at Bolt Holes
Bearing failure occurs in the connected steel material (the plate or web), not the bolt itself. It can manifest as tear-out (the bolt rips through the edge of the plate) or excessive hole deformation (the hole elongates into an oval).
Bolt Bearing Capacity (Standard Holes)
Calculates the nominal bearing capacity considering both tear-out and hole deformation.
$$
R_n = 1.2 L_c t F_u \\le 2.4 d t F_u
$$Resistance Factor (): 0.75
Why 2.4 d t Fu?
The upper limit equation () controls when the bolts are spaced far apart or far from the edge. In this case, tear-out will not occur, but the material behind the bolt will crush and deform excessively. The 2.4 factor limits this deformation to acceptable levels. If the clear distance () is small, the tear-out equation () governs.
- Slip Resistance (Slip-Critical Connections)
For connections that must not slip, the friction force must exceed the applied shear force.
Slip-Critical Connection Resistance
Calculates the frictional resistance preventing plates from slipping.
$$
R_n = \\mu D_u h_f T_b n_s
$$Resistance Factor ():
- 1.00 for standard holes and short-slotted holes loaded perpendicular to the slot.
- 0.85 for oversized holes and short-slotted holes loaded parallel to the slot.
- 0.70 for long-slotted holes.
Block Shear Strength in Connections
A critical tearing failure mode at the ends of coped beams or tension members.
Block shear often governs the capacity of bolted connections where a block of material can tear out. It involves shear yielding/rupture along the line of bolts parallel to the force, combined with tension rupture along the line of bolts perpendicular to the force. See the Tension Members module for the block shear equation ().
Bolted Connection Limit States
Shear Capacity0.0 kips
Bearing Capacity0.0 kips
Governing Strength (φR_n)
0.0 kips
FAIL (DCR = Infinity)
* Note: Bearing capacity assumes standard edge distance and hole spacing. Block shear and tensile rupture of the member must be checked separately.
Key Takeaways
- Connections must be designed for both the bolt's shear capacity AND the connected material's bearing capacity.
- The thread condition (N vs. X) significantly impacts bolt shear capacity, as threads reduce the actual steel area available to resist shear forces.
- Bearing-type connections allow minor slip into bearing under load (snug-tight). Slip-critical connections rely entirely on clamping friction and require full bolt pretension.
- To ensure constructability and prevent material tearing (tear-out), bolts must satisfy minimum spacing (typically ) and edge distance requirements.
- Prying action dramatically increases the tensile force on bolts connecting flexible elements, requiring thicker plates or stronger bolts to safely resist the added load.
- A325 and A490 are the primary high-strength bolts used in modern structural steel design, replacing older A307 and rivets.