Engineering Guide to Through Bolting: Anchor Bolts vs Through Bolts

The Crucial Role of Preload (Axial Tension)

The effectiveness of Through Bolting hinges on the controlled application of torque to achieve a specific bolt Preload. This preload tension keeps the joint members compressed together, ensuring that any external shear force applied to the connection is resisted primarily by the **static friction** between the clamped surfaces before the bolt itself is stressed in shear.

Grip Length and Joint Relaxation

The Grip Length—the total thickness of the material being clamped—is an essential factor. A longer grip length generally results in a more elastic joint, which is better at sustaining the preload over time without significant relaxation. This is vital in timber structures or joints with shims, where the material itself may settle or compress after the initial tightening. Engineers use the Torque-Tension relationship to calculate the precise torque needed for the specified bolt material (e.g., ASTM A325).

Load Transfer: Shear vs Tensile Loading

In a well-designed Through Bolting application, the bolt's primary job is to maintain the preload, making the joint's load-carrying capacity a function of friction. However, if the shear force exceeds the friction limit, the joint slips, and the bolt bears the load directly in shear. In applications involving direct tension (pull-apart forces), the bolt shank and its threads carry the full load, making proper material grade selection mandatory.

Heavy Construction: Timber Framing and Steel Connections

• Steel-to-Steel Connections: Used for primary moment and shear connections in steel-frame buildings, often utilizing **high-strength structural fastener assemblies** (bolts, nuts, and washers).
• Mass Timber Framing: Essential for joining glulam beams and heavy timber posts, where the bolt must distribute forces across the entire wooden member. Galvanized bolts are common here for corrosion resistance.
• Base Plate Fixing: Although anchor bolts are common, Through Bolting is sometimes used when fixing to existing structures where full access to the underside (e.g., through a hollow section) is available, offering unparalleled pull-out resistance.

Industrial Machinery: Vibration-Resistant Through Bolting

Machinery and equipment subject to high dynamic loads—such as large motors, pumps, crushers, and conveyors—rely on Through Bolting to maintain alignment. The high preload achieved prevents relative motion, which is the primary cause of fatigue failure and bolt loosening. Special attention is paid to using **Lock Nuts** or **Chemical Thread Lockers** to ensure the nut does not back off under constant vibration.

Automotive Engineering: Chassis and Suspension Integrity

In automotive design, Through Bolting secures safety-critical components like engine mounts, suspension control arms, and major chassis components. These joints are subjected to complex, multi-axial stresses (tension, shear, and bending), making the reliability of a fully clamped connection paramount. These often require precise torque-angle control during installation to guarantee the exact preload specified by the manufacturer.