Understanding Rivet Pitch in Structural Joints

In structural engineering, particularly in riveted connections, the 'pitch' refers to the distance between the centers of two adjacent rivets along the direction of the applied force (either tension or compression). Proper pitch is crucial for distributing the load effectively among the rivets and preventing excessive stress concentration in the connected plates. Codes like IS 800 specify minimum and maximum pitch requirements based on rivet diameter and load conditions to ensure the joint's integrity and prevent premature failure, such as shear of rivets or tearing of the plate.

The Scenario of Exceeding Maximum Pitch

The question describes a specific situation where the distance between the centers of adjacent rivets exceeds the maximum allowable pitch. This situation might arise due to various practical reasons, such as simplifying the arrangement of fasteners, accommodating specific structural configurations, or managing long grip lengths where plates are thick.

Identifying Tacking Rivets

When the maximum pitch limit is surpassed, engineers often introduce additional rivets. These extra rivets are specifically placed to hold the connected members together securely, especially where the standard spacing is exceeded. The key characteristic highlighted in the question is that these additional rivets are *not subjected to the calculated stress*. Their primary role is not load carrying but rather to maintain the proximity of the connected parts, prevent buckling of thinner plates under compression, or ensure the joint remains stable during assembly or under service loads where primary stress calculation doesn't account for them.

These types of rivets, used under the condition of exceeding the maximum pitch and not carrying calculated stress, are known as tacking rivets.

Distinguishing Tacking Rivets

Let's look at why the other options are not the correct fit for this specific definition:

• Auxiliary rivets: While the term 'auxiliary' suggests a supplementary role, 'tacking rivets' is the more precise term for rivets used specifically when the maximum pitch is exceeded and they don't carry calculated stress. Tacking rivets are a specific type of auxiliary fastener.
• Long grip rivets: These rivets are used when joining members that have a significant combined thickness (a 'long grip'). They are designed to transfer calculated stress and are chosen based on their length, not primarily on pitch issues or lack of stress carrying capacity.
• Packing rivets: Packing rivets are used in built-up sections, such as joining flange plates to webs or cover plates to main plates, where a packing piece (spacer) is present. These rivets are intended to carry calculated stress and are essential for the structural performance of the built-up member.

Therefore, based on the definition provided – additional rivets used when maximum pitch is exceeded and not subjected to calculated stress – the correct term is tacking rivets.