Riveted Joint Efficiency at Minimum Pitch according to IS 800

A riveted joint is a connection between two or more structural members (like plates or sections) using rivets. The efficiency of a riveted joint is a measure of its strength compared to the strength of the original, uncut member. It is defined as:

\(\text{Efficiency} = \frac{\text{Strength of the riveted joint}}{\text{Strength of the solid plate}} \times 100\%\)

The strength of the joint is determined by the minimum of the following:

• Strength of the joint in shear failure of rivets.
• Strength of the joint in bearing failure of rivets or plate.
• Strength of the plate in tension failure between rivet holes.

The strength of the solid plate is its tensile strength calculated based on its gross area.

Understanding Minimum Pitch in Riveted Joints

According to IS 800, the minimum pitch (distance between centers of adjacent rivets in a row) should generally not be less than \(2.5\) times the nominal diameter of the rivet (\(2.5d\)). This minimum pitch requirement is primarily to ensure there is enough material between rivet holes to prevent premature failure due to bearing or shear and to facilitate proper riveting.

Efficiency at Minimum Pitch

When the minimum pitch of \(2.5d\) is used, the critical mode of failure often becomes the tensile failure of the plate between the rivet holes. The strength of the plate in tension between holes is reduced because of the presence of the holes.

Let:

• \(p\) be the pitch of the rivets.
• \(d_h\) be the diameter of the rivet hole (usually \(d + 1.5\) mm for rivet diameter \(d \le 25\) mm, and \(d + 2\) mm for \(d > 25\) mm).
• \(t\) be the thickness of the plate.
• \(\sigma_{at}\) be the allowable tensile stress in the plate.

The tensile strength of the plate in one pitch length between rivet holes is given by:

\(P_t = (p - d_h) \times t \times \sigma_{at}\)

The tensile strength of the solid plate in one pitch length is:

\(P_{\text{solid}} = p \times t \times \sigma_{at}\)

The efficiency based on tensile strength between holes is:

\(\eta_t = \frac{(p - d_h) \times t \times \sigma_{at}}{p \times t \times \sigma_{at}} = \frac{p - d_h}{p} = 1 - \frac{d_h}{p}\)

Substituting the minimum pitch \(p = 2.5d\) and considering \(d_h \approx d + 1.5\) mm, the efficiency calculation gives a value around 60% or slightly more, depending on the rivet diameter and hole tolerance used for calculation. IS 800 effectively limits the efficiency of a lap joint or a single-riveted butt joint with minimum pitch to approximately 60% due to this tensile capacity limitation between holes.

While other failure modes (shear and bearing) also contribute to the overall joint strength, the minimum pitch specifically impacts the net area available for tension, often making tensile failure between holes the governing factor for efficiency at this spacing.

Therefore, as per IS 800 guidelines and standard practice for a riveted joint designed with the minimum pitch, the efficiency is typically around 60%.

Let's look at the options provided:

• 60%
• 70%
• 40%
• 50%

Based on the discussion regarding the minimum pitch and its effect on the tensile strength between holes as per IS 800, the efficiency is approximately 60%.