In case of staggered or zigzag riveting in the design of tension members of steel structures, the net cross-sectional area along the chains of rivets is:

Explanation:

In double cover butt joint, the bolts or rivets can be placed in the following manner:

1. In multiple rows having zero staggered pitch called chain riveting as shown in the following figure:

2. In multiple rows having either non-uniform pitch or having staggered pitch i.e. bolts are placed in a zig-zag pattern as shown below:

As per IS 800:2007, the tensile strength based on the rupture of the net section of the plate is given
as:

\(\begin{aligned} & T_n=\frac{0.9 f_u A_{\text {net }}}{\gamma} \\ & A_{\text {net }}=\left\{B-n d_n+\sum\left(\frac{s^2}{4g}\right)\right\} \times t \end{aligned}\)
Where,
f_u  = Ultimate strength of the plate
A  = Minimum net area of the section
γ = Factor of safety for plate

t is the thickness of the plate joined.
d  is the diameter of the hole
s is staggered pitch, g is gauge distance, b is the width of the plate
To calculate A_net, all possible failure paths (straight as well as zig-zag) are required to be considered and the corresponding net area is to be computed for each possible failure path and the minimum net area is to be selected.

It is clear from the above formula that A_net in staggered riveting is increased by an amount equal to \(\rm \frac{s^2t}{4g}\)