Maximum spacing of lacing bars is an important consideration in structural design to ensure the stability and strength of the main member between consecutive lacing connections. The maximum slenderness of the main member is determined by the maximum spacing of lacing bars.

The slenderness of a member is defined as the ratio of its effective length to its radius of gyration. The effective length is the length at which the member is effectively restrained against lateral buckling. The radius of gyration is a measure of the member's resistance to bending.

To prevent excessive buckling of the main member between lacing connections, it is important to limit its slenderness. This is because a slender member is more prone to buckling under compressive loads.

The maximum slenderness of the main member between consecutive lacing connections depends on the maximum spacing of lacing bars. The lacing bars provide additional stiffness and restraint to the main member against lateral buckling.

To determine the maximum slenderness, we can use the Euler's buckling formula:

Pcr = (π^2 * E * I) / (K * L^2)

Where:
Pcr = critical buckling load
E = modulus of elasticity
I = moment of inertia of the member
K = effective length factor
L = effective length of the member

The effective length factor depends on the end conditions of the member. For a member with both ends pinned, the effective length factor is 1. For a member with one end fixed and one end pinned, the effective length factor is 0.7. The effective length can be determined based on the support conditions and the member's geometry.

By rearranging the Euler's buckling formula, we can determine the maximum slenderness:

L / r ≤ √(π^2 * E) / (Pcr * K)

Where:
L = effective length of the member between consecutive lacing connections
r = radius of gyration of the member

The maximum spacing of lacing bars should be such that the maximum slenderness of the main member between consecutive lacing connections is not more than a certain value. In this case, the correct answer is option 'C' which states that the maximum slenderness should not exceed 50. This means that the maximum spacing of lacing bars should be determined such that the effective length of the member does not exceed 50 times its radius of gyration.

By limiting the slenderness of the main member, we can ensure its stability and prevent buckling under compressive loads. This is important in structural design to ensure the safety and integrity of the structure.