Steps for Design:

The subsequent steps are to be taken while designing compound compression members with lacing:

(a) Select a suitable arrangement of structural members (e.g. one as shown in Figure 10). The size of the open columns usually lies between 1/9 to 1/14 of the column length.

(b) Supposing a reasonable value of λ, find the corresponding σ_ac from Table 2 and find the total area of the column A=   P/ σ_ac.

From your assumed value of λ, also find r_min = l_eff / λ.

(c) From the total area A, find the individual area of each element and select suitable structural sections. From the tabulated values of r_x′x and r_y′y and A′, of each component find r_xx and r_yy of the compound column, so that both are approximately equal. In this, help of Table 3 may be taken.

(d) Finally, for the selected structural sections, calculate the exact A, r_min, l_eff, λ_max etc. From the last data find the corresponding σ_ac from Table 2 and check up with your assumptions in step (b). If there is a discrepancy repeat the process.

(e) Select the angle of inclination of the lacing bars (40^o < θ < 70o; generally 45^o), their maximum spacing, width, thickness, etc. (If the single-lacing system gives a large value of l′ such that θ > 70^o, then adopt double-lacing system.)

(f) Design the lacing for a transverse load of V = P/40. Divide total V between the numbers (N) of lacing systems provided. The forces (compressive or tensile) in the lacing bars will be V/ N cosec θ as shown in Figure.

Figure: Forces Acting on Lacings and Tie Plate

(g) End tie plates are given at end of the column and are designed as battens

Bending moment in tie plate = VC/2 N, and

Shear force = VC/aN

where, C = Spacing of lacings, and

a = Centre to centre length of tie plate.