The step is a rectangular prism: $3$ ft $\times$ $1.5$ ft $\times$ $1$ ft.

$\text{Volume} = 3 \times 1.5 \times 1 = 4.5 \text{ cubic feet}$

$\text{Bags} = \frac{4.5}{0.4} = 11.25 \approx 11.3$