  The connected part of a metric space $E$ is defined to be the union of
non-trivial connected components of $E$. We proved that for a class of
self-affine sets called slicing self-affine sponges, the connected part of $E$
either coincides with $E$, or is essentially contained in the attractor of a
proper sub-IFS of an iteration of the original IFS.This generalize an early
result of Huang and Rao [L. Y. Huang, H. Rao. \emph{A dimension drop phenomenon
of fractal cubes}, J. Math. Anal. Appl. \textbf{497} (2021), no. 2] on a class
of self-similar sets called fractal cubes. Moreover, we show that the result is
no longer valid if the slicing property is removed. Consequently, for a
Bara\'nski carpet $E$, the Hausdorff dimension and the box dimension of the
connected part of $E$ are strictly less than the Hausdorff dimension and the
box dimension of $E$, respectively. For slicing self-affine sponges in $\mathbb
R^d$ with $d\geq 3$, whether the attractor of a sub-IFS has strictly smaller
dimensions is an open problem.
