At high temperatures the spatial correlation functions in QCD can be
described in terms of dimensionally reduced effective theory. In
dimensional reduced effective theory we have $SU(3)$ gauge fields
$A_i$ coupled to scalar $A_0$ field in adjoint representation and the
massive fermions are already integrated out. Observables which are
dependent on magnetic($g^2T$) scale remains non-perturbative even at
very high temperatures due to matsubara zero modes in the gauge
sector. Spatial Wilson loop (or spatial Wilson line correlator) which
is sensitive to magnetic d.o.f can be calculated in lattice
non-perturbatively. The spatial Wilson loops obey area law at all
temperatures, and this is characterized by the spatial string tension.
We extract the spatial string tension for $2+1$ flavour QCD with HISQ
action at temperatures $T\equiv[T_c:4T_c]$ for $N_t=8,10$ lattices.
The calculations shows the dimensional reduction picture becomes valid
at $T\geq1.5~T_c$ ($T > 240$ MeV), while for
$T<240$ MeV the spatial string tension is equal to the zero
temperature (vacuum) string tension.