  We present structural parameters and morphological properties of faint 450-um
selected submillimeter galaxies (SMGs) from the JCMT Large Program, STUDIES, in
the COSMOS-CANDELS region. Their properties are compared to an 850um selected
and a matched star-forming samples. We investigate stellar structures of 169
faint 450-um sources (S450=2.8-29.6mJy; S/N>4) at z<3 using HST near-infrared
observations. Based on our spectral energy distribution fitting, half of such
faint SMGs (LIR=10^11.65+-0.98Lsun) lie above the star-formation rate
(SFR)/stellar mass plane. The size-mass relation shows that these SMGs are
generally similar to less-luminous star-forming galaxies selected by NUV-r vs.
r-J colors. Because of the intrinsic luminosity of the sample, their rest-frame
optical emission is less extended than the 850um sources (S850>2mJy), and more
extended than the star-forming galaxies in the same redshift range. For the
stellar mass and SFR matched sample at z~=1 and z~=2, the size differences are
marginal between faint SMGs and the matched galaxies. Moreover, faint SMGs have
similar Sersic indices and projected axis ratios as star-forming galaxies with
the same stellar mass and SFR. Both SMGs and the matched galaxies show high
fractions (~70%) of disturbed features at z~=2, and the fractions depend on the
SFRs. These suggest that their star formation activity is related to galaxy
merging, and the stellar structures of SMGs are similar to those of
star-forming galaxies. We show that the depths of submillimeter surveys are
approaching the lower luminosity end of star-forming galaxies, allowing us to
detect galaxies on the main sequence.
