  Due to their characteristic temperature and density, loop structures in
active regions (ARs) can be seen bright in extreme ultraviolet (EUV) and soft
X-ray images. The semiempirical determination of the three-dimensional (3D)
distribution of basic physical parameters (electronic density and temperature,
and magnetic field) is a key constraint for coronal heating models. In this
work we develop a technique for the study of EUV bright loops based on
differential emission measure (DEM) analysis and we first apply it to AR
structures observed by the {Atmospheric Imaging Assembly} (AIA) on board the
{Solar Dynamics Observatory} (SDO). The 3D structure and intensity of the
magnetic field of the observed EUV loops are modelled using force-free field
extrapolations based on magnetograms taken by the {Helioseismic and Magnetic
Imager} (HMI) on board SDO. In this work we report the results obtained for
several bright loops identified in different ARs. Our analysis indicates that
the mean and width of the temperature distributions are nearly invariant along
the loop lengths. For a particular loop we study its temporal evolution and
find that these characteristics remain approximately constant for most of its
life time. The appearance and disappearance of this loop occurs at time-scales
much shorter than its life time of $\approx 2.5$ hours. The results of this
analysis are compared with numerical simulations using the zero-dimensional
(0D) hydrodynamic model, Enthalpy-Based Thermal Evolution of Loops (EBTEL). We
study two alternative heating scenarios: first, we apply a constant heating
rate assuming loops in quasi-static equilibrium, and second, we heat the loops
using impulsive events or nanoflares. We find that all the observed loops are
overdense with respect to a quasi-static equilibrium solution and that the
nanoflare heating better reproduces the observed densities and temperatures.
