The filamentation of femtosecond pulses in air is investigated experimentally and numerically for beam powers accessing several hundreds of critical powers for self-focusing. First, evolution of the filament patterns is approached by an averaged-in-time (2D+1)-dimensional model derived from the standard propagation equations for ultrashort pulses. Elementary processes such as soliton generation, dissipation by multiphoton absorption and coalescence of filamentary cells are discussed. Second, the 2D model is employed for reproducing filament patterns of femtosecond pulses delivered by a mobile TW laser source (TERAMOBILE). Beam propagation is shown to be driven by the interplay between intense, robust spikes aggregated around the defects of the beam and random nucleation of light cells.