Strength of silica fibers heat treated in humid air is affected by swelling strains. In the absence of externally applied stresses swelling is isotropic. Under uniaxial stress an anisotropy of swelling strains has to be expected with maximum expansion in stress direction. In order to compute the maximum possible anisotropy, a Finite Element analysis is carried out. Under the assumption of a spherical nano-pore generated by a single bond breaking event, an averaged axial swelling strain was computed that is by a factor of >2 larger than the volume swelling strain.