The effect of geometric stiffness on vibration frequencies of bulkhead frames of pressurized aircraft fuselages

Resumo

Aircraft fuselages are structured by, among other elements, planar portal frames called bulkheads.
Among their main loads, is the internal pressurization, which causes considerable traction efforts. It is well known,
from the Matrix Structural Analysis theory, that the stiffness of frame elements is composed of two parts, the
elastic stiffness and the geometric stiffness. The latter depends on the level of axial forces acting on the members.
If in traction, it increases stiffness, and, consequently, raises the frequencies of free vibration of the structure
related to it. This effect is widely explored in so-called tensile structures, such as inflated blimps. If of compression,
it decreases the total stiffness and lowers the frequencies, leading, in the limit, to the buckling of the element. In
this work, a bulkhead portal frame of a fictitious aircraft is numerically analyzed, demonstrating that the presence
of high levels of traction that such structures support, due to internal pressurization, considerably increase their
frequencies.