Civil Engineering Reference
In-Depth Information
Fig. 14.20
Example of a load function for a Boeing 747, mass 380 t, v
¼
175 m/s, comparison
between calculation results and idealized function
the target. After t
400 ms all masses of the fuselage are compressed except those
in the rear section. The aircraft has come to rest. An examination of the contact
forces to the target over time shows that initially only the spring extending along the
fuselage axis transmits a force. Only after approx. 130 ms the wings hit the obstacle,
at which point the springs located there take part of the load. The sum of all contact
spring forces is the integral impact force that acts upon the target or structure.
For the Boeing 747 with an initial speed of 175 m/s the impact duration is
approx. 400 ms. Within the framework of the investigation the mass and bursting
load distributions were varied with respect to magnitude and distribution in differ-
ent computational runs. A variation of the bursting load showed only insignificant
differences in the end result: the impact load-time function. Within the scope of the
overall uncertainties of the model and the assumptions these can be ignored. The
mass distribution therefore plays the deciding role at speeds of about 175 m/s.
Figure
14.20
presents the impact load-time function as the result of a calculation
for a given set of conditions (mass and bursting load distributions). It shall be noted
here that the function shown is only to be considered as an example. With different
boundary conditions and other mathematical models different load function curves
can be determined. An impact load-time function that is calculated in this way is
badly suited for use in further calculations of the structural behaviour. It would have
to be specified as a polygon with many nodes. It also includes many high frequency
oscillation which are completely irrelevant for the loading of a massive structure.
Therefore the loading function can be well described by an idealized function with
only a few nodes. The peak value of the load, the duration and the momentum as an
integral over the time of the impact load-time function should largely agree. With
these considerations in mind, Fig.
14.20
also shows an idealized function. The
momentum of the idealized function shown is equal to I
¼
66.75 MNs.
Within the scope of parameter studies taking into account the site-specific
factors different impact situations can be simulated. For example, the straight
target can be replaced by one with a cylindrical geometry to correspond to the
¼
