Civil Engineering Reference
In-Depth Information
The structural behaviour and the resistance of safety enclosures of a nuclear
power plant in the case of an aircraft impact shall be examined here using the
example of the containment of a pressurized water reactor of the convoy type. The
outer form of the containment consists of a cylindrical shell with a superimposed
dome with an outer diameter of 66.8 m. This rotationally symmetric form is only
disrupted by the materials lock, its front building and the fresh steam and feed water
armature chambers. The wall thickness of the reinforced concrete construction is
1.80 m. The internal structures such as the inner steel containment etc. are only
connected to the outer structure via the foundation.
Figure
15.1
schematically shows the design of a reactor building of the convoy
type. In the sectional cut the 1.80 m thick reinforced concrete shell is clearly visible.
The structural design of the outer shell is not uniform for all locations. The
spectrum of the flexural reinforcement of the high-strength reinforcing steel BSt
1100 ranges from approx. 26 cm
2
/m to 60 cm
2
/m per side and flexural direction,
depending on the impact location (cylinder or sphere) and the location of the
nuclear power plant. The shear reinforcement of the ductile stell BSt 420/500 varies
between 44 and 66 cm
2
/m
2
. For illustration of the geometric dimensions of an
aircraft impact see Fig.
14.2
.
15.2 Local Structural Behaviour: Resistance to Penetration
The impact load of an aircraft is a dynamic effect with a very short duration. For
military aircraft this effect lasts approx. 70 ms, for commercial aircraft up to
approx. 400 ms. Due to inertia effects only the directly impacted areas are initially
affected, the rest of a large building does not “experience” anything as a result of
the impact in this phase. The entire building only responds after a certain time lag.
This applies especially to the internal structures, which are separated from the
impacted outer shell and only coupled to it by the foundation. It is therefore possible
to analyze the local behaviour independent of the overall structure.
Figure
15.2
shows how a steel reinforced concrete structure behaves in principle
in the case of an impact. In the case of a soft impact, which can be assumed for an
aircraft impact, local shear failure (punching) or flexural failure is possible. In the
case of a hard impact on the other hand, such as in the case of a massive engine
component, penetration or perforation can occur depending on the ratio of the plate
thickness to the diameter of the impacting body in addition to spalling on the front
and rear side of the target. The term “inherent protection” is used when no spalling
occurs on the reverse side of the impact area.
To determine the resistance of the safety enclosures against the penetration of an
aircraft different models are available ranging from simple equivalent static
approaches against punching to complex non-linear finite element analyses.
In the design praxis for the impact of a military aircraft a simplified static method
was predominately used to examine the local resistance. Figure
15.3
shows the
assumptions of the model used to examine punching according to DIN 25449 [
4
].
