J03_2

8/13/2019 J03_2

1/7

Variants of Analysis of the Load Case Airplane Crash

Fritz-Otto Henkel1)

, Dietrich Klein2)

1) Managing Director, Woelfel Beratende Ingenieure, Hoechberg, Germany

2) Senior Engineer, Woelfel Beratende Ingenieure, Hoechberg, Germany

ABSTRACT

In Germany, the load case airplane crash was introduced into the design criteria of new nuclear power plants inthe early 1970s since in those days a huge number of military jets was positioned on both sides of the iron curtain. In

these years intensive research and testing activities were started to develop load criteria and analysis methods for

this new load case. This phase ended with the issue of the KTA 2203 draft and the Sandia experiments in the late1980s. The September 11, 2001 attacks, however, prompted new activities and investigations of airplanes impacting

nuclear power plants. The activities are concentrated on three tasks:

Derivation of load time functions for airplane crash Analysis and design of the impacted structures Development of building concepts against airplane crash

This paper outlines the actual state of art of the methods for the analysis of the load case airplane crash. The

existing building concepts against airplane crash are described and an outlook on the recent and future activities

which were launched after the terrorist attacks of September 11th

2001 is given.

LOAD TIME FUNCTIONS OF AIRPLANE CRASH

The first milestone in the derivation of airplane loads is the work of Prof. Riera [1] in 1968 who developed a

method for deriving load time functions which is still used today. He started from the mass distribution of the air

plane and developed a spring-mass system which could be solved analytically, Fig. 1. Drittler/Gruner [2] extend thismethod for the derivation of the load time function for the crash of a fast flying military aircraft of the type Phantom

on a rigid structure, Fig. 2. The smoothed load time function is used for the design of nuclear plants in Germany.

The validity of this function was verified by large scale tests in Sandia [3].

Fig. 1: Spring-mass system of air craft Fig. 2: Load time function of Phantom RF-4E

Fig. 3 shows some selected load time functions for different military jets, private business aircrafts and commer-

cial passenger air planes which were in use at the time.

1

SMiRT 19, Toronto, August 2007Transactions, Paper #J03/2

8/13/2019 J03_2

2/7

Fig. 3 Selected load time functions of airplane crashes

STRUCTURAL ANALYSIS METHODS; STATE OF THE ART

A first article about the structural point was given by a paper of Schalk and Woelfel at the 3rd

SMiRT in London

[4]. Their paper deals with the airplane induced vibrations of the building which cause loadings for the secondary

systems (equipment) in the structure. In the years 1974 to 1985 large scale slab tests in Meppen [5] and impact tests

during the HDR safety program [6] were performed. In the Meppen tests the damage behavior of reinforced concrete

slabs under impact of hard missiles was investigated. In the HDR safety program impact tests at the high pressure

reactor in Kahl, Germany, were performed. These tests allowed insight in the non-linear dynamic behavior of rein-

forced concrete structures under short impact loads. The results were used to develop simplified but nevertheless

realistic analysis methods for the design of reinforced concrete structures. They finally led to the draft of the KTA

2203 standard [7] for the design of nuclear power plants against airplane crash.

For design of a nuclear power plant against airplane crash we have to differentiate between three main tasks for

the analysis:

The local analysisof the directly hidden outer walls of the structure. The global analysisof the stability of the structure and the strength of the stiffening elements. The vibration analysisof the secondary structural parts and equipment in the structure.(Fire scenarios and the effect of the burning fuel after an airplane crash, which also are of major interest, are no part

of this paper.)

Local AnalysisIn the local analysis of an impact we have to distinguish between two different physical impacts:

The hard impact of a quasi rigid, concentrated missile, for instance the turbine with the turbine shaft The soft impact of a deformable missile, for instance the tubular body of the plane

In the hard impact we have at first spallation of concrete at the impact side, followed by scabbing at the opposite

side after progress of the impact wave through the wall thickness. The missile will penetrate the concrete and finally

will perforate the wall if the thickness is not sufficient. A lot of semi-empirical formulas are available for calculationof the minimum thickness required to prevent perforation. The codes normally define the required thickness for

inherent protection, that is perforation and spallation of concrete fragments at the backside are prevented.

In the case of a soft impact a large part of the impact energy is dissipated by plastic deformation of the missile

itself. The missile does not penetrate into the structure. The impacted concrete wall, however, may fail by combinedbending and shear. This requires a design with regard to the non-linear dynamic behavior of the wall. Excessive

cracking of the concrete with yielding of the reinforcement is allowed. The failure is defined by limits of the strains

of reinforcement and concrete. For the analysis a classical, simplified 2-mass-model is used, as shown in Fig. 4 for

the roof of a cubicle reinforced concrete structure. The two masses represent the mass of the punching cone M Sand

the active bending mass MB. The masses are supported by non-linear spring-damper-systems. The non-linear charac-

teristics of the springs for bending as well as for shear are modeled by tri-linear load deformation curves. The first

part represents the initial linear behavior until cracking of concrete. The second part accounts for the reduced stiff-

2


8/13/2019 J03_2

3/7

ness of the cracked concrete and the last part with ideal plastic behavior stands for yielding of the reinforcement.

Failure is defined by limit deformations.

Fig. 4 Simplified mass-spring-damper model

The 2-mass-model is generally sufficient for the basic design of plane concrete structures with simple geometry

and boundary conditions. For structures with more complicate geometry and curved shells, high sophisticated finite

element programs for non-linear dynamic analysis of the composite material concrete and reinforcement are avail-

able.

Global AnalysisIn the global analysis the stability of the structure, that is the prevention of sliding on the ground or overturning,

is proven, the stiffening elements of the structure including base plate are designed and the maximum displacements

to prevent contact with neighboring structures are calculated. Linear dynamic analysis with 3D finite element mod-els is state of the art. Fig. 5 shows the model of the cubicle reinforced concrete structure.

Fig. 5 Global finite element model of the structure

3


8/13/2019 J03_2

4/7

The response of the structure is calculated by use of the time history modal analysis (THMA) for all relevant

Fig. 6 Investigated load ases for airplane crash

The results are the time histories of the displacements, accelerations and section forces in the structural parts (for

Fig. 7: R equired rei ent in cm2/m

ibration analysisalysis includes the calculation of acceleration response spectra of the structure which are again

impacts. In this example about 30 different impacts were investigated, see Fig. 6. For hard impacts, that is for

impacts on the corners and edges which are stiffened by the adjoining walls, the common load time function for the

impact of the Phantom on a rigid structure is used. For soft impacts, that is for impacts on the roof and the walls,

the maximum impact force is reduced to account for the local non-linear behavior of the structure and the impacttime is extended so that the impetus of the impact is unchanged.

c

plate). For each component of the section forces the maximum and minimum values in the time histories are stored

together with the belonging force components at the same time point. This results in 2x8 sets of data (three mem-

brane force components, three bending moment components and two shear force components) for each element and

impact case. These forces are superposed to the static results of the operation loads (dead weight, life loads, tem-

perature etc.). For each resulting set of force combinations the reinforcement is designed and enveloped over all

impact cases. The result is the required reinforcement in each finite element, see Fig. 7.

nforcem

V The vibration an

the input for the vibration analysis of the secondary structural parts and the safety relevant equipment in the struc-ture. The same model as for the design of the structure in the global analysis is used. The time histories of the accel-

erations at each relevant point in the structure are calculated for all impact cases and the belonging response spectra

are derived. The design spectra then are the appropriate envelope of the spectra for all impact cases. Fig. 8 showsexemplarily the acceleration time histories and spectra for two selected impact cases.

4


8/13/2019 J03_2

5/7

Fig. 8 Floor response spectra for airplane crash

UILDING CONCEPTS AGAINST AIRPLANE CRASH

ready summarized by Henkel [8] see Fig. 9. The firstne shows different variations of single shell structures. Concept no. 1 shows a common building with usual thick-

ssical double shell concept.

ion behav-

B

The existing building concepts against airplane crash are alli

ness of the walls. It is evident that such a structure is not safe against an impact of a plane. The simplest way tomake the building safe against airplane crash is to strengthen the outer wall, concept no. 2. The German code re-quires a wall thickness of 1.25 m and more to prevent perforation and major scabbing at the rear side. Concept no. 3

utilizes the plastification potential of the structure arranged in front. It provides protection only in the so-called hard

core. Components and inventory with high activity must be placed inside this hard core.

The single shell concepts may prevent against penetration. No precautions, however, are taken against induced

vibrations. This is possible by the double shell concepts in the next lines. No. 5 is the cla

The functional inner part of the building is separated from the protecting outer shell. This kind of structure is the

state of the art for all aircraft protected buildings at the time. The problem of the induced vibrations, however, is not

solved satisfactorily by this concept. The vibrations are transmitted to the inner part of the building by the common

foundation. The vibrations can be reduced by separation of the outer shell and internal building also in the founda-

tion slab, concept no. 6. This, however, reduces the mass that counterbalance the attacking airplane load.

The spring concepts in the third line may be taken as consequences of high response spectra. These concepts

reflect a higher technology because special spring-damper elements were introduced to modify the vibrat

ior of the structure in a desired way. Only special rooms with very sensitive equipment, concept no. 7, or the com-

plete inner part of the building, concept no. 9, may be mounted on the spring-damper elements. Concept no. 8 is thewell known a-seismic spring concept, which, however, has no positive effect on the load case airplane crash. Only

the inertia forces counterbalance the attacking airplane load, which may result in even higher accelerations of the

internal building than by direct foundation on the soil.

Fig. 9: Building concepts against airplane crash

5


8/13/2019 J03_2

6/7

NEW ACTIVITIES AND INVESTIGATIONS

rompted new activities and investigations of airplanes impacting

uclear plants. New ideas beside the existing building concepts against airplane crash were born to prevent the

ners which

elled by shell elements with equivalent

The terrorist attacks of September 11th2001 p

n

impact at the plant, for example to obscure the plant by fog or to erect additional structures in front of the plant, see

for example Eibl [9]. Because all these investigations are still under work, no final results can be shown.New technologies originating from automotive crash analysis have been developed and are used by few re-

searchers to investigate the crash of a plane into a building. The Airbus 320 represents the type of city li

are most frequently flown. This airplane was chosen for a crash simulationFig. 10 shows the finite element model of the plane. The outer surface corresponds to the real structure. The

stiffness of the different surface components as fuselage or wing is mod

thickness. Because no detailed information about the structural parts is available from the airplane producers, most

of the stiffening components are represented in a simplified manner, whereas important stiffening parts, e.g. wing

box, are modelled realistically. The turbines are defined as rigid body with point mass.

FFig. 10: Finite element model of Airbus 320

In the first step of the computation aluminium was defined as material for all parts. To fulfill the requirements

f a crash analysis, the plasticity and the damage behavior of aluminium is considered. Before the crash simulation

reflects

second one displays

o

an eigen-frequency analysis was carried out to validate the setup of the model. The results of this analysis reflect

with eigen-values of 2.36 Hz (wings in phase) and 3.07 Hz (wings out of phase) the values out of literature.

The so far executed computations simulated a crash between the airplane structure and a wall modeled as rigid

body. As initial boundary condition all nodes of the airplane were charged with a velocity of 125 m/s which

an early state of the landing approach. The wall was fixed in all directions. Non-linear effects out of the material

definition and out of finite displacements are considered within the simulation. To keep the computation stable a

constant time increment of 8.1e-4 ms was used. Altogether a crash time of 0.2 s was simulated.

Fig. 11 shows three different states of the simulation. In the first plot the effects just after the first contact (t =

0.036s) with a collapse of the cockpit and the buckling of the fuselage and wing are shown. The

the situation after the first impact of the turbine (t = 0.072 s), destroying the wing due to the backlash. The last plotdemonstrates the end of simulation, where almost two-thirds of the fuselage is collapsed. The right wing is broken

into two halves. While one part is still connected to the fuselage, the other shows a tailspin behavior. This asymmet-

ric behavior for a symmetric model and load is caused by small numerical effects, which have a great influence onto

the buckling behavior. This leads finally to the effect that the right turbine has contact before the left one what

causes the cut of the right wing.

6


8/13/2019 J03_2

7/7

Fig. 7 Different states of the crash simulation

This example shows that the advanced analysis tools for crash analysis as they have been used for many years in

the automotive industry can be applied successfully and in feasible time for the case of an airplane crash. Further

investigations require a better data base of the structure of the airplane. The investigations then can be extended forcoupled analysis of airplane and structure.

REFERENCES

1. Riera, J.D., On the stress analysis of structures subjected to aircraft impact forces, Nuclear engineering and

design,Vol 8, 1968, pp 415-426

2. Drittler, K., Gruner, P.,Zur Auslegung kerntechnischer Anlagen gegen Einwirkungen von aussen; Teilaspekt

Berechnung von Kraft-Zeit-Verlufen beim Aufprall deformierbarer Flugkrper auf eine starre Wand, Institut

fr Reaktorsicherheit, Wissenschaftliche Berichte, IRS-W-14/, April 1975

3. Riesemann, W.A. et al, Full-scale aircraft impact test for evaluation of impact forces; Part 1: Test plan, test

method and test results, Transactions of the 10thSMiRT, Vol. J, Anaheim 1989

4. Schalk, M., Woelfel, H., Response of equipment in nuclear power plants to airplane crash, Transactions of the

3rdSMiRT, paper K5/7, London, 1975; andNuclear Engineering and Design, Vol 38, No. 3, pp 567-582, Sept.

1976

5. Nachtsheim, W., Stangenberg, F., Selected results of Meppen slab tests State of interpretation, comparison

with computational investigations, Transactions of the 7th SMiRT, paper J8/1, Chicago Aug. 1983

6. HDR Sicherheitsprogramm, Erdbebenuntersuchungen am Heissdampfraktor in Kahl, Ergnzende Sprengver-

suche und Stoversuche auf niederer Anregungsstufe, Technischer Fachbericht PHDR 15-18, Kernfor-

schungszentrum Karlsruhem, April 19807. Brandes, K., Steuer, J. Auslegung kerntechnischer Anlagen gegen Flugzeugabsturz, Stand der Auslegung bau-

licher Anlagen bei vorgegebenen Lastannahmen, DIN-Fachbericht NKe FB 3 Nr. 1-93, DIN Deutsches Institut

fr Normung e. V., Dec. 19928. Henkel, F.-O., Woelfel, H., Building concepts against airplane crash, Transactions of the 7th SMiRT, paper

J9/1, Chicago Aug. 1983, andNuclear Engineering and Design, Vol 79, No. 3, pp 397-409, June 1984

9. Eibl, J., Airplane impact on nuclear power plants, Transactions of the 17th SMiRT, paper J03-6, Prag Aug.

2003

7


J03_2

Documents

Transcript of J03_2