A review of procedures for the analysis and design of concrete structures to resist missile impact effects

Concrete containment walls and internal concrete barrier walls are often required to withstand the effects of missile impact. Potential missiles include external tornado generated missiles (steel rods, steel pipes, wooden poles, and automobiles), aircraft crash, and internal accident generated missiles (turbine blade, and steel pipe missiles resulting from pipe break). Impacting missiles can be classified as either ‘hard’ or ‘soft’ depending upon whether the missile deformability is small or large relative to the target deformability. This paper only deals with the effects of ‘hard’ missile impact. Missile velocities between 100 and 1500 ft/sec are emphasized. ‘Hard’ missile impact results in both local wall damage and in overall dynamic response of the target wall. Local damage consists of spalling of concrete from the front (impacted) face and scabbing of concrete from the rear face of the target together with missile penetration into the target. If damage is sufficient the missile may perforate or pass through the target. This paper reviews the various empirical procedures commonly used for determining penetration depth, perforation thickness, and scabbing thickness for concrete targets subjected to ‘hard’ missile impact. Results obtained from these procedures are compared with test data results for low velocity impacts (200–1500 ft/sec). Design recommendations to prevent detrimental local wall damage are presented. Overall dynamic response of the target wall consists of flexural deformations and a potential flexural or shear failure if the strain energy capacity of the wall does not exceed the kinetic energy input to the wall by the striking ‘hard’ missile. Simplified procedures are defined for determining the dynamic response of the target wall and for preventing overall failure of the wall. Included are procedures for defining the effective target mass to be used in determining the fraction of the total missile kinetic energy which is transferred or ‘input’ into the target wall. Also included are procedures for defining the total strain energy capacity of the target wall as determined from the moment and rotational capacities of flexural yield hinges and the yield line deformation pattern of the wall. Lastly, criteria for preventing a premature shear failure are presented.     

Local damage to reinforced concrete structures caused by impact of aircraft engine missiles Part 1. Test program, method and results

Structural damage induced by an aircraft crashing into a reinforced concrete structure includes local damage caused by the deformable engines, and global damage caused by the entire aircraft. Local damage to the target may consist of spalling of concrete from its front face together with missile penetration into it, scabbing of concrete from its rear face, and perforation of missile through it. Until now, local damage to concrete structures has been mainly evaluated by rigid missile impact tests. Past research work regarding local damage caused by impact of deformable missiles has been limited. This paper presents the results of a series of impact tests of small-, intermediate-, and full-scale engine models into reinforced concrete panels. The purpose of the tests was to determine the local damage to a reinforced concrete structure caused by the impact of a deformable aircraft engine.     

Model tests of turbine missile impact on reinforced concrete

The results of 25 impact tests on 1/11-scale models of reinforced concrete nuclear plant walls are presented. These tests determined experimentally the maximum velocity at which postulated turbine missiles are contained by typical reinforced concrete walls. The parameters varied were missile weight, velocity, orientation, and impact angle, as well as target design and thickness. The results showed that the NDRC perforation formula used extensively in current practice is overly conservative, whereas a newer empirical formula (CEA-EDF) gave reasonably conservative predictions of the test results. All but the most energetic postulated missiles are stopped by containment wall models, and steel liners on these walls are effective in suppressing backface concrete scabbing.     

Local effects of impactors on concrete structures

Available formulae for predicting the penetration depth, scabbing thickness, and perforation thickness of concrete structures impacted by solid missiles are summarized, reviewed, and compared. Based on quadratic and cubic regression analysis of existing data, two new formulae have been proposed for predicting the penetration depth of concrete due to the impact by solid missiles. The new penetration equations are compared statistically with NDRC penetration formula and two other recent penetration formulae. Also, new simple formulae have been proposed for predicting the scabbing thickness and perforation thickness of concrete walls.     

Local response of reinforced concrete to missile impacts

An experimental and computational study was undertaken to determine the response of reinforced concrete walls to impacts from postulated tornado and other missiles. The study included laboratory-scale missile impacts, experiments to characterize concrete, computational model development, and two-dimensional simulations of missile impacts. Impact experiments with rods and pipes on small reinforced concrete walls showed crushing, cratering, spalling, radial cracking, and plug formation. The mechanisms governing this material response appear to be crushing, shearing, and tensile fracture. Static triaxial and dynamic plate impact experiments were used to determine the material properties. Dynamic strengths were higher than static; tensile strengths were ten times as high. A CAP constitutive model developed for concrete described compaction, Mohr-Coulomb yielding, and tensile separation following tensile strain accumulation. Model parameters were derived separately from the dynamic and the static data. Two-dimensional computational simulations were made of a rod impact experiment with threshold cracking using both static and dynamic parameters. The correct locations of fractures were predicted with the static parameters, but penetration and severity of failure were overpredicted. Penetration distance was correctly given with the dynamic parameters, but fracture was underpredicted. A model combining dynamic shaer and compaction properties with intermediate-rate tensile properties may be appropriate.     

Response of buried pipes to missile impact

This paper presents the methodology and results of the analyses carried out to determine an effective layout and the dynamic response of safety related cooling water pipes, buried in backfill, for the Alto Lazio Nuclear Power Plant in Italy, subjected to missile impact loading at the backfill surface. The pipes are composed of a steel plate encased in two layers of high-quality reinforced concrete.The methodology comprises three steps. The first step is the definition of the ‘free-field’ dynamic response of the backfill soil, not considering the presence of the pipes, through a dynamic finite element direct integration analysis utilizing an axisymmetric model.The second step is the pipe—soil interaction analysis, which is conducted by utilizing the soil displacement and stress time-histories obtained in the previous steps. Soil stress time-histories, combined with the geostatic and other operational stresses (such as those due to temperature and pressure), are used to obtain the actions in the pipe walls due to ring type deformation.For the third step, the analysis of the beam type response, a lumped parameter model is developed which accounts for the soil stiffness, the pipe characteristics and the position of the pipe with respect to the impact area.In addition, the effect of the presence of large concrete structures, such as tunnels, between the ground surface and the pipe is evaluated.The results of the structural analyses lead to defining the required steel thickness and also allow the choice of appropriate embedment depth and layout of redundant lines. The final results of the analysis is not only the strength verification of the pipe section, but also the definition of an effective layout of the lines in terms of position, depth, steel thickness and joint design.     

Tests of spinning turbine fragment impact on casing models

Ten 1/11-scale model turbine missile impact tests were conducted at a Naval spin chamber test facility to assess turbine missile effects in nuclear plant design. The objective of the tests was to determine the effects of missile spin, blade crush, and target edge conditions on the impact of turbine disk fragments on the steel casing. The results were intended for use in making realistic estimates for the initial conditions of fragments that might escape the casing in the event of a disk burst in a nuclear plant. The burst of a modified gas turbine rotor in a high-speed spin chamber provided three missiles with the proper rotational and translational velocities of actual steam turbine fragments. Tests of bladed, spinning missiles were compared with previous tests of unbladed, nonspinning missiles. The total residual energy of the spinning missiles, as observed from high-speed photographs of disk burst, was the same as that of the nonspinning missiles launched in a piercing orientation. Tests with bladed missiles showed that for equal burst speeds, the residual energy of bladed missiles is less than that of unbladed missiles. Impacts of missiles near the edge of targets resulted in residual missile velocities greater than for central impact.     

Loading time history for tornado-generated missiles

Nuclear power plant structures in the USA are designed for impact by tornado-generated missiles. The design load for flexure and shear can be obtained from the deceleration of the missile on impact. The paper gives a simple method to determine the deceleration of the most critical pipe missile. Results, obtained by the simple method, are compared with full-scale test results. The comparisons between the predicted and actual deceleration time histories show excellent correlation.     

Hard missile impact on reinforced concrete

New penetration, scabbing and perforation formulae are derived for use in the design of reinforced concrete barriers to withstand impact by hard missiles. This is done by using dimensional analysis together with physical theories for the various impact processes. This leads to impact formulae with unknown coefficients which are then determined by an analysis of all available test data. The new formulae so derived are simple and, because of their parametric formulation, have a range of applicability easily definable in terms of impact parameters. The analysis indicates that some recently proposed impact formulae are not safe from the point of view of barrier design because the test data used for their derivation was affected by global movement of the barriers which reduced the measured local damage.     

Piping response testing associated with pipe rupture

EPRI has sponsored an experimental program in the pipe whip impact and pipe rupture and depressurization areas. Sixteen pipe whip tests were performed with 3 in Schedule 80 (or 10) carbon steel pipes impacting on rigid target or concrete slab. The major testing parameters include distance, impact location, pipe rupture location, and concrete slab thickness and strength. The piping crushing at impact correlates with impact force and target response behavior. Conservatism was established by comparing measured and calculated impact forces. The pipe rupture and depressurization tests were carried out using 6 in stainless steel and carbon steel pipes under either PWR or BWR fluid conditions. These tests are of axial crack with initial machined-in surface flaw. It was found that pipe rupture would occur only if a long unstable through-wall crack was embedded in a sufficiently long unstable part-through crack (in the pipe wall). All other flaw configuration tested led to pipe leakage only. Reaction forces were measured which show conservatism of simplified method for fully ruptured condition. No good crack propagation information was obtained.     

Nuclear shielding of openings in ITER Tokamak building

The external walls of the Tokamak building, made of thick concrete, provide the nuclear shielding for operators working in adjacent buildings and for the environment. There are a series of openings to these external walls, devoted to ducts or pipes for ventilation, waveguides and transmission lines for heating systems and diagnostics, cooling pipes, cable trays or busbars. The shielding properties of the wall shall be preserved by adequate design of the openings in order not to affect the radiological zoning in adjacent areas. For some of them, shielding properties of the wall are not affected because the size of the network is quite small or the source is far from the opening. But for most of the openings, specific features shall be considered. Even if the approach is the same and the ways to shield can be standardized, specific analysis is requested in any case because the constraints are different.     

Analytical studies on local damage to reinforced concrete structures under impact loading by discrete element method

This paper proposes a new analytical approach for assessing local damage to reinforced concrete structures subjected to impact load, by applying the discrete element method (DEM). It first outlines the basis concept and analytical formulation of the DEM. Next, it discusses the results of simulation analyses of concrete material tests, uni-axial compression tests and tensile splitting tests conducted to determine appropriate analytical parameters such as material constants, failure criteria and strength increase factors depending on strain rate. Finally, the adaptability of the DEM to local damage to reinforced concrete structures impacted by rigid and deformable missiles is verified through simulation analyses of various types of impact tests. Furthermore, the various impact response characteristics and failure mechanisms, such as impact forces, penetration behavior, reduction in missile velocity and energy transfer process, which are difficult to obtain experimentally, are analytically evaluated by the DEM.     

Overview of EPRI research in structural integrity

This paper is an overview of the structural integrity research within the Nuclear Safety and Analysis Department of the Electric Power Research Institute. This research addresses structurally related safety issues in light water reactors. Five major technical areas are covered: Analysis/Design Methods, Seismic/Vibratory Response, Fluid/Structure Response, Impact/Impulse Response, and Structure/Component Performance. Each technical area is briefly described and research results are highlighted. This paper puts in perspective the research and development work described in this special issue of the journal in addressing such safety and licensing issues as soil-structure interaction, seismic response of piping systems, hydrodynamic loads in pipes and vessels, pipe rupture and whip, jet impingement, missile impact, and concrete containment integrity.     

Impact load for tornado-generated missiles

Nuclear power plant structures are designed to resist impact loads from tornado-generated missiles. The paper gives a method to determine the contact pressure at the interface of the missile and the target, and the velocity and deceleration time-histories of the missile. It also gives a method to obtain the design load due to impact by tornado-generated missiles. The calculated design load compares very well with the total support reaction from a full-scale dynamic test. The comparison between the predicted deceleration and the deceleration recorded in a test is excellent. The method of determining the deceleration has also been used with very good results for earth-penetrating missiles.     

Local damage to Ultra High Performance Concrete structures caused by an impact of aircraft engine missiles

The impact of an aircraft engine missile causes high stresses, deformations and a severe local damage to conventional reinforced concrete. As a consequence the design of R/C protective structural elements results in components with rather large dimensions.Fiber reinforced Ultra High Performance Concrete (UHPC) is a concrete based material which combines ultra high strength, high packing density and an improved ductility with a significantly increased energy dissipation capacity due to the addition of fiber reinforcement. With those attributes the material is potentially suitable for improved protective structural elements with a reduced need for material resources.The presented paper reports on an experimental series of scaled aircraft engine impact tests with reinforced UHPC panels. The investigations are focused on the material behavior and the damage intensity in comparison to conventional concrete. The fundamental work of [Sugano et al., 1993a] and [Sugano et al., 1993b] is taken as reference for the evaluation of the results. The impactor model of a Phantom F4 GE-J79 engine developed and validated by Sugano et al. is used as defined in the original work. In order to achieve best comparability, the experimental configuration and method are adapted for the UHPC experiments. With ‘penetration’, ‘scabbing’ and ‘perforation’ all relevant damage modes defined in [Sugano et al., 1993a] and [Sugano et al., 1993b] are investigated so that a full set of results are provided for a representative UHPC structural configuration.     

Comparative assessment of impact resistance of SC and RC panels using finite element analysis

In this paper, the impact resistance of steel-plate concrete (SC) and reinforced concrete (RC) panels is evaluated using the commercial software LS-DYNA. The structural components and their contacts are fully modeled in the analysis, and the material nonlinearity and strain rate effect for concrete and steel are considered. The analysis results of SC and half steel-plate concrete (HSC) panels under impact loading are compared with the test results conducted in previous research in order to determine the main factors influencing the analysis. The impact analyses as per four different concrete thicknesses with five different steel ratios are performed in order to compare the impact resistance of the SC and RC panels. Failure mode, damage size, and displacement of the SC and RC panels are investigated. The results show that the SC panel has better impact resistance than the RC panel. Finally, the impact analyses for optimal panel design are performed, and the optimal concrete thickness and steel ratio are recommended.     

Research needs and improvement of standards for nuclear power plant design

The need for research and improvement of code requirements, for both economy and safety reasons, is discussed for the following topics relevant to nuclear power plant structural analysis: Earthquake definition; dynamic behavior of reinforced concrete structures under impact loads; design for postulated pipe rupture; code requirements for loading combinations for concrete structures, reinforcing steel splicing, reinforced concrete structural design for thermal effects.     

Numerical studies of impact on reinforced concrete beam of hard missile

The safety design of concrete containment structures in nuclear power stations has thus far covered only accidents due to internal pressure, temperature loading and earthquake loading. Recently, designers and researchers have become interested in the important effects of the impact load of a projectile on nuclear power stations. This paper develops an FEM model for analyzing the collision of a hard missile against reinforced concrete structures and compares the results with impact tests conducted at our institute.     

Applicability of structural wall test results to seismic design of nuclear facilities

A review of tests on earthquake-resistant reinforced concrete structural walls is presented. Laboratory tests of isolated walls and construction joints are discussed. Where appropriate, design recommendations are given. The review indicates only few experimental data are available for short walls which are directly applicable to nuclear power plant design. In particular, tests of short rectangular walls subjected to load reversals are needed. Tests are also needed to determine the damping and frequency characteristics of cracked short walls. Analytical and experimental results should be correlated so that the hysteretic response observed in tests can be realistically related to the analytical response “demand” of nuclear power plant structures.     

Critical impact energies for scabbing and perforation of concrete target

In this paper, the influences of the relative target thickness (H/d) on those critical impact energies, at which local damage of various forms in concrete targets are initiated, are explored. The empirical formulae developed in the R3 Impact Assessment Procedure [BNFL, 2003. Reinforced Concrete Slab Local Damage Assessment, R3 Impact Assessment Procedure, vol. 3, Appendix H. Magnox Electric plc & Nuclear Electric Limited] are rationalized by different methods. A dimensional analysis was conducted to identify influential non-dimensional numbers, which were subsequently employed in the analyses of the experimental results relevant to scabbing and perforation by flat nosed missiles.The relationships between the non-dimensional impact energy at failure and the non-dimensional target thickness H/d are presented for all of the relevant experimental data in the “World Impact Data” collection [Bainbridge, P., 1988. World Impact Data—S.R.D. Impact Database Version Pre 3i, CCSD/CIWP(88)107(P)]. This collated hundreds of experimental data on local damage in concrete targets due to missile impact from various sources of nuclear industries, as well as experimental data from the UK electrical power industry used to develop empirical formulae in the R3 Impact Assessment Procedure [BNFL, 2003. Reinforced concrete slab local damage assessment, R3 Impact Assessment Procedure, vol. 3, Appendix H. Magnox Electric plc & Nuclear Electric Limited]. The experimental data in Bainbridge [Bainbridge, P., 1988. World Impact Data—S.R.D. Impact Database Version Pre 3i, CCSD/CIWP(88)107(P)] are compared with empirical and semi-empirical formulae for scabbing and perforation in order to examine the effects of H/d on the critical non-dimensional impact energy for these two local failures. An analytical formula based on a penetration-plugging model is employed to give the relationship between the critical impact energy and target thickness for perforation by a flat-ended projectile. Comparisons between these formulae and experimental data are presented.