CONTAINMENT EVALUATION FOR IMPACT AND IMPULSE
The containment must be designed for impulse and impact from accident loading, tornado missile and aircraft impact. Impact loading results from tornado missile, aircraft, pipe whip or other missiles. Impulsive loads result from jet impingement, blast pressure, compartment pressurization, and pipe whip restraint reactions.
ASME BPV III Codes do not give detailed guidance on design for impact and impulse loads. The Code is silent on the utilization of the dynamic increase factor (DIF) for impact and impulse loading. Therefore, the design used need to be justified through detailed analysis and/or testing. For concrete containments, ACI 349-06 may be referred to for some basic guidance on the subject. Recognizing the extreme nature of these loads, local damage/yielding and plastic behavior of the containment is allowed in order to dissipate the energy of the impact and impulse loads. For concrete, the level of plasticity is controlled through allowable ductility factors for impulsive and impactive loads prescribed in ASME Section III, Div 2.
For a containment vessel of steel or concrete, a shield building (or secondary containment) is provided to serve as a missile barrier against external impact and impulse, thus the containment vessel itself need only to be designed for internal impact and impulse load. The analysis and design rules for impact and impulse loads are essentially same for both the shield building and the primary containment. Herein, the term “containment” is used in this section for both primary containments and shield buildings unless specified otherwise.
Missile impact effects are generally assessed in terms of local damage and global response. Local damage (damage that occurs in the immediate vicinity of the impact area) is assessed in terms of perforation and spalling. Spalling (or scabbing) is defined as displacement of material from the back face of the containment. Evaluation of local effects is essential to ensure that components within the containment would not be damaged directly by a missile perforating the containment, or by secondary missiles such as spall particles. Empirical formulae are used to assess local damage.
Evaluation of global response is essential to ensure that protected items are not damaged or functionally impaired by deformation or collapse of the impacted structure. Global response is assessed in terms of deformation limits, strain energy capacity, structural integrity, and structural stability. Structural dynamics principles are used to predict structural global response.
The general procedures for analysis and design of containments for missile impact effects include:
- (a) Defining the missile properties (such as type, material, deformation characteristics, geometry, mass, trajectory, strike orientation, and velocity).
- (b) Defining the containment and its properties, (such as geometry, section strength, deformation limits, strain energy absorption capacity, stability characteristics, and dynamic response characteristics).
- (c) Determining impact location, material strength, and thickness required to preclude local failure (such as perforation for steel containments and spalling for concrete containments).
- (d) Determining global response of the subject containment considering other concurrent loading conditions.
- (e) Checking adequacy of structural design (stability, integrity, deformation limits, etc.) to verify that local damage and global response (e.g., maximum deformation) will not impair the function of safety-related items.