by Nabil Rahman
In this article, we discuss blast analysis of structures as set out in US building codes and standards. The purpose of the article is to provide readers interested in performing blast analysis and new to this subject, a starting point to get a better understanding of the requirements. Before doing any type of analysis, it is important to have a clear understanding of the objectives, problem definition, design requirements, methodology, acceptance criteria, assumptions, and limitations of the analysis and design evaluation. We hope designers will utilize advanced structural analysis methods in addition to existing experimental testing methods, experience, and product performance track record to efficiently make use of cold-formed steel in design and expand its use in building construction.
Exterior blast can be the result of an accidental explosion or a terrorist attack, and can release a wide range of energy. The exterior walls and/or the roof of buildings constitute the components that first experience the blast pressure and therefore need to be protected from its effects. The main objectives of building design against blast is to prevent the occurrence of disproportionate collapse and to reduce the possibility of a breach through the exterior walls or the roof that may cause injury to the building occupants.
When a blast incident pressure wave hits a surface of a building that is not parallel to its travel direction (such as an exterior wall or a roof), the wave reflects and creates a stronger pressure called peak reflected pressure. The main design parameters of a blast wave are the peak reflected pressure, Pr, the positive phase duration, tp, and the positive phase impulse, Ip. Other design parameters of a blast wave that may be considered based on the application are dynamic pressure, shock wave velocity and blast wave length [1].
Dynamic analysis is the primary engineering method to evaluate structural components and systems under blast loads. For most building use and occupancy, it is impractical to design the building to withstand a blast load without any damage. Therefore, the objective of dynamic modelling is to predict the damage level of the building after the blast. The dynamic response of framing members or systems made from cold-formed steel to blast pressure is a function of the force generated from the blast, the mass, and the stiffness or resistance of the member or the system. It can be reasonably assumed that the dynamic response of cold-formed steel members and panels is dominated by flexural modes. However, end shear and end bearing are part of the flexural response and must be considered.
It is permitted in blast dynamic analysis to consider available additional strength that can be quantified in the material. Test data show that average yield strength of cold-formed steel being installed is typically greater than the specified minimum values by the code. In addition, cold-formed steel, similar to hot-rolled steel, exhibits higher strength under rapid applied load or rapid strain rate. As a result, the flexural capacity of cold-formed steel members and systems loaded dynamically is permitted to include the effects of a Static Increase Factor and a Dynamic Increase Factor, applied to the steel minimum yield strength.
ASCE Standard 59 [2] and PDC TR-06-08 [3] give a whole building or a building’s component blast performance criteria in the form of a Level of Protection, LOP, and an expected component damage. A “Very Low LOP” reflects extensive damage, but no building collapse, with safe evacuation of occupants. A “High LOP” reflects fully functional building with only superficial component damage and continuation of occupancy. Table 1 gives recommendations for the expected component damage for each LOP and whether the component is primary, secondary or non-structural.
Table 1: Expected damage versus level of protection
| Level of protection | Primary structural component | Secondary structural component | Non-structural component |
| Very Low | Heavy | Hazardous | Hazardous |
| Low | Moderate | Heavy | Heavy |
| Medium | Superficial | Moderate | Moderate |
| High | Superficial | Superficial | Superficial |
Response limits should be defined for various structural applications in order to accept or reject the results of a dynamic analysis. ASCE Standard 59 and PDC TR-06-08 define the response limits for individual components dominated by flexural mode in the form of a maximum ductility ratio, μ, and a maximum support rotation, θ. The ductility ratio μ is the ratio of the maximum deflection of a component to its maximum elastic deflection. The support rotation θ is the angle through which a flexural component subject to blast loading has rotated at its supports when it achieves its maximum dynamic deflection. Table 2 gives the maximum allowed μ and θ for cold-formed steel wall studs, corrugated panels and wind girts and purlins in flexure with various end connection conditions and various expected damage level, which has been addressed in the previous section. Higher response limits can be achieved when tension membrane is utilized; however end connections must be designed to deliver matching shear and bearing capacity.
Table 2: Response limits for cold-formed steel components
| Component | Superficial damage | Moderate damage | Heavy damage | Hazardous damage | |||||
| μ | θ | μ | Θ | μ | θ | μ | θ | ||
| Wall studs | Top slip track | 0.5 | — | 0.8 | — | 0.9 | — | 1.0 | — |
| Connected top & bottom | 0.5 | — | 1.0 | — | 2.0 | — | 3.0 | — | |
| Endsanchored to develop full tensilemembrane capacity | 0.5 | — | 1.0 | 0.5o | 2.0 | 2.0o | 5.0 | 5.0o | |
| Corrugated panel (one way) | Full tension membrane | 1.0 | — | 3.0 | 3.0o | 6.0 | 6.0o | 10.0 | 12.0o |
| Some tension membrane | 1.0 | — | — | 1.0o | — | 4.0o | — | 8.0o | |
| Limited tension membrane | 1.0 | — | 1.8 | 1.3o | 3.0 | 2.0o | 6.0 | 4.0o | |
| Girt or purlin | 1.0 | — | — | 3.0o | — | 10.0o | — | 20.0o | |
See the authors article in Structure Magazine for more information. In addition to the cited references herein, the following codes and standards provide additional information to help with blast analysis methods:
Federal Guidelines
Department of Defense
- Unified Facilities Criteria (UFC) 4-010-01, DoD Minimum Antiterrorism Standards for Buildings, 2018.
- Protective Design Center (PDC) TR-06-02, ‘User’s guide for the single-degree-of-freedom blast effects design spreadsheets (SBEDS)’, 2008.
General Services Administration (GSA)
- GSA Alternate Path Analysis & Guidelines for Progressive Collapse Resistance, 2016.
- PBS-P100 Facilities Standards for the Public Buildings Service—Chapter 8, Security
Department of State
- Physical Security Standards Handbook, 1998.
- Structural Engineering Guidelines for New Embassy Office Buildings, 1995.
Private Sector Guidelines
- Blast Effects on Buildings, 2nd Edition by David Cormie, Geoff Mays and Peter Smith. London: Thomas Telford Publications, 2009.
- Modern Protective Structures, Theodor Krauthammer, CRC Press, 2008.
References
- ASCE (2010), ‘Design of blast-resistant buildings in petrochemical facilities’, Second edition, American Society of Civil Engineers, USA.
- ASCE (2011), ‘Blast Protection of Buildings’, ASCE 59-11 Standard, American Society of Civil Engineers, USA.
- DoD (2008), ‘Single degree of freedom structural response limits for antiterrorism design’, PDC TR-06-08, Protective Design Center, Army Corps of Engineers, USA.