Chapter 2. Literature review
2.2 Previous research on blast and shock loading
An explosion is a major hazard that can severely damage the building structures.
It is typically categorized as a physical, nuclear, or chemical event based on its origin.
Physical explosions are consequences of events such as the eruption of a volcano or catastrophic failure of pressure vessels. A nuclear explosion happens due to the energy
Impact loadSeismic loadsBlast & shock loadsMetal foamHoneycombPolyurethane foam
Blast and shock loading on RC structures Slabs & Shear walls Low-velocity impactHigh-velocity impact
Beam-column jointBeamsColumns Exterior beam-column jointAluminum honeycomb Impact Mitigation Structure: Honeycomb Shielded Exterior Beam-Column Joint
released during the redistribution of protons and neutrons within different interacting atomic nuclei. The chemical explosions occur when the fuel elements of an explosive compound rapidly oxidize, a reaction known as combustion. Most of the practical explosives are solid or liquid in form, also known as condensed explosives. During the reaction process, the hydrogen and carbon atoms comprising the fuel element of the explosive disintegrate violently, releasing heat and high-pressure gas. When the velocity of the high-pressure gas is significantly higher than the speed of sound, the explosive reaction is referred to as detonation and results in a high-intensity shock wave known as a blast wave (Smith and Hetherington 1994). The brief explanation of blast and shock mechanism and its fundamentals are detailed in Chapter 3.
The use of explosives by terrorist groups around the world that target civilian buildings and other structures is a growing problem in modern societies. The terrorist activity of WTC Towers blast and collapse scenario can be seen in Figure 2.2.
Structural failures like member collapse and the shattering of glass can increase the fatalities in addition to the immediate effect of the blast wave pressure. Therefore, structural integrity has to be ensured by the engineers under unexpected extreme loading situations.
Figure 2.2 World Trade Center (WTC) Tower with antenna-Blast and collapsed scenario:
Tower I (Bangash 2006)
The blast loading is complex to quantify since it depends on the nature of the explosive charge, weight, distance from the target structure, geometry of the building facades, angle of incidence of the shock front, and the level of confinement of the explosion. The wide range of explosives can be represented in terms of the equivalent weight of Trinitrotoluene (TNT). UFC 3–340–02 (2008) gives the various blast load categories and the estimation of the blast parameters of some common explosives.
In the past few decades, considerable research is performed on explosive technique, quantification of blast load on various structures, and design of blast resistant structures. A brief summary of the research investigation on the quantification of blast wave loading upon interaction with various geometries of the target structure is presented below.
Remennikov (2003) studied the methods for predicting blast effects on buildings when a single building is subjected to blast loading produced by the detonation of a high explosive device. An extensive literature is available on various analysis methods to predict the loads and blast pressure from a high explosive blast on buildings. Their research was mainly focused on finding out the ways for protecting structures against bomb attacks. Further, numerical techniques, including Lagrangian, Eulerian, and finite element methods are used for accurate prediction of blast loads on commercial and public buildings.
Remennikov and Rose (2005) studied the different empirical relationships for determining blast loading. These relationships assumed that there is no obstacle between charge and target. In real scenarios, the actual blast load can be reduced due to the adjacent building geometry or can be enhanced due to the presence of other buildings. An approach to determining the enhancement factor was described.
Numerical simulation using a computational fluid dynamics (CFD) code Air3D was used for determining blast effects on the building
Naito and Wheaton (2006) presented the methodology for the performance of structural elements under blast loads. They conducted finite element analysis to study the pushover analysis and to calculate the blast resistance of the existing shear wall subjected to an external explosion. Pressure impulse curves were generated to evaluate the blast resistance of the wall relative to various levels of damage. The resulting pressure-impulse curves provide information regarding the damage done by a particular intensity of the blast. This analysis is very useful for the designers to retrofit the structure to withstand the blasts.
Ngo et al. (2007) gave an overview of the explosives and their effects on the structure. Mechanism of the blast wave and nature of explosions was explained. They demonstrated different methods for finding out blast loads and the calculation of structural response. They pronounced that the comprehensive overview of the analysis and design of the structures subjected to blast loads require a detailed understanding of blast phenomena and the characterisation of its parameters. They suggested the design consideration against extreme events like bomb blast is very important and the
design of the building under blast load should be included in the building regulations and design standards.
Koccaz et al. (2008) studied the blast resistant design theories along with the effect of explosives in both architectural and structural design process. Study of explosion and explosives give better understanding to make blast resistant building design. They suggested that cast-in-situ reinforced concrete floor slab are the preferred option for blast resistant buildings, but it may be necessary to consider the use of precast floors in some cases. Lightweight roofs and particularly glass roofs should be avoided and a reinforced concrete or precast concrete slab is preferred. In the case of explosion, columns of reinforced concrete structure are the most important members and should be protected. Wrapping with steel belts and wrapping with carbon fiber-reinforced polymers (CFRP) is provided to protect the columns.
Draganić and Sigmund (2012) studied the method for determining the blast load on structures and explained the method with an example of a structure subjected to a blast load. A comprehensive study was conducted on the blast loading to improve the guidelines provided in the UFC and EUROPEAN code. They modelled a structure in finite element software subjected to three different blast loads with same standoff distance. The main point of the analysis of the structure elements exposed to blast loading was to check their demand ductility. From the results it was evident that non- linear analysis is necessary and simple plastic hinge behavior is satisfactory.
Karlos and Solomos (2013) presented their report in which calculation of the external explosion loads to be considered in the blast protection design of the structure.
Empirical formulae for the prediction of the blast loads were proposed for the design of the structure. Several formulae and graphs were included in the report as a design aid. Case studies are presented to understand more efficiently the problem and calculation of blast load.
Mirgal et al. (2014) studied the effect of blast loading on a structure from architectural point of view. Structural form is a parameter that affects the blast loads on the building. Arches and domes are the types of structural forms that reduce the blast effects on the buildings. Single story buildings are more blast resistant compared with multi-story buildings. Internal layout of the building is another parameter from architectural point of view for blast resistant building. Lobby area should be protected with reinforced concrete walls; double doors should be used and the doors should be arranged eccentrically within a corridor to prevent the blast pressure entering the internals of the building.
Hamra (2016) proposed an easy way to examine the type of damage introduced into the building when one compartment is subjected to blast. Initially the whole frame was not studied and individual component of structure like beam, column was studied separately. The materials are assumed elastic - perfectly plastic. For the blast loading, the blast pressure is uniformly distributed along each structural member. For the analysis of simple frame blast loading, two scenarios were involved in the study. The first corresponds to quasi-static blast loading while the others refers to a dynamic blast
loading. The effects of strain rate on the yield strength and the ductility capacity of the structural elements were neglected.
UFC 3–340–02 (2014) Unified Facilities Criteria is a manual titled “Structures to resist the effects of accidental explosions,” widely used for analysis and design of structures. UFC manual provides guidelines and standards on the evaluation of (i) Blast, fragment and shock loading, (ii) Types of explosions (iii) Principle of dynamic analysis of blast (iv) Reinforced concrete design (v) Structural steel design, and (vi) Special considerations in explosive facility design. UFC code has proposed a set of design values for given structural steel and reinforced concrete elements based on the dynamic increase factor (DIF). It is defined as the ratio of the dynamic stress to the static stress. Strain rate effect was also considered in the DIF values. However, the DIF values are based on the average strain rate value of the cross-section whereas, the strain rate is time dependent and also varies upon the change in the depth of the cross-section along the length of the beam. These DIF values may also be independent of the blast loading scenario and can be un-conservative in some special cases.
To summarize the section, detailed literatures related to quantification of explosive loads, methods of analysis and structural response to blast loading are presented in Table 2.1.
Table 2.1 Summary of literature review on blast and shock loading