Steel

As opposed to concrete, steel possesses the ductility as well as strength to resist blast loading. However, due to its cost, steel structural elements tend to be slender and less massive than concrete structures. As a result, failure modes such as global and local buckling are common failure modes of steel structures which are subjected to blast loading. In general, steel structure members that are subjected to blast loading have three failure modes: (a) Mode I, large ductile flexural deformation failure, (b) Mode II, tensile tearing at support failure and (c) Mode III, transverse shear failure at supports, as illustrated in Figure 2-16 (Jones, 1989).

Figure 2-16 Different modes of failure of beams under blast loading: (a) Mode I, (b) Mode II and (c) Mode III

Currently, the most common approach in designing steel structure members is through the SDOF approach that has been introduced through design manuals (TM5-1300, 1990) and further encouraged through practicing engineers and researchers (Biggs, 1964; Krauthammer, 1999; Longinow and Alfawakhiri, 2003; Ngo el al., 2007). Similar to the research on RC structures, the focus on research has been based on the analysis of the response of key elements and one of them is the research on columns. Unlike beams, columns are subjected to both lateral and axial loads thus the ductility limits assigned by TM5-1300 for steel may not be always applicable (TM5-1300, 1990; Barker, 1993). Liew and Chen modeled and analysed the effects on a single steel column which is subjected to blast loading (Liew and Chen, 2005) and that model is further improved with the addition of moment-thrust load to define a failure surface that characterises the effects of shock on the component (Leigh and Earls,

(c) (b) (a)

2008). Other methods and standards can also be found in literature for the design of columns (Kang, 2005; Godinho, 2007) but the standardisation in the design is far from being concrete. Experiments on columns which are subjected to blast loading are scarcer as compared to other components due to the fact that it is very hard to impose an axial load on the member whilst loading it under an explosion. However, Morrill et al. conducted such a complex experiment to test the beam-column connections under blast loading (Morrill et al, 2006). Exact details of the setup are not fully evaluated. Other researchers have also focus on the local effects of blast through the evaluation of the local buckling of steel columns (Lee et al, 2009).

Another steel component that received massive attention is the wall panel. Although concrete may be more suitable as barriers, they are simply too massive to be used in applications such as marine and offshore structures. Therefore, steel is used in such situations. Early research in blast resistant steel wall is geared towards the ship construction design (Houlston et al, 1987; Houlston and DesRochers, 1989; Slater, 1994). Numerical and experimental studies have studied the use of stiffeners to improve the blast-resistance of these panels. Such design was then extended to offshore platforms which are prone to hydrocarbon explosions. Analytical solutions were proposed in the design of such panels (Louca et al, 1996) and experiments and numerical simulation were carried out to show the adverse effects of over-stiffening such panels (Yuen and Nurick, 2005). Different configurations of stiffeners were used and results showed that by increasing the stiffness either by increasing the thickness or number of stiffeners will results in a lower displacement. However, it should also be noted that the panel response also changed from a flexural response to tensile tearing response with the increase. Similar tests were conducted based on localised

blast effects and they showed that these loadings may lead to an increase in the threshold impulse up to a factor of 4 as compared to uniform distributed load (Langdon et al, 2005). It is also observed that stiffeners play a much larger role in reducing the deformation in localised blast.

Corrugated and profiled panels were also studied and in view of the difference in boundary conditions of such panels, new methodologies such as introducing new transformation factors and the incorporation of rigid-plastic assumption had to be proposed to obtain more accurate results (Liang et al., 2007). Experiment and numerical simulation were carried out on stainless steel panels in which the study focused on the effect of the boundary condition design on the overall dynamic response of the structure (Langdon and Schleyer, 2005; Langdon and Schelyer, 2006; Schleyer et al, 2007). It was observed that the end connections which form the boundary condition may affect the onset of the membrane effect. Subjected to large impulsive loading and producing large displacements, such panels, which yield with large in-plane forces, require optimisation to ensure that these forces do not cause extensive damage to the primary structures. Buckling resistance is also appraised to perform well for corrugated panels. To further improve these panels, an innovative passive barrier system is introduced to reduce the membrane effects and minimise tearing of the horizontal welds at the ends (Boh et al, 2005). In these numerical simulations, strain rate effects and failure criteria were considered in the model to produce agreeable results.

Other novel ideas include a sandwich system which positions a layer of corrugated plate between two flat steel plates (Liang et al., 2001) and other systems that use

metallic square honeycomb and I-core as core materials to improve the blast resistant of these steel panels (Liang et al., 2007)