Blast Analysis of Steel Framed Buildings
Steel-framed buildings are a popular form of construction due to their ease of design, speed of construction, and good levels of sustainability throughout their life-cycle. It is critical that the performance of steel framed structures can be accurately assessed against a wide range of load cases including blast loading.
Steel framed buildings range from multi-story buildings, to singular story structures and portal frames. DGA have decades of experience in assessing these structures under blast loading. Historically, engineers have been restricted to SDOF or MDOF analysis tools, however, rise in the use of finite element analysis (FEA) within the construction industry has meant that the ability to implement FEA can give rise to detailed analyses, value engineering and design optimisation. DGA have considerable experience in applying both implicit and explicit FE solver technologies to these building designs.
DGA have been commissioned on numerous projects to ascertain the response of steel-framed buildings to blast effects. Global sway checks can be performed using established SDOF methods. However, more complex cases where the distribution of stiffness and mass within a structure can have significant influence on response, and performance, a finite element model is a fast-running tool to provide accurate answers for clients. These global sway design criteria often involve limiting the peak deflection as a function of story height in order to limit the influence of second order effects.
Changes to the building dimensions or section sizes can be efficiently implemented within the calculations. The same model can be used to investigate different load cases, i.e. roof loading, rolling blast wave loading and charge size and location. Key members can be evaluated with respect to maximum stress, ductility ratio, and peak joint rotations following design guidance from within UFC-3-340-02.
A wide variety of complex geometries and boundary conditions can be modelled using the finite element process. These bespoke geometries can be difficult and time-consuming to model by hand. DGA can model these structures in different level of details depending on the nature of the analysis using a combination of beam elements, cable elements, shell elements, or solid elements. An example of this is a domed structure, where DGA performed a series of Computational Fluid Dynamics (CFD) calculations to define the blast loadings on the structure and extract a series of pressure band loads staggered up the structure to represent the blast wave enveloping the building. This loading was subsequently assigned to an Explicit Finite Element (FE) model to predict deflections and stress levels within individual members as well as globally. This model is then used to investigate the potential for progressive collapse within the structure by the removal of one element and the application of a quasi-static load case.
A combination of materials and structural elements can be built up to create a full picture the behaviour of a structure, often identifying local areas requiring either strengthening, or weakening.
The beam elements used in these analyses have the section properties attached to each element, either standard sections or customised. The results produced include detailed cross sectional stresses, deflections, bending moments, shear stresses/forces, torsional stresses/forces as well as end connection force time histories which can be used to design connections, base plates and anchor systems as well as the foundation themselves.
Typically, the global sway analyses will run through a number of load increments in the transient dynamic analysis whereby static loadings (dead/permanent and live/variable loads) are incorporated, followed by the short time duration blast pressures and finally the structure is allowed to settle down to its post event state. The incorporation of the static loadings is important in terms of locking in stress stiffening effects prior to the subsequent blast event.
The finite element method allows for very accurate predictions of the response of steel framed buildings to blast events allowing proposed designs to be value engineered and optimised to resist events that are impossible to test physically. With the increased prevalence of BIM platforms including 3-dimensional digital models of these structures it is now possible to create workflows connecting these BIM models directly to FE models enabling the iterative design process to proceed much more speedily that in the recent past. These models allow for multiple stories of a building to be simulated, which can lead to significant cost saving for the client, alongside smaller sections and less material usage to help provide a more sustainable solution.