Blast and Impact Loading and Response of Structures
The need for sustainable construction has led to more focus being directed towards renewable building materials, such as wood. Whereas engineered wood products (EWPs) have been used for decades in low-rise construction, a change in regulations permitting the construction of mid- to high-rise structures using wood combined with advancement in manufacturing technologies to produce high-performing EWPs, has allowed wood to become a viable material option for much larger structures. While wood is not typically seen as a blast resistant material due to its low mass and energy dissipation capabilities, the presence of glulam beams and columns in high-risk high-profile structures (e.g. educational institutions, high density residential/offices) could potentially be subjected to blast loads. The vulnerability of glulam beams and columns to extreme loads could lead to severe damage potentially resulting in progressive collapse and loss of life.
Although EWPs are included in both the American and Canadian Blast Design Standards as a material option, the current provisions for wood are based on limited data stemming primarily from light-frame structures using dimensional lumber. Fundamentally, even though glulam consists of two or more layers of dimensional lumber joined together with glue and finger-joints (JFs) in the parallel-to-grain direction, the behaviour differs significantly in part due to the layers being stress-rated resulting in tighter control on the individual laminations. An experimental program involving the testing of seventy unretrofitted and retrofitted glulam beams and columns under both static and dynamic loads was undertaken in order to provide design and analysis methodologies for glulam beams and columns subjected to blast loading. Three distinct phases were developed to address shortcomings found in the current blast standards: 1) Development of a dynamic increase factor (DIF) on the flexural strength, 2) The effect of combined lateral-blast and axial gravity loads, 3) Retrofit of glulam beams using fibre-reinforced polymers (FRP).
Key results aimed to assist a designer in conducting his design and analysis of glulam beams subjected to blast loads will be discussed. Details about the DIF, which was found to be significantly lower than that provided by the codes and dependant on failure modes, will be discussed and implemented into a material model capable of predicting the flexural resistance curves of both unretrofitted and retrofitted beams and columns. Due to the observed lack of post-peak resistance, unretrofitted glulam beams and columns should be designed to remain elastic. Designing a structural element to remain linear-elastic under blast loading could have significant cost implications, and therefore, the option of retrofitting the beams using both unidirectional and bidirectional FRPs were investigated. Ductility ratios approaching the current response limit of blowout corresponding to a ductility ratio of 4 were achieved. Further, proper detailing of the considered retrofits will be discussed to achieve desirable performance.