BLAST HARDENING: STEEL STRAPS AND CORRUGATED STEEL PANELS

By Eamonn Ryan, derived from an ACI podcast

A March 2024 American Concrete Institute (ACI) podcast by Marlon Bazan, a principal at Protection Engineering Consultants, shared insights on effective blast hardening retrofit concepts covered in the ACI 370R report. This is Part 3 of a three-part series

Continuing his exploration, Bazan introduces steel straps and corrugated steel panels as retrofitting alternatives akin to FRP but using different materials. Steel straps, similar to FRP, reinforce masonry walls by adding external support, requiring anchors to develop composite action effectively. Meanwhile, corrugated steel panels, applied either on the exterior or non-loaded sides of walls, act similarly to FRP reinforcement, enhancing structural integrity against blast forces. These methods find frequent application in industrial settings like petrochemical facilities, where aesthetic concerns are secondary to functionality and resilience.

Bazan demonstrated retrofit applications such as adding steel posts to CMU walls for enhanced structural support. These applications illustrate how retrofitting techniques evolve to meet specific structural demands, particularly in industrial and critical infrastructure sectors. These posts are strategically installed to horizontally span the wall, providing additional structural support against blast loads. Crucially, each post must be securely connected at both the bottom to the floor and at the top to the diaphragm of the building. This connection ensures that the posts effectively hold the wall in place without compromising its integrity. While placing the posts on the non-loaded side of the wall is simpler due to direct bearing, positioning them on the loaded side requires careful anchoring to achieve sufficient capacity and stability.

He notes ongoing efforts within the ACI 370 committee to develop comprehensive guidelines for blast-resistant designs using FRP, reflecting the industry’s continuous pursuit of standardised and effective retrofit solutions.

Another highlighted retrofit approach discussed briefly in Bazan’s presentation is the bonded membrane catch system. Unlike traditional retrofits that reinforce the wall itself, this system focuses on catching debris in case of wall failure during a blast event. The concept involves installing an unbonded membrane that acts as a tension membrane to absorb and contain debris, preventing it from dispersing and causing secondary damage. Critical to its effectiveness are the meticulously detailed connections at the top and bottom of the membrane to ensure it can withstand and manage tension forces effectively. Bazan emphasises that careful detailing of these connections, including rounding the edges of anchoring plates to prevent tearing, is essential for maximising the system’s performance.

Material options and considerations

Bazan also explores various material options used in these retrofit systems, including geotextiles, steel sheets, and even heavy polymer materials similar to those used in industrial applications like heavy-duty shower curtains. Each material offers distinct advantages depending on the specific retrofit application and blast load intensity. However, Bazan cautions that the effectiveness of any retrofit solution must be evaluated relative to the specific blast conditions and performance criteria required, emphasising the need for nuanced consideration in retrofit design and implementation.

Reiterating the key considerations for blast hardening retrofit designs, Bazan says: “While these retrofits generally enhance flexural capacity, attention must be paid to addressing potential weaknesses in shear capacity and ensuring robust connections. Additionally, the added mass from some retrofit materials may impact structural dynamics and distribution effects, requiring careful analysis during design.”