Blast Retrofit of Historic Buildings

Featured image source: http://www.ft.com |

Written By: Hinman Team |

Historic buildings like the U.S. Capitol Building in Washington D.C. or the Houses of Parliament in London function as cultural landmarks that attract millions of visitors every year. With the increase in traffic to iconic historic buildings, the buildings become more vulnerable to explosive threats; however, historic building construction does not inherently provide a high degree of blast protection. As historic buildings are retrofit for conventional loading such as gravity or seismic loading, enhanced blast protection is the next step in protecting historic landmarks and the people who visit and work within them.

Here at Hinman we have come across a wide range of challenges when working on historic blast retrofit projects. Every blast retrofit is unique because existing conditions in historic buildings vary widely. We often visit each building and do extensive studies to evaluate the existing conditions and determine risk and damage acceptance. Retrofits projects may focus primarily of the building exterior, building structure, or a combination of the two. In this post, we will focus on strategies for specifically retrofitting the building exterior.

One of the easiest ways to improve the blast performance of the building envelope is to improve the performance of the windows. Historic buildings typically have windows made of untreated monolithic glass panes that perform poorly under blast loading. Furthermore, windows in many historic buildings from the late 19th century and early 20th century are supported by wooden frames that have minimal blast resistance. The National Institute of Building Sciences has a publicly-accessible guide that details methods for retrofitting existing buildings to resist explosive threats. Buildings with a historic designation often require a high degree of effort on maintaining the pre-retrofit appearance of the building.

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This San Francisco historic building contains one of the first recorded curtain walls on the west coast. Source: Hinman Consulting Engineers

The simplest way to improve the performance of the glass is to apply anti-shatter film to the inside face of the windows. The film is applied to the clear area of the window, without attaching the film to the mullions or supporting structure; this application method is often called “daylight” installation. The anti-shatter film improves post-event behavior by holding the fragments of glass together, thus preventing them from projecting far into the building interior. This retrofit method does not increase the demand on the window support structure (i.e., mullions, frames, and anchorage), making it a good option for buildings where the support structure is difficult to determine or insufficient to resist increased loading.

 “Daylight” film is often recommended for historic buildings from the late 19th century to the early 20th century because “daylight” application does not increase loads on existing wooden or aluminum mullions, frames, or anchors. This is also a good option for buildings with strict historic preservation requirements because the film does not significantly alter the appearance of the building.

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Wooden mullions and frames support a window in an early 20th century building. Source: Hinman Consulting Engineers

One step up from “daylight” application of anti-shatter film on glazing, is to attach the film at the edges of the window. This method increases the demand on the supporting structure, because the glass can resist more blast loading than in the original condition. In retrofits of this kind, the supporting structure is analyzed for the increase in loading. This analysis requires more intensive investigation of current conditions, which can be an issue because architectural and structural drawings are sometimes hard to obtain for historic buildings. Typically, destructive and non-destructive testing procedures can be used to determine the in-situ conditions when drawings are insufficient.

Attached anti-shatter film is only feasible in buildings where the supporting structure is sufficient or nearly sufficient to resist increased loading. If window mullions, frames, or anchorage are nearly sufficient to resist the increased loading, strengthening procedures can be developed as part of the retrofit strategy. This retrofit strategy is often recommended for historic buildings built in the mid- to late-20th century with steel or aluminum window framing. One obstacle to installing attached film is that the operability of windows is often sacrificed. This sacrifice can be an obstacle in buildings that rely on open windows for air circulations and temperature control. Installing attached film to windows on a building with an insufficient HVAC system, may trigger costly mechanical system upgrades along with the blast retrofit.

In retrofit projects where blast performance is a high priority, there are more robust ways to increase blast performance beyond anti-shatter film application. Several high-profile federal buildings in Washington, DC have had success with installation of secondary interior windows systems behind the existing windows. Glazed systems manufacturer, TERMOLITE, has performed three case studies on retrofits of historic buildings in Washington, DC. Blast tests have shown that in the event of an explosion, the secondary glazing does not break out of the frame and is expected to “catch” the original glazing. Although there are secondary systems that do not transfer additional load onto the supporting structure, typically the frames and anchorage require strengthening to resist the additional capacity of the window system. One big advantage to installing a secondary system is that the finishes on the windows can be matched to the original windows, making the retrofit almost undetectable. As with the attached film retrofit strategy, operability of windows is often sacrificed when adding a secondary window system.

Secondary glazing
Secondary glass installation options. Source: Chicago Window Expert

The non-glazed exterior of historic buildings is often constructed of stone masonry, which can be very challenging to retrofit for explosive loading. Historically, masonry buildings have been constructed out of materials locally available during the time of construction. These practices lead to difficulty in matching the materials for retrofits and lead to variability in the material properties of masonry blocks. Historic masonry walls are often unreinforced, which adds to the difficulty of retrofit. Abass Braimah with the Infrastructure Protection and International Security Department of Civil and Environmental Engineering in Carleton University has performed a literature review summarizing the research done on blast loading effects on historic masonry buildings. Braimah’s conclusion is that there is a lack of publicly available information about the blast effects on masonry walls and that additional research is required to adequately analyze and design retrofit solutions for historic masonry walls.

Historic masonry walls have been analyzed with a variety of modeling techniques, including rule-of-thumb methods, rigid block rotation, and finite element analysis. However, most models cannot capture the true behavior of historic walls without a good understanding of the material properties and behavior. For this reason, destructive testing is often required to quantify the material properties of masonry walls. Blast retrofit strategies for historic masonry walls are similar to methods used in seismic retrofits. Retrofit strategies include supplementary steel backup structures, fiber reinforced plastic (FRP) backing, and adding steel reinforcement inside the existing masonry cavities.

Overall, the degree of blast retrofit is dependent of the condition of the existing building and risk assessments quantifying threats to the building. There is a wide range of blast retrofit strategies that can be matched to the level of risk of a particular historic building.

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