This presentation is part of Preserving U.S. Military Heritage: WWII to the Cold War, Fredericksburg, Texas, June 4-6, 2019.
by Ron Anthony and Doug Porter
Originally constructed for the U.S. Navy during its lighter-than-air program in the 1940s, 17 “blimp” hangars were constructed using large built-up timber trusses that serve as arches. Each hangar is approximately 1,072 feet (327 m) long by 292 feet (89 m) wide by 192 feet (59 m) tall with an unobstructed floor area of approximately 7.4 acres (3 hectares), making them some of the largest timber structures in the world at the time. An initial survey of Hangars 2 at Moffett Field, California, USA, in 2013, which was to undergo adaptive reuse, led to a detailed investigation of the condition of the structural elements of the hangar. The documentation and condition assessment project was possibly the largest condition assessment ever conducted on a timber structure, providing detailed information on approximately 20,000 timber elements. The timber assessment protocol included:
• visual inspection of individual timbers in each parabolic arch truss,
• species identification to determine appropriate allowable design properties,
• strength testing to determine duration of load effects and strength loss due to fire-retardant chemicals after 70 years of service,
• digital radioscopy of split rings and shear plate connections at truss panel points to determine internal condition of the wood and possible corrosion of the steel, and
• in-situ visual grading of the lumber and timbers to determine appropriate allowable design properties.
Digital radioscopy was developed, in part, with funding from NCPTT. Digital radioscopy allows for non-intrusive examination of hidden construction and material conditions. The equipment is portable and field rugged, making it practical for historic preservation projects, including examination of bolted connections at the truss nodes. The x-ray source and imaging plate can be positioned to obtain images of complex configurations under a range of field conditions, such as those found in the hangar.
An in-situ grading protocol was also developed, in part, with funding from NCPTT. Lumber grade is determined by species, size of the member(s), and growth characteristics, such as knots and slope of grain. Knots and slope of grain tend to be the grade-limiting characteristics for lumber and timber in older buildings. Measurement of knots and slope of grain on the lumber and timbers making up the trusses indicated the allowable structural grade for each wood member. Knot size and location and slope of grain were measured from boom lifts on all visible faces of individual wood members of the trusses. Documentation of the truss members, including recording survey data, observations, and digital photos, was achieved in real time using hand-held tablets and customized software (Tablet PC Annotation System, or TPAS). This paper describes the development and implementation of the wood assessment protocol using nondestructive testing to determine material condition and behavior.
Ron Anthony, wood scientist, received an M.S. in Wood Science and Technology from Colorado State University. Prior to forming Anthony & Associates in 1999, he conducted research and consulted on wood properties and the use of wood in construction applications. Mr. Anthony’s research activities, several projects that have been conducted through NCPTT, have focused on nondestructive evaluation and materials testing to better understand how wood interacts with other materials and performs over time. He participates in historic preservation projects, conducts forensic investigations, and assists with timber design issues. Mr. Anthony is the 2002 recipient of the James Marston Fitch Foundation Grant for his approach to evaluating wood in historic buildings. He is a Fellow in APT International and the 2018 recipient of APTI’s Harley J. McKee Award.
Doug Porter, an architectural conservator, focuses on investigation, stabilization, and repair of culturally significant sites and structures in cooperation with academic, federal and non-profit partners. Projects involve condition assessment, materials analysis, structural modeling, laboratory and field-testing of conservation treatments, and treatment implementation. Porter holds a research faculty position in the School of Engineering, University of Vermont. He is on the Board of Directors of the ICOMOS International Scientific Committee on Earthen Architectural Heritage (ISCEAH), and an expert member of the ICOMOS International Scientific Committee on the Analysis and Restoration of Architectural Heritage (ISCARSAH). Recent projects include assessment and repair of the Lost Horse Mine and Mill (CA), the Reiling Dredge (CO), the Bartlett Cabin (NM), the Silver Bell Mine (CA), the Live Oak Mine (CA), trestles at Golden Spike National Historic Site, (Utah), Keane Wonder Mine Aerial Tramway (Death Valley National Park), Cable Mountain Draw Works (Zion National Park), and the Breeding Barn at Shelburne Farms (Vermont).