By Kenneth Ogorzalek, PE, SE; Blake Dilsworth, PE, SE; and Shakhzod Takhirov, Ph.D, PE
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Have you ever walked into a garage or a shed and discovered timber defects in structural elements that made you think twice about needing to fix them? Often, we ignore these early warning signs simply because it is easier to turn a blind eye and close that garage door with the hope of standing on conservative designs and inherently redundant structures. Now, imagine that garage was 173 feet tall, 300 feet wide, and 1,000 feet long with over 4 million board feet (FBM) of lumber; and it was designated as a historic structure in the National Register of Historic Places (NRHP) while maintained by the U.S. Navy and subsequently, NASA. Observations of timber checks, splits, warps, and any other defect that crosses your mind have a whole different meaning when staring into an abyss of wood. This was exactly the case when KPFF stepped into the Moffett Federal Airfield (MFA) Hangars 2 and 3 in 2013.
Project History
Hangar 2 (H2) and Hangar 3 (H3) were built in 1943 in Mountain View, California, by the U.S. Navy to aid WWII efforts and the Lighter-Than-Air (LTA) program. Constructed in 12 and 7 months, respectively, H2 and H3 are 2 of 17 similar hangars constructed along the west and east coast of the U.S. (Figure 1). The hangars primarily housed the U.S. Navy blimp fleet for submarine patrol, as well as dirigibles (rigid airships) when needed. A single hangar consists of 51 parabolic timber arched trusses spaced at 20 feet on-center and on top of two-story portal frame concrete bents supported by concrete pile caps and timber piles. Each truss was constructed from erection units containing timber chords and web members ranging from 3×8 to 6×14 in size, and were connected to one another by split rings, shear plates, and/or bolts. Independent door structures consisting of concrete towers connected by a spanning box beam are at each end of the hangars. A seismic joint separates the main hangar from the door structures.
All 17 LTA hangars were built from the same structural drawings and specifications, with some minor optional variations. LTA was part of the Accelerated Public Works Program of the Navy in aid of the war efforts, so the hangars were designed in a way to maximize efficiency and speed. At the time in 1942, this meant that the project structural engineer was able to use slightly higher member and connection capacities while simultaneously using slightly lower force demands when compared to code requirements. In addition, it was acceptable to use improperly seasoned timber to expedite construction.
The hangars were created strictly to support WWII efforts, so these notions were approved since their life necessity was only 5 years. Remarkably after 82 years of service, six of these hangars still stand (Figure 1). In November 2023, one of the Tustin, California, hangars tragically burned down. This hangar put up a good fight and in certain on-site photos it was observed that the west side of all 51 wood arches still stood after the fire. This speaks to the level of redundancy within the original structural system relative to a 5-year design life, which is exactly what we have also observed in the MFA hangars.
H2 and H3 are not new to being studied. More recently, Rutherford & Chekene (1992), Neal Engineering Associates (1993), and Degenkolb (2006) have all provided their expertise. Starting in 2013, KPFF designed a voluntary upgrade to the portion of hangar H2 to ASCE 41-13 seismic demands per Life Safety (S-3) performance objective under the BSE-1N hazard level, as well as to ASCE 7-10 wind demands for a 700-yr event (95 mph). Given the complexity of the hangar, extensive condition assessment by visual inspection and 3D laser scanning was conducted on H2 to accurately document all in-field conditions. This allowed for better representation in our site-specific seismic time history analyses, site-specific wind tunnel testing, and progressive collapse emergency repairs for hangar H3 (Figure 2). One upgrade item that was installed in the hangars included the strengthening of various arch truss wood connections, which is the focus of this article.
Wood Connection Strengthening
A single hangar contains 3,774 primary truss panel points, so strengthening of existing timber arch connections was a top concern. During the condition assessment phase of the project (Figure 4), numerous timber connection defects such as longitudinal splits, shear plugs, net section rupture, and bearing failures were identified. These connections are traditionally strengthened by anti-check bolts, clamps, or robust sistered members with bolted steel gusset plates, all of which can be time consuming and costly to install within the hangars because of difficult access constraints (Figure 3). Some of these strengthening techniques can even harm a structure by restraining the connection and the in-framing elements from translating or rotating during a wind or seismic event.
KPFF developed a simple, elegant, and highly strategic concept to repair the defects as well as enhance the strength and ductility of undamaged split ring and shear plate connections while accommodating inherent limitations. The concept is an extension from research by Mohammad and Quenneville (1999, 2000) and Bejtka and Blaß (2005). The solution uses modern-day mass timber self-tapping SWG ASSY VG Plus fully threaded screws manufactured by MTC Solutions (formerly MyTiCon) and is designed to be installed within seconds rather than hours. These specific fully threaded screws are lightweight, have a high withdrawal capacity, do not require pre-drilling (confirmed by KPFF in-field drilling tests) nor prep work, and do not require complete spatial access around a connection for installation. The historic aesthetic impact of screw installation is also minimal since the screw head and washer, if used, are the only exposed surfaces.
Although the SWG ASSY fully threaded screws currently have an ICC-ES evaluation approval, KPFF conducted full-scale experimental testing at the University of California at Berkeley, Structures Lab, to more accurately quantify the increased strength and ductility of different screw strengthening configurations for both split ring and shear plate connections. A typical hangar arch truss connection was used to develop the testing specimen, and five different strengthening concepts were investigated: the historic anti-check bolt versus an equivalent single fully threaded screw, as well as a variety of different fully threaded screw orientations (Figure 5 presents four out of the five strengthening concepts for brevity). A total of 73 specimens were tested, handled, and fabricated according to ASTM D1761/D4442/E2126 and ASCE 41-13. The testing included monotonic (ASTM protocol) and cyclic (CUREE protocol) loading, thereby representing wind and seismic effects, respectively. A majority of the specimens were fabricated with new Select Structural Douglas-Fir wood (typically two 3x8s connected to a central dapped 4×8), 4” Ø SAE 1010 hot rolled carbon steel split rings that were galvanized per ASTM A123, 4” Ø malleable iron shear plates per ASTM A47/D5933, and A307 steel bolts. However, a portion of the testing program investigated and used extracted wood and connectors from Hangar 3.
The experimental testing results were precisely what KPFF had envisioned. It was evident after the first few tests that the fully threaded screws provided strength where wood is weak in cross-grain tension, similar to the way steel rebar functions in reinforced concrete. All split ring and shear plate connections enhanced with fully threaded screws, as well as the anti-check bolt, exhibited increased strength and ductility when compared to an “unstrengthened” connection, while also simultaneously resisting perpendicular-to-grain splitting and minor secondary stresses of prying. Key takeaways and KPFF recommendations from the experimental testing program include:
- “Existing” unstrengthened connections exhibited ASCE 41-13, Fig. 7-4, Type 3 brittle failure force-deformation curves for both monotonic and cyclic tests (Figure 6). Unstrengthened connection tests turned out to be an expensive way to split wood for a bonfire, but all screw and anti-check bolt strengthened connections exhibited ASCE 41 Type 1 ductile behavior.
- The single screw strengthening proved to have similar response to the historic anti-check bolt strengthening (Figure 7). However, the single screw option is more efficient with installation time and therefore is recommended to use.
- The 45° screw strengthening proved to have the largest strength and ductility increase for both split ring and shear plate connections under monotonic and cyclic loading, as this was the intent of this strengthening option. The 45° screw orientation was implemented to take advantage of the screw’s high withdrawal capacity, which is about three times larger than the screw lateral shear capacity. Even under cyclic loading, the 45° screw connection strength increased by up to 34% and with a ductility factor of 9 (Figure 8).
- Fully threaded screw head pull-through failures were observed. However, the pull-through occurred at deformations well beyond code requirements. KPFF recommended installation of washers under the screw head (for cylinder heads) in order to increase bearing resistance against member side grain and ultimately to achieve a higher connection capacity.
- Internal and external hydrogen embrittlement of the fully threaded screws was not directly studied during the experimental program. However, KPFF considered these factors during the hangar upgrades and provided a slightly more conservative design threshold for strengthening in-field hangar connections, even though the in-field screws are in a constant state of relatively dry conditions since installed on the inside of the hangars. KPFF recommends contacting your structural fastening hardware supplier for more information about individual internal hydrogen embrittlement management policy.
Simple-to-Use Wood Connection Health Monitoring System
Prior to testing various specimens, the wood connections were littered with random black dots drawn by hand with a permanent marker (any size and any shape was acceptable). Photographic still images were continuously taken throughout the duration of testing by a Cannon 6D camera that had been calibrated for lens distortion by a checkerboard concept. Each digital image consisted of 20.2 mega-pixels, and by using Matlab (MathWorks) each pixel was then evaluated per its RGB color distribution. This allowed the centroid of each black dot to be determined and tracked throughout testing. The vector representing a distance between any of the dots in a 2D plane was then calculated, thereby providing a relative displacement or strain field across the specimen that was correlated to the measured axial load. This method allowed the team to discover and track wood cracks as small as 0.007 inches (0.18 mm) and larger simply through a series of photographic images (Figure 9). Since the process turned out to be simple and precise, KPFF installed numerous tracking dots (colored thumb tacks to preserve the historic nature of the hangar wood) within the MFA hangars that function as a passive health monitoring system. Readily available commercial products and software use similar techniques presented here. However, KPFF recommends exploring crack detection first through day-to-day software used in practicing firms, such as Matlab or Microsoft Office VBA, and a camera.
Conclusion
Only a portion of the experimental results and health monitoring system are presented here. The proposed screw repair and strengthening concept has been approved by NASA through the Alternate Means or Methods of Construction (AMMC) process. To date, thousands of fully threaded screws have been installed in both hangars H2 and H3 to repair or strengthen existing wood connections that well exceed ASCE 41-13 S-3 performance objective, along with ASCE 7-10 site-specific wind demands (Figure 10). The experimental testing program and developed screw strengthening concepts proved to be quite beneficial for project budget and to enhance the strength and ductility of the hangars when subject to wind and seismic hazards. We look forward to these strengthening measures and the health monitoring system to be applied to the renovation or new design of other timber structures. ■
About the Authors
Kenneth Ogorzalek, PE, SE, is an Associate at KPFF in San Francisco, CA, and specializes in Performance Based Design and Resiliency of new and existing buildings in high-seismic regions. (kenneth.ogorzalek@kpff.com)
Blake Dilsworth, PE, SE, is the Managing Principal for KPFF’s San Francisco office and leads the structural design and management of many of their highest profile projects, including dozens of Design/Build and Integrated Project Delivery (IPD) projects. (blake.dilsworth@kpff.com)
Shakhzod Takhirov, Ph.D, PE, is Director of Operations (Structures Lab and Center for Smart Infrastructure) at the University of California at Berkeley and has extensive expertise in structural testing and structural health monitoring.
(takhirov@berkeley.edu)
KPFF is the SEOR for the renovation of all three hangars at MFA and hasve been working on them for more than a decade. The team would like to thank CBRE Director of Project Management, Alex Saleh P.E., for his project oversight, Dr. Ben Brungraber and Dr. Richard Schmidt from Fire Tower Engineered Timber (FTET) for their continuous peer review during the experimental testing program, Power Engineering Construction for fabricating all wood connection specimens, the personnel at the Structures Laboratory, University of California at Berkeley for conducting the experimental testing, and all other members of the design team.
References
Bejtka, I., and Hans J. Blaß (2005). Self-Tapping Screws as Reinforcements in Connections with Dowel-Type Fasteners. Paper No. CIB-W18/38-7-4. International Council for Building Research Studies and Documentation, Working Commission W18, Timber Structures.
MATLAB Release R2024a, The MathWorks, Inc., Natick, Massachusetts, U.S., 2024.
Mohammad, M., and Quenneville, J.H.P., (1999). Effectiveness of Anticheck Bolts in Split Ring Connections Repair. Journal of Performance of Constructed Facilities, Vol. 13 (4): 157-62. doi:10.1061/(ASCE)0887-3828(1999)13:4(157).
MTC Solutions., https://mtcsolutions.com/category/structural-fasteners/
Quenneville, J.H.P. and Mohammad, M. (2000). Anti-Check Bolts as Means of Repair for Damaged Split Ring Connections. Proceedings of the World Conference on Timber Engineering, BC, Canada.
RWDI, https://rwdi.com/
U.S. Navy, H2 Construction and Aerial View of H2 and H3, NAS MFA Photo Lab, NAS Moffett Field, CA, 1943, 1968.