Shoring and Monitoring a Damaged Masonry Building

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In September 2022, an SUV made an unplanned excursion through a small tree and into the wall of the Guggenheim Building, an unreinforced masonry building on the University of Northern Colorado (UNCO) campus in Greeley, Colorado. The impact occurred near a building corner on a load-bearing wall that supported two floor levels and a long-span roof structure. The masonry wall was displaced by over 4 inches and required reconstruction. However, appropriate selections for the extent of replacement and the methods used for temporary support were critical to avoid further damage or instability in the structure. Innovative techniques were used to evaluate the damaged wall conditions and to carefully provide temporary support to the nearly 2-foot-thick brick masonry wall during reconstruction operations.

The Guggenheim Building was constructed in 1910 and is one of the oldest and most iconic buildings on the UNCO campus. Following renovation in 2007, it became the first building on any university campus to receive the CI Silver sustainability rating from the Leadership in Energy and Environmental Design (LEED) rating system. It currently houses the School of Art and Design, including office, classroom, and multipurpose spaces.

The vehicular impact resulted in a large area of damage to an unreinforced clay brick bearing wall near the southeast corner of the three-story structure (Fig. 1). The floor and roof framing generally consisted of wood lumber joists and rafters with plank wood decking. Access to this portion of the building was restricted, and within days, steel strongbacks were bolted through the damaged wall area to reinforce the wall. After the temporary stabilization was in place, the investigative engineering team began to identify and quantify the damage. Light Detection and Ranging (LiDAR) scanning (which is a laser-based remote sensing technique to create 3D models) was used to capture the interior surface profile of the damaged wall, and post-processing software was used to evaluate displacements and to define the areas requiring reconstruction.

The scope of engineering work for the masonry repair project included assessment, temporary support consulting, and reconstruction design. The initial phase of the project included site observations, precise measurements, and the use of LiDAR scanning technology to identify areas of displacement and deterioration.

Due to the delicate nature of the damaged area, the structural engineering team worked hand-in-hand with the contractor to design the temporary shoring required for removal and reconstruction of the damaged masonry. This included placement of a temporary foundation for the shoring frames, establishing deflection limits for the temporary supports, and providing preloading parameters to limit movement of the temporary supports.

A repair design was developed in close collaboration with the University, contractor, and design team. The primary goal of the design phase was to restore the masonry wall as closely as practical to its original condition while maintaining the original structural load paths.

Construction proceeded in challenging conditions, including snowy winter weather. Great care was taken to minimize disruption to ongoing classes, with scheduling designed to limit impact on University activities. The repair work also focused on maintaining the historical accuracy of the materials used, ensuring that all repairs matched the original construction. Throughout the construction phase, regular inspections and monitoring were conducted to verify that the work met the design intent and to promptly address any issues that arose.

Temporary shoring at the Guggenheim Building presented several complex challenges, primarily due to the nature and location of the damage and the critical need for careful handling of the sensitive historic masonry structure. Given its unreinforced brick masonry construction and the significant lateral displacement, stabilizing the masonry both temporarily and permanently was paramount to prevent further damage.

The first challenge lay in accurately calculating the loads on the damaged section of the wall. This required analysis of the load paths, considering live and dead loads (roof, floor, and masonry wall), and snow loads. The temporary shoring system had to be designed to support these loads without causing excessive deflection or cracking in the brittle masonry, as even minor deflections could result in further damage.

The need for a shoring system that could handle this brittle nature of the masonry added complexity. The masonry was highly susceptible to cracking, so even slight variations in the load distribution during reconstruction needed to be minimized. Temporary support was carefully planned to ensure that masonry deflections were kept to a minimum. The shoring system also required support both at a temporary exterior foundation and at the building interior, between floor joists spaced 12 inches on-center (Fig. 2). Moreover, there were potential conflicts with the existing ceiling system and fire sprinkler lines at the building interior that added further complexity to the shoring arrangement in terms of where supports could be placed.

The primary method of temporary support involved needle beam shoring, which utilized four wide flange steel beams placed at 2-feet-on-center spacing above the damaged masonry and below the second floor joist bearing. This system was designed by a shoring contractor in collaboration with the design team to manage the vertical loads from the masonry wall, floors, and roof structure above. The beams were supported by shoring towers on both the interior and exterior of the building (Fig. 3). A cast-in-place concrete foundation extending below frost depth supported the exterior towers, while cribbing on the soil floor of the over 100-year-old (and apparently undisturbed) crawl space supported the interior towers.

An important detail of the needle beam shoring design was ensuring proper transfer of loads from the masonry wall to the needle beams. To achieve this, dry pack material was used at the bearing surface to distribute the load evenly across the masonry. In addition, the beams were designed with robust sections to prevent rolling and localized buckling. Careful detailing was performed to ensure stability of the shoring frame and needle beam elements. The shoring system included individual screw jacks that were adjusted to fine-tune the leveling of the structure. These adjustments occurred simultaneously on both the interior and exterior shoring supports, with continuous monitoring to ensure the beams remained level and stable throughout the process.

An innovative approach was used at the temporary shoring system to apply a specific preload to needle beams prior to demolition of the supporting masonry below. The objective was to approximately match the estimated actual load that was to be transferred from the damaged masonry to the needle beam shoring when the brick below was removed. Applying this preload by pushing the needle beam up on the supported masonry helped to ensure that minimal sagging occurred once the damaged masonry was removed. However, excessive preload could also lift the building above and cause cracking and damage to the adjacent masonry.

A novel approach was adopted for preload monitoring utilizing scales originally developed for use in brewery tanks. The preload conditions for the needle beams located below the window opening are significantly different than those at the solid wall area. To monitor these varying preload conditions, one scale was placed at each end of the needle beams, with a total of eight scales used in the system. Four of these scales were positioned on the exterior side of the beams, and four on the interior side. The digital scales were connected to a single monitoring station that provided real-time load tracking. The screw jacks were turned simultaneously at the interior and exterior to apply a preload to match the anticipated load transfer, calculated by the design engineers. This system of scales was left in place and monitored throughout the construction process, allowing for ongoing load evaluation that could provide warning of shifts, sags, or instabilities in the shoring system.

Additionally, a surveyor was tasked with closely monitoring any movements in the structure throughout the demolition, shoring, and reconstruction processes. Survey points above and around the repair area were surveyed regularly to ensure that settlement or other movement was not excessive.

The real-time tracking of both the masonry elements by the surveyor and the needle beam shoring loads with the scale system ensured that the loads remained evenly distributed and that the shoring system remained stable under the various conditions during the demolition and rebuilding phases. A summary of the needle beam loading during the project is provided in Figure 4.

The masonry reconstruction process involved carefully removing damaged masonry using hand tools to prevent damage to surrounding materials and unbroken units that could be used in the reconstruction. The face brick replacement units were sourced to ensure a close match in size, color, and texture with the original masonry. The reconstruction used compatible mortar materials with careful attention given to the stiffness along with the color and gradation of the sand aggregate to match the original mortar (Fig. 5).

Once the masonry was reconstructed, the final step involved removing the temporary shoring system. This was done after ensuring the masonry had sufficiently cured and the load-bearing capacity of the repaired wall was adequate to support the structure. The holes created by the needle beams were filled with brick masonry, restoring the wall to its original condition.

The temporary shoring and masonry reconstruction at the Guggenheim Building was a successful, carefully coordinated effort. A collaborative design process and meticulous monitoring ensured that the needle beams and shoring system effectively supported the structure while the damaged masonry was removed and rebuilt. The installation and preloading of the shoring, along with ongoing movement monitoring, were closely tracked to maintain safety and stability throughout the process. As a result, the repairs were completed seamlessly, and the final outcome is virtually invisible (Fig. 6), restoring the building to its original condition without any visible signs of the extensive work that was done. ■