Architect: FFKR Architects, Salt Lake City, UT
Engineer: Reaveley Engineers + Associates, Salt Lake City, UT
General Contractor: Jacobsen Construction Company, West Valley City, UT
Concrete Contractor: Jacobsen Construction Company, West Valley City, UT
Construction Management: Jacobsen Construction Company, West Valley City, UT
Reinforcing Bar Fabricator: Harris Rebar, Salt Lake City, UT
Western States Rebar, Pleasant View, UT
Pacific Coast Steel, Draper, UT
Award: 2016 CRSI Award Winner – Cultural & Entertainment Facilities Category
Photography: Reaveley Engineers + Associates, Salt Lake City, UT
In the early morning of December 17, 2010, a misplaced electrical light started a fire in the attic of the Provo Tabernacle. The fire completely destroyed the building except the exterior masonry walls and the four corner stair tower roofs. The Tabernacle has been a center for the community’s activities for over 100 years.
The Provo City Center Latter Day Saints Temple was built from the burned-out shell of this historic structure. The design goal was to restore and repurpose the building in the design language of the era the building was originally constructed.
The first major challenge was to preserve the exterior masonry walls of the original structure and make them part of the new Temple. The walls were to remain in place with the aid of temporary shoring until a permanent system could be installed. Since this was the first phase of the project to be designed and detailed, it had to take into account all future work that would take place that relates to the above grade walls (such as floor framing connections and roof connections and diaphragm tie-ins).
Another major challenge was taking a 35,000 square foot historic structure to a modern 85,000 square foot house of worship without losing historic details. In order to double the size of the building, the project team knew excavation would be required to accommodate a basement, sub-basement, and parking. Since the original building only had a crawlspace of a few feet below the main level, a system had to be engineered to support and reinforce the existing masonry walls while excavation could take place.
A substantial parking garage was needed to accommodate the large number of visitors expected to visit the temple. The owner requested the grounds have a beautiful, peaceful, and meditative appearance for its patrons and visitors. To this end, the large parking area needed to be underground to allow for landscape creativity at grade.
For this project site, the height of the groundwater was above the lowest levels. The owner requested that a permanent dewatering system not be used. Since the building was about 14 feet deep into the groundwater table it had to be designed to keep water out. The sub-grade structures and parking structure would be subject to hydrostatic pressures that are imposed on both the walls and bottom floor of the structures.
STRUCTURAL FRAMING SYSTEM
The use of shotcrete reinforced-concrete walls worked well with the remainder of the structure. Reinforced concrete walls were used below the shotcrete as the new foundation and these walls were anchored to a new mat footing below the footprint of the structure. As a result, all systems of the structure are congruent and allow for a continuous system.
Besides the need to use reinforced concrete for portions of the new structure that would be built below-grade, the ability to preserve intact the remaining historic brick walls with the use of reinforced concrete was the single most driving factor in the use of the material.
SUSTAINABILITY OBJECTIVES. Portions of the project were designed with a longer than standard design life to meet the long-term use requirements of the facility. These portions utilized reinforced concrete to meet those objectives. A concrete compressive strength of 5,000psi for footings, larger clear cover distances to rebar and the use of epoxy-coated reinforcing steel (rebar) are ways in which the long-term objectives were met.
UNIQUE STRUCTURAL AND/OR ARCHITECTURAL DESIGN FEATURES
Space inside the building was at a premium and the shotcrete walls needed to be as thin as possible. In many areas, the thickness of the new shotcrete shear walls and boundary elements within the walls did not comply with prescriptive code requirements. To gain owner confidence and buy-in the thinner walls would perform as desired during an earthquake, Reaveley completed a performance-based design using non-linear analysis techniques. The results indicated the thinner shear walls would perform as expected by the code requirements and meet the owner’s performance goals.
Because of the reduced thickness of the shotcrete shear walls, the reinforcing steel within the walls was heavier than normal. Extra care was required by the contractor to place the shotcrete in the walls while avoiding voids behind and between the layers of reinforcing steel. An enhanced degree of inspections were conducted to maintain quality control of the shotcrete placement.
Mechanical rebar couplers were placed at the bottom of vertical reinforcing bars within the shotcrete shear walls. These couplers were then used to splice reinforcing steel at the interface between the bottom of the shotcrete shear walls and the top of the reinforced concrete foundation walls.
The next phase was to support the stabilized building shell so excavation could begin. The new composite shell would be supported by micropiles while the earth was removed below it. A series of eight-inch diameter micropiles were drilled on each side of the walls to a depth of about 90 feet. Some were installed with a batter to resist the temporary lateral loads during construction.
Holes were then excavated through the 1880s era rubble stone foundation walls at the micropile locations. Steel beams (referred to as needle beams) were placed through the foundation wall holes and connected to the micropiles.
A series of hydraulic jacks were placed between the needle beams and the shotcreted walls. The jacks were then loaded to the amount needed to effectively remove the load from the existing foundation walls. Excavation of the surrounding soils then commenced gradually with daily monitoring of the elevation of the structure.
Steel braces were added between the micropiles as the grade of the existing soil continued to drop to reduce their unbraced lengths. When the bottom of excavation was reached, the footings and then foundation walls were placed. The foundation walls were blocked out tightly to the needle beams.
Once the foundation walls reached their specified concrete strength, the needle beams were removed and the shoring micropiles were cut down and cast into the mat footings for the subgrade structures that surrounded the now re-supported structure.
The micropiles serve several purposes: first, they temporarily supported the brick/concrete walls; second, they serve to support the footings for the heavy loads of the new structures; third, they helped in mitigating the potentially liquefiable soils that exist on the site, and; last, they act as uplift anchors to prevent the structures from lifting due to the buoyancy forces from being so deep into the water table.
REASONS FOR CHOOSING REINFORCED CONCRETE
The brick walls are attached to new concrete shear walls by shotcreting the interior surface of the brick. Two of the five wythes of brick were removed and that space was replaced with the concrete – so essentially no floor space was lost. The brick and concrete walls were permanently attached together by installing 14- to 16-inch long spiral steel anchors, or helical ties, into the bricks from inside prior to shotcreting the concrete walls.
In the completed building the shotcrete walls function as reinforced shear walls to resist the lateral earthquake and wind loads imposed on the building. The shotcrete also stabilizes the existing brick walls against lateral earthquake and wind forces perpendicular to the plane of the wall.
Since the stabilization of the historic brick walls was the first phase of the project, accommodations needed to be made to tie the above-grade walls into the foundation that would be placed later. To accomplish this, rebar couplers were used at the bottom of all vertical wall bars. The couplers were identified for their use, whether jamb bar or wall bar, so the proper splice length and location of ties could be placed in continuity from walls above to walls below.
Reinforced concrete allowed underground requirements for the location of the parking garage to be met especially with the heavy soil and water fountain feature loads. The beams were designed with post-tension stresses higher than normal to reduce beam depth as much as reasonably possible. The result was an increased number of post-tensioning tendons and steel reinforcing bars within the beams. Careful detailing of the tendon anchorage plates was required to verify the plates could fit within the profile of each of the beams. To reduce congestion of the steel reinforcing bars within the beams, grade 75 rebar was specified.
In order to meet the hydrostatic pressure demands on the structures below the groundwater table, a reinforced concrete mat footing was constructed below both Temple and parking areas to resist these loads on the lower floors.
The mat footing ranges from 18″ to 24″ thick and was anchored at regular spacings with micropiles about 30 feet deep to keep the structures anchored from the buoyancy forces of the water table height.
Through careful coordination, a waterproofing layer was designed below the mat footing to keep water out. As a back-up, a system of sumps and a gravel (drainage) layer between the top of the mat footing and the slab on grade was installed.