Gathering Strength

 

Located in Salt Lake City, the Tabernacle on Temple Square is distinguished by a domed roof structure, above, supported by 44 exterior sandstone piers. These 3 by 9 ft (0.9 by 2.7 m) pillars, which vary in height, are oriented perpendicular to the external wall of the building. During the seismic retrofitting of the structure, the piers were connected by means of a steel belt truss 12 in. (610 mm) deep composed of 292 steel members. Sections of the oval-shaped dome were removed, opposite, so that the new steel arch trusses could be placed next to the original wooden king trusses. Since only a certain percentage of the total roof diaphragm could be removed at a time, the endeavor was undertaken in phases.

Courtesy of the Church Archives, The Church of Jesus Christ of Latter-day Saints, all photographs

Approximately 140 years after its construction, the Tabernacle on Temple Square, in Salt Lake City, has undergone a significant seismic retrofitting and renovation. In view of the fact that a large seismic fault runs along the nearby foothills in the Salt Lake Valley, the leaders of the Church of Jesus Christ of Latter-day Saints determined that the structure should be preserved as a community icon and a symbol of religious heritage. They also wanted it to be able to continue to function as a meeting hall, auditorium, concert hall, and broadcast studio for the Mormon Tabernacle Choir.

On July 24, 1847, when the first party of pioneers belonging to the church entered the Salt Lake Valley, they beheld what amounted to little more than a desert. For them, however, it was a place they could finally call home after hostile mobs expelled them from their homes in Illinois in early February 1846, triggering a 1,300 mi (2,100 km), 17-month exodus across the midwestern plains and through the Rocky Mountains. One week after their arrival, under the direction of their new leader, Brigham Young, they constructed a 40 by 28 ft (12 by 8.5 m) bower—composed of a roof of willow branches supported by wooden poles—beneath which they congregated, worshipped, and listened to sermons by the church’s leaders.

The bower was the first structure the Mormon pioneers built in the valley, and it reflected the way that members of the church worshipped, according to Richard Oman, the senior curator of the church’s Museum of Church History and Art—located in Salt Lake City—and a history consultant for the renovation project. He says that because Mormons believe in living prophets, they tend to congregate in a single mass to hear such individuals speak. As a result, large structures have always been an integral part of the religion’s culture.

Young knew that a more permanent structure would be needed to comfortably accommodate all of the church’s members. He therefore decided to construct what is now referred to as the Old Tabernacle, an adobe structure that was partially underground. Its wooden floor was sloped, which provided the audience with proper sight lines, and its walls and barrel vault roof provided shelter from the weather. However, the Old Tabernacle often flooded during heavy rains, and the timbers in the floor became damp and moldy. The structure’s foundation was later raised to ground level, and a rounded band shell was added at one end, which improved the acoustics tremendously. But as more pioneers arrived in the valley, the congregation soon outgrew the structure.

Again, Young recognized that something greater was needed. He wanted a structure big enough to accommodate a sizable congregation but free of the large columns that would block sight lines. The acoustics also had to be superior. Shortly after the renovation of the Old Tabernacle, he announced a plan to construct “the Great Tabernacle.”

Young approached an engineer by the name of Henry Grow, a member of the church from Pennsylvania who at the time had been commissioned to construct a bridge over the nearby Jordan River based on techniques he learned when he worked for the Remington Company, of Philadelphia. Grow’s design for the bridge included wooden trusses with long chords in which the web members formed a lattice. Young asked Grow if it would be possible to construct such trusses in an arch shape to support the roof of the new tabernacle. Grow affirmed the possibility, concluding that the trusses could support a span of approximately 150 ft (46 m).

Young drew on his experiences with the bower and the Old Tabernacle to develop plans for the new edifice. The result of Young’s vision and Grow’s labor is a structure that is oval shaped in plan and has a domed roof supported by nine curved full-arch wooden trusses that span the structure’s 150 ft (46 m) width. Moreover, 13 radial trusses are located at each end. The latter are approximately half as long as the full-arch trusses and converge at the center of one of two full-arch trusses called king trusses. The radial and full-arch trusses are supported by 44 exterior sandstone piers. These 3 by 9 ft (0.9 by 2.7 m) pillars—which vary in height from 11 to 19 ft (3.3 to 5.8 m)—are oriented perpendicular to the external wall of the building and are supported by stone and mortar footings 4 to 5 ft (1.2 to 1.5 m) deep.

The interior of the tabernacle formed a large hall with a pulpit, choir seats, and, at the west end, an immense pipe organ. The pulpit overlooked rows of wooden pews and a horseshoe-shaped balcony that was supported by small pillars. Perhaps the most breathtaking feature of the structure’s interior was its plaster ceiling, which followed the curvature of the arch trusses and gave the hall its renowned acoustical properties.

Nearly a century and a half after the completion of the tabernacle, in 1867, the church interviewed several engineering firms and selected Reaveley Engineers & Associates, headquartered in Salt Lake City, to carry out a structural evaluation. During their initial seismic study, the firm’s engineers discovered that the sandstone piers were susceptible to overturning under significant earthquake loads owing to their shallow foundations and narrow stone footings. They also found that there were no connections between the stone piers and the trusses. “The biggest deficiency in the whole structure was that nothing was really tied together, especially the roof,” says Jeff Miller, P.E., M.ASCE, a structural engineer and Reaveley principal. In fact, according to Miller, the only thing that had prevented the arch trusses from slipping off the piers for 140 years was friction. The engineers also discovered that the king trusses were significantly overstressed both vertically and laterally.

To alleviate the vertical stresses on the king trusses, the engineers recommended that steel arch trusses be placed next to them. They also recommended that all of the structural elements be tied together so that the edifice would move as a single mass in the event of a major earthquake. But this was no easy task given the desire of church leaders to preserve as much of the original structure as possible. “The major challenge was to preserve the feel and character and the historic integrity of the building and to not let this work overshadow the building and its history,” says Roger Jackson, an architect with FFKR Architects, of Salt Lake City, the firm hired to review the engineers’ structural evaluation and determine the effect of an upgrade on the tabernacle’s architecture.

The first phase of the project involved reinforcing the exterior sandstone piers that support the arch trusses. The stone footings supporting the piers are 4.5 to 5 ft (1.4 to 1.5 m) wide and approximately 11 ft (3 m) long. The piers are perpendicular to the exterior wall of the structure and are spaced approximately 13 to 16 ft (4 to 5 m) apart to provide additional support for the trusses and to maximize the space for exterior doors and windows.

According to Miller, the piers needed to be reinforced and the footings extended to prevent overturning. However, the engineers also wanted to preserve the original foundation. So they designed a larger concrete footing that would encase the existing footing and extend it 20 in. (508 mm) into the site’s granular soil. Six micropiles approximately 8 to 10 in. (203 to 254 mm) in diameter were embedded in the new concrete footing and driven 20 to 30 ft (6 to 9 m) into the ground. After cores 4 to 6 in. (102 to 645 mm) in diameter for vertical reinforcement had been drilled into the piers, threaded steel rods were placed in the cores and grouted.

The footings are tied together by means of concrete grade beams. The ground-floor diaphragm of the tabernacle was tied to these grade beams “so they function as a shear collector for the floor diaphragm,” Miller says. The main floor’s diaphragm was connected to the grade beams by means of a wood ledger.

To connect the roof structure to the piers and, subsequently, the foundations, the engineers designed a steel belt truss that rests upon the piers. As originally conceived, the belt truss required the removal of some of the timber elements, including the timber bracing between the trusses and the piers. After reviewing that design, however, the architects requested that the engineers design the truss so that the existing timber bracing could be retained. In response, the truss was redesigned to be 12 in. (610 mm) deep and to have a total of 292 steel members. The belt truss extends between the piers and then connects to a three-sided steel box that is placed between the timbers in the cribbing, which connect to the arch trusses and the vertical steel rods inside the piers. Because of the hundreds—perhaps thousands—of highly flammable dry timber members in the attic, the belt truss and all of the steel components were bolted together rather than welded.

Grow’s radial and full-arch trusses are composed of 2.5 by 12 in. (63.5 by 305 mm) timbers arranged in two sets of two chords that sandwich web members. Each chord is composed of two layers of timber sections, each approximately 14 ft (4.3 m) long, that are staggered and spliced throughout the length of the arch truss.

The chord members were not bent into position; rather, they were cut to the radius of the designed arch. Because the ends of the chord members were not connected at the splices, only one of the two layers of timber resisted the load at the splice locations. An analysis of the trusses indicated that at many locations both layers of the chord members needed to be fully effective in supporting loads. At these locations the ends of the chord members needed to be connected together to deal with compressive or tensile loads. Where chord members would be in compression, an epoxy material was inserted in the gaps between the ends of the chord members. Where the chord members would be in tension, a fiber composite material was placed across the ends of the chord members to tie the chord members together and make both layers effective in resisting loads.

Roof Frame in Plan View

During the tabernacle’s construction, in what at the time was virtually a desert, supplies were scarce. Constructing an edifice as large as the tabernacle thousands of miles from the nearest major city required not only stamina but also a massive recycling effort. For example, the two layers of timber that make up each chord are bolted together using bolts and washers fashioned from the metal that was used for wagon parts and ox shoes. (The settlers had traveled across the country primarily in ox wagons.)

According to Oman, the oxen also provided another key element to the trusses. As the timber members began to dry out after they had been assembled, they would start to crack. When the workers began to see such cracks, they wrapped the members with strips of wet rawhide. As the rawhide dried, it contracted and acted as a clamp to prevent further cracking. “The tabernacle has hundreds of rawhide straps on [the timbers]; they are still up there and are as hard as iron,” Oman says.

Given the paucity of iron, Grow decided to use wooden dowels to hold all six layers of the arch trusses together. Four 2.5 in. (63 mm) diameter holes were drilled in a diamond pattern and a 17 in. (432 mm) long wooden dowel was inserted into each. Then, to ensure that the dowel would not slide out, wedges were inserted at each end of the dowel.

To alleviate the high stresses on the king trusses—which in some places were 3.5 times as much as current codes would allow—the engineers decided to place two steel arch trusses next to the king trusses. Among the greatest challenges for the engineers was designing the steel trusses so that they would act in conjunction with the existing wooden trusses, roof, and ceiling of the structure. The steel trusses are approximately 7 ft 1 in. (2.2 m) deep and span 140 ft (43 m).

The engineers—together with Jacobsen Construction Company, of Salt Lake City, the contractor for the project—decided that the only way to insert pieces of the new steel arch trusses without disturbing the existing trusses was to open several sections of the roof. But the structure “only allowed [the contractor] to take out a certain percentage of the roof diaphragm at one time,” Miller says. As a result, the truss erection was undertaken in several phases.

The steel trusses were constructed from bottom to top, beginning at the belt truss and then moving upward toward the building’s crown. Because the weight of the steel trusses could not be handled by the existing king trusses alone during their construction, shoring towers were constructed in the interior of the tabernacle to support the king trusses at midspan. Miller says that only a small portion of the acoustically important plaster ceiling was removed to provide access for the shoring towers.

The contractors used a series of hydraulic jacks to transfer the loads from each king truss to each new steel truss. As Miller explains, they “lifted the existing truss about three-eighths to one-half inch, which pulled the new steel truss down.” Each of the two king trusses now works together with its accompanying steel truss to handle the loads.

Arch Truss Elevation

The structural engineers modeled the trusses—using the structural program SAP2000, developed by Computers and Structures, Inc., of Berkeley, California—to calculate the properties of the various sections. According to Miller, it was a wide-ranging process owing to the vast number of structural members located in the attic. The engineers then applied the section properties to create a model of an equivalent arched beam for the earthquake analysis. “There were so many structural members in the [attic], we had to simplify, especially the roof trusses, for the earthquake model,” Miller says. The simplified model enabled the structural engineers to determine the structure’s seismic behavior.

Although the steel trusses relieved the vertical stresses imposed on the king trusses, significant lateral loading from the radial trusses had caused the apexes of the king trusses to deflect laterally 4 to 5 in. (102 to 127 mm). To support the king trusses against those loads, a 100 ft (30 m) long, 7.5 ft (2.3 m) wide horizontal steel brace was placed along the crown of the building perpendicular to the steel trusses so that the lateral loads at both ends of the tabernacle will counteract each other.

The project design and management team decided to replace the roof, which made construction easier for the contractor, which was installing the steel arch trusses and performing repairs to the roof diaphragm. If the roof had not been replaced, its diaphragm would have had to be repaired from the building’s interior, a much more challenging endeavor. The new roof’s exterior is clad with shiny aluminum panels.

According to Jackson, the curved ceiling provides exceptional acoustics. (A person standing at the east end of the tabernacle can hear a pin drop at the podium, which is at the other end, he says.) As a result, the repair work on the ceiling had to be meticulous to preserve the tabernacle’s “signature sound,” Jackson says.

The old ceiling is composed of three layers of plaster over wood lath. As part of the renovation, wooden joists suspended from the diagonal bracing between the bottom chords of the trusses connect to a bracket that in turn connects to blocking, which is attached to the lath by means of an adhesive. “The earthquake force in the plaster ceiling [will] be transferred through the adhesive to the blocking that extends in both directions and then will deliver [the load] back to the brace,” Miller explains.

As workers cleaned and repaired the ceiling, they found 14 layers of paint as well as areas where leaks had occurred and where the ceiling had been repaired improperly. In one area, for example, they discovered that maintenance workers used the same material that is used to fill dents in automobiles. To repair the ceiling, the architects tested several different plaster materials to determine which would match the one used in the original construction—a lime-based material containing animal hair. One material they found worked well acoustically but would not adhere to the original plaster. Eventually they hit upon a plaster that matched the original almost perfectly. Once the architects completed the repairs, they applied a thin finish coat of that plaster to the entire ceiling.

When the tabernacle first opened, in 1867, it had no balcony. As a result, when anyone spoke from the pulpit, the sound waves followed the arch of the ceiling until they reached the floor. Reflections from the floor and ceiling caused reverberation. Several years later, in an effort to improve the acoustics and increase seating capacity, Young decided to commission the construction of a balcony along the building’s interior perimeter that would prevent the reverberation.

The balcony that was constructed is inclined and is supported by columns. To enable the sound to reach the space between the bottom of the balcony and the floor, a 2.5 ft (0.8 m) gap is located between the back of the balcony and the wall. The structural engineers discovered that if the balcony were allowed to move independently of the rest of the structure during an earthquake, it could cause significant damage or even collapse. Therefore, to maintain the hall’s acoustic integrity while meeting seismic requirements, the engineers designed a bracing system that used horizontal steel rods 0.75 in. (19 mm) in diameter to connect a new plywood diaphragm on the balcony through the wall to the belt truss.

In addition to retrofitting the building to meet seismic requirements, church leaders decided to make other changes to the building to help it meet its current needs. According to Jackson, the tabernacle hosts approximately 600 religious meetings, performances, and other events every year. It also functions as a live television and radio studio for Music and the Spoken Word, the Mormon Tabernacle Choir’s 75-year-old weekly national broadcast. 

To make things more comfortable for audience members, the architects decided to increase the slope of the balcony to improve sight lines. The balcony was also stepped in such a way that the benches would be farther apart, making the seating more comfortable. The original wooden floor of the balcony was temporarily removed and then put back in place on the new diaphragm.

The configuration of the stairs leading to the balcony also was changed to help manage crowds and meet fire codes. In the original construction, the stairs from the balcony led outside. Those who entered the main floor and found it too crowded therefore had to exit the building to gain access to the balcony. The new configuration of the stairways makes it possible for people to move directly between the main floor and the balcony.

According to Jackson, keeping the original stairways and moving them to a new position was a complicated endeavor. Conceding that it would have been much easier to remove them and construct new stairways, he notes that “we wanted to keep and preserve them. They were very uniquely built.” The stairs were constructed of matchboard and were supported by finished sidewalls. “The stairs were attached to these walls from the bottom support all the way to the top,” Jackson says. They were reconfigured so that their bottoms would be perpendicular to the doors at the main floor.

Steel arch trusses have been placed next to the king trusses to alleviate vertical stresses. The steel trusses are approximately 7 ft 1 in. (2.2 m) deep and span 140 ft (43 m).

The original benches were made of white pine, which was painted to resemble oak. But these faux oak benches contained lead paint and were rather uncomfortable, according to Jackson. Moreover, because they were painted with such detail, they were difficult to repair and maintain. As a result, a decision was made to replace them with oak benches, which are more comfortable.

Perhaps the most widely known element of the Tabernacle on Temple Square—its giant organ—was constructed in 1885 by a carpenter by the name of Joseph H. Ridges. Young wanted the instrument to be sufficiently large to accompany the tabernacle’s entire congregation as they sang hymns. The organ originally featured 700 wooden and metal pipes, but over the years the number has grown to 11,623.

Miller says that the main casing and cabinetry of the organ needed to be seismically braced. He says that the engineers designed an irregularly shaped horizontal bracing system that has been attached to the rear of the organ’s woodwork at one end and transfers loads through the wall to the belt truss at the other. The engineers employed this type of bracing system so that the acoustics would not be impaired. During the renovation, several parts of the organ were repaired and replaced.

According to Jackson, the tabernacle’s podium is designed to have three different configurations: one for large conferences and meetings, another for small meetings, and a third for concerts. Each configuration requires different lighting and sound and a different arrangement of certain physical features at the west end of the building. Before the renovation, changing from one configuration to another was by no means an easy process, and it had to be done many times each week to accommodate the church’s busy schedule. To ease the transitions, the engineers redesigned the lectern and the organ console to make their removal possible. They also added a 14 ft (4 m) long, 23 ft (7 m) wide lift shaft that is 24 ft (7.3 m) deep and makes it possible to move the organ console, parts of the lectern, a grand piano, and other elements between the basement and the main floor.

Before the engineers could design the lift and the new offices that were to be placed beneath the choir seating, which is directly in front of the organ, they had to investigate and map the support structure for those portions of the building. The Mormon Tabernacle Choir has grown from 30 to more than 360, and the seating for its members has been expanded and raised over the years. But since there are no original blueprints for that area, the engineers were forced to crawl into spaces beneath the seating to map out the structural framing by hand. According to Miller, they found upper frames constructed on top of lower frames constructed on even lower frames. “It was the most interesting piece of construction I have ever seen,” Miller says. To meet seismic requirements, most of the framing had to be tied together, and a steel frame was placed beneath the multi-layered wooden structure.

Perhaps the largest casualty from the standpoint of preservation was a timber framing system that was used to support the original podium for the tabernacle. This framing system had to be removed to accommodate the lift despite the efforts of the architects and engineers. “It was hopelessly in the way,” Jackson says. “That was a piece of old fabric that would have been really nice to keep,” he says, despite the fact that it was not serving any structural purpose.

A basement was added in the 1960s to provide space for the organ’s air-handling system as well as rehearsal space for the choir. To make room for the new offices and a new air-conditioning system for the choir seats and the podium, a subbasement was constructed. The architects and engineers had considered placing air-conditioning systems throughout the tabernacle, but such a step would have been too costly and would have made it difficult to maintain some of the building’s original elements.

The project was completed in late January. For approximately six weeks afterward the acoustics of the hall were tested and the organ was allowed to settle into its new surroundings. On March 31, 2007, a broadcast from the tabernacle during the church’s 177th annual general conference marked its reopening to the public.

The reopening has also been celebrated by a new exhibit at the Museum of Church History and Art showcasing such artifacts from the renovation as square-headed nails, ceiling plaster, joists, benches, organ pipes, and a pulley mechanism that was used to suspend objects from the ceiling. The exhibit also features a full-scale reproduction of one of the arch truss sections and a reproduction of the original pulpit.