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The Loma Prieta earthquake that hit northern California on October 17, 1989, measured 6.9 on the Richter scale. The intensity of the quake was quite severe in downtown Oakland, which lies 20 miles east of the San Andreas Fault, and many unreinforced masonry structures were badly damaged. However, the Hotel Oakland survived with only architectural damage to the exterior and interior walls. The fact that the damage sustained was minimal can be attributed to seismic strengthening made in 1979 during conversion of the building into an apartment complex for the elderly.
The architects and engineers who had renovated the structure in 1979 were again retained to restore the building to its full use. During these repairs, this team had the rare opportunity to study the effectiveness of the strengthening they had made 10 years previously.
Originally constructed between 1910 and 1912 with funds exceeding $3,000,000, the block-square hotel (Figure 1) became a prominent social center during the next decade. Presidents Wilson, Coolidge, and Hoover were guests at the facility, as were other celebrities, including Amelia Earhardt, Sarah Bernhardt, Jean Harlow, and Mary Pickford (Scott 1959).
During the 1930's, the hotel was forced into bankruptcy several times as the result of the depression and management difficulties. In 1943, the U.S. Army took possession of it for use as a hospital. All furnishings were auctioned off, including irreplaceable chandeliers of which only photographs remain. Following World War II, several unsuccessful attempts were made to reopen the hotel for public use. The Veterans' Administration eventually occupied the facility as a hospital until August of 1963. For the next 15 years it stood vacant. In 1978 a Boston-based developer obtained possession and remodeled it into a housing project for the elderly. It remains in this use today.
Currently, the exterior of the building and all two-story spaces on the main floor are on the National Register of Historic Spaces. These grand, ornately decorated rooms include the main entrance lounging room; the Corinthian-columned, 5,000-ft2 ballroom; the dining room; and the cafe, which has 30-ft-high oak-paneled walls and a finely detailed plaster ceiling.
The building was designed as a steel-frame construction with a reinforced-concrete foundation supported on spread footings on sandy-silty material at the basement level (Wooser 1981). Reinforced concrete was also used for the floor slabs. The columns were fabricated of Bethlehem Steel "H" sections. The exterior 13-in.-thick walls were nonload-bearing and consisted of thre wythes of brick. The face brick was described as Carnegie Pressed Brick of a creamy-beige color. Brick was used to fireproof the columns, while concrete encased the beams and girders. All partitions in the building were of hollow clay tiles covered with a plaster finish.
After the 1979 earthquake, a survey of the structure indicated that the steel frame and concrete foundations were in good shape. The building had been designed to resist wind forces, but it fell far short of complying with seismic code requirements. The unreinforced exterior brick walls were especially subject to failure in an earthquake. Of prime importance in the rehabilitation of the hotel was the need to develop a cost-effective system of earthquake bracing that would reduce the life-safety hazard.
The survey indicated two major deficiencies in earthquake situations: the first was the potential for brick-wall collapse and the second was the need for major vertical shear walls throughout the building to strengthen it. Although steel buildings do not usually collapse in an earthquake, unreinforced masonry walls do; so the hotel's three-wythe brick walls had to be tied to the steel frame to guard against collapse and detachment from the steel framework.
The purpose of the renovation was to develop a maximum number of one-bedroom and efficiency apartments for the elderly. At the same time, it was determined that the building should be reinforced to reduce life-hazard exposure to a minimum in the event of an earthquake. The evaluation indicated that the building provided resistance to 60 percent of the forces required by the 1973 California Uniform Building Code. The limitation to 60 percent was controlled by the overturning forces, but in many respects the structure had the capacity to resist much higher forces.
The frame added significant seismic-resisting capability to the building and provided a completely independent system for support of gravity loads. The steel frame acted as a backup system and provided additional ductility, continuity, and redundancy to the structure. No building with a complete structural steel frame has been known to collapse in an earthquake. Significant experience with similar structures was gained during the 1906 earthquake in San Francisco, where numerous steel-frame buildings over 10 stories in height survived with reportedly little earthquake-induced damage.
The renovation plans stipulated that all interior walls from roof to basement would be demolished. Exceptions to this decision were the concrete floors, the exterior walls, and the historical rooms. Removal of the hollow clay tile partitions, which would shatter in an earthquake, eliminated substantial seismic hazard (Figure 2). This removal also reduced the total mass of the building, thus lowering the effective earthquake inertia forces.
The exterior brick was part of the historic charm of the building, and any attempt to replace it with another cladding would have been prohibitively expensive. Therefore, one of the requirements in the repair solution was that this brick would remain.
Because of their great length, the existing brick walls provided stiffness to the building to resist minor and moderate earthquakes, but major earthquakes could cause severe damage to the brick. In resisting lateral forces whether from wind or earthquake, the brick walls had to resist forces normal to their surface as well as those produced in the plane of the wall. That is, they had to be able to resist floor-to-floor normal forces and act as shear walls for in-plane forces. At low force levels, brick walls have the capacity to do this. However, major earthquakes create stresses that will exceed the brick strength, particularly for out-of-plane bending. The shear-wall response is complicated by the number of openings in the walls (i.e. window openings), which subject piers and spandrels to flexural as well as shear stresses.
In an effort to determine the shear strength of the existing masonry, bead-joint shear tests were performed on 6-in.-diam cores removed from the exterior walls. The 15 samples tested showed an average shear strength of 50 psi. The testing indicated that the walls were not designed for resistance to the high force levels experienced in an earthquake. However, if the brick walls could be held in place, they would be effective even after major cracking since the crushing of the brick along fracture surfaces during earthquake movement would absorb a great deal of energy. To this end, a basketing system was developed to stabilize the brick walls if cracking occurred.
The system devised was incorporated into the new wall furring that was applied within the steel frames of the exterior walls (Figure 3 and Figure 4). Heavier structural studs were mixed in with the basic stud-furring system and were spaced so that wall anchors could be secured on approximately 3-ft centers in both vertical and horizontal directions. The wall anchors were 1/2-in.-diam bolts that were long enough to extend from the structural studs through the two interior wythes of the exterior wall and into the face brick. The bolts were inserted into the wall through holes drilled into the brick to a depth including partial penetration in the face brick. They were then anchored to the brick with polyester-resin epoxy cartridges.
Prior to the actual use of this system, tests were conducted to determine the strength of the epoxied bolts in the brick. Three anchors were epoxied into brick test panels and were loaded to failure. One failed at 7,500 lbf, and the other two at 9,000 lbf, with all failures occurring in the anchor, not in the brick or in the bond between the brick and the anchor. There were 4,900 bolts used in the wall renovation, and 520 of these bolts were subjected to pullout testing. The anchors were loaded to a magnitude of 1,000 lbf and held at that level for 1 min. Only 34 anchors failed the proof loading.
To complete the wall anchorage, the bolts were attached to plates that spanned adjacent structural studs. The stud system then provided a positive anchorage detail to the floor framing above and below. Thus, the exterior wall system was reinforced with steel studs having the capacity to brace the walls against out-of-plane forces after failure of the brick. The system was intended to hold the brick in place, reduce potential falling hazard, and use the crushing of the brick during an earthquake for its energy-absorbing value.
Reinforcement of the exterior brick walls was important to the performance of the building in an earthquake, but it was only part of the story. To provide additional strength and ductility to resist major earthquakes, a new system of reinforced-concrete shear walls was designed to be added around the stair and elevator shafts (Figure 5). This system was well distributed around the building in the upper stories and was supplemented by additional shear walls from the second floor down to the foundations.
Several functions were served by this system: the needed seismic shear resistance was provided by the new walls; the shafts (stairwell, elevator, etc.) would remain accessible and operable (free of debris that would result from use of a more brittle material); and a 4-hr fire-resistive environment was provided in the shafts.
The new shear-wall system worked well within the confines of the existing structural framing system. The walls were tied into steel floor beams, which served as collectors to deliver diaphragm forces, and into the steel columns, which acted as chord members to resist the tension and compression from the cantilever action of the wall. Nelson studs were added to the existing structural steel members to develop the forces. The new walls were reinforced for the shear stresses and for resistance of net tension forces at the steel columns, supplementing the capacity of the column splices.
The most critical aspect of the shear-wall design was the overturning effect. Although the interior stresses within a shear wall were readily accommodated, enough gravity load had to be mobilized in the walls to enable them to resist overturning. To this end, the walls were tied into the load-carrying columns in the upper stories. In the lower stories, the walls were extended to embrace adjacent columns, creating a bigger base and providing stability for each shear-wall setup to resist overturning effects. In areas where the existing steel columns could not transfer all the uplift into the foundations, additional reinforcing steel was provided, and the new foundations were interconnected with the existing footings to provide a new composite system.
During the 1989 Loma Prieta earthquake, the structure performed as the engineers anticipated. While the measures taken had not brought the building up to current codes or structural standards, they did provide a life-safety performance level that prevented collapse and protected human life.
Shortly before the 1989 earthquake, the hotel had been acquired by a local real estate development firm with a long history of successful rehabilitation development. Two days after the quake, the new owners contacted the architects and engineers who had renovated the building in 1979. In assessing the damage and estimating the costs of repair, the owner and design team enlisted the assistance of an experienced general contractor. The architects, engineers, and general contractor worked as a team from the initial investigation until completion.
Although the 1989 Loma Prieta earthquake did no damage to the integrity of the structure, there was considerable architectural damage to the exterior brick masonry walls and to a number of apartments. Substantial amounts of brick had fallen to the street from the southwest corner of the building, exposing some of the apartments and the steel framing (Figure 6). Except for 30 apartments, the remaining 185 units could be occupied while the remaining units were being repaired.
The most significant damage sustained by the hotel was cracking of some of the exterior brick walls. Although not load-bearing, these brick walls were the stiffest elements in the building, and they resisted the main thrust of the earthquake force. The damage was manifested as diagonal (X) cracks in the wall piers with virtually no cracking in the horizontal spandrels (Figure 6).
In the most severely damaged wing, the cracks were over 1/2 in. wide, with complex fracturing through the entire thickness of this wall. There were also areas where the face brick had fallen and other areas where it was loose. The damage in this area was enough to create a potential life-safety hazard that could not be ignored. An aftershock could dislodge portions of the wall that could fall on people below. It appeared that the brick in this area would have to be removed and replaced.
Only very minor damage (hairline cracks) occurred in the reinforced concrete interior walls constructed in 1979. Additional cracks were reported in the concrete floor slabs and may have been a result of the earthquake.
It is interesting to note that during the 1979 renovations, reinforced concrete stair towers were built into both the east and west wings of the building. In the east wing, the stair tower was built with its longitudinal axis perpendicular to the longitudinal axis of the wing itself or parallel to the facade of the end wall of the wing. In the west wing, where the major brick damage from the 1989 earthquake occurred, the stair tower had been built with its longitudinal axis parallel to the longitudinal axis of the wing due to space considerations. The east wing experienced significantly less damage to the brick facade as a result of this additional stiffening.
The corner areas of the building where the wings meet the main body of the building were damaged, especially in the upper floor. The concrete floor slabs exhibited some cracking starting at the corner and extending into the main floor area. The steel floor framing in these areas did not line up directly with the walls and allow for a direct load path between the floor diaphragms and the walls. To strengthen the floor slab, new steel drag elements were added to interconnect the steel framing below slab; this would relieve stress concentrations in the slabs and help collect and deliver the tributary loads to the new shear walls.
In consultation with building department officials, the owners decided that the new strengthening elements of the building would be designed to meet 75 percent of the current Uniform Building Code design force level for new buildings. The owners were interested in fulfilling the requirements of the local building officials and in providing further assurance of life safety to the occupants. In addition, the new strengthening measures had to consider the historic character of the building.
The structural strengthening system was designed to provide seismic resistance within the confines of reinforced-concrete walls arranged in a relatively uniform manner to minimize diaphragm stress and earthquake forces on the brick walls. In areas where the concrete walls were damaged, new reinforced-concrete walls were added to the inside face of the exterior brick walls such that total resistance was provided by the new and existing reinforced concrete.
In the first-story historic spaces, the strengthening walls were located behind the existing historically sensitive wall surfaces to keep the architecture of the space clean (Figure 7). In the upper stories, the additional wall thickness encroached into the living spaces, and the apartment units had to be remodeled (Figure 8). The work space for placement of the new concrete shear walls against the existing brick was very tight; therefore, the contractor devised a resourceful forming system. Stay-in-place formwork was used and held in place by metal studs that were in turn supported by steel dowels carefully placed and epoxied into the facia brick. The studs supported the forms and anchored the brick to the new concrete walls. The studs were stiffened by wood blocking at each stud and laterally supported by whalers. This method allowed observation of the concrete placement through the formwork and greatly simplified and accelerated the process. The metal studs that remained in place were used as furring for the interior finishes, so there were no forms to remove, only the wood blocking and whalers.
Because of the landmark status of the hotel, the new shear walls had to accommodate the existing window and door openings. Historic preservation considerations did not allow for filling the door and window openings, so a solution was needed that could meet these constraints. The concrete shear-wall detailing provisions of the Uniform Building Code requires tightly tied reinforced boundary elements in the walls and diagonal shear reinforcing in some of the spandrel beams between the window openings. To reduce some of the reinforcing requirements and to minimize construction costs, higher strength concrete was specified in these shear walls.
Where the brick wall was removed, a reinforced concrete wall was constructed and finished with brick veneer to match the existing brick work. Major new foundation work was required to carry the additional weight of the new concrete walls and, more importantly, to resist the overturning effects of earthquake forces.
The exterior brick walls required significant work to repair the cracks caused by the earthquake. The cracks were injected with epoxy to restore original strength, and the finish was then patched to match the adjacent surfaces as closely as possible. The restoration process was complicated by the effects of cleaning on the existing material and on the new face brick and mortar.
The seismic strengthening system for the Hotel Oakland was selected only after several alternative systems had been considered, including steel bracing, interior shear walls, and shotcreting the entire exterior wall system. The final choice was based on a combination of structural effectiveness, minimal disturbance to the residents, preservation of historic significance, and relative cost, including cost of displacement.
The architectual and engineering work and the construction documents were completed in mid-1991. Financing was in place and the construction loan closed in early 1992, with construction starting shortly thereafter. Construction was completed in August 1993.
This project has afforded a unique opportunity for observation of the performance of a seismically strengthened historic building in an earthquake and to confirm that criteria used in earlier work have performed as projected. It also presented the challenge of revisiting the damaged building and working within the constraints of current codes, modifying it for prevailing structural code compliance, for life-safety, and within an affordable budget.
A total of 315 apartments were developed, including a portion of the ground floor and the mezzanine floor. At completion, 272,000 ft2 of residential floor area has been remodeled at a cost of $11.8 million, averaging $44/ft2; 315 efficiency and one-bedroom apartments have been created; and 50,000 ft2 of first-floor public space has been developed, part of it for tenant use and part to be historically restored for leasing as commercial space.
This effort demonstrates the importance of consistent and cooperative effort between a developer with development and financing expertise, a contractor who can successfully work with the unknowns and complexities of an old building in constructing new concrete shear walls in almost inaccessible locations, and an architect-engineer team with the knowledge and technical ability to creatively solve the attendant problems.
Photographs included in this article are courtesy of the Roberts-Obayashi Corp., Danville, CA.
For additional information, contact Ed O'Neil at (601) 634-3387 (e-mail oneile@ex1.wes.army.mil).
Scott, M. (1959). The San Francisco Bay Area, a Metropolis in Perspective. University of California Press.
Wosser, T.H. (1981). "The Hotel Oakland Comes to Life," paper presented at American Concrete Institute, Quebec.
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