A Saving Isolation

Victor Muschetto

The 80-year-old City Hall in Pasadena, California, recently underwent a seismic upgrade and major renovation that included the installation of a double off-grid isolation system. Together with other improvements, the work should extend the structure’s life well into the 21st century. By James Guthrie, S.E., Stephen Marusich, S.E, and Geoff Bomba, P.E.

he Pasadena City Hall—the dominant structure among a series of government, institutional, and cultural buildings that form Pasadena’s downtown civic center complex—opened in 1927. Designed in the California Mediterranean style by architects Bakewell and Brown, of San Francisco, the structure features a massive circular tower that rises above a rectangular office edifice. Two levels of rounded arches support a massive dome—26 ft (8 m) high and 54 ft (16.5 m) in diameter—that, in turn, supports a wood-framed cupola that rises to the structure’s maximum height of 206 ft (62.8 m). But this beautiful and elegant building, which in 1980 obtained a listing in the National Register of Historic Places, has also been subjected to numerous earthquakes throughout its life, including four moderate earthquakes in the past 25 years. Most recently, the Sierra Madre (1991) and Northridge (1994) earthquakes caused limited structural damage, including concrete cracking and pounding between elements, as well as widespread nonstructural damage. The damage prompted the city to seek alternatives for a seismic evaluation and upgrade. Forell/Elsesser Engineers, a San Francisco–based structural engineering firm, was selected for the project on the basis of its recent experience repairing and seismically upgrading city halls in San Francisco and Oakland. Those structures too were prominent buildings of historical importance that had suffered significant earthquake damage, albeit from the Loma Prieta temblor (1989).

The main sections of the Pasadena City Hall form a U-shaped structure that surrounds a landscaped interior courtyard and fountain. The westward-facing building is composed of several major structural elements: a central tower with a dome and cupola, which together weigh 20 million lb (9.072 million kg); L-shaped office wings with three 15 ft (4.6 m) high floors above grade on either side of the tower and a 12 ft (3.6 m) deep basement below; and large stair towers at the four corners of the courtyard.

At the rear of the building a one-story arcade structure extends between the eastern ends of the office wings. The architects had originally intended this arcade to be temporary, but a planned fourth wing to replace it was never constructed. The total gross floor area of the building is 190,000 sq ft (17,650 m²).

The office wings are constructed of reinforced concrete with a 5.5 in. (140 mm) thick one-way slab and beam floor framing system supported by 16 in. (406 mm) thick perimeter concrete walls and 21 in. (533 mm) square interior concrete columns that are founded on trapezoidal spread footings approximately 7 ft (2.1 m) square at the base. The central tower structure is framed up to the sixth floor with structural steel columns and beams, 4 in. (102 mm) thick cast-in-place concrete slabs, and 14 ft (4.3 m) square concrete piers at the corners. Above this level, the dome structure is offset inward in plan and features concrete piers along the perimeter to support the concrete dome and wood-framed cupola above.

The stair towers at the four corners of the courtyard are constructed of 32 in. (813 mm) thick reinforced-concrete bearing walls. Both the dome and the stair towers are supported on mat slabs that are respectively 5 ft (1.5 m) and 3.5 ft (1 m) thick.

To determine the potential seismic deficiencies in the structure, Forell/Elsesser used the SAP90 software system, developed by Computers & Structures, Inc. (CSI), of Berkeley, California, to perform a series of lateral analyses. Designers constructed and analyzed a three-dimensional elastic model of the entire structure to evaluate the overall building dynamics and the distribution of lateral forces to the various resisting elements. Using the same software, the firm also performed separate nonlinear pushover analyses on the dome, the office wings, and the stair towers to evaluate their individual seismic performance.

The seismic hazards were based on response spectra for the site that had been developed by Geomatrix Consultants, Inc., of San Francisco. The response spectra were influenced by a variety of nearby faults but were largely determined by the San Andreas Fault. Two sets of spectra were developed: an upper-level earthquake with a 475-year return period and a lower-level earthquake with a 72-year return period. The ground motion for the lower-level spectrum corresponded roughly to the ground motion measured near the site during the Northridge earthquake.

Victor Muschetto

The building’s vaulted entranceway received special care. A number of the plaster rosettes had come loose or had cracked over time, and the plaster and hollow clay tile colonnades also required attention. Although the exterior plasterwork was in reasonably good condition, the slender, hollow clay tile walls of the interior did not provide sufficient lateral support. After studying several options, the design team determined that shotcrete would impart the added stiffness and strength necessary to limit the movement of these otherwise brittle elements.

On the basis of the seismic evaluation, the existing structure was determined to have poor seismic performance. Moreover, several components posed life-threatening hazards. Severe damage was predicted for the concrete walls at the perimeter of the office wings, which were minimally reinforced and lacked ductile details at the wall piers and spandrels. The different dynamic characteristics of the dome, the stair towers, and the adjacent office wings resulted in high diaphragm stresses in the concrete floor slabs that the evaluation predicted would result in local structural instability or partial collapse. Severe damage was also expected at the offset between the dome and the support tower at the sixth floor. Finally, the arcade was considered likely to collapse because of its limited lateral strength and stiffness in the transverse direction.

To address these deficiencies, three seismic upgrade alternatives were considered, each providing a progressively higher level of seismic performance:

• Ensuring life safety: This alternative would seek to prevent a structural collapse during a major earthquake, although the building would probably suffer extensive damage. 
• Limited disruption: This approach would still see significant damage from a major earthquake, but the damage would be less. Thus, repairs would probably be accomplished more easily and the building could return to operation more quickly.
• Ensuring uninterrupted operation: Here the structure would be protected in such a way that only minor damage would result and the building could be returned to operation after only a short interval.

The life safety seismic upgrade would primarily have added new concrete shear walls from the roof to the foundation in the office wings and in the dome support tower to limit the building drifts so that the existing lateral and gravity elements did not exceed their inelastic deformation capacities. The upgrade concept also involved adding steel collectors to connect the stair towers to the office wings, bracing the dome, and strengthening the arcade. Nonstructural elements were to be braced only if they represented a significant safety hazard.

The upgrade to ensure limited disruption was similar in concept to the life safety approach but was significantly larger in scope. Its goal would be to reduce the building drifts even further to permit more cost-effective repairs after a major earthquake.

By contrast, the upgrade to ensure uninterrupted operation would seismically isolate the entire structure. It would require replacing the existing foundation and adding both new seismic isolators and a suspended basement floor framing system. To prevent independent movement of the office wings, a below-grade concrete tunnel would be constructed beneath the arcade to link the ends of the office wing basements. New concrete walls also would be required in the basement. But in contrast to the first two schemes, these walls would primarily be intended as vertical transfer elements rather than as lateral elements. The scope of nonstructural anchorage also was expanded under this proposal to preserve the original features of the building.

Although the option for ensuring uninterrupted operations would require significant work in the basement, only minimal structural work would be necessary above. That work would include the addition of steel collectors at the stair towers, the strengthening of the central support tower at the dome interface, and the addition of concrete walls at the ends of the office wings.

The aboveground work would be minimal because the isolators planned for the basement would reduce the elastic seismic demand to approximately 25 percent of that for the fixed-base building. Base isolation can accomplish this reduction in two ways. First, the isolators can reduce the overall building stiffness while the weight of the new suspended basement slab adds mass. This combination would increase the fundamental period of the building by roughly a factor of 3. Second, the nonlinear hysteretic behavior of the isolators increased the damping so that it was two to three times greater than that of the fixed-base structure. Together, these effects would lower the overall seismic demand on the structure above.

Isolator Placement

Forell/Elsesser Engineers, Inc.

On the basis of the conceptual design information developed, cost estimates were prepared for each of the three seismic upgrade alternatives, estimates that included the initial costs of construction as well as the costs and duration of postearthquake repairs. Although the estimated construction cost of the upgrade that would ensure uninterrupted operation was initially greater than that of the other two alternatives, the total cost of the base isolation option turned out to be less than the other two schemes when the estimates also included the costs associated with future repairs and the length of time that the building would not be usable.

A series of public meetings were held as part of the city’s review of the alternatives. During this process, city officials determined that their goals for the project were, first and foremost, to protect the occupants of the building. They also wished to fully renovate the structure by installing new mechanical, electrical, and plumbing (MEP) systems and repairing and restoring the building’s original finishes and features. Thus, they wanted to minimize any structural intrusions into the building. Finally, they determined that the building should not be out of operation for a lengthy period after a major earthquake.

On the basis of these goals, the base isolation alternative was selected for the seismic upgrade and the decision was also made to proceed with the architectural and MEP services renovation of the building. Leading the design team was the Architectural Resources Group, of San Francisco, which over the years had performed other renovation work and architectural services on the building. Forell/Elsesser continued as the structural engineer, assisted by Gerald Lehmer, a Pasadena-based structural engineer. DMJM H&N, of Los Angeles, was selected as the construction manager.

To address the city’s seismic goals for the project, the design team proposed two sets of structural performance objectives. The first was to significantly limit damage caused by the earthquake with the 475-year return period. The second was to ensure the safety of occupants for the maximum considered earthquake, which had a return period of about 2,500 years. These design objectives were best defined by the Federal Emergency Management Agency (FEMA) publication Prestandard and Commentary for the Seismic Rehabilitation of Buildings (FEMA 356), the predecessor of ASCE’s Seismic Rehabilitation of Existing Buildings (ASCE 41-06). Consequently, FEMA 356 was selected as the rehabilitation guideline for the project.

As the design work began, a number of key decisions were made regarding the isolation system, among them the type of isolator to be used, the isolator configuration, and the method of isolator procurement.

Initially, three types of seismic isolators were considered for the project: lead-rubber isolators, rubber isolators with high damping capacity, and friction pendulum isolators.

Friction Pendulum Base Isolation System

Forell/Elsesser Engineers, Inc.

Lead-rubber bearings consist of circular layers of natural rubber laminated to alternating rings of steel with a small-diameter—usually 6 to 8 in. (152 to 203 mm)—cylinder of lead in the center. Deformation of the lead core damps the earthquake energy while the rubber provides the restoring force.

Rubber bearings with a high damping capacity are similar in construction to lead and rubber isolators except that there is no lead core and the rubber compound is modified to provide the damping.

Friction pendulum isolators have two concave metal bearing surfaces separated by a specially coated articulating slider assembly. The coating provides the specified friction and the primary damping. The radius of the dish provides the restoring force.

Victor Muschetto The one-story arcade structure that extends between the eastern ends of the office wings was originally intended to be temporary, but a planned fourth wing to replace it was never constructed. Because the arcade was seen as likely to collapse during an earthquake, a consequence of its limited lateral strength and stiffness in the transverse direction, a below-grade concrete tunnel was constructed beneath the arcade to link the ends of the office wing basements.

To determine the required axial load and lateral stiffness properties for each isolator type, a series of nonlinear response history analyses were performed on the entire structure. The older SAP90 model was converted into an SAP2000 model (both created using CSI software), and different models were constructed for each isolator type because of their various stiffness and damping properties. Because of the potentially long lead time (up to nine months) for the fabrication and delivery of the isolators, the owner sometimes purchases the isolators directly from the manufacturer as an early package. This can shorten the construction schedule when isolator procurement is on the critical path. However, it does pose some risk to the owner. Problems with delivery as well as evolution of the design after issuance of the isolator package can result in change orders or delays.

Because of the significant demolition work and the construction of a new foundation for the Pasadena project, it was not necessary to acquire the isolators early. City officials therefore decided to consider all three isolator designs through the construction documents phase and then let the contractor determine which type to use through open bidding.

One of the primary challenges of any seismic isolation upgrade is altering the path of the vertical and lateral loads at the foundation level. In a conventional installation the isolators are located directly below each column. This “on-grid” installation approach involves a complex sequence of shoring, demolition, and construction. The conventional method requires jacking and shoring the columns to remove the load in the lower portion of the column and foundation. The bottom of the column and foundation are then removed to create space for the new construction. Once the new foundation is complete, the isolators are installed and the floor framing above is constructed. Then the jacks are removed and the existing column comes to bear on the new isolator-supported framing.

During the schematic design phase, this conventional isolation approach was proposed along with a “double off-grid” system, wherein new isolators are placed between the column grids in both plan directions. The major advantage of the system is that the existing walls and columns do not need to be shored during construction. The sequence of construction begins with the installation of the new concrete foundations, isolators, and concrete floor girders between the existing foundations using conventional construction techniques. A series of 5 ft (1.5 m) wide transfer beams that span 12.5 ft (3.8 m) between the girders are then cast around the existing columns. Once the framing is complete, the alternative gravity load path is in place, and the lower portion of the existing column and foundation can therefore be removed.

With the conventional approach, an isolator would have had to be located under every vertical support. For Pasadena City Hall, that would have required four isolators for each framing bay in the office wings. But this is not necessary with the double off-grid approach. Thus, the design team studied alternative isolator configurations to fully capitalize on this new approach. Ultimately, the team determined that three isolators per framing bay constituted the optimal choice to balance the lateral and vertical load requirements; one would be located at each end of the bays and one would be in the middle.

Without an isolator beneath every column, the floor framing had to be designed to limit vertical deflections. The solution was to use posttensioned girders to balance the appropriate dead load. The posttensioning also preloaded the new foundation and thus further limited the total vertical movement of the building. The transfer beams that encapsulated the existing columns also were posttensioned to limit potential slippage between the new and the existing concrete.

After reviewing all of these factors, the design team selected the double off-grid concept for the Pasadena City Hall project because of the scheduling and cost advantages it conferred over its conventional counterpart. In particular, the aforementioned lack of temporary shoring and relatively simple construction sequence were seen as significant scheduling advantages. The fact that the double off-grid approach required fewer isolators also helped to reduce the overall cost. With the double off-grid approach, a total of 240 isolators were needed for the project, compared with the 290 isolators that would have been needed for the on-grid alternative.

Although the off-grid system made it less complex to resupport the office wings, resupporting the dome was much more challenging, mainly because of the massive (14 ft [4.3 m] square) concrete piers at the corners of the dome’s base. Installing the isolators by undermining large portions of the piers was considered, but that approach was not deemed safe for the workers or conducive to the goal of preserving the building’s architectural elements. Instead, a hybrid approach that combined indirect and direct support was used.

The indirect support involved jacketing the existing piers with 24 in. (610 mm) thick concrete walls at the basement level. To transfer the 2 million lb (907,200 kg) weight of the piers to the new jacket walls, posttensioning rods were cored through the piers and embedded into the jacketed walls. The rods were then stressed to clamp the jackets to the piers, limiting the potential slippage between the new and the existing concrete. Targeted portions of the piers were then removed so that the isolators could be installed.

The isolators were mainly located under the jacketed walls to provide the indirect support; they were also located partially under the existing piers to provide the direct support. Once the isolators were in place, the remainders of the pier bottoms were demolished. The original foundation made this task simpler: Because of its size and significant reinforcing, the original mat foundation provided sufficient strength and stiffness to be incorporated into the new design.

Although these measures had addressed the lower portion of the dome tower, there were still weaknesses in the structure at the sixth-floor offset. To mitigate this deficiency, the interconnection of the dome and the support tower was strengthened by adding two layers of reinforcing dowels spaced 12 in. (305 mm) on center between the concrete piers above and the steel support girders below. The bars were epoxied to the concrete above and hooked around the steel framing below. The reinforcing was then encased in concrete for moisture and fire protection. Some of the steel girders at the sixth floor also required strengthening to support the large overturning forces from the dome above. This was achieved by adding steel channels 4 to 8 in. (102 to 203 mm) deep to the underside of the beams and welding the existing riveted double-angle shear connections.

The stair towers were resupported in similar fashion, but their foundations required additional work. According to the original drawings, the stair towers were supported on a 3.5 ft (1.1 m) thick concrete mat. During the design phase, the team discovered a supplemental drawing that revealed that a second 3.5 ft (1.1 m) thick mat had been placed on top of the original one to address a design or construction issue. Because the upper mat was located just below the finished floor of the new basement slab, the existing mat foundation of the stair towers could not be reused. Instead, it was replaced with a new mat slab via a complex sequence of shoring, demolition, and reconstruction.

To complete the load path between the stair towers and the surrounding diaphragms, structural steel collectors were added between the tower walls and the surrounding office wing slabs. This had the added benefit of strengthening the corners adjacent to the towers. The stitched double-channel collectors were anchored to the walls using 5 ft (1.5 m) long epoxy-grouted high-strength rods and to the underside of the slab with expansion anchors.

Because of the building’s U shape, the design team was concerned about the possibility of independent movement at the ends of the office wings. To address this potential problem, the arcade between the office wings was demolished so that a 14 ft (4.3 m) square concrete tunnel, which would also serve as a utility chase for the new MEP services, could be constructed below grade to connect the office wing basements. Once the tunnel construction was complete, the arcade was rebuilt above the tunnel using conventional steel framing to match the original concrete structure. Steel moment-resisting frames were installed to resist lateral loads in each direction.

To allow a base-isolated building to move in an earthquake, a continuous gap, or moat, is required between the isolated structure and the surrounding ground. The nonlinear response history analyses indicated that a moat with a minimum width of 25 in. (635 mm) was required to accommodate the peak building displacement from the maximum considered earthquake.

An existing light well approximately 7 ft (2.1 m) from the perimeter of the building provided the perfect location for constructing this moat without significantly altering the original architectural design. Permanent cantilevered steel soldier piles were installed just behind the existing walls; after these walls were demolished, permanent 8 in. (203 mm) thick shotcrete lagging was installed to complete the structure. The top of the moat was left open so that the basement would continue to receive natural light.

A similar process was used on the courtyard side of the structure, although there the moat was concealed beneath composite aluminum and concrete covers that were installed approximately 4 in. (102 mm) below the finished grade. The covers were attached to the building with articulating pin assemblies so that when the structure moves toward the moat, the covers can ride up on the top of the moat wall. Then, when the building pulls away, the covers can slide back via the concrete corbels that were cast into the moat wall. To match the rest of the courtyard, the panels were covered with decomposed granite.

One of the most significant challenges during the design of the moat involved the routing of the new MEP services into and out of the building. To accommodate the maximum building movement in any direction, a series of seismic ball joints and pipe loops were constructed between the points where the MEP services connected to the ground and to the structure. Because the ball joints allow only a few degrees of rotation, lengths of pipe had to be installed between three of these joints to accommodate the total displacement. The locations of suspended pipes, conduits, and ducts also had to be coordinated to maintain a minimum clearance of 25 in. (635 mm) between these elements and the isolators in order to prevent damage during an earthquake.

As part of the overall rehabilitation of the building, the entire exterior of the structure was refurbished. This work ranged from patching plaster and repairing cracks to simple cleaning and painting. There were also numerous exterior ornamental elements that had to be preserved in order to maintain the character of the California Mediterranean architecture.

To evaluate the seismic performance of these ornamental elements, the design team used the three-dimensional nonlinear analytical models to generate floor spectra for each level of the building. Many of the elements, including the cupola, finials, cartouches, and other features that were damaged in previous earthquakes, required repairs and a more secure anchorage. Other ornamental stone elements, for example, the balustrade parapets, crowns, and lion heads, had to be reinforced to prevent their detachment from the structure.

The building’s grand main entranceway also received special care. Many plaster rosettes in this vaulted entry had come loose or had cracked over time, and the plaster and hollow clay tile colonnades also required attention. Although the exterior plasterwork was in reasonably good condition, the slender interior hollow clay tile walls did not provide sufficient lateral support. After several options were studied, interior shotcrete was deemed the best solution because it would provide the added stiffness and strength necessary to limit the movement of these otherwise brittle elements.

City of Pasadena

To prevent the ends of the office wings from moving independently, the arcade between the wings was demolished so that a 14 ft (4.3 m) square concrete tunnel could be constructed below grade. The tunnel also serves as a utility chase for the new mechanical, electrical, and plumbing services. After the tunnel had been constructed, the arcade was rebuilt above it using conventional steel framing to match the original concrete structure.

The Pasadena City Hall project was put out for bids in late 2004, and Clark Construction Group, L.L.C., of Bethesda, Maryland, was selected as the general contractor. Construction began in March 2005 after the building’s occupants were moved to temporary locations for logistical and employee safety reasons.

One of Clark Construction’s first tasks was selecting the type of isolators that would be used. The firm chose friction pendulum isolators manufactured by Earthquake Protection Systems, of Vallejo, California, after considering not only the cost of the isolators themselves but also the associated costs of the isolator pedestals and the installation of anchor bolts.

Because of the aggressive construction schedule proposed by the general contractor, timely delivery of the isolators was critical so that they could be installed easily on top of the new foundations before the basement floor framing was constructed above. Still, the construction work sometimes outpaced the availability of the isolators. Fortunately, the contractor developed a method of installing the isolators after the basement concrete work was completed. This involved providing blockouts in the concrete girders so that the top anchor bolts could be installed without inhibiting the placement of the isolator. The formwork below the basement framing had to be removed and the posttensioned girders were reshored for access. Next a series of rails were constructed to slide the isolators into position. The anchor bolts were then set and the blockouts grouted.

The historical importance of the building also presented significant construction challenges for the contractor. For instance, the courtyard trees and fountain could not be disturbed during construction, and original floors, ceilings, and other elements of the building that were not scheduled to be renovated had to be protected throughout the project.

But because the base isolation approach limited the amount of work required on the building’s superstructure, the contractor was also able to save considerable time by concurrently scheduling the structural work in the basement and the architectural and MEP services work for the building’s upper floors. This top-down, bottom-up sequencing significantly reduced the overall duration of construction and made it possible for the project to be completed in May 2007, two months ahead of schedule.

The Pasadena City Hall was officially rededicated on July 10, 2007. The results of this renovation and seismic upgrade, which have been well received by the city’s residents, should ensure that the structure remains a source of pride to the community for many generations to come. The Pasadena City Hall Seismic Upgrade and Rehabilitation Project was a merit award winner in the competition for ASCE’s 2008 Outstanding Civil Engineering Achievement Award.  

Project Credits
Owner: City of Pasadena, California
Construction manager: DMJM H&N, Los Angeles
Architect: Architectural Resources Group, San Francisco
Structural engineer of record: Forell/Elsesser Engineers, Inc., San Francisco
Consulting structural engineer: Gerald Lehmer Associates, Pasadena, California
Geotechnical engineers: Geomatrix Consultants, Inc., San Francisco, and Hydrologue, Inc., Pasadena, California
Mechanical/plumbing engineer: Glumac International, San Francisco
Electrical engineer: F.W. Associates, Inc., San Francisco
Civil engineer: GKC Engineering Corp., Irwindale, California
Landscape architect: Melendrez Design Partners, Los Angeles
General contractor: Clark Construction Group, L.L.C., Bethesda, Maryland