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EXCERPT FROM:    The Seismic Retrofit of Historic Buildings Conference Workbook

David W. Look, Editor,

Western Regional office of the National Park Service / Federal Emergency Management Agency / Western Chapter of the Association for Preservation Technology

1991


 

THE SEISMIC STRENGTHENING

OF THE PARDEE HOME MUSEUM

 

by Sven Thomasen1 and Randolph Langenbach

 

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ABSTRACT

This paper will present the design for the earthquake strengthening of the Pardee Home Mansion located in downtown Oakland, as an example of a system which preserved the unreinforced masonry foundation, and also which has been tested with a major earthquake shaking The work was completed in 1986, and the Loma Prieta Earthquake caused significant shaking at the Pardee Home site in 1989. Many major buildings around the Home, including Oakland's City Hall, remain closed to this date because of earthquake damage.

The Pardee Home was constructed in 1868-69 as the home of Dr. Enoch H. Pardee. It remained in the Pardee family for four generations until it was left in trust to be maintained and opened as an historic house museum. In order for the building to open as a public museum, a change of use permit was necessary, requiring a seismic upgrade of the structure. Randolph Langenbach became involved with the project when the museum's future was threatened by an engineer's cost estimate of approximately $180,000 for replacing the original 5-foot deep unreinforced brick foundation. Mr. Langenbach joined with Sven Thomasen to explore alternatives that would be more cost effective, and less destructive to the historic fabric of the house.

The Pardee Home, a two story building with a cupola, is an excellent example of the mid-nineteenth century Italianate Style home. It is of balloon frame construction with horizontal wood siding rusticated to resemble stone. The building rests on a brick foundation surmounted by a 3-foot high cripple wall. The interior of the house is filled with the artifact of the four generations of the Pardee Family, and the seismic retrofit had to avoid disturbing both the interior wallpapered walls, and the artifacts themselves, which remained in-situ and on display throughout the work.

In the analysis of the structure, it was found that the original brick foundation was adequate for the force levels anticipated, and could be preserved in conformance with the State historical Building Code. The house was successfully strengthened without disturbing the foundation or the interior by concentrating the work on the upgrading of the cripple wall, and limiting the work above to a renailing of the exterior siding with stainless steel nails. The strengthening included:

(1) The bolting of the house to the brick foundation,

(2) The jacketing of the interior cripple walls with plywood,

(3) The connection of the cripple walls to the first floor diaphragm and the installation of plywood stiffening under portions of the first floor, and

(4) The strengthening of the exterior walls of the building to the second floor by adding a schedule of new stainless steel nails to strengthen the connection between the existing siding and sheathing to the balloon frame studs.

The structural configuration of the Pardee Home is typical of many Victorian Period buildings in the seismically active areas of California. The project is an example of how these structures can -be strengthened at moderate cost without destroying their original fabric. During the Loma Prieta Earthquake, the building suffered only very minor plaster cracking, whereas a significant number of Victorian Homes in the nearby neighborhoods were seriously damaged or destroyed.

SUMMARY

This paper presents the design for the earthquake strengthening of the Pardee Home Museum located in downtown Oakland. The Pardee Home, constructed in 1868 as the home of Dr. Enoch H. Pardee, remained in the Pardee family for three generations. In 1981, the two unmarried daughters who had lived there since their childhood, arranged for the house to be left in trust to be maintained and opened as a historic house museum. In order for the building to be opened as a public museum, a change of use permit was necessary. This required a code conforming seismic upgrade of the structure.

Before the Authors were engaged to design the seismic upgrading of the Pardee Home, a report by another engineer and architect had stated that the unreinforced masonry foundation had to be replaced, with an estimated cost of $180,000. In our analysis of the structure, it was found that the original brick foundation was adequate for the force levels anticipated, and could be preserved in conformance with the State Historical Building Code. The house was successfully strengthened without disturbing the foundation or the interior finishes by concentrating the work on the upgrading of the crawl space cripple walls, and limiting the work above the first floor level to the renailing of the exterior siding with stainless steel nails. The cost of the project was less than one third of the original estimate.

The Pardee Home is an excellent example of the mid-nineteenth century Italianate Style and, although a uniquely outstanding large Victorian house, this building has many characteristics common with most other late nineteenth century timber frame buildings in the Bay Area. With its unreinforced masonry foundation, and high cripple walls, the building was vulnerable to damage in a severe earthquake. Many houses in the area surrounding the Pardee Home were severely damaged in the Loma Prieta Earthquake, which fortunately occurred after the seismic retrofit described in this paper had been completed. Although the house was in the heart of the area most severely affected by the Loma Prieta Earthquake, it suffered only minor damage to the plaster walls and to the as yet unstrengthened chimneys. There was no structural damage to the building, and the plaster damage was quite limited.

HISTORIC BACKGROUND OF THE PARDEE HOME

The Pardee Home Mansion at 11 th and Castro in downtown Oakland was built in 1868 by Dr. Enoch H. Pardee. He had come to the Bay Area during the gold rush and he later became mayor of Oakland. His son, Dr. George C. Pardee, continued the tradition of public service by first becoming Oakland's mayor and later Governor of California. In that capacity he mobilized the disaster recovery effort for the City of San Francisco in 1906 and he later was active in the establishment of the regional water system for the East Bay.

The mansion was the home of the Pardee family for four generations. In 1981, when the last survivor of the pioneer family died, the mansion was put into a trust to be maintained as a Victorian House Museum. The mansion, designed by the San Francisco architectural firm of Hoagland and Newsom, is a fine, remaining example of the Bracketed Italianate Villa style. It is listed on the National Register of Historic Places and has City Landmark status. It also has been recorded by the Historic American Building Survey (HABS Cal-1899). The magnificently preserved Victorian home has become the cornerstone of Oakland's Preservation Park which was established immediately to the north as an enclave of houses preserved from the site of a freeway. The Pardee Home is one of a very few structures in the area which remains on its original foundation.

DESCRIPTION OF BUILDING

The Pardee Home, a two story building with a cupola, is of balloon frame construction with diagonal sheathing under horizontal wood siding, rusticated to resemble stone. The freestanding building rests on a brick foundation, and the house appears to have been constructed completely of Redwood (Figure 1).

The foundation, constructed of unreinforced brick masonry, has continuous grade beams under the exterior and the interior walls and spread footings under the intermediate post that supports the floor joists at mid span. The four feet high crawl space has cripple walls of 2x6 (2 inches by 6 inches) studs with sheathing on the exterior walls only, but with some diagonal bracing. The floors are framed with 2x12 (2 inches by 12 inches) joists and straight sheathing with finished wood flooring. The exterior walls are of balloon frame construction with straight horizontal cladding over diagonal board sheathing. The interior partitions are full height and have plaster finishes over wood lath. The partitions connect directly to the plaster ceilings.

DEVELOPMENT OF SEISMIC RETROFIT DESIGN

When the Pardee family established the foundation in order to preserve the mansion as a historic house museum, a change of use permit was required to open the mansion as a public facility. This necessitated a seismic upgrade of the structure. A report prepared by an engineer engaged by the foundation stated that the building had to be upgraded by replacing the original unreinforced brick foundation with reinforced concrete footings. The following is the findings from the engineer's report.

"Replacement of existing footings: .... There is no reinforcing in the brick and no apparent attachment between the sill plates and the brick. The poor condition of the brick foundation coupled with its potential ineffectiveness in resisting seismic loading makes the replacement of footings a necessity. Continuous and spread footings of reinforced concrete should replace existing brick foundations. These footings would be founded approximately 18" below grade."

The preliminary cost estimates for this work was $180,000. This cost, in addition to other restoration work that would be necessary in order to open the museum, almost forced the newly established foundation to abandon the project. The Trust, which had been established by the Pardees to preserve the house as a museum, did not have the resources to carry out the work, nor the prospect of raising funds on that scale. If they had been unable to receive the permit necessary to open as a museum, the house eventually may have been destroyed.

In 1986 the Authors were engaged by the foundation to evaluate the seismic retrofit. An inspection showed that the framing of the structure was sound and that, while there was erosion and powdering to the brick and mortar in the area of the boiler room, the condition of the foundations taken as a whole was excellent. The mortar and the bricks had suffered from significant efflorescence and deterioration only in the vicinity of the boiler, where the wicking of moisture would have expected to occur. Below grade inspection of the typical three feet deep brick foundations revealed that the mortar was not only sound, but unusually strong.

The basic premises of the author's seismic design were:

  1. The retrofit should rely on the inherent strength of the original construction while adding strength to the weak sections.

  2. Any changes to the house, whether visible or not, should be kept to a minimum.

  3. The intervention should avoid significant spaces and features by locating seismic strengthening elements in less conspicuous areas of the building.

  4. The retrofit should conform to the requirements of the City of Oakland Building Code (UBC 79).2

  5. Since the mansion is a qualified historical building the alternative structural regulations in the State of California Historical Building Code (SHBC)2 could be applied in lieu of or in combination with the local code provisions.

The regulations in the SHBC apply to all structures on existing or future National, state and local registers. The intent of this code is to provide means for the preservation of California's architectural heritage by recognizing the unique construction problems inherent in upgrading historical buildings. The purpose of the alternative structural regulations in SHBC is to provide a reasonable level of life safety. It is not the intent to protect the property and, by so doing, adversely affect the authenticity of the historic building. The SHBC requires conformance to basic engineering procedures, and additional strengthening shall be provided where a lack of stability is indicated. Archaic materials are acceptable and the designer may assign allowable stresses, which in his or her professional judgment represent the quality and condition of the archaic materials. When using archaic materials to resist lateral forces considerations is given to their past performance under actual earthquakes.

The adoption of the alternative structural regulations in SHBC in the seismic retrofit design made it possible to maintain the unreinforced brick foundation and to avoid interventions into the ornate interior spaces of the mansion.

SEISMIC RETROFIT DESIGN

The survey of the building and a preliminary evaluation indicated that the structure was in good condition with almost no decay or deterioration. It also indicated that the weakest link in the seismic resisting load path was the framing in the crawl space, where the exterior stud walls had nailed sheathing while the interior cripple walls only had let-in braces. The survey also indicated that any, interventions in the interior of the building would destroy the original ornate finishes. Therefore, to preserve historic fabric and to reduce the cost of the retrofit, the preferred strengthening should, where possible, avoid disturbing the interior museum spaces. The selected seismic retrofit design included the following elements:

  1. The original brick foundation was left in place. Deteriorated mortar joints were repointed with a compatible lime-based mortar. The sills were bolted to the foundation with threaded rods grouted into the masonry.

  2. The crawl space walls were strengthened by jacketing selected interior cripple walls with plywood panels. The existence of open stud walls resting on brick foundations across the interior of the crawl space, made it possible to avoid installation of plywood on the inside face of the outside walls. Thus all studs and sill plates were exposed on one side for ease of inspection, ventilation and repair. (On other, more modest homes, with fewer internal divisions of the crawl space area, the strengthening of outside walls may be necessary.)

  3. The first floor diaphragm was stiffened by installing plywood shear panels below portions of the floor framing

  4. The connection of the cripple walls to the new first floor diaphragm was strengthened.

  5. The exterior walls of the mansion were strengthened by a schedule of new stainless steel nails, attacking the existing siding to the balloon frame studs.

The second floor and the roof were judged to have adequate strength and stiffness to act as diaphragms, and the interior partitions and the ceilings at the first and second floor were left unchanged. All seismic strengthening was confined to the crawl space and the exterior, and the house remained occupied, with all museum objects on display during the construction process.

The condition of the individual seismic resisting elements and their strengthening was as follows:

Foundation

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The foundation consists of continuous unreinforced masonry footings under the exterior and the interior walls, with spread footings under intermediate posts in the basement. The soil on which the house is built is a dry sand. The area under the building is unexcavated, except for a 12' by 12' boiler room where the foundation is exposed for its whole height.

Excavations found the footings were about 3 feet deep with a width of two courses corbelled out at the bottom (Figure 2). The lower portion of the masonry was generally sound with solid mortar joints but some portions of the upper bricks and mortar were soft. The main area of deterioration was in the walls surrounding the boiler room, where the moisture had been wicked through the walls from the soil on the other side. The soft bricks (under-fired bricks) were replaced and the deteriorated mortar joints were repointed.

The analysis of the brick foundation, using allowable values from SHBC of 4 psi with a 1/3 increase for masonry shear without testing, showed that the existing foundation was acceptable for the anticipated lateral forces. The only alterations to the foundation which were required was the installation of 5/8 inch threaded anchor bolts at three feet on center. The bolts were set within a cement grout, in two feet deep holes drilled directly through the redwood sill into the brick foundation.

Crawl Space Walls, Interior

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The interior crawl space walls were originally constructed in line with the upper building partitions. They were constructed as cripple walls without any cladding, but with some let-in braces. They were located over continuous footings with no anchor bolts between sill and masonry.

The lateral shear capacity of the walls was increased by addition of plywood panels on one or, rarely, at both sides of the studs (Figure 3). New blocking and nailing were installed for shear transfer at the connections to the foundation and to the new first floor diaphragm (Figure 4). The existence of the interior cripple walls and foundations made it possible to avoid installing the plywood strengthening on the inside face of the ou tside walls, with its attendant risk of enhancing and obscuring moisture and fungal decay of the exterior sill. In the Pardee Home, none of the outside walls have been covered by plywood shear panels.

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First Floor Diaphragm

The first floor has straight sheathing and finished wood flooring over wood joists. The floor distributes the lateral loads from interior partitions to the basement cripple walls. To improve this diaphragm action the floor was stiffened with plywood panels. The panels were nailed to the bottom of the floor joists to avoid disturbing the floor finish and the interior partitions. Furthermore, the panels were not applied over the entire floor but only in areas of high diaphragm shear stresses (Figure 5). The anticipated demand on the diaphragm also necessitated a strengthening of the connection to the existing floor sheathing and sheet metal angles were installed between floor joist and floor sheathing. The angles were fastened with screws rather than the customary nails because of the tight working space between the floor joists.

Second Floor and Roof Diaphragm

The second floor has straight sheathing and finished wood flooring and the roof has straight sheathing. Both were judged as adequate to act as diaphragms based on the SHBC and no strengthening was required.

Interior Walls

The first and the second floor partitions am wood stud framed with plaster over wood lath on both faces. Based on the SHBC, no strengthening was required.

Exterior Walls

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The exterior walls are wood stud framed. Their exterior face has diagonal redwood sheathing below straight horizontal boards, while the inside face, except in the crawl space, has a plaster finish over wood lath. The shear capacity of the exterior walls around the cupola and below the second floor was increased by additional nailing of the horizontal boards to the balloon frame studs (Figure 6). The large width of the boards allowed for the installation of up to three nails in the top and the bottom of each board. The power-driven stainless steel nails were installed with slightly recessed nail heads. The recess was then caulked and the wall repainted.

THE PERFORMANCE OF THE PARDEE HOME DURING LOMA PRIETA EARTHQUAKE

The 1989 Loma Prieta Earthquake occurred less than two years after the seismic retrofit had been completed. The quake caused considerable damage to many structures near the site of the Pardee Home, including the collapse of several homes in the West Oakland Area (Figure 7). The Pardee Home is less than one mile from the Cypress Freeway that collapsed. In fact, the Museum Curator, David Casebolt, was waiting at the traffic light to drive onto the lower deck of that freeway when the earthquake struck!

Unable to get home David Casebolt returned to the Pardee Home to check on its condition. It was immediately evident that the house had experienced significant shaking because many of the loose objects, including the sculptures and historical artifacts, had been thrown to the floor and broken, and some tables had overturned. An inspection of the mansion immediately after the quake showed:

1. The foundation and the cripple walls showed no evidence of damage or distress from the earthquake.

2. No damage was seen in the ornate plaster ceilings but the interior plaster wall finishes had cracked in several locations. The cracks were mostly at wall corners and at doorframes, and they varied from hairline to 1/8 inch in width (Figure 8).

3. The horizontal siding at the exterior walls was intact and had no evidence of nail popping.

4. The mansion has three chimneys with stacks extending up to five feet above the roofline. These unreinforced brick stacks had not been braced or strengthened. Two of the chimneys developed cracks in the mortar joints and a piece of rendering spalled (Figure 9). The stacks did not shift and none of the bricks came loose. The brick chimneys were undamaged aged below the roofline but cracks had developed in the finish wall plaster where it adjoined the brick.

     

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COST OF SEISMIC RETROFIT

The objective for the seismic retrofit of the Pardee Home was to reduce life safety hazards in a cost effective way, with a minimum of destruction to the historic fabric of the mansion. Focusing the retrofit on the exterior walls and the crawl space area avoided repairs and replacement of expensive interior finishes. Strengthening the first floor diaphragm below the floor preserved the original floor finishes.

The construction cost of the retrofit in 1987 was:

1. MASONRY: Repointing of brick foundation; replacing loose bricks; installing anchor bolts between sill plate and foundation .........................    $13,920.

2. CARPENTRY: Installing plywood shear panels at cripple walls; strengthening first floor diaphragm with plywood panels below the floor joist; connections between cripple walls and first floor diaphragm; renailing exterior walls; repainting exterior walls .....   $44,175.

TOTAL CONSTRUCTION COST FOR SEISMIC RETROFIT ..................................$58,095.

There were additional costs for rewiring and upgrading the electrical distribution in basement ($2,715), and relocating alarm system in basement ($968). The cost per square foot for the seismic retrofit (not including the misc. electrical work) was $9.00 per square foot.


While it is difficult to evaluate what would have happened had the earthquake struck the Pardee Home before the retrofit had been carried out, there is evidence from other structures in the surrounding area that the building would have been prone to damage to the cripple wall area. While it is unlikely that the house would have been knocked over, the damage could have been costly to repair. It is also likely that the building would have been red tagged had it been thrown out of alignment, as had other adjacent houses, and the inability for it to be occupied until the damage was corrected could have jeopardized the museum by leaving it prone to theft and vandalism. In fact, losses in many of the damaged buildings in the immediate surroundings, of the Pardee Home were from post-earthquake vandalism, rather than from the earthquake itself.

This experience only reinforces the need for earthquake mitigation programs on historic buildings to make sure the buildings remain in service after an earthquake, even if some damage takes place. The jeopardy to the historic resource in the earthquake's aftermath may be as profound, as from the earthquake itself.

THE SEISMIC RETROFIT OF VICTORIAN TIMBER FRAME HOUSES, FOLKLORE AND FACT.

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The structural configuration of the Pardee Home is typical of many Victorian Period buildings in the seismically active areas of California. This retrofit project is an example of how these structures can be strengthened at moderate cost without damaging their original fabric. During the Loma Prieta Earth quake, the building suffered only very minor plaster cracking, whereas a significant number of Victorian Homes in the nearby neighborhoods were seriously damaged.

Before, and especially since, the Loma Prieta Earthquake in 1989, the owners of many Victorian period houses in the Bay Area have been bombarded with information supporting the belief that unreinforced masonry foundations should be replaced. Some of this comes from contractors interested in the lucrative work, but it also is frequently backed up by statements from engineers and architects who believe that brick foundations make timber frame houses vulnerable to damage or collapse in a major earthquake. For most owners, untutored in the performance of construction systems in earthquakes such claims seem intuitively correct.

The unfortunate result is that many owners end up doing nothing to improve their homes. The cost of replacing masonry foundations is usually beyond the reach of most homeowners, whereas the much more effective reinforcing of the cripple walls, such as that undertaken at the Pardee Home, is comparatively cheap. It is even possible for a homeowner with minimal carpentry experience to carry out the work himself. For the standard Victorian home, the cost of replacing a foundation can fall within the range of $50,000 to $75,000, whereas the cost of bolting the house to the existing foundation, and plywood jacketing of the cripple walls (adding basement crosswalls as necessary) can be done for between $5,000 to $15,000.

The Loma Prieta Earthquake itself has provided the best evidence to indicate where the most cost effective repairs, to Victorian Period homes are needed. This earthquake provided a chance to inspect the damage caused by two very different types of ground motion. The first, in the area near the epicenter in Santa Cruz, was a very short period ground motion typical of what one might expect in the Bay Area along the Hayward or San Andreas Faults. The second type was experienced where the distant earthquake resonated with specific areas of soft soil with a long period (about 1 sec.) tremor.

In both of these areas many houses were damaged or destroyed. It was interesting to note that the type of damage to the Victorian Period timber frame buildings was very similar, regardless of the distance from the epicenter. Of those houses which fell off of their foundations, or which suffered partial or total collapse, the failure originated with the overturning of the cripple wall between the elevated 1st floor and the top of the foundation. All of this provided strong evidence that the "weak link" in the seismic resistance of Victorian homes is the cripple wall below the first floor, not the foundations. In no structure of the many that the authors inspected, did the damage or collapse originate from failure of the unreinforced masonry foundation. In fact, even with those houses where the cripple wall had completely separated from the foundation and had collapsed, the brick foundations remained undisturbed.

ACKNOWLEDGEMENTS

The Contractors for the retrofit were Fitzgibbon Masonry of San Francisco, and Winans Construction, of Emeryville. The assistance of David L. Casebolt, Pardee Home Museum Administrator and Paul Winans of Winans Construction Co. are gratefully acknowledged.


ENDNOTES

1. Sven Thomasen, Affiliated Consultant, Wiss, Janney, & Elstner, Associates, Inc.

2. International Conference of Building Officers, Uniform Building Code 1979 Edition, Whittier, California.

3. State of California Administrative Code Title 24, Part 8, State Historical Building Code, State of Califomia Documents Section, North Highland, CA.

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