B09_A1_Attach_18_Seismic_Eval_Degenkolb_05_10_10_0001.PDF

Degenkolb Engineers

235 Montgomery Street San Francisco, California 94104-2908

392.6952 phone

981.3157 fax 41

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THE AHWAHNEE COMPREHENSIVE REHABILITATION PLAN

The Ahwahnee

Yosemite National Park

Seismic Evaluation and Rehabilitation Alternatives

CIF Project 4714

PMIS 1549103

May 10, 2010 Degenkolb Job Number A9138006.00

SEISMIC EVALUATION AND REHABILITATION ALTERNATIVES

Contents

Executive Summary ........................................................................................... vi

1.0 Introduction

2.0 Project Information

2.1 Building Descriptions

2.1.1 Main Building

2.1.2 Guest Cottages

2.1.3 Employee Dormitory

2.2 Previous Studies

2.3 Project Scope and Intent

2.4 Documents Reviewed

3.0 Evaluation Procedures

3.1 Evaluation and Rehabilitation Methodologies

3.2 Performance Levels

3.2.1 Life Safety

3.2.2 Collapse Prevention

3.2.3 Immediate Occupancy

3.3 Seismic Hazard Level

3.4 Material Testing and Site Investigations

3.4.1 Site Investigations

3.4.2 Geotechnical Engineering Study

3.4.3 Forensic Investigation

3.5 Detailed Evaluation Methods

4.0 Evaluation Results

4.1 Seismic Evaluation

4.1.1 Main Building Seismic Evaluation

4.1.2 Dining Room Seismic Evaluation

4.1.3 Kitchen Seismic Evaluation

4.1.4 Entry Gallery Seismic Evaluation

4.1.5 Porte Cochere Seismic Evaluation

4.1.6 Guest Cottages Seismic Evaluation

4.1.7 Employee Dormitory Seismic Evaluation

4.1.8 Expected Performance in BSE-1 Event

4.1.9 Expected Performance in BSE-2 Event

4.2 Kitchen Floor Deterioration

4.3 Dining Room Roof Snow Loads

5.0 Conceptual Seismic Strengthening Alternatives

5.1 Alternative 1 – Minimum Code Requirements

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5.1.1 Mitigation Work

5.1.2 Expected Performance in BSE-1 Event

5.1.3 Expected Performance in BSE-2 Event

5.2 Alternative 2 – Additional Measures for Historic Preservation

5.2.1 Mitigation Work

5.2.2 Expected Performance in BSE-1 Event

5.2.3 Expected Performance in BSE-2 Event

5.3 Alternative 3 – Mitigating Liquefaction-Induced Settlement

5.3.1 Mitigation Work

5.3.2 Expected Performance in BSE-1 Event

5.3.3 Expected Performance in BSE-2 Event

5.4 Alternative 4 – Base Isolation for Immediate Occupancy

5.4.1 Mitigation Work

5.4.2 Expected Performance in BSE-1 Event

5.4.3 Expected Performance in BSE-2 Event

6.0 Peer Review Process

7.0 Conclusions

8.0 Figures

9.0 References

Appendix A Ground Motion Scaling

Appendix B Detailed Evaluation Methods

Appendix C Main Building Evaluation Results

Appendix D Dining Room Evaluation Results

Appendix E Kitchen Evaluation Results

Appendix F Porte Cochere Evaluation Results

Appendix G Entry Gallery Evaluation Results

Appendix H Employee Dormitory Evaluation Results

Appendix I Guest Cottages Evaluation Results

Appendix J Peer Review Correspondence

Appendix K Treadwell & Rollo Geotechnical Investigation

Appendix L Plant Construction Company Forensic Exploration and Materials Testing Report

List of Figures Figure 1.1 – The Ahwahnee Site Plan

Figure 1.2 – Mitigation Alternative Matrix

Figure 2.1 – Key Plan of The Ahwahnee

Figure 2.2 – Construction Photograph of the South Wing of the Hotel: Courtesy of National Park Services

Figure 2.3 – Southwest Exterior of Hotel

Figure 2.4 – Granite Stone Columns outside the Great Lounge

Figure 2.5 – South Wall of Dining Room

Figure 2.6 – Porte Cochere

Figure 2.7 – Employee Dormitory

Figure 3.1 – BSE-1 Response Spectrum

Figure 3.2 – BSE-2 Response Spectrum

Figure 3.3 – Dining Room Truss Connection to North Wall

Figure 3.4 – Gypsum Block Walls Adjacent to Great Lounge Fireplace

Figure 3.5 – Employee Dormitory Crawl Space

Figure 3.6 – Hotel Ceiling Above Mezzanine

Figure 3.7 – Kitchen Floor Slab Deterioration

Figure 3.8 – Granite Mortar Core Displaying Excellent Bond to Granite

Figure 3.9 – Void Found During Borescope Exploration of Granite Masonry

Figure 3.10a – Analysis Model: Main Building

Figure 3.10b – Analysis Model: Main Building

Figure 3.10c – Analysis Model: Main Building

Figure 4.1 – Drift Results Summary iv May 10, 2010 p:\project.a09\138\a9138006.00\reports\seismic evaluation and rehabilitation report\100510rpt ahwahnee hotel comprehensive v May 10, 2010 p:\project.a09\138\a9138006.00\reports\seismic evaluation and rehabilitation report\100510rpt ahwahnee hotel comprehensive

Design Team/Acknowledgements

Owner: National Park Service

Don Neubacher, Yosemite National Park Superintendent

David V. Uberuaga, Former Acting Superintendent (January 2009 – March 2010)

Structural Engineer: Degenkolb Engineers

Chris Poland S.E., M.ASCE, NAE, Principal in Charge

Loring Wyllie S.E., Dist.M.ASCE, NAE, Project Principal

Cale Ash S.E., Project Engineer

Jennifer Ahlport P.E., Designer

Architect: Hornberger + Worstell

Mark Hornberger FAIA, Principal

John Davis AIA, Project Manager

Jim Degener, Project Architect

Stephen Li, Job Captain

Structural Peer Review:

David Bonowitz S.E.

Martin Button PhD, P.E.

EXECUTIVE SUMMARY

AHWAHNEE HOTEL COMPREHENSIVE REHABILITATION PLAN

Executive Summary Project Scope and Intent

The Ahwahnee is a National Historic Landmark and its design features and use of structural materials has endured over its first 83 years in operation. That it was built primarily with structural steel and concrete, rather than timber and masonry, represents a significant advantage for both its longevity and its seismic performance. Public opinion of National Park stewardship has been evolving since the formation of the National Park Service in 1916. Preservation and protection of the current resource represents one path toward stewardship of The Ahwahnee for future generations.

As part of its commitment to seismic safety and historic preservation, the National Park Service has commissioned other evaluations over the past ten years to understand the expected building performance in a seismic event. Previous studies completed by Martin/Martin Consulting Engineers in 1999 and by URS Corporation in 2002 identified a number of significant deficiencies that caused concerns about the safety of people in and around the buildings in the event of a major earthquake. The current evaluation expands on these studies by utilizing the most recent displacement-based advanced analysis techniques to best understand the inherent strengths and weaknesses in The Ahwahnee. These evaluation results have been related to expected seismic performance from a standpoint of both life safety and damage to historic features of the hotel.

This peer-reviewed seismic evaluation and development of rehabilitation alternatives expands upon the previous studies by using the most recent national codes and standards and differentiates between those mitigation measures required for code compliance from those that are opportunities for protecting the resource and operational capability. They are based on detailed forensic investigations and advanced analysis techniques to estimate the behavior of the existing building and assess likely damage resulting from earthquake ground shaking. The damage that is expected to occur during a major earthquake was used to form the basis for the required code mitigation and the additional opportunities for reducing the expected damage. A total of four conceptual design rehabilitation alternatives are presented, with Alternative 1 representing code-required mitigation. Beyond that, Alternatives 2, 3, and 4 are ordered sequentially with regards to increasing seismic performance and decreasing damage to historic features and post-earthquake downtime. In addition to the seismic evaluation and development of conceptual seismic strengthening alternatives, both the deteriorated Kitchen floor slab and the Dining Room trusses were assessed relative to their current condition and ability to resist required gravity loads.

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The forensic investigation included four distinct activities aimed at determining the best possible understanding of conditions at the site. They included site investigations of the as-built conditions by the engineering teams, a comprehensive and site specific evaluation of the seismic hazard, a geotechnical investigation of the site that included additional borings, and a forensic testing program to understand the quality of the construction and strength of the in place materials.

Evaluation Results

For evaluation purposes, The Ahwahnee is divided into several individual structures that are expected to respond independently in a seismic event. The main hotel is comprised of the Main Building, Kitchen, and Dining Room. Adjacent to the hotel are the Entry Gallery and Porte Cochere. Separate structures include the employee Dormitory and Guest Cottages.

The Main Building and the Kitchen generally meet code-required seismic performance though there are a number of falling hazards that could cause casualties - serious injuries or death – involving people in and around the building. The Dining Room does not meet code requirements as it may separate from the adjacent Main Building and Kitchen and may dislodge stones from the granite veneer and/or shatter glazing in the windows. Either occurrence could injure building occupants; however, the Dining Room is expected to remain standing during the largest expected earthquake at the site.

The Entry Gallery and Porte Cochere are timber framed structures that are independent of each other and combine to serve as primary egress paths. As a result, the performance of both is important in maintaining access for emergency personnel. Both the Entry Gallery and the Porte Cochere are expected to remain standing during the largest expected earthquake with only moderate damage and thus meet code requirements for seismic performance.

The eight Guest Cottages are similar in construction to small wood-framed houses with masonry chimneys. The Guest Cottages are expected to perform well in an earthquake;

however, the masonry chimneys may shake independently of the roof and could result in partial collapse. As a result, the Guest Cottages do not meet minimum code requirements for life safety performance.

The employee Dormitory is a one story wood framed building that was originally intended to be a temporary structure. As a result it was not constructed with a complete perimeter concrete foundation and the sill plates in contact with the soil have experienced decay and rot. This deterioration combined with the cripple wall configuration could result in partial collapse from a seismic ground shaking. The building location is very near to the active talus slope geohazard zone and may be impacted by earthquake-induced rock falls. As a result of the collapse potential and rock fall hazard, the employee Dormitory does not meet code requirements.

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The kitchen floor slab has experienced significant deterioration due to regular wash water intrusion. Based on observed slab deterioration, the kitchen floor slab represents a safety risk as it will continue to degrade any may experience excessive deflections and local collapse. Temporary shoring or repair is recommended to prevent this from occurring in the short term. Additional forensic survey to determine extent and severity of damage, and appropriate long term repair are recommended.

The Dining Room roof trusses were studied in 2003 to determine their capacity to support expected snow loads. This report concluded a 35 pound per square foot (psf) capacity, which is less than the 60 psf required by Mariposa County. The roof has performed adequately over the life of the structure and has passed the “test of time” as permitted by the Historic Building Code. However, modification that would serve to increase the load would invalidate this test and should not be done without strengthening the trusses.

Conceptual Seismic Strengthening Alternatives

While the mitigation required to meet minimum code requirements is not extensive, additional seismic strengthening and rehabilitation opportunities exist and are presented with the goal of both reducing damage to historic features of the hotel and improving post-earthquake recovery time. Refer to Section 5.0 for detailed descriptions of the strengthening work required for each alternative and Table 1.2 for a matrix summarizing the expected performance and operational downtime associated with each alternative.

Alternative 1 – Minimum Code Requirements

Alternative 1 represents the minimum level of seismic strengthening required per code.

The work includes stabilizing the Dining Room roof with a partial braced frame and diaphragm from the Kitchen, repairing or replacing the Kitchen floor slab, securing granite stone veneer above exit paths, bracing chimneys in the main building and Guest Cottages, bracing selected interior walls, adding film to large windows, and constructing a new foundation for the employee Dormitory. The main hotel, Guest Cottages, and employee Dormitory will meet the minimum code requirements for the seismic performance of existing buildings after this work has been completed and the people in and around the buildings are not expected to be subject to serious injury or death. The hotel may still experience damage to the Kitchen structure, historic finishes, and mechanical systems that may require closure of the hotel to complete repairs.

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Alternative 2 – Additional Measures for Historic Preservation In addition to the code-required mitigation actions included in Alternative 1, the mitigation actions contained in Alternative 2 will provide additional protection of the historic features of the hotel and will allow the hotel to return to operation sooner after a damaging seismic event. The work includes bracing the Kitchen roof with a full braced frame and diaphragm from the Kitchen, adding concrete shear walls at selected interior locations in the main hotel, securing granite stone veneer throughout, and bracing all equipment and systems needed for hotel operations. Mitigation of liquefaction-induced settlement risk is not addressed in Alternative 2.

Alternative 3 – Mitigating Liquefaction-Induced Settlement Liquefaction-induced settlement is predicted in the largest expected level of ground shaking at the site and will result in damage and cracking to historic features of the hotel.

In addition to the strengthening presented in Alternatives 1 and 2, liquefaction-induced settlement risk can be mitigated by constructing a new foundation system comprised of new piles and grade beams for the main building. This will allow the hotel to return to operation sooner after a damaging seismic event.

Alternative 4 – Base Isolation for Immediate Occupancy Alternative 4 is intended to provide Immediate Occupancy performance in The Ahwahnee and allow the hotel to return to operation within days after an earthquake although other damage or infrastructure failure within Yosemite Valley may preclude an immediate resumption of hotel operations. The work includes adding a new foundation system that includes a complete network of isolators located within the existing crawl space of the main building.

1.0: INTRODUCTION

1.0 Introduction

This report presents the Seismic Evaluation and Rehabilitation Alternatives in support of The Ahwahnee Comprehensive Rehabilitation Plan. The Ahwahnee consists of the Hotel, Porte Cochere, Entry Gallery, employee Dormitory, and eight Guest Cottage Buildings containing a total of 24 guest accommodations. See Figure 1.1 for The Ahwahnee site plan. The results of the seismic evaluation are included in this report, along with four rehabilitation alternatives addressing seismic deficiencies in The Ahwahnee. The report also summarizes the distinct forensic investigation activities undertaken to understand the existing conditions at the site.

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2.0: PROJECT INFORMATION

2.0 Project Information

2.1 Building Descriptions

The Ahwahnee is located at the eastern end of Yosemite Valley. The hotel started construction in August, 1926, and finished construction in July, 1927. Throughout its 83-year history the building has undergone changes, but the primary structural system of the building has remained unchanged. Some of these changes include relocating the Porte Cochere shortly after construction began from where The Ahwahnee Bar currently resides to the current location at the north end of the building, rebuilding the Entry Gallery in 1990, and re-roofing the hotel in 1990 and 2005.

2.1.1 Main Building

The hotel is Y shaped in plan, as seen in Figure 2.1 (see Section 8.0), with each wing radiating from the center at a 60 degree angle relative to the other wings. The building is approximately 40,000 square feet in plan. Although there are no seismic separations between the sections of The Ahwahnee, three separate portions of the hotel were identified as having unique structural systems: the Main Building, the Dining Room, and the Kitchen. The Main Building and the Kitchen share construction materials and methods, but are only physically connected through a single concrete wall and stairway.

The Dining Room separates these two structures, and is built primarily out of wood instead of concrete.

The Main Building consists of eight total floors including a partial mezzanine above the first floor and a mechanical penthouse for elevator equipment at the top of the building.

Above the second floor there are various setbacks in the plan size such that, when viewed in elevation, the hotel appears to mimic a geological form with balconies analogous to cliffs and drop offs. The mezzanine covers much of the main hotel floor plan except the Great Lounge, Solarium, Dining Room, and southern half of the Kitchen. There is a partial basement housing mechanical equipment underneath the main tower section of the building and crawl spaces approximately 4 to 6 feet deep underneath the rest of the building. The first floor and mezzanine levels are a mix of public, back of house, and guest room areas, and the guest rooms and back of house support occupy the spaces on the floors above.

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The hotel was built with a full steel frame which was then partially encased in concrete;

Figure 2.2 is a photograph taken during construction which shows the steel frame before the concrete was added. The amount of concrete covering the steel frame varies from fully encased columns in a box-like section to a light spray-applied coating to serve as fire proofing. As seen in Figure 2.3, the hotel was built to look like it was made of wood;

on the exterior of the building, rough formwork was used to give the concrete the texture of wood and the concrete was stained to take on the appearance of wood. Concrete walls occur at some locations both around the perimeter and in the interior of the building to provide resistance to lateral loads. Granite stone columns, shown in Figure 2.4, are present at many key locations in the building: the Great Lounge, the Solarium, the Dining Room, and The Ahwahnee Bar. These exterior columns consist of approximately a 24 inch thickness of granite stones surrounding either a central column of reinforced concrete, concrete-encased steel, or, in the case of the Dining Room columns, a central void. This same granite stone masonry was used as a veneer on many of the exterior walls of the building. This veneer ranges from 15 to 18 inches in thickness.

The foundations of the entire hotel, Main Building, Kitchen, and Dining Room, are connected by a 12 inch thick concrete perimeter wall around a 4 foot to 6 foot deep crawl space. The perimeter wall is supported by a grade beam. The columns are supported by spread footings. An excavation and review of one of the footings in the Dining Room was conducted by BSK in 2003. From the information gathered in this report, the information shown on the drawings and the information gathered on the site visit, approximations of the footing sizes and make up were determined.

The floors are constructed of 2 ½ inch thick concrete slabs on wire mesh that are supported by open web joists spaced at 24 inches on center. The roof is made up of a 2-inch thick concrete slab poured against a wire mesh. This slab is covered by plywood sheathing and slate roofing.

The Dining Room is a one story structure in the West wing of the “Y” plan and shares columns with the main building along the east wall. It was constructed of sugar pine logs and is approximately 140 feet long and 50 feet wide. While the columns at the east end of the Dining Room are similar in construction to those elsewhere in the Main Building, the columns at the west end of the Dining Room are hollow. The south wall, shown in Figure

2.5 has double columns spaced at 15 ½ feet on center throughout the length of the side.

The north wall consists of single wood columns spaced to match the columns on the south wall, and a 6 inch thick concrete shear wall that is shared with the Kitchen. The connection between the concrete shear wall and the wood columns is not clear. The columns are connected by wooden trusses in the transverse direction, which provide lateral support in that direction.

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The Kitchen is a two story structure with only a partial mezzanine and is approximately 110 feet long and 90 feet wide. It was built with the same construction means and materials as the main building. The center half of the Kitchen has a steel truss-supported clerestory roof higher than the eastern and western portions. The roof consists of a 2 inch thick concrete slab poured against metal lath, which is then covered by plywood and slate roofing to match the rest of the hotel. The higher roof concrete diaphragm is then connected to the lower concrete diaphragms through 6 inch thick concrete walls at the perimeter of the clerestory.

The Porte Cochere, shown in Figure 2.6, is located in the Northwest corner of The Ahwahnee. The ground floor of the East wing, where The Ahwahnee Bar currently resides, was originally planned to be the location of the Porte Cochere, but it was relocated shortly after construction began. It is a one story open framed building with a hipped roof, granite stone column at each corner that support the hipped roof, and wood columns on the north and south sides. The granite stone columns serve as buttresses to resist lateral forces.

The Entry Gallery connects the Porte Cochere with the Main Building. It is an open frame structure. The west side of the gallery is adjacent to the gift shop portion of the Main Building. Wood columns are spaced every 15 feet on center and connected in the East-West direction by wood trusses. Lateral support is provided in the North-South direction through knee-type diagonal braces at the top of each column. The Porte Cochere and Entry Gallery share columns at their intersection but, as discussed in Section 4.1, respond seismically as separate structures as their diaphragms are at different elevations.

The Porte Cochere and the Entry Gallery were reroofed in 1993 by RSE when the existing 2x tongue and groove sheathing was reinforced with ¾ inch thick plywood. This plywood was covered with slate roofing to match the rest of the hotel.

2.1.2 Guest Cottages

The Guest Cottages were built in 1928 and are located southeast of The Ahwahnee. There are a total of eight Guest Cottages with three different floor plan arrangements: five two-room, one four-room, and two five-room. They are all wood single story buildings with shallow concrete foundations and contain construction details similar to a wood-framed residential house. The two room and four room configurations are rectangular, and the five room configuration is shaped like an H in plan with stone chimneys.

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2.1.3 Employee Dormitory

The employee Dormitory, shown in Figure 2.7, was built during World War II when The Ahwahnee was temporarily changed from a hotel into a hospital for war veterans by the US Navy, and is located northwest of the hotel. It is a one story wood frame building, 34 feet by 160 feet in plan. The floor and roof diaphragms are made up of diagonal sheathing and the perimeter cripple wall is sheathed with straight sheathing. The employee Dormitory has small concrete foundations at only a few locations and lacks a continuous concrete perimeter foundation. The wood will plates are in contact with the soil and have experienced significant rot and decay.

2.2 Previous Studies

The Ahwahnee has been the subject of two other detailed seismic evaluations over the past ten years. Previous studies completed by Martin/Martin Consulting Engineers in 1999 and by URS Corporation in 2002 identified a number of significant deficiencies that caused concerns about the safety of people in and around the building in the event of a major earthquake. In the interest of public safety and protection of this national resource, seismic mitigation was made one of the cornerstones of the Comprehensive Rehabilitation Plan.

The current seismic evaluation and development of rehabilitation alternatives expands upon the previous studies by using the most recent national codes and standards. It differentiates between those mitigation measures required by code and those that are opportunities for improving the seismic performance of the building and further protecting historic features and finishes. As such, this report expands on the work presented in the Martin/Martin and URS reports both by implementing their recommendations for site specific geotechnical engineering studies and a forensic investigation program and by using the more advanced analytical techniques available in the most recent national standards.

2.3 Project Scope and Intent

The structural scope of the Comprehensive Rehabilitation Plan consists of a seismic code analysis, detailed seismic evaluation, peer review, and development of conceptual structural strengthening alternatives for The Ahwahnee. The detailed seismic evaluation uses current national standards to best understand the behavior and performance of the existing building and assess likely damage from earthquake ground shaking. Likely damage states are then compared against limit states for existing buildings and mitigation options were developed to meet these standards. In addition to the seismic evaluation both the deteriorated Kitchen floor slab and Dining Room trusses were assessed relative to their current condition.

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A total of four conceptual design rehabilitation alternatives have been developed for seismic strengthening of The Ahwahnee. All alternatives are presented in this report.

Alternative 1 is the minimum rehabilitation required to meet Federal standards for life safety and collapse prevention. Alternatives 2, 3, and 4 represent increased seismic performance and improved protection of historic features. All conceptual seismic strengthening alternatives are described in more detail in Section 5.0.

2.4 Documents Reviewed

1. Historic Structures Report (Page + Turnbull, 1997)

2. FEMA 178 – Seismic Evaluation Report, The Ahwahnee Hotel, Final (Martin/Martin, 1999)

3. Seismic Rehabilitation Alternatives for The Ahwahnee Hotel. NPS Seismic Safety Program (Includes Earthquake-Related Geological and Geotechnical Hazards Assessment, URS, 2002)

4. Geotechnical Investigation Ahwahnee Hotel (Treadwell & Rollo, 2009)

5. Sense of Place: Design Guidelines for Yosemite Valley (NPS, 2005)

6. ARG Outline for Historic Structures (ARG, 2009)

7. Original Historic Architectural drawings of Ahwahnee Hotel (Underwood, 1926)

8. Documenta Survey Drawings: “As-Found” Architectural Documentation (Documenta, 2005/2006)

9. Photos of Drawings for Ahwahnee stored in El Portal Archives

10. Historic Construction Photographs (1926-1927)

11. Original Historic Architectural Drawings: Cottages (E.T. Spencer, 1928)

12. Original Historic Architectural Drawings: Dormitory (U.S. Navy)

13. Original Historic Architectural Drawings: Indian Bar (ANCO, 2000)

14. Original Historic Architectural Drawings: Porte Cochere and Walkway Reroofing

(RSE, 1993)

15. Original Historic Architectural Drawings: Hotel Reroofing (RSE, 1990)

16. Exterior Restoration of the Ahwahnee Hotel Drawings (W.M. Sontheimer, 1977)

17. Foundation Review: Ahwahnee Hotel Dining Room (BSK Assoc, 2003)

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7 May 10, 2010 p:\project.a09\138\a9138006.00\reports\seismic evaluation and rehabilitation report\100510rpt ahwahnee hotel comprehensive mite National Park (RJA/Siemens, 2001) al tures Report Draft (Architectural Resources Group, 2009)

23. The Ahwahnee – Forensic Exploration and Materials Testing (Plant Construction Company, 2009)

18. Geotechnical Report: Yosemite Valley Integrated Utility Project Phase 3B (Kleinfelder, 2009)

19. Report on Life Safety Conditions and Fire Detection and Alarm Systems Installed at the Ahwahnee Hotel, Yose

20. Ahwahnee Hotel Dorm Foundation Design CIF Project 4811 – Yosemite Nation Park (Waring Group, 2009)

21. Proposed New Foundations for Existing Dormitory (Geotechnical Engineering Investigation Report, Moore Twining Associates, 2009)

22. 75% Historic Struc

3.0 EVALUATION PROCEDURES

3.0 Evaluation Procedures

3.1 Evaluation and Rehabilitation Methodologies

Requirements for seismic evaluation and seismic rehabilitation of The Ahwahnee are governed by RP6, ICSSC Standards of Seismic Safety for Existing Federally Owned and Leased Buildings. All existing Federal buildings undergoing repairs or alterations that total more than 30% of the building replacement value must be seismically evaluated and rehabilitated per the provisions of RP6 which requires a minimum seismic performance of Life Safety Performance in the BSE-1 event and Collapse Prevention Performance in the BSE-2 event. Refer to Section 3.3 for definitions of the BSE-1 and BSE-2 seismic events.

RP6 references Federal Emergency Management Agency (FEMA) Handbooks 310 and 356 for the Seismic Evaluation of Buildings and Seismic Rehabilitation of Buildings, respectively. These FEMA Handbooks have been superseded by consensus documents from the American Society of Civil Engineers (ASCE). ASCE 31 replaces FEMA 310 and ASCE 41 replaces FEMA 356.

The methodology used to evaluate The Ahwahnee is based on the American Society of Civil Engineers Standard of Seismic Evaluation of Existing Buildings (ASCE 31), which employs a three-tiered approach to seismic evaluation. Tier 1 is a checklist-based evaluation phase intended to quickly identify seismic deficiencies common to a given construction material and building class. At the end of a Tier 1 evaluation, deficiencies and recommendations for additional studies are presented. The next phase, Tier 2, consists of a more-detailed evaluation approach where a complete computer model of the building is used to refine the results obtained during a Tier 1 study. Results from Tier 2 can be used to verify or discount Tier 1 findings and are frequently the basis of a seismic strengthening approach. A Tier 3 evaluation may be used where additional analysis may be beneficial in reducing the seismic strengthening scope of work. Certain buildings may be adequately analyzed by only a Tier 1 or Tier 2 evaluation while more complex buildings may be benefitted by a Tier 3 evaluation.

For comparison, the Martin/Martin evaluation roughly corresponds to a Tier 1 evaluation while the URS evaluation roughly corresponds to a Tier 2 evaluation. The current evaluation corresponds to a Tier 3 evaluation and uses the procedures of the American Society of Civil Engineers – Standard for Seismic Rehabilitation of Existing Buildings (ASCE 41). All rehabilitation alternatives also follow the procedures of ASCE 41, which is consistent with the intent of RP6 requirements.

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Federal RP6 requirements for seismic performance of existing buildings are more stringent than those in the California Building Code (CBC) and the California Historic Building Code (CHBC) and must be followed for The Ahwahnee Comprehensive Rehabilitation Plan. However, since RP6 does not address modifications to gravity elements, the CBC and CHBC will be used for gravity load evaluation of existing structural element. The National Park Service decision to adopt the California state codes is addressed in a memo dated January 13, 2010.

3.2 Performance Levels

ASCE 41 defines the building performance level as a limiting damage state for the building, considering structural and nonstructural components, used in the definition of the rehabilitation objective. The expected performance of The Ahwahnee has generally been compared to two discrete performance levels defined by ASCE 41 and as described below. The actual expected building performance during a particular earthquake depends on the magnitude and duration of shaking that occurs at The Ahwahnee site. This report attempts to communicate expected building performance in two distinct earthquake ground shaking levels described in Section 3.3.

3.2.1 Life Safety

The Life Safety Performance Level is the minimum required in the BSE-1 event and results in damage to structural components but retains a margin against onset of partial or total collapse. There may be severe damage to some structural elements and to nonstructural components, but this has not resulted in large falling debris hazards within or outside the building. The overall risk of life-threatening injury is expected to be low, although injuries may occur during the earthquake. Egress routes within the building are not extensively blocked, but may be impaired by debris. Damage to HVAC and plumbing systems may result in loss of function and local flooding due to ruptured piping. Repair of the structure should be possible but may not be economically feasible, and restoration of the nonstructural components may take extensive effort.

3.2.2 Collapse Prevention

The Collapse Prevention Performance Level is the minimum required in the BSE-2 event and results in damage to the building structure in which the building is on the verge of partial or total collapse. There is widespread and substantial damage to the building structural and nonstructural components. Although the building is not expected to collapse from the primary shock; aftershocks could induce collapse. Building occupants experience significant risk of injury due to falling debris. Egress paths will likely be blocked and some occupants may require rescue. The structure may not be technically practical to repair and will not be safe to reoccupy.

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3.2.3 Immediate Occupancy

The Immediate Occupancy Performance Level exceeds RP6 minimum standards for The Ahwahnee and has limited damage to structural components and the building remains safe to occupy. The strength and stiffness of the lateral-force-resisting system have been retained and there is little to no permanent lateral deformation of the structure. The capacity of the structure to support vertical loads has not been impaired. Nonstructural components may be damaged, but building access and life safety systems such as doors, stairways, elevators, emergency lighting, fire alarms, and fire suppression systems generally remain operable.

3.3 Seismic Hazard Level

The minimum seismic performance level for The Ahwahnee, as required by RP6, is the Basic Safety Objective (BSO) per ASCE 41. The BSO contains the dual performance criteria of Life Safety in the Basic Safety Earthquake 1 (BSE-1) and Collapse Prevention in the Basic Safety Earthquake 2 (BSE-2). The BSE-1 event has a 10% chance of exceedance in 50 years and a return period of approximately 500 years. The BSE-2 earthquake, also known as the Maximum Considered Earthquake (MCE) has a 2% chance of exceedance in 50 years and a return period of approximately 2,500 years. The 2007 California Building Code (CBC) uses the BSE-1 as the basis for seismic design of new buildings.

The Ahwahnee is located in a region of the Sierra Nevada characterized by moderate seismicity with expected seismic ground motions much less than coastal California. The nearest faults are the Hartley Springs and Mono Lakes faults, each approximately 30 miles from the site and capable of Moment Magnitude 6.5 earthquakes. Other possible seismic sources, their magnitudes, and distances from The Ahwahnee are presented in the Treadwell & Rollo report.

Treadwell & Rollo also selected seven earthquake ground motion records for use in the nonlinear dynamic analyses performed in accordance with ASCE 41. The ground motions were selected from sites with similar soil conditions and fault distances as The Ahwahnee and include six California earthquake records and one Japanese earthquake record. The California records include two each from Loma Prieta (1989) and Northridge (1994), and one each from Imperial Valley (1979) and Superstition Hills (1987). The Japanese record is from the 1995 Kobe earthquake. Each earthquake record consists of a pair of acceleration values representing ground shaking in two orthogonal directions. The entire suite of seven ground motion pairs was scaled according to ASCE 41 procedures to match the site-specific response spectra for each of the BSE-1 and BSE-2 hazard levels as shown in Figures 3.1 and 3.2, respectively. Additional information on the ground motion scaling procedures is presented in Appendix A.

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3.4 Material Testing and Site Investigations

The following sections summarize the site and forensic activities performed as part of the Comprehensive Rehabilitation Plan in an attempt to best understand the characteristics of the existing building configuration, the geological setting and seismic hazard, and the hotel construction materials and techniques. The results and findings were used to develop conceptual seismic strengthening alternatives that take advantage of the strengths inherent in The Ahwahnee while quantifying the expected damage to historic features.

3.4.1 Site Investigations

Limited documentation exists regarding the specific details of the structural systems present within The Ahwahnee. The available 1926 Underwood drawing show general sizes of existing structural elements but do not fully detail the structural steel framing or the concrete and its reinforcing. Original structural drawings were no longer available. In order to understand the as-constructed conditions, several site investigations were conducted in the summer and fall of 2009.

The initial site investigations were performed to understand the current conditions present in the hotel and included gaining access to crawl spaces, attics, ceilings, mechanical rooms, and various public and private rooms within the hotel. Refer to Figures 3.3 – 3.6 for various conditions observed during the site investigations. In general, the hotel construction matches the available documentation. Specific details such as steel framing sizes, concrete thickness, reinforcing details, and foundation sizes could not be verified during the site investigation site visits. As a result, a forensic investigation program was implemented to determine more information about material properties and construction details. Refer to Section 3.4.3 for more details on the forensic investigation.

Several small construction projects have been implemented since the hotel opened in the 1920’s, however; the main structural systems have not been the subject of any significant restoration work or modification since construction was completed. Deterioration of concrete and structural steel framing was observed in several locations, primarily in and below the kitchen floor where repeated washing water has migrated through the elevated concrete slab and corroded the reinforcing steel. As a result, the concrete cover has spalled off the bottom of the slab and several locations were observed where patches were attempted as shown in Figure 3.7. The slab has deteriorated to the point that replacement may be the most feasible option. In addition to the slab deterioration, the ends of the roof eaves, overhangs, and cantilever brackets that supported outdoor light brackets were covered with fiberglass in 1977 due to deterioration.

As the various rehabilitation projects on The Ahwahnee move forward, additional on-site investigation and documentation will be necessary and become feasible as access is gained to currently concealed spaces. This additional information will become useful to further refine the construction details and verify assumptions that have been made during the current analysis.

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3.4.2 Geotechnical Engineering Study

A geotechnical engineering investigation of soils at The Ahwahnee was conducted by Treadwell & Rollo in support of The Ahwahnee Comprehensive Rehabilitation Plan. The resulting geotechnical report develops the site-specific response spectra and selects the earthquake time histories that are used as input in the nonlinear dynamic analyses developed later in this seismic evaluation report. Further discussion of the ground motion scaling procedures is provided in Appendix A and the geotechnical engineering report is provided in Appendix K.

An important component of the seismic evaluation was gaining an accurate understanding of the site geology and seismicity. Treadwell & Rollo completed a geotechnical engineering investigation in October 2009 that included three soil borings to assess soil conditions at The Ahwahnee. The borings were located around the perimeter of the hotel with one in the service yard, one west of the Dining Room, and one south of the Solarium.

Treadwell & Rollo utilized Next-Generation Attenuation relationships (NGA) in determining the site-specific seismic hazard. NGA relationships use the latest knowledge gleaned from recent earthquake ground motion recordings to improve engineering knowledge on the reduction or attenuation of earthquake ground motions relative to epicentral distance. In Yosemite Valley, use of NGA relationships serves to reduce expected seismic intensities by over 30% relative to seismic design values previously used for evaluation. The NGA design values are consistent with the values provided in the next edition of the CBC. Combining NGA relationships with a thorough understanding of site geological conditions was beneficial in improving the understanding of the expected seismic performance of The Ahwahnee relative to previous reports.

Previous geotechnical engineering studies reported the possibility of liquefaction-induced settlement at and around The Ahwahnee site. The three additional soil borings performed as part of the Treadwell & Rollo study confirmed the presence of liquefiable layers of saturated unconsolidated gravels and sands 30-60 feet below the current ground surface.

These layers were deposited as Lake Yosemite was filled with stream-carried debris, ultimately forming the current valley floor. Liquefaction-induced settlement is anticipated in the BSE 2 only and settlements are expected to range from 0” in the north service yard, to 3” near the elevators and 6” at the Solarium. No liquefaction-induced settlement is predicted for the BSE 1.

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3.4.3 Forensic Investigation

A forensic investigation program was conducted in October 2009 to quantify existing material strengths and determine construction details for seismically-susceptible building nonstructural elements. The forensic investigation was completed by Plant Construction Company and Technicon Engineering Services and a full report of their findings is provided in Appendix L.

Material strength tests were obtained for both concrete and granite mortar compressive strength testing. Concrete cores were taken throughout the hotel in both wall and slab locations to account for inherent variability in concrete placed at different times during the hotel construction. The concrete quality may be characterized as average and a mean compressive strength of 2600 psi (pounds per square inch) was obtained from the forensic test results. The mortar quality may be characterized as excellent and a mean compressive strength of 1350 psi was obtained from the forensic test results.

Cores were taken through mortar joints in the granite stone veneer both to obtain compressive strength specimens and to observe as-constructed conditions. A borescope was used to explore material conditions within the depth of the granite stone veneer and verify the interface between concrete backup wall and the granite stone veneer. Some voids were observed at the interface between mortar and granite stones but, in general, the mortar displayed excellent bond to the granite stones as shown in Figure 3.8. Voids were also observed at the interface between the granite stone veneer and the concrete backup wall for some of the core locations, however there was no indication that rain water had infiltrated this interface nor was any deterioration observed. See Figure 3.9 for a borescope image showing a void observed within the granite masonry.

3.5 Detailed Evaluation Methods

All of the buildings were evaluated initially using ASCE 31 Tier 1 procedures, which were supplemented with Tier 2 procedures as appropriate. The Tier 1 and 2 evaluation procedures were sufficient to identify and report on deficiencies for the Entry Gallery, Porte Cochere, employee Dormitory, and Guest Cottages. For the Main Building, an ASCE 31 Tier 3 evaluation was performed using the procedures available in ASCE 41.

The models created for the Main Building were analyzed using a suite of time history runs, described in more detail in Appendix A.

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The Main Building was analyzed with three-dimensional analysis software and two models were created to both explore the effects of the existing conditions of the building and to manage modeling uncertainty by bounding the analytical solution. In addition, an analysis model representing potential rehabilitation alternatives was also developed to validate the conceptual seismic strengthening measures. These models were used to simulate the lateral resistance of the building using different structural stiffness assumptions. These elements included the concrete shear walls at several locations around the perimeter of the building, granite stone columns and steel columns that were either encased within the concrete walls, or steel columns that were directly underneath discontinuous shear walls. Further information about the analysis methods used can be found in Appendix B.

The lower-bound model was used to simulate the minimum lateral resistance that was expected from the building. The upper-bound model increased the stiffness, and in turn the lateral resistance, of the building by including the granite stone veneer located around the perimeter of the building. A stiffer building will tend to attract higher forces, which in turn may produce larger accelerations within the building. Nonstructural damage is sensitive to both acceleration and drift so the upper bound model was used to estimate an approximate upper value of the accelerations to which the building contents may be subjected. The rehabilitation concepts model investigated the benefit of adding concrete shear walls within existing furred walls in four locations near the Great Lounge as discussed in Section 5.2.

Forensic testing was performed to determine appropriate concrete strengths as described in Section 3.4.3. Based on results from previous evaluations, it was assumed that the walls in the building would yield from the expected seismic demands of the building. As a result, the walls were modeled with nonlinear elements up to the second floor, and linear elements above this level. An exception to this were the spandrels underneath the guest room windows which were modeled as nonlinear frame elements to monitor the rotational demands imposed by adjacent shear wall piers.

In order to simplify the model, subcomponent models of the granite stone columns were evaluated using more detailed three-dimensional computer models.