Bldg 1365 Seismic Assessment Final_30 Dec 2005.pdf

Prepared By:

Building 1365

Seismic Assessment Report Travis Air Force Base, California

Prepared For:

TRAVIS AIR FORCE BASE

Contract No. F04626-03-D-0101 Delivery Order 5012 30 Dec 2005

Final Submittal

180 Grand Avenue, #400 Oakland, CA 94612 phone: 510-835-2761 fax: 510-835-9839

TSC Project No. P501050095

Seismic Assessment Report Building 1365, Travis Air Force Base, California

TOC

Table of Contents

I. Introduction A. Overview B. California Code of Regulations C. Documents Reviewed

II. Geological Seismic Hazards A. Site Location and Seismicity B. Liquefaction Potential Evaluation

III. Building Condition A. Building Construction and Earthquake Design B. Structural Conditions Noted

IV. Assessment of Possible Risk A. Seismic Risks B. Interpreting Probable Maximum Loss (PML)

V. Summary

Appendix A: Original Construction Drawings

Appendix B: Photographs

Appendix C: Structural Risk Calculations

Appendix D: Seismic Upgrade Concept Plan

Appendix E: Conceptual Cost Estimate

I. Introduction

A. Overview

The objective of this study is to provide a seismic assessment of the existing Building 1365 located at Travis Air Force Base, California, in accordance with California Code of Regulations, Title 22, Section 66264.31, which includes the following:

Perform seismic assessment to determine facility performance during and following a maximum credible earthquake.

Perform assessment of structural building elements.

Perform assessment of geological risk.

Prepare report to summarize the findings from the assessment, including identifying the geological earthquake hazards affecting the site, description of the present structural system, and calculating the probable maximum loss (PML).

Provide recommended corrective actions if facility poses high seismic risk.

Provide conceptual cost estimate for seismic upgrade where recommended.

This review is cursory in nature and focuses on identifying high-risk items. Therefore, the review process is aimed at checking the existence of various important building systems and features rather than providing an in-depth analysis of detailed design.

The seismic assessment report is based primarily on the design information shown in original construction documents since the structural members are often covered by other building materials and some structural hardware such as reinforcement is typically not visible. Selective field measurement and investigation of the structural system were performed to confirm that construction conforms to the drawings and assess condition of any major modifications or additions.

B. California Code of Regulations

California Code of Regulations, Title 22, section 66264.31 requires facilities to be located, designed, constructed, maintained, and operated to minimize the possibility of a fire, explosion, or any unplanned sudden or non-sudden release of hazardous waste or hazardous waste constituents to air, soil, or surface water which could threaten human health of the environment. This includes a seismic assessment to determine facility performance during and following a maximum credible earthquake. The analysis must include an assessment of facility soil types, footing design, and such details as the connections between the building foundation and frame.

C. Documents Reviewed

As a part of this review, the following information regarding the building and the design professionals of record was provided to TranSystems by Travis AFB CE:

Original construction drawings of Building 1365, Multi-Cubicle Magazine, Sheets 1 through 10, dated 5 April, 1962, prepared by George E. Goodall Co.

Consulting Engineers. A copy of the original construction drawings reviewed are included as Appendix A.

Other than those specifically described above, the following items were unavailable to TranSystems for review:

Design calculations.

Project manual (specifications).

Modification or addition drawings other than those listed above, specifically including any drawings, calculations, or other information concerning the steel frame addition of east and west vestibules and roof over all.

Based on a cursory review of the above documents that were available, the quality of the structural design appears to be consistent with the state of practice at the time of design and the use of the building.

II. Geological Seismic Hazards

A. Site Location and Seismicity

Building 1365 is located in a flat region within the northeast portion of Travis Air Force Base, in Fairfield, California. There are a number of active seismic faults in this region.

The site is located approximately 10 miles northeast of the Concord-Green Valley Fault and 17 miles northeast of the West Napa Fault, approximately 32 miles northeast of the Hayward Fault, approximately 42 miles north of the Calaveras Fault, and approximately 52 miles northeast of the San Andreas Fault. All of these faults are seismically active, with small earthquakes occurring frequently. In addition, numerous significant earthquakes have occurred in the past. These faults have the potential to generate very large earthquakes.

This report considers three earthquake scenarios: Maximum Probable Earthquake (MPE) (50% chance of exceedance in 50 years or 73 years average return period), Design Basis Earthquake (DBE) (10% chance of exceedance in 50 years or 475 years average return period), and Maximum Credible Earthquake (MCE) (2% chance of exceedance in 50 years or 2,475 years average return period).

The United States Geological Survey (USGS), through its World Wide Web site, gives estimates of peak ground accelerations (PGA) for bedrock at this site for the DBE and for the MCE. Similarly, the Web site gives values for the 0.2-second and 0.3-second Spectral Acceleration, which are typically similarly numbers. The average of the 0.2-second and 0.3-second Spectral Accelerations are used as Effective Peak Ground Acceleration (EPGA). Estimates for the MPE are not given. Values for the MPE have been extrapolated from the values given on the site for use in this report. It should be noted that the MPE probability is well outside the range of probabilities estimated on the Web site, so that the extrapolated values are of limited reliability. The model for Probable Maximum Loss (PML) used in this report is based on the PGA from the 1996 data provided by the USGS. The values for DBE and MCE and the extrapolated values for MPE are:

MCE DBE MPE

PGA: 0.91g 0.48g 0.06g EPGA: 1.92g 1.12g 0.50g

Information furnished by the government included an extract of a deterministic study of a nearby site. Deterministic values are different from the probabilistic values used in this report. They are less relevant to projecting maximum damage to be expected in a major earthquake and therefore was not used.

The site is not located within an Alquist-Priolo Special Studies Zone. There are no known active or inactive faults crossing the site as mapped and/or recognized by the State of California. The risk of fault rupture occurring at the site in an earthquake is low.

B. Liquefaction Potential Evaluation

According to CDMG regional data, the site is not located in an area where there is a high potential for ground failure, such as liquefaction and settlement. Other than regional geological data, no specific information related to the geological seismic hazards of this site was available to us in our review. No in-depth study of ground shaking, fault rupture or ground failure risk has been provided for this site. The estimated risk in this study is based on the maximum shaking intensity described above.

In order to further assess risk for ground failure, a detailed geotechnical investigation would be required. Such an investigation would involve specialized borings, soil sampling, and analysis, which are beyond the scope of this report.

III. Building Condition

A. Building Construction and Earthquake Design

The concrete multi-cubicle magazine portion of Building 1365 was probably designed to satisfy the requirements similar to those of the 1961 edition of the Uniform Building Code. That edition of the code had seismic requirements that were less stringent than those required by the current code. Since the date of construction of the steel shed is unknown, the probable seismic design requirements for it are also unknown.

It should be noted that the philosophy of the Uniform Building Code is to prevent life loss and injuries and that code compliance does not necessarily protect against property damage.

B. Structural Conditions Noted

Structural Description

The structure consists of a concrete multi-cubicle magazine which was built circa 1962, and a steel shed providing a roof over the then-existing magazine and an ante-room on the two entry sides of the original magazine.

The multi-cubicle magazine consists of a concrete mat foundation with concrete dividing walls between the cubicles and roof panels set on the walls (and beams in the center of wide bays). The original roll-up doors have been removed as the entire structure is now under the steel shed.

The steel shed is principally cold-formed steel framing consisting of joists between steel tube columns at the perimeter to the edge of the concrete structure and across it. The joists are supported at the edge of the concrete structure by short columns over the concrete columns and angles spanning between the short columns. Purlins transverse to the pitch of the roof and steel deck with flutes in the direction of the pitch complete the roof structure. The exterior walls are steel girts and steel panels between the columns.

General views of the building are shown in the following photographs, included as Appendix B:

Photo 1: North elevation.

Photo 2: West elevation.

Photo 3: South elevation.

Photo 4: Interior view–south side.

Structural Conditions General Condition: Fair.

Concrete Walls: No major problems noted where visible.

Concrete Floors: No major problems noted.

Concrete Panels: See below.

Conditions Noted:

At least two “blow-out panels” have been damaged by fork-lift operation. Both are in place and present no immediate hazard.

IV. Assessment of Possible Risk

A. Seismic Risks

Construction Type: Magazine: Cast-in-place concrete walls with precast roof panels;

Steel shed: cold-formed framing with steel deck.

Probable Maximum Loss (PML):

For the concrete magazine: MPE: 4%; DBE: 17%; MCE: 26%;

For the steel shed: MPE: 0%; DBE: 3%; MCE: 4%.

(Note: As the steel shed structure is supported by the concrete magazine, the PML for the concrete magazine governs the risk of the facility as a whole.)

Seismic Risk: Moderate.

Conditions Noted/Recommended Seismic Upgrade:

According to CDMG regional data, the site appears to be located in an area where there is a low potential for ground failure, such as liquefaction and settlement. Based on our review of the geotechnical reports that are available for the site, the near-surface soils are largely stiff clay or stiff sandy clay, which are not typically liquefaction-susceptible soils.

The structural seismic risk for the building is moderate, due to one distinct vulnerability: the walls are connected to the foundation or other walls at two adjacent sides, and unbraced at the other two sides, so that one corner is free to move laterally, restrained only by the bending stiffness of the wall itself. If two adjacent walls move in opposite directions, the roof panels may slip off of their supports and fall. This in turn would cause the steel structure above to be unsupported. The bearing length is six inches at each end, and the walls are 12 inches thick with a height of eight feet and a length of 17 feet. Sufficient differential motion to drop the panels is unlikely. Connection of the free corners of the walls together with a steel tie would ensure that the walls would deform together.

The concrete roof panels (blow-out panels) are not attached to the concrete walls. Since the original function of the structure is munitions storage, the panels were designed to move in an explosion. However, the lack of attachment is a seismic vulnerability. The panels may slide or jump during an earthquake. The panels should be bolted to the walls.

The steel shed is almost completely unbraced. Small braces appear to be attached to the “red iron” frames supporting the joists. These braces appear to be inadequate for today’s seismic standards. Shear connection of the steel roof structure to the concrete structure is recommended at the following locations:

1. Provide a light gauge steel stud and metal panel shear wall above the “spine” of the concrete structure to the steel roof (longitudinal center line).

2. Provide transverse light gauge shear walls above transverse concrete walls to steel roof at the center and the end of the building.

3. Provide strengthening of miscellaneous connections so that the structure can act as a whole.

Soil failure under the concrete structure is unlikely. The eight-inch mat slab with top and bottom reinforcing in both directions will apply such light loads to the soil including overturning effects due to a seismic event that the total pressures will be within allowable values for any soil. Partial or total collapse from soil failure would be very unlikely.

Information describing the footings under the steel columns at the perimeter of the added steel shed was not available. Based on field observation, the connection details of the columns suggest bearing only with no overturning resistance intended. These“pin”connections do not appear to be able to function as part of a lateral resistance system.

Structural risk calculations are included in Appendix C. Location of the recommended upgrade is shown on the seismic upgrade concept plan in Appendix D.

Conceptual cost estimate of the recommended structural upgrade is included as Appendix E.

B. Interpreting Probable Maximum Loss (PML) The PML figures are derived from the method given by Thiel and Zsutty in Earthquake Spectra, 1987. This value is actually intended to be a prediction of the performance of a portfolio of buildings of substantial size, and not a prediction for a specific building. It should be noted that losses due to earthquakes are always possible, regardless of a building's design or how it has been strengthened. PML does not represent possible maximum loss. It is possible to incur earthquake losses that equal or exceed the replacement cost.

The PML applies only to the building and interior finishes. It does not include loss to specialized building contents, such as tenant-installed equipment or furnishings, and does not include loss due to earthquake interruption of facility use. It also does not include loss due to possible earthquake-induced fire.

Seismic resistance of building contents, such as equipment and furniture (other than those specifically mentioned), was not analyzed and is beyond the scope of this study.

The potential earthquake loss of building contents could exceed that of the structure in some buildings. The PML does include consideration of replacement of normal tenant improvements.

It should be noted that decisions which are based on probable loss estimation will always have a degree of risk, due to the possibility of an unfavorable outcome. A more in-depth investigation is warranted if quantitative and accurate results are required.

Such an investigation would involve detailed design analysis, calculations and cost estimation.

V. Summary

In conclusion, we believe that the building is of moderate seismic risk. Structural upgrade to include connection of free corners of the concrete magazine structure walls and of the steel shed to the concrete walls would be recommended. These recommended upgrades would address existing vulnerabilities that could lead to partial collapse or severely damaged conditions during the maximum credible earthquake.

The remedial cost to mitigate problems is summarized in the Conceptual Cost Estimate.

Appendix A Original Construction Drawings

Appendix B Photographs

Appendix C Structural Risk Calculations

Appendix D Seismic Upgrade Concept Plan

Appendix E Conceptual Cost Estimate