National Park Service

Building 107

Boston National Historical Park Charlestown Naval Yard Charlestown MA.

Field Investigation and Assessment Draft Report

January 26, 2022.

Submitted by:

Lead Professional

BBIX, LLC.

102 South Street, Suite 3 Boston, MA. 02111

Sub-Contractor Simpson Gumpertz & Heger 480 Totten Pond Road Waltham, MA. 02451

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EXECUTIVE SUMMARY

Building 107 at the Charlestown Navy Yard is a three-story steel-framed building with concrete floors, a brick facade, and slate roofing constructed in 1904. Building 107 historically served as the Public Works facility, and currently houses offices and museum collections and storage. Building 107 is connected to

Building 108 with a three-story Connector constructed similarly to Building 107. Building 108, which is owned by the City of Boston, is scheduled to be demolished in 2022; Building 107 and the Connector will remain. The roofing and roof structure at Building 107 and the Connector are in poor condition.

The purpose of this project is to determine the condition of the existing roofing and the structural concrete roof slab at Building 107 and the Connector, provide repair options to address deterioration, and prepare construction documents for the repairs. Our recommendations will comply with the Secretary of the

Interior's Standards for the Treatment of Historic Properties. Although the windows and masonry are in poor condition, NPS has limited the project’s scope to the roofing and roof structure only. The project does not include repairs of the windows and does not include permanent masonry repairs such as replacing deteriorated brick or mortar.

The project team visited Building 107 and the 107/108 Connector between 15 and 17 November 2021.

During these visits, the project team performed a visual assessment of the slate roofing, copper gutters, down leaders, and the brick corbel of the south elevation of the building. The team also reviewed exploratory openings in the slate roofing and performed a visual and limited hands-on assessment of the underside of the roof structures by hammer sounding.

During the visit, the project team collected samples of the slate roof shingles and samples of the concrete roof slab and roof truss encasement. We classified the slate in accordance with ASTM C406. We determined the density, chloride content, and depth of carbonation in the concrete roof slab. We also confirmed that the concrete does not contain asbestos fibers.

We recommend NPS maintenance personnel perform visual assessments of the roof slab and truss encasement at least annually to monitor its condition until the repairs are completed.

We also recommend NPS install shoring of the roof slab at the bays discussed on

19 November 2021 and indicated in Appendix F. Additionally, NPS should restrict access to the Building

107/108 Connector and require appropriate personal protective equipment for anyone who enters the space. For Building 107 and the Connector, we recommend NPS remove and replace the roofing

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- ii -assembly and demolish and replace portions of the roof slab at locations indicated in Appendix E. The replacement structure could consist of formed, cast-in-place concrete or conventional galvanized roof deck with plywood infill on top of the deck and gypsum wallboard on the underside of the deck.

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Table of Contents

EXECUTIVE SUMMARY

CONTENTS Page

1. BACKGROUND 1

2. SCOPE OF WORK AND PURPOSE 3

3. REVIEW OF RELEVANT EXISTING DOCUMENTS 4

3.1 Building 108 Demolition Drawings 5

4. RELEVANT BUILDING CODES AND REFERENCE STANDARDS 6

4.1 Review of IEBC 2021 Provisions 6

5. INFORMATION FROM NPS PERSONNEL 11

6. LABORATORY TESTING 12

6.1 Slate Testing 12

6.2 Concrete Testing 12

7. SITE VISIT OBSERVATIONS AND DISCUSSION 14

7.1 Roofing Observations 14

7.1.1 Visual Assessment of Roofing 14

7.1.2 Roofing Exploratory Openings 20

7.2 Structural – Building 107/108 Connector 22

7.2.1 Structural Observations 22

7.2.2 Structural Discussion 26

7.3 Structural – Building 107 27

7.3.1 Structural Observations 27

7.3.2 Structural Discussion 30

7.4 Other Observations 34

8. RECOMMENDATIONS 35

8.1 Maintenance 35

8.2 Recommendations for Further Study 35

8.3 Immediate Repairs 35

8.4 Short-Term Repairs 35

8.4.1 Roof Slab Repair Option – Replace In-Kind 36

8.4.2 Roof Slab Repair Option – Metal Roof Deck 36

8.5 Long-Term Repairs 37

8.6 Summary of Recommendations 38

9. COST ESTIMATE 39

ILLUSTRATIONS

Photos 1 through 32

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Figure 1

APPENDIX

APPENDIX A – Original Third-Floor Drawings APPENDIX B – Building 108 Demo Drawings APPENDIX C – Lab Results APPENDIX D – Roof Opening Locations APPENDIX E – Structural Field Notes APPENDIX F – Shoring and Closure Plan APPENDIX G – Repair Sketches APPENDIX H – Construction Cost Estimate

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1. BACKGROUND

Building 107 in the Charlestown Navy Yard is a three-story steel framed building with concrete floors, a brick facade, and slate roofing constructed in 1904 (Photo 1). Building 107 historically served as the

Public Works facility, and currently houses offices and museum collections and storage. Building 107 is connected to Building 108 with a three-story Connector constructed similarly to Building 107. (Photo 2).

Building 108, which is owned by the City of Boston, is scheduled to be demolished in 2022; Building 107 and the connector will remain. Refer to Figure 1 below, taken from the 2004 NPS Cultural Landscape

Report for Charlestown Navy Yard.

Photo 1: South facade of Building 107.

Photo 2: South facade of Building 107/108 Connector.

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Figure 1: Location of Building 107 and the Building 107/108 Connector.

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2. SCOPE OF WORK AND PURPOSE

The Design Criteria prepared by the GSA and NPS, as revised 22 September 2022, state that NPS intends to replace the roofing at Building 107 and the Building 107/108 connector, and that the scope of this project includes the roofing and roof structure only; it does not include windows or the masonry, except as related to the gutters. The purpose of the investigation was to determine the condition of the existing roofing (including gutters and down leaders) and the structural concrete roof slab.

This project has an investigation phase followed by a design phase. The investigation tasks include:

Performing a visual and limited hands-on assessment of the roof of Building 107. The investigation consists of a visual assessment of the slate roofing, the gutters, and the masonry directly beneath the gutters only; review of exploratory openings in the roofing; visual examination of the underside of the concrete roof slab; and hands-on assessment via hammer sounding of representative areas of the underside of the concrete roof slab. The investigation does not include probing of any masonry or testing for hazardous materials.

Evaluating the condition of the existing concrete using petrographic analysis and testing for chlorides of existing spalls in the concrete that we retain from the investigation.

Evaluating and commenting on the potential impact of the demolition of Building 108 on the Building 107/108 Connector.

Providing a construction cost estimate.

Providing a report that describes the existing conditions, prioritizes options for repairs, and discusses historic considerations.

Meeting with NPS and other stakeholders to discuss the report.

The design phase tasks include:

Preparing repair drawings and specifications including a memorandum for pricing.

Meeting with NPS and other stakeholders periodically to discuss the drawings.

Preparing sealed bidding documents and an initial construction control affidavit.

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3. REVIEW OF RELEVANT EXISTING DOCUMENTS

The project team reviewed the following documents that we received from NPS. The drawings provided by NPS for Building 107 dated 1 March 2012 do not provide information on the roofing materials or assembly. The existing structural and architectural drawings are titled, “Navy Yard, Boston, Mass” and were approved by the Chief of Bureau, Yards and Docks, and are dated between 1902 and 1907.

Document Date Description

Drawing Set #457/125326 by NPS 1 March 2012 North facade rehabilitation drawings Drawing Set #457/125328 by NPS 1 March 2012 South and west facade rehabilitation drawings Existing 107 Sheet 24 10 February 1902 Building 107 architectural sections and elevations Existing 107 Sheet 41 Unknown Building 107 structural column and foundation plan Existing 107 Sheet 62 1 February 1904 Building 107 first- and second-floor architectural plans Existing 107 Sheet 68 30 April 1907 Building 107 third-floor structural framing plan.

See Appendix A Environmental Remediation and Demolition of Building 108, Charlestown Navy Yard by City of Boston and Weston and Sampson

24 February 2020 Drawing set provides details for the demolition of Building 108. See Appendix B.

The existing drawings show that Building 107 is a three story, steel framed building, with concrete floor and roof slabs. The building measures approximately 58 ft – 4 in. the north-south direction and has three bays—the northern and southern bays are 18 ft – 8 in., and the center bay is 21 ft. (The Building 107/108

Connector is slightly narrower in the north-south direction.) In the east-west direction, the building has 14 bays and measures 366 ft – 4 in. long. The typical east-west bay is 28 ft while the hipped end bays are 15 ft – 2in.

The columns are I-shaped and are built up with steel plates and angles riveted together. The columns are typically spaced 14 ft on center longitudinally along the exterior of the building, and 28 ft on center longitudinally in the interior. Every other longitudinal column along the exterior stops at the second floor while the remaining columns continue to the third floor. All exterior columns are encased in brick masonry.

The third floor of Building 107 consists of 2-1/2 in. one-way concrete slab on 1/2 in. metal Ferroinclave sheets. (Ferroinclave sheets were a propriety corrugated steel deck produced by the Brown Hoisting

Equipment Company in the early 1900s.) The concrete floors are topped with a 1/2 in. granolithic surface;

the underside of the Ferroinclave sheets are filled with a layer of cement mortar. The composite slabs span in the east-west direction to steel I-beams (10I25 and 12I31.5 sections) that in turn frame to steel girders (20I80 sections). The steel beams are I-shapes (10I25 and 12I31.5), have riveted connections, and

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- 5 -are spaced approximately 7 ft on center. The steel beams around the perimeter of the building are encased in brick masonry. The original 1907 drawings show that the third floor had openings in the center bay;

these openings have since been filled in.

The roof structure for Building 107 consists of a sloped 4 in. one-way concrete slab that spans in the north-south direction to steel trusses that span 28 ft in the east-west direction to primary trusses on the column lines that span in the north-south direction The roof trusses are built-up with steel angles riveted together. The top chords of the trusses are encased in the concrete slab. The spacing of the east-west trusses varies between 5 ft – 3 in. and 6 ft – 2 in. Large steel-framed skylights and ventilator openings are distributed along the roof’s ridge. The freight elevator between Lines 19 and 20 appears to have been a later addition.

3.1 Building 108 Demolition Drawings

The City of Boston, which owns Building 108 and controls the lower two stories of the Building 107/108

Connector, plans to demolish Building 108 in 2022. The Connector will remain after the demolition work is complete, and the City will turn control of the lower two stories of the Connector to NPS. The engineering firm Weston & Sampson prepared the City’s Demolition drawings. The drawings provide the scope of the demolition work and give schematic direction to the demolition contractor on how to shore the Connector during the work. As part of its means and methods, the demolition contractor is responsible for designing and installing the shoring. In other words, the design of the shoring is delegated to the

City’s demolition contractor. As currently shown, the Weston & Sampson documents indicate that the demolition contractor will:

Abate all hazardous materials in the Connector

Install shoring towers to support the floors and roof slab of the Connector

Install channel bracing along the multi-wythe brick wall party between the Connector and Building 108. The channels are intended to stabilize the wall and restrain it from separating from the Connector’s floor and roof diaphragms. The schematic design also calls for vertical braced frames to laterally support the walls.

Contain and stabilize the north and south brick masonry cornices

Sawcut the existing multi-wythe brick wall party wall at the southeast and northeast corners of the Connector to separate it from Building 108.

Because the City has not fully engaged the demolition contractor, we have not yet had an opportunity to review the contractor’s delegated design submissions.

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4. RELEVANT BUILDING CODES AND REFERENCE STANDARDS

The following building codes and reference standards are applicable to the proposed repairs in accordance with the list of the design standards published on the NPS website as updated 12 November 2021:

International Building Code (IBC) 2021 – general building code for new construction.

International Existing Building code (IEBC) 2021 – building code for alterations, changes of use, repairs, and additions to existing structures.

American Society of Civil Engineers, ASCE 7-16, Minimum Design Loads for Buildings and Other Structures. This standard provides criteria for wind, earthquake, snow, and live loads.

Although local building codes generally do not apply to Federal projects, the project will also follow the requirements of the Massachusetts State Building Code, Ninth Edition (MSBC), which provides amendments to the 2015 editions of IBC and IEBC.

Given that the building is listed on the National Register of Historic Places, and that government funds will be used for work on the building, the Secretary of the Interior’s Standards for the Treatment of

Historic Properties will apply to all work on the building.

Our recommendations will also comply with the Secretary of the Interior’s Standards for the Treatment of

Historic Properties. Given the repair and alteration scope of this project, the Standards for Rehabilitation are applicable.

4.1 Review of IEBC 2021 Provisions

The IEBC requires that the project team choose one of three compliance methods: the prescriptive compliance method, the work area compliance method, or the performance compliance method. In the following analysis, we assume that the project will follow the work area method. The following table summarizes the relevant provisions of IEBC as they pertain to the proposed structural work.

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Code Provision Comment

101.2 – Scope IEBC provisions apply to the repairs of Building 107.

101.4 – Applicability IEBC applies because the project involves repairs to an existing building in Massachusetts.

Chapter 2 – Definitions

This chapter defines technical terms in the code.

Chapter 2 defines a “Flood Hazard Area” as “the area within a flood plain subject to a 1-percent or greater chance of flooding in any year.”

Building 107 is not within the FEMA 100-year flood zone, and therefore does not need to need to comply with the flood protection requirements of IBC/IEBC.

Chapter 2 defines a “Historic Building” as a building “designated as historic under an applicable state or local law” or “certified as a contributing resource within a National Register, state designated or locally designated historic district.”

Chapter 2 defines a “Repair” as “the reconstruction, replacement or renewal of any part of an existing building for the purpose of its maintenance or to correct damage.”

Chapter 2 defines a “Roof Repair” as the “reconstruction or renewal of any part of an existing roof for the purpose of correcting damage or restoring the predamage condition.”

Chapter 2 defines “Substantial Structural Damage” as a condition where “the capacity of any structural component carrying snow load, or any group of such components, that supports more than 30 percent of the roof area of similar construction has been reduced more than 20 percent from its pre-damage condition, and the remaining capacity with respect to dead, live and snow loads is less than 75 percent of that required by the International Building Code for new buildings.”

This project includes structural and non-structural roof repairs to a historic building.

301.1 – General

A project must follow the Prescriptive Compliance Method, the Work Area Compliance Method, or the Performance Compliance Method.

As described above, we assume that the project will follow the Work Area Compliance Method.

301.3.2 – Work Area Compliance Method

We assume that the project will follow the Work Area Compliance Method. Chapters 6 through 12 of the IEBC contain the requirements for this method. This project does not need to comply with the provisions of IEBC Chapter 5, Chapter 13, and Chapter 14.

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Code Provision Comment

405.2.1.1 – Snow Damage

Structural components with damage caused by or related to snow load effects are to be repaired/replaced meeting the snow load provisions in IBC Section 1608.

Although the damage to the roof slab of Building 107 and the Building 107/108 Connector is not likely caused by or related to snow load effects (as discussed below in Section 7.2 and Section 77.3), the repair design will comply with IBC Section 1608.

405.2.4 – Substantial Structural Damage to Gravity Load-Carrying Components

This section states that gravity load-carrying components that have sustained substantial structural damage as defined in IEBC Chapter 2 are to be rehabilitated to comply with the applicable provisions for dead, live, and snow loads in the IBC.

Building 107 is historic and is not required to comply with this section for the repair of substantial structural damage per the exception in IEBC Section 1205.1.

405.2.5 Substantial

Structural Damage to Snow Load-Carrying Components

This section requires the structural roof slab to be repaired, replaced, or retrofitted to satisfy the requirements of IBC Section 1608 where substantial structural damage is caused by or related to snow load effects.

Building 107 is historic and is not required to comply with this section for the repair of substantial structural damage per the exception in IEBC Section 1205.1.

Section 602 – Alteration – Level

This section states that Level 1 Alterations include removal and replacement of existing materials, elements, equipment, or fixtures using new materials, elements, equipment, or fixtures that serve the same purpose.

The replacement of the roofing of Building 107 and replacement of the structural roof slab are a Level 1 Alteration and must comply with the provisions in IEBC Chapter 7.

Section 603 – Alteration – Level 2

This section states that Level 2 Alterations include the reconfiguration of space, the reconfiguration or extension of any system, or the installation of any additional equipment. Projects that are Level 2 Alterations must also comply with the requirements of Level 1 Alterations.

The repairs to Building 107 are not a Level 2 Alteration and need not comply with the provisions in IEBC Chapter 8.

Section 604 – Alteration – Level 3

If the work area (i.e. area of reconfigured interior space) exceeds one-half of the aggregate building area, the alteration is classified as a Level 3 Alteration.

Because the renovation area of the existing building is less than one-half of the aggregate area of the building, this project does not include Level 3 Alterations and does not need to comply with IEBC Chapter 9.

Section 605 – Change of Occupancy

This project does not include a change of occupancy and does not need to comply with provisions in IEBC Chapter 10.

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Code Provision Comment

Section 606 – Additions This project does not include additions and does not need to comply with provisions in IEBC Chapter 11.

Section 607 – Historic Buildings Building 107 is a historic building and, therefore, needs to comply with the provisions in IEBC Chapter 12.

705.1 – General This section states that methods of application used for replacing an existing roof need to comply with the provisions in IBC Chapter 15.

705.2.1 – Roof Replacement

This section states that a roof replacement includes the removal of all existing layers of roof coverings down to the roof deck.

This project will comply with this section.

706.2 – Replacement of Roofing

Any existing gravity load-carrying structural element in which the roof replacement causes an increase in gravity loads by more than 5 percent will need to meet the provisions of gravity loads required by the IBC for new structures.

This project will not increase the dead, live, or snow loads on the building.

1201.2 – Report

A historic building undergoing an alteration is to be investigated and evaluated. A written report needs to be prepared by a registered design professional.

This report addresses the investigation and evaluation of Building 107 and its compliance with the provisions in the IEBC. Because the building is assigned to Seismic Design Category C, the evaluation does not need to include the lateral force-resisting system.

1201.4 – Flood Hazard Areas Building 107 is not within the FEMA 100-year flood zone, and therefore does not need to meet the flood protection requirements of IBC/IEBC.

1201.5 – Unsafe Conditions

This section requires conditions deemed unsafe by the code official to be remedied.

This project will remedy unsafe conditions in the roof structure and at the deteriorated brick masonry cornices at the Building 107/108 Connector.

1202.1 – General Repairs to Building 107 are permitted with original or like materials subject to the provisions of IEBC Chapter 12.

1202.2 – Replacement Replacement of existing or missing features using original materials is permitted. Replacement of glazing in hazardous locations must comply with the requirements of IBC Chapter 24.

Section 1204 – Change of Occupancy

This project does not include a change of occupancy and does not need to comply with the provisions of Section 1204.

1205.1 – General

This section states that historic buildings need to comply with the provisions in Chapter 4 or Chapter 5 except for the repair of substantial structural damage. Repair of substantial structural damage is to meet the provisions in IEBC Section 405.2.1 and need not comply with Sections

405.2.3 and 405.2.4.

This project will replace the concrete roof slabs in-kind with modern concrete materials or with metal roof deck as described in Section 7.3.2.2 of this report.

Section 1206 – Relocated Buildings

Building 107 is not to be relocated and does not need to comply with the provisions of Section 1206.

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The Building 107 and Building 107/108 Connector site is not within the 100-yr flood zone as defined by the FEMA Flood Insurance Rate Map. In addition, because the building is on the National Register of

Historic Places and contributes to the historic district, it is exempt from flood resistance requirements of

IEBC and ASCE 24-14. Similarly, the Boston Planning and Development Agency has identified the site as being within the Coastal Flood Resilience Overlay District with a base flood elevation (BFE) of 19.5 ft measured in accordance with the Boston City Base (BCB). (The grade elevation near Building 107 is approximately El. 16 BCB.) Because the building is historic and relatively small (less than 100,000 sq ft), the building is exempt from compliance with the City’s flood resistance requirements,

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5. INFORMATION FROM NPS PERSONNEL

After our site visits, NPS reported that three windows had fallen from the building prior to our site visits and that they observed leaks at many of the second-floor window heads. We understand the windows are not currently in the scope of work, but NPS understands the windows need repair/replacement. Refer also to Section 7.4 for additional information about our observations on the windows.

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6. LABORATORY TESTING

6.1 Slate Testing

We collected twelve slate roof shingles that we submitted to our laboratory for testing to be able to classify the slate in accordance with ASTM C406. The laboratory reports and results are attached as

Appendix C. Using the average results for flexure, water absorption, and depth of softening, the slate can be classified as Grade S2. Grade S2 slate is expected to have a service life of between forty and seventy-five years. The average absorption is the only metric that does not meet the classification for Grade S1, and it narrowly misses this mark. Thus, we can expect that the existing slate has a reasonable amount of service life left; extrapolating the absorption result, the slate likely has approximately sixty-five years of service life remaining.

6.2 Concrete Testing

We collected nine samples of concrete from the Building 107 and the Building 107/108 Connector roof slabs and truss top chord encasements that we submitted to our laboratory for testing. Our laboratory tested the concrete for density, chloride content, and depth of carbonation. We also retained an outside laboratory to test for the presence of asbestos fibers.

The density test results indicate that the roof slab concrete weighs approximately 125 pcf. This density is considerably lower than modern normal-weight concrete (approximately 145 pcf) but slightly higher than modern lightweight concrete (approximately 115 pcf). Repair options described below assume lightweight concrete for all roof slab and encasement repairs, and thus, the repair material will be lighter than the original roof structure.

The concrete chloride test is in accordance with ASTM C1152/C1152M-04(2012)e1. The results indicate that the chloride content of the roof slab is approximately 0.3% of the samples by weight. This percentage is much lower than the recommended corrosion threshold limit of chloride content of 0.05% by weight.

The concrete carbonation testing is performed to test the reaction of the components of the cement paste in the sample concrete with carbon dioxide in the atmosphere. Carbonation of the concrete reduces the pH value over time. The pH reduction inhibits the passivating function of the concrete paste which protects the reinforcement against corrosion. Without the high pH passivating effect, the reinforcement is prone to corrosion when exposed to moisture from humid air or condensation. The carbonation testing results

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- 13 -indicate full-depth carbonation of each sample tested indicating that the carbonation depth has reached the reinforcing steel.

Hazardous material testing indicates that the concrete samples do not have asbestos-containing materials.

Fibrous materials discovered in the concrete samples are likely to be goat hair used as reinforcing fibers per the original drawings.

Additional testing indicates that the volume of permeable pore space of the roof samples is high at approximately 25%. This indicates that the concrete is relatively porous and permeable. The porosity makes the concrete more prone to moisture penetration aggravating the corrosion potential created by the carbonation.

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7. SITE VISIT OBSERVATIONS AND DISCUSSION

The observations in this report are based on the project team’s site visits on

15 – 18 November 2021. We performed a visual assessment of the slate roofing, copper gutters and down leaders, and the brick corbel of the south elevation of Building 107 and the Building 107/108 connector from a boom lift. We performed a binocular survey of these components on the north and west elevations where we were unable to access the facade from the boom lift. We also reviewed exploratory openings in the slate roofing (Photo 3).

Photo 3: Overview of Building 107 roofing.

During our site visit, we sounded portions of the underside of the roof slab and concrete encasement on the top chords of the roof trusses using a mason’s hammer. We were unable to access some portions of the underside of the slab because of stored materials on the third floor and limitations of our lift equipment and ladders. We also observed the condition of the trusses and exposed columns. We did not make destructive openings to expose steel columns or portions of trusses embedded in the brick masonry along the eaves.

7.1 Roofing Observations

7.1.1 Visual Assessment of Roofing

During our visual assessment of the roofing assembly, we observed the following:

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The slate shingles generally appear to be in good condition. We noted isolated areas of cracked or missing slate (Photo 4).

Photo 4: Cracked and missing slate.

The copper gutter is not sloped to drain. On the south elevation, the gutter drains to internal down leaders; on the north elevation, the gutter drains to external down leaders, many of which are missing or damaged (Photo 5 and Photo 6). The gutter at the Building 107/108 connector appears to have been removed. The gutter fasteners are missing or disengaged at many locations and the remedial coating on the gutter interior is cracked (Photo 7).

Photo 5: Overview of gutter on south facade.

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Photo 6: Missing down leader on north facade.

Photo 7: Cracked remedial coating in gutter.

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Missing or cracked skylight glazing at multiple locations (Photo 8).

Photo 8: Missing portion of skylight glazing.

Localized damage of the copper flashing at the skylight perimeter and roof ridge (Photo 9). At one location, the ridge flashing is loose and is able to move in light wind.

Photo 9: Damaged copper ridge flashing.

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Localized areas of eroded or missing mortar and efflorescence at the brick corbel of Building 107 (Photo 10).

Photo 10: Missing and eroded mortar and efflorescence on the brick masonry corbel.

The brick corbel at the Building 107/108 Connector is in poor condition (Photo 11). The mortar is heavily eroded and many bricks are loose or missing. Birds appear to be nesting in the corbel, and we observed vegetation on the top surface of the damaged corbel.

Photo 11: Displaced brick masonry at the south brick corbel at Building 107/108 Connector.

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At the Building 107/108 Connector, the top portion of one of the exhaust vents has fallen and is sitting on the edge of the roof (Photo 12). The exhaust vent is within the portion of the roof that will remain after the City demolishes Building 108.

Photo 12: Portion of exhaust vent on roof edge.

Note that the snow guard anchors are not integrated into the roofing assembly.

Partially collapsed fence on roof of Building 107. The fence is restrained on two sides by cables, which may be a remedial effort to prevent it from falling off the roof (Photo 13).

Photo 13: Partially collapsed fence restrained by cables.

One location where the roofing assembly is visibly sagging on the south elevation. This location corresponds with the severe deterioration of the concrete structural roof slab below.

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7.1.2 Roofing Exploratory Openings

We directed Standard Buildings to make and repair several exploratory openings in the existing slate roofing. We reviewed four openings in the roofing assembly; opening locations are shown in the attached

Appendix D. We observed the following:

The roofing system consists of slate shingles nailed directly to the structural concrete roof slab.

No underlayment is present between the slate shingles and the concrete roof slab (Photo 14).

Photo 14: No underlayment beneath the slate shingles.

While the exposed surfaces of the slate shingles generally appear intact and free of surface scaling and other deterioration, the concealed faying surfaces of the slate shingles at all exploratory openings generally exhibit very shallow, surficial material erosion.

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The top surface of the concrete structural roof slab is generally in poor condition (Photo 15). At three of the four exploratory openings, the slab is cracked and deteriorated. This deterioration generally corresponds with locations of noted damage on the underside of the slab. Refer to Sections 7.2 and 7.3 for additional information about the condition of the concrete slab.

Photo 15: Cracked concrete roof slab.

While the NPS Design Criteria anticipated that an entire roofing replacement is necessary, we sometimes find in our experience with similar buildings that the damage to a roofing system is localized and contained to the most vulnerable areas (e.g., perimeter conditions and penetrations). Accordingly, we chose our slate roofing openings within the field of the roof and areas that may be in better condition to confirm whether complete replacement is required or whether localized repair might be a viable option. In this case, the lack of underlayment between the slate and the concrete slab has contributed to widespread deterioration of the concrete slab. All of the slate shingles must be removed to repair the slab and provide a more robust and appropriate roofing system with continuous underlayment to protect the portions of the slab that remain. Additionally, the lack of slope in the gutter necessitates a new gutter. The existing skylights are deteriorated beyond repair and the subsequent leakage is further contributing to the roof slab deterioration.

Given the classification of the existing slate, reusing the existing slate in a roofing replacement is a reasonable option.

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7.2 Structural – Building 107/108 Connector

7.2.1 Structural Observations

The project team visited the Building 107/108 Connector briefly on 15 July 2021 and again over the course of 15 – 18 November 2021. At the Building 107/108 Connector, we observed the following at the underside of the concrete roof slab and the supporting steel trusses:

The steel trusses are painted and typically appear to be in good condition (Photo 16).

Photo 16: Steel trusses at the northern side of the Building 107/108 Connector.

The southernmost truss is severely deteriorated at its eastern end (Photo 17). Large portions of the top and bottom chords are corroded and show loss of section. The connections and rivets are heavily corroded and cracked (Photo 18). This deterioration is atypical.

Photo 17: Deteriorated steel truss at the eastern end of the southernmost truss in the Building 107/108 Connector.

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Photo 18: Cracked and corroded top chord connection at the truss shown in Photo 17.

The structural roof slab is severely deteriorated and has spalled along the eaves of the Connector roof. Large pieces of spalled concrete are sitting on the Connector floor slab, exposing the expanded steel mesh reinforcement in the mid-thickness of the slab (Photo 19 and Photo 20). In areas where the slab has not spalled, it is delaminated from the eave to the ridge on the north and south slopes. The delaminated concrete poses a falling object hazard to people standing on the third floor.

Photo 19: Spalled concrete on the Building 107/108 Connector floor slab.

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Photo 20: Exposed reinforcement at the underside of the Connector roof slab.

The concrete encasement around the top chord of the trusses is cracked, damaged, or missing throughout the Connector (Photo 21).

Photo 21: Photo of damaged and missing encasement at the Connector trusses.

At the time of the site visit, there was standing water on the Connector floor and in several buckets placed to catch water leaking through the roof (Photo 22).

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Photo 22: Standing water and water collection buckets on the Connector floor slab.

The concrete curbs at the skylight sills are deteriorated (Photo 23).

Photo 23: The concrete curb at the south side of the skylight is delaminated.

The skylight framing is severely corroded and has lost much of its cross section in many areas.

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Photo 24: The skylight framing is severely deteriorated.

7.2.2 Structural Discussion

The skylight and roof slabs in the Building 107/108 Connector are in poor condition. As we recommended to NPS during a conference call on 19 November 2021, NPS should restrict access to the third floor of the Connector immediately. The condition of the roof slab and the skylight pose a potential falling object hazard to people standing on the third floor. Anyone who enters this space should wear a hard hat, safety glasses, mask, long-sleeved garments, and sturdy shoes. We sent an email to NPS on 22

November 2021 with our recommendations.

Based on our observations, it does not appear that any portion of the roof slab or the skylight can be salvaged and repaired. The slab and skylight should be replaced in their entirety within the Connector.

Section 7.3.2.27.3.2 describes repair approaches.

The east end of the southernmost truss will need to be repaired and supplemented with new steel angles spliced to the existing, deteriorated angles. This work can be done after the existing roof slab is removed.

Part of the City’s demolition scope for Building 108 is shoring the brick party wall between the Building

107/108 Connector and Building 108 as well as shoring the roof slab of the Connector and the cornices.

The City’s demolition contractor has not yet submitted designs for the shoring. The project team discussed the Building 107 rehabilitation project with the City’s engineer. We informed the engineer that the Connector’s roof slab should be demolished and replaced and that the north and south brick cornices should be rebuilt. The City’s demolition contractor will need to consider the condition of the roof slab and cornice in the shoring design.

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7.3 Structural – Building 107

7.3.1 Structural Observations

The third-floor attic space of Building 107 is used primarily for storage by several NPS departments. We accessed the underside of the slab from a telescoping lift and ladders except within the archive space west of Line 10 where a door threshold prevented our use of the lift. Areas with large obstructions restricted access to the underside of the roof slab, and, consequently, we observed some areas visually only (Photo

25).

Photo 25: Obstructions on the third floor in Building 107.

At Building 107 we worked around the obstructions and made the following observations.

The red marks on the structure indicate delaminated concrete. Where feasible, we removed loose concrete using small tools to eliminate a falling object hazard for people standing on the third floor.

The concrete encasement around the top chord of the trusses is cracked, damaged, or missing throughout the length of the building (Photo 26; refer also to our field notes in Appendix E).

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Photo 26: Damaged encasement exposing the top chord of the truss.

Large portions of the roof slab sounded delaminated during hammer sounding, mainly along the eaves of the building and in the areas between roof skylights (Photo 27 and Photo 28; refer also to our field notes in Appendix E).

Photo 27: Delaminated slab at the eave of Building 107.

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Photo 28: Delaminated or deteriorated roof slab between skylights.

Large portions of the roof slab along the southern eave of the building in the museum storage space is spalled and, in locations, the slab is completely perforated (Photo 29). Potions of the slab are bowing into the attic space and need immediate shoring. The project team reported this condition to NPS during a meeting on 19 November 2021 and in a shoring and closure plan on 22 November 2021 (refer to Appendix F).

Photo 29: Deteriorated and bowing slab along the southern eave in the museum storage space.

Areas of the roof slab around the ice guard anchorage are typically damaged and show signs of water ingress (Photo 30).

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Photo 30: Deteriorated slab around the ice guard roof anchors.

The steel trusses supporting the roof slab in Building 107 are painted and show no signs of corrosion or deterioration (Photo 31).

Photo 31: Typical steel roof truss in Building 107.

7.3.2 Structural Discussion

7.3.2.1 Causes of Deterioration

Large areas of the roof slab are deteriorated, particularly along the eaves and around the skylights and ventilators near the ridge. The slab, which is 4 in. thick, is reinforced with a single layer of expanded steel mesh at approximately its mid-thickness.

The products of corroded steel (rust) occupy approximately eight to ten times the volume of the original steel. The expansion of the rust creates tensile stresses in the surrounding concrete that the cement paste is unable to resist. As a result, the concrete around the corroded reinforcement cracks. The concrete cover surrounding the reinforcement delaminates and, ultimately, spalls leaving the reinforcement exposed as we observed in Building 107 and the Building 170/108 Connector.

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Chloride ions in the cement paste can accelerate the corrosion of steel reinforcement. In the first half of the Twentieth Century, builders often used calcium chloride to accelerate the set of fresh concrete, particularly in cold temperatures. Our laboratory testing did not indicate high levels of chloride ions on the cement paste. Chlorides are not a contributing factor to the corroded steel mesh.

As described above in Section 6.2, the concrete mix for the slab is very permeable and does not include any large aggregate.

The Portland cement paste in fresh concrete is basic—it has a pH of approximately 12 to 13. The high pH of the paste creates a passivating layer around the steel reinforcement which protects the steel from corroding. Over time, the cement paste in concrete exposed to air absorbs carbon dioxide from the atmosphere in a process called carbonation. As the concrete carbonates from the exposed surfaces inward, the pH reduces. When the carbonation depth reaches the reinforcement, the reinforcement is more susceptible to corrosion. Our laboratory testing described in Section 6.2 indicates that the concrete in the samples is carbonated through its thickness to at least the depth of the steel mesh reinforcement. With the pH of the concrete reduced, the atmospheric oxygen and water vapor inside Building 107 have likely contributed to the corrosion of the reinforcement.

Because the roofing assembly does not include an underlayment between the slate shingles and the slab, any water that finds its way under the shingles will be in direct contact with the concrete slab. It follows that the most severely deteriorated concrete is along eaves, both since this area sees the highest volume of water flow, and because water trapped above the gutters by ice dams or the snow guards may back up on the roof and finds a path to the slab near the eaves. The anchorages for the snow guards along the southern eave were punched through the slab and are not flashed (Photo 12), providing a direct path for water to saturate the slab and corrode the steel mesh. The new roofing system will address this concern with the use of a self-adhered membrane underlayment.

The skylights are supported on short concrete curbs that rise about 6 in. above the plane of the slate shingles at the sill and only about 2 in. above the ridge flashing as shown in Photo 8 and Photo 9. The low curbs—and the correspondingly low flashing height—create areas that are vulnerable to water ingress.

Similarly, the copper roof ventilators do not appear to be watertight, as shown in Photo 9. In the repair design, the skylight curb heights should be increased to accommodate taller flashing, and the ventilators should be integrated with the underlayment.

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Based on our observations, the condition of the concrete encasement along the top chords of the east-west…