Attachment_8,_RSA_Shaw_Remediation_Report.pdf

REMEDIATION OF THE

ROBERT GOULD SHAW MEMORIAL

FEBRUARY 12, 2015

Prepared For:

Friends of the Public Garden

69 Beacon Street

Boston, MA 02108

617-723-8144

Prepared By:

Robert Silman Associates

111 Devonshire Street

Boston, MA 02109

617-695-6700

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EXECUTIVE SUMMARY

The Robert Gould Shaw Memorial, erected in 1897, is now suffering deterioration due to sustained environmental exposure. The face stone joints were repointed in the fall of 2014, and while this does provide some limited protection, it is evident that further remedial action will be necessary to ensure public safety and the monument’s longevity. This report makes specific recommendations for further investigation, repair, and structural upgrades, with the intention of extending the life of the memorial by at least another 50 years.

The monument’s marble facing stones are not anchored to the back-up masonry, nor is there sufficient anchorage between the stones themselves. Furthermore, the mortar joints are not watertight and the flashing is defective, allowing water to seep behind and between the stones. Over the years, cycles of freezing and thawing have displaced and rotated many of the stones, causing damage both to the bronze sculpture and to the stones themselves, as well as to the brick wall upon which the sculpture rests. As a further matter, the sculpture and the masonry have no lateral support to provide stability in a seismic event.

The plaza in front of the monument rests on steel framing above a crawl space. Poor drainage and inadequate waterproofing at the junction between the monument and the plaza have allowed water to infiltrate the crawlspace, which has in turn has led to varying degrees of corrosion of the underlying steel structure.

Structural analysis of the framing reveals that the steel beams do not possess adequate capacity to support reasonable design live loads. Furthermore, the presence of two historically significant elm trees complicates any attempt to remedy these deficiencies.

To address the above-mentioned problems, the following remedial actions are considered necessary for the long-term, permanent stabilization of the monument and the plaza, and are listed in order of priority.

For the monument:

1. Dismantle the monument, replace the underlying brick wall with a new concrete wall, and install anchorage of stones to the back-up wall, or between the stones where there is no back-up wall.

2. Replace cap flashing under coping stones, properly detailed to prevent ingress of water, but hidden from view.

3. Install a new steel moment frame (i.e. a steel frame in which the beams are fully welded to the columns) to support the sculpture and face stones against seismic loads.

For the Plaza:

1. Excavate the crawlspace and make additional probes to determine the extent of steel framing corrosion.

2. Install new steel framing to support the existing framing under the plaza.

3. Apply cathodic protection or new coatings to all steel framing under the plaza to arrest further corrosion.

In case it is not possible to accomplish some or all of these tasks in a reasonable time frame, alternatives are suggested in the body of this report.

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INTRODUCTION

The Friends of the Public Garden retained Robert Silman Associates (RSA) to provide structural engineering services, perform an investigation, and present a pre-schematic level report containing recommendations for remedying the deficiencies of the Shaw Memorial in the Boston Common. RSA worked with Ivan Myjer’s firm, Building and Monument Conservation (B&MC), on this effort.

The Memorial to Robert Gould Shaw and the Massachusetts

Fifty-Fourth Regiment, erected in 1897, displays a bronze relief sculpture by Augustus Saint-Gaudens, and sits across the street from the Massachusetts State House in the Boston

Common. The sculpture, pictured on the cover of this report, portrays the Massachusetts 54th Volunteer Infantry, America’s first documented African American regiment, marching down Beacon Street on May 28, 1863. The bronze is comprised of several small sections, joined together to form a unified panel 11 feet high and 14 feet wide. The setting and masonry surround of the bronze relief was designed by Charles McKim of McKim Mead and White. As depicted in Figure 1, the ground level behind the monument is eight feet lower than the front, and the facing stones below are granite, whereas the stones above are marble.

Over the years the stonework and the bronze have undergone deterioration due to weathering and occasional maintenance work has been performed. B&MC recently made some modest repairs to the monument, but it has been over thirty years since the last major preservation campaign was designed by Ann Beha Architects.

This report will first present the environmental damage to and future risks for the monument and plaza structures, and afterward will provide a detailed and prioritized list of recommendations for further action.

MONUMENT

Conditions

A campaign to make interim repairs to the stone façade of the monument has recently been completed by

B&MC. Comments and findings written by Ivan Myjer of B&MC follow:

Front wall under the bronze:

Eight of the ten units directly under the bronze are displaced. The horizontal course directly below the bronze is both lifted and pushed out at the center three units. The five vertically laid stones under the horizontal course are pushed out and tilted forward.

The movement of the units appears due to the expansion of ice forming in the gaps between the back of the stones and the brick support wall behind the stones. The center stone of the horizontal course is cracked in the middle where the stone has been lifted the most. The end units of this course however are through wall units that remain in their original locations.

Figure 1 – Rear of Shaw Memorial

Figure 2 – Displacement and Cracking of Front Wall Stones

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Issues arising from the movement of the front wall stones:

The lifting of the center units is putting a strain on the bronze relief directly above. The gap at the top of the bronze is directly above the highpoint of the lifted units of the course directly below the bronze. The original carved details are no longer in alignment. The movement of the stones below the bronze is responsible for the cracking of these units as well as the adjacent units. The front wall stones were not anchored to each other or to the back-up masonry. The movement will continue if water enters the core of the monument.

Rear Tennessee Marble wall at the level of the bronze relief:

The major source of water infiltration into the upper monument appears to be thru the failed mortar joints in the units directly below the coping stones. The coping stones were originally detailed so that rain water that flows over the coping stones is directed into a skyward facing joint located at the low point. In addition, water running off of the coping stones flows into the joints between the pilasters and the wings of the carved eagle.

This water flows down through the gap between the back of the bronze and the back of the stone units and collects on either side of the brick support wall. Since there is no back- up masonry wall between the bronze and the stone, it is not clear what, if anything, the rear wall stones were anchored to. It is possible that the rear wall stones are anchored to the back of the bronze relief with some sort of straps. There is however no evidence for this. This would mean that the only thing holding the units in place is the mortar in the joints.

The upper granite course with the wave motif is lifted and bowed. The cause of the lifting of the granite course is the jacking of corroded steel anchors installed on the top surface of the units.

The lifting of the granite course is responsible for most of the displacement of the Tenn. Marble Balustrade.

RSA’s comments continue. On October 24, personnel from B&MC removed one marble stone from the base of the monument at the plaza side to reveal the brick core behind, pictured in Figure 5. The brick was not in good condition: much of the surface was spalled and friable, and pieces could be removed by hand. It could not be determined how deep into the brickwork this decay persisted.

Additionally, under the bronze monument there were only pieces of loose brick and mortar, instead of a solid grout bed.

Since October 24, B&MC repointed many of the stonework joints to inhibit water intrusion. While this will certainly reduce the rate of future deterioration and movement of stones for the near future, there are some systemic shortcomings in the details of the monument that need to be addressed. Many marble stones at the upper part of the monument have moved significantly, dragging

Figure 3 – Cracking at Top of Bronze Figure 4 – Vertical Crack at Center of Bronze

Figure 5 – Brick Core of Monument Support

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along adjacent stones and cracking their edges—see Figure 6. There is no known anchorage of the facing marble stones to the back-up brickwork. There were, however, several ferrous cramps between facing stones, but even these were irregularly spaced.

Furthermore, the structure has very poor resistance to seismic forces.

Boston is an active seismic zone, and with nothing to anchor the granite stones or the sculpture itself, even a moderate earthquake could cause significant damage to the monument and even pose a risk to anyone standing in its vicinity.

Recommendations

The following scope of work, listed according to priority, is considered necessary to extend the life of the monument by 50 years.

1. Dismantle all marble face stones from the monument and securely store the stones offsite.

Temporarily move bronze sculpture laterally a short distance off its present location atop the brick wall. Demolish the brick back-up wall and pour a new 24” reinforced concrete wall in its place.

Reassemble the facing stones, installing anchorage to the new concrete back-up wall or laterally between the stones where there is no back-up wall. Reinstall the bronze, anchoring it properly on all sides to the new structure. Use only stainless steel anchors. Install necessary waterproofing at the joint between the monument and the plaza. Install cap flashing under the coping stones, properly detailed to prevent ingress of water, but hidden from view. (If a lateral support system is to be installed, this should be done before reassembly of the facing stones. See item two for details.)

2. The following work should be interposed into the sequence of item one, before reassembly of the facing stones: install a new stainless steel moment frame on top of the new concrete bearing wall mentioned in item one to support the sculpture and the stones against seismic loads. Cut four full-height, eight-inch thick, four-inch deep slots in front side of the rear granite wall to encase the steel framing, and reassemble.

Of primary concern is the cracking of the bronze. As discussed in the comments by B&MC above, the supporting masonry elements directly below the bronze have failed and the bronze itself is now beginning to crack. This may be reason enough to accelerate major repairs.

With regard to item two, it may be worth noting that, if this were a building project subject to the

Massachusetts Building Code, then a full seismic upgrade would be required as part of the scope of item one.

Alternatives

Understandably, the resources necessary for carrying out the foregoing recommendations may not be available. In that case, one viable alternative would be to conduct ongoing, periodic maintenance—such as repointing the stone joints as necessary to minimize water intrusion, regularly monitoring the deflections of the stones and the bronze, and re-setting any highly displaced stones. Such a program of modest repairs and adjustments to the memorial every few years may be able to hold major deterioration at bay. However, the extent of damage already suffered by the underlying brick structure would remain unknown, and it is possible that minor cracking of the bronze could persist.

Figure 6 – Spalling of Marble Due to Displacement of Adjacent Stones

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PLAZA

Conditions

On November 12, personnel from RSA and B&MC entered the crawlspace under the memorial plaza to make probes, take measurements, and observe the condition of the plaza structure. The framing of the plaza, depicted in Figure 7, consists of north-south spanning steel beams which bear on brick masonry at the south end and connect to an east-west spanning steel beam at the north end, which in turn bears on stone piers.

Two tree wells, also shown in Figure 7, frame two historic elm trees which are believed to be over 200 years old. Complications arising from the presence of these trees will be discussed later in this report.

Figure 8 shows how brick arches rest on the bottom flanges of the beams; the arches are approximately 10½ inches thick at the springing and five inches thick at the crown.

Above the brick arches is a six inch thick concrete slab, reinforced with welded wire fabric, and directly adhered to the top of the slab is a layer of waterproofing. Finally, a herringbone pattern of brick pavers is set in sand on top of the slab.

Figure 7 – Plaza Framing Plan

Figure 8 – Cross Section of Plaza Build-Up

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The first and most striking observation upon entry into the crawlspace was the presence of numerous stalactites formed on the underside of the steel beams (pictured in

Figure 9) which indicate that water is infiltrating from the plaza to the beams. The presence of water has corroded the steel in some locations, and in fact a quarter-inch thick piece of exfoliated pack rust was easily removed from the bottom flange of the steel beam that frames the east side of the west tree hole. A probe was made into the brick surrounding a point where one of the beams bears on the south wall, and at this location it was determined that corrosion is not substantial, even though there were numerous stalactites at this location. Unfortunately, the extent of corrosion is not known; the ground slopes within the crawlspace from about three feet at the south end to almost no space at all at the north end, making a thorough investigation impossible unless the soil is removed. It is highly probable, however, that some significant loss of section has occurred in several locations.

It is worth noting that the stalactites were more numerous at the south end of the beams (near the monument) and less numerous or even absent at the north end (near Beacon Street) which indicates that water intrusion is a more serious problem at the south end of the plaza than at the north end. This suggests that the ingress of water is not due to failure of the waterproofing on the surface of the plaza but rather from a lack of drainage from the plaza and from seepage at the junction of the monument itself with the plaza, where waterproofing and flashing is very difficult to achieve.

Finally, a structural analysis of the steel beams, loaded with the build-up of dead load shown in Figure 8 and a crowd of people or a small vehicle, and neglecting the effects of corrosion, does not give an adequate factor of safety against failure. Of course, the results of the analysis would be worse still if the effects of corrosion were found to be significant.

Figure 9 – Stalactites Forming Under Plaza

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Recommendations

The presence of the two historic elm trees is a complication for performing work on the plaza. Ideally, if there were no risk of damage to the historic trees, the crawlspace could be excavated to allow access for investigation of existing conditions and making repairs. In reality, the trees’ roots are prevalent throughout the soil, and any excavation of the crawlspace risks damaging the trees. However, if the risk to the trees associated with excavation is acceptable, then to extend the life of the plaza by 50 years the following prioritized scope of work is recommended.

1. Excavate the crawl space uniformly to four foot clear depth, exercising extreme care at the two existing elm trees. Pour a new reinforced concrete sheeting wall at Beacon Street to retain the soil.

Underpinning will likely be necessary.

2. Investigate the extent of corrosion of all the steel beams.

3. Install a new steel girder, placed to support the north-south steel beams at mid-span for the full plaza width; support the girder on new steel columns; support the columns on new concrete footings.

4. Determine the best method of permanent corrosion protection (e.g. cathodic protection, scraping and painting), and apply to the steel.

Alternatives

As mentioned, a major drawback to the foregoing recommendations is the risk to the trees associated with excavation. One point to consider is that if the trees were to be removed, there would be no need to install new framing under the plaza—at that point, the crawlspace could simply be backfilled with soil under a new concrete slab on grade. However, assuming it is undesirable to remove or risk damage to the trees, the following is suggested as a temporary solution.

1. Install temporary shoring of the steel beams under the plaza. The shoring shall consist of 6x6 pressure-treated posts placed to support each of the 11 north-south beams at mid-span. Place the posts on a mud sill. Some minor excavation may be necessary to allow access to the shoring locations and to provide a level surface for the mud sill, but this should be minimized to avoid damaging the tree roots.

It is worth emphasizing that the extent of steel corrosion is unknown, and since this scope of work does not entail any further investigation, the extent of the steel corrosion will remain unknown.

CONCLUSION

The primary recommendations of this report are considered necessary for the long-term health of the memorial. While alternative courses of action were suggested, for the sake of minimizing expenses and protecting the two elm trees, these alternatives are ultimately incapable of securing the longevity of the memorial. In any case, while a program of regularly scheduled maintenance and monitoring of the monument may be sufficient for the short-term, it is certain that the framing under the plaza must be supported as soon as possible.

The problems with the monument and with the plaza are systemic; the underlying structures of both have suffered damage, the extent of which is uncertain. At some point, if the monument is to be preserved for future generations to enjoy, a full upgrade of the structure will be necessary.