Completed Projects: Cathedral Church of the Nativity

Project Background
The Cathedral Church of the Nativity in Bethlehem, Pennsylvania, was constructed in the late 19th century through substantial funds from the leading industrialist families of the time. Philanthropic individuals including Asa Packer, Robert Sayre, Elisha Wilbur, and Garrett Linderman—names which are common to colleges, hospitals, and charitable trusts throughout the East—were the founders and benefactors of this Episcopal parish community.

The Cathedral Church’s bell tower stands approximately 85' tall and is one of the most prominent structures in Bethlehem. The bell tower, church, and other parish buildings are constructed of quartzite stone, with decorative elements including traceries made from Indiana limestone and parapet caps, buttress caps, belt courses, and water tables made of Pennsylvania blue sandstone.

The Problem: The effects of nearly a century of natural weathering, combined with acid rain, freeze-thaw cycling, and previously improper repairs had resulted in severe deterioration of the structure.

Nearly three tons of new steel beams and channels were hoisted up into the bell tower and welded, bolted, and grouted in place. Holes were then core-drilled through the 28" thick walls. Stainless steel tie rods and decorative star plates were used to restrain the masonry and transfer loads to the new structural steel. (Fig. 2)
Parapet walls were then disassembled, the location of individual stones being catalogued for re-installation. New lead-coated copper thru-wall flashing was added to provide long-term watertight integrity. Parapet was then re-laid using a historically correct high-lime mortar mix. (Fig. 3)
The bluestone battlement caps and the decorative niche stones of the parapet were deteriorated beyond reuse. MPS salvaged the best pieces of cap and stone for molding and forming purposes, then cast replacement pieces using a precast concrete mix with a coloring agent to match the appearance of the original bluestone. Each piece, some weighing 500 pounds, had to be hoisted back to the top of the tower and built into or set on top of the parapet wall. (Fig. 4)
The limestone traceries at the bell level displayed severe cracking, resulting from corrosion of embedded steel anchors and undue stresses created by adjacent displaced masonry. MPS removed the tracery limestone pieces, core-drilled out the offending steel, and reset the limestone using stainless steel anchors. Beyond structural repair areas, these repairs were important from the standpoint of aesthetics and the maintenance of the watertight integrity of the structure.
The Cathedral Church’s main wall element is the random-range quarry-faced quartzite which is laid in an ashlar pattern. At some point in the past, an acrylic water repellent solution had been applied to the walls in an attempt to reduce water penetration. This material was degraded by UV light, and where it was still extant, it had turned white. This material was removed from the surface of the stone through high-pressure water blasting combined with abrasive grit blasting. (Fig. 5)
Naturally occurring iron deposits within the quartzite had rusted and bled down the walls, and heavy carbon deposits covered the stones as well. These stains were removed with an ammonium bi-fluoride solution.
Weak occlusions within certain pieces of quartzite had rendered them into a powdery, crumbling mass. These stones were removed and replaced. (Fig. 6)
The Cathedral Church’s original mortar joints extended out onto the rough face of the quartzite, hiding the irregular and uneven edges of the rockwork. A narrow, raised ribbon of mortar was applied on top of the underlying wide mortar and struck plumb and level to simulate the appearance of higher quality cut stonework. In general, original mortar joints were cracked, eroded, and missing. The mortar mix that was used in previous repair campaigns had a high Portland cement content, which does not typically provide good long-term performance in Northeastern climates. MPS removed all surface mortar and painstakingly rebuilt the joint profile with a historically accurate high lime mortar. (Fig. 7)
The blue sandstone was the worst performing of the stone types on the building, exhibiting extreme exfoliation, delamination, cracking, and widespread loss of detail and section. The sandstone was chipped back until a sound substrate was uncovered. Missing stone sections were rebuilt using a proprietary stone patching material. Overhanging or deep section replacements were reinforced with epoxy-anchored stainless steel pins. (Fig. 8)
Upon completion of the bluestone repairs, a breathable potassium silicate mineral coating system was applied to even out any color variations resulting from the patching process. Stainless steel bird-proofing was added at ledges in order to discourage roosting of pigeons and avoiding the buildup of associated dirt and staining.

Additional Project Notes
All work was performed from MPS system scaffolding. This project was very close to busy sidewalks and a major vehicular thoroughfare, so the scaffolding was wrapped with heavy-duty netting in order to prevent any accidents or injuries as a result of falling debris. The scaffolding was engineered to withstand the additional wind loads caused by the netting. The scaffolding was also designed to cantilever out over the adjacent church roof so as to not bear on and damage the historic slate roof.

This project involved over 11,000 working hours and was completed with no lost-time accidents.

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