Masonry structures are one of the oldest forms of building. In the late 1800s, a desire for more unusual and remarkable buildings, along with an increased emphasis on newer materials and a consequential decreased use of masonry, led to a gradual decline in masonry expertise.
Atlantic Engineering Services saw problems with two turn-of-the-century masonry churches that are related to this poor understanding of masonry materials and structures.
The First Presbyterian Church of Pittsburgh presented two "simple" problems. The first problem appeared where rainwater was running off the limestone elements and onto the sandstone surfaces below, particularly at limestone pinnacles where large surface areas were exposed to water and weather. Rainwater, the pH of which had been increased by contact with the limestone, became a solvent for the matrix binder in the sandstone, eroding the sandstone immediately below the limestone. Water repellants to prevent the runoff from being chemically modified by the limestone are being evaluated.
The second problem encountered - a minor problem structurally but a major issue for the owner - was deterioration of the ornamental stonework attachments. This building's ornate exterior was decorated with hundreds of stone ornaments. The decorative elements, however, extend down to slender bases that had to be drilled and pinned to the support with steel pins. As the pins corrode, they expand and crack the ornament or the base stone. There was no way to effectively prevent additional failures of the ornament bases. The only solution was to fabricate new ornaments and install them with fiberglass or stainless steel pins.
At an average cost of $1,500 apiece for nearly 300 ornaments, the owners decided simply to remove all of the ornaments at this time. With detailed records kept, however, the long-term plan is to restore the ornaments when funding is available.
Saint Mary of the Angels, a small parish church in western New York, revealed two other problems.
First, cracks extended up the length of each of the corners of its twin octagonal spires. It appeared that the octagonal sides were separating along their points of contact. The corners were simply failing because there was no bond between the wall panels. The exterior stone wythe served as the tie between the wall surfaces; but as the mortar joints in this wythe began to suffer deterioration and loss of strength through aging, they cracked, allowing water penetration to accelerate the joint failure.
To tie these corners together, a backup layer of reinforced shotcrete was applied against the inside surface of the original rough stone backup and mechanically tied to the face stones at regular intervals on each of the exterior panels. This created some connectivity between the spire surfaces. The exterior joints were then cut out and repointed.
The second problem involved the purely decorative flying buttresses. With the spire walls steeply built to reduce lateral thrust, they did not require the balancing force of the buttresses. The designers did not understand that the mass of the vertical buttresses (or piers), against which the flying buttresses bore, was essential to balancing the thrust from the spires. The vertical buttresses were capped very low and did not therefore have enough mass to overcome the lateral thrust. In addition, the arches of the flying buttresses were scaled up and flattened, increasing their own lateral-thrust component.
Several of the flying buttresses were failing, with large open cracks and shifting of the arch and pier stones. We developed three potential courses of action:
reconstruct the buttresses with hidden reinforcement
- completely remove the buttresses, reconfigure the piers and camouflage the buttress arch landings with masonry reconstruction
- reconstruct the arches and piers and the addition of new pinnacles to the piers to balance the lateral load
The church requested the second option. This was so successful that many parishioners were pleased with the church's new appearance and even failed to notice the absence of the flying buttresses.
As these examples illustrate, structural problems can arise from neglect of the basic elements of masonry design.
The article includes a diagram and explanation of flying buttresses.