CROSS Safety Report
Structural misunderstanding leads to historic church tower nearly being demolished
A historic church stands at the center of a charming town. Highlighted among many striking features is the iconic 170-foot (52m) bell tower, which looms over the town. For decades, the tower has become the centerpiece for cultural events in addition to being the original community center of worship.
Key Learning Outcomes
For owners of historic churches
- Regularly inspect the facades of old stone churches and similar buildings and look for cracks and displacements
- Consult a suitably experienced and qualified structural engineer if there is ongoing deterioration
For structural engineers
- Thoroughly investigate the form of historic towers and load paths to determine the reasons for cracking or other defects
- Restore the facades using original type materials and techniques whenever possible
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A historic church stands at the center of a charming town. Constructed in 1888, the church celebrates architectural influences from the heritage of the original parishioners. Highlighted among many striking features is the iconic 170-foot (52m) bell tower, which looms over the town, offering a welcoming beacon to visitors. For decades, the tower has become the centerpiece for cultural events in addition to being the original community center of worship.
The community regularly gathered in the lawns around the steeple to attend free concert offerings from a twelve-bell carillon housed within the tower. The idyllic tradition was recently cast into shadow when caretakers noted the existence of cracks, localized spalling, and general masonry disrepair. These concerns led parish leaders to adopt the understanding that the tower was falling apart. The presumed culprit? Vibrations and other dynamic forces from the motions of carillon bells. This justification led leadership to suspend concerts, eliminate all carillon usage, close the facility, and begin planning for the demolition of the historic building.
These steps were taken without any input from outside the church organization. In hope of saving the building, concerned staff and community members sought an independent technical opinion from a structural design professional. Grade-level exterior inspections revealed a series of cracks that existed in the exterior brick masonry at windows, doors, and near building corners. Presenting at areas of expected stress concentration throughout the building, these cracks are common in masonry of this vintage and are not related solely to structural behaviors of the tower.
Significant gaps between the exterior masonry wall of the tower and the perpendicular flying buttresses of the adjacent side aisles were observed from windows of adjacent structures. Previous reports and assumptions had asserted that these gaps were specifically attributable to the usage of the carillon bells. Implementing some respectful skepticism of established assumptions, however, the structural design professional climbed several flights of stairs and a few seldom-used ladders, which afforded admission to the rarely accessed carillon belfry. One notable feature became immediately apparent: the aged timber structure supporting the bells was structurally isolated (Figure 1).
Figure 1: Timber structure supporting bells is structurally isolated from the exterior masonry walls
Over a century ago, the original designer astutely determined to keep the support of the carillon independent from the surrounding masonry facade. This was likely more challenging and less efficient for the builders, constructing one frame inside the other, but allowed for two completely isolated structures.
A second critical observation was that the bells were permanently affixed in a stationary position, either by design or decades-old retrofitting. Instead of a bell rotating/swinging and sounding with an internal clapper, the ringing of each bell is activated by a side-mounted, electronically-activated hammer. Since they are stationary, the bells do not create significant dynamic forces within the tower. Further, whatever minimal forces are initiated are not transferred to the masonry walls due to the as-designed structural isolation. Combined, these two realities painted the originally voiced concerns as potentially costly misconceptions.
Though this review occurred well beyond the original design timeline, the grasp of the current situation featured misunderstandings that led to significant problems. Incorrect assumptions grossly mischaracterized common masonry problems and led to misinformed decisions. The actual designer, of course, could not have been consulted, but grave consequences for a historic structure and the surrounding community were quickly unfolding.
As structural design professionals, it is critical to ask difficult questions, challenge the presented ‘facts,’ and take extra steps to physically verify the actual conditions. Assuming the provided information is correct can be a mistake. Instead, skeptical review and verification are often appropriate.
Fortunately, in this case, the involvement of a structural professional furnished the church with a fact-based assessment and detailed explanations regarding typical exposure-related masonry cracking and separations due to long-term differential building movements. Armed with renewed insight, a phased masonry restoration program was initiated, the carillon restrictions lifted, and the community gathered around the spire once again to hear the bells ring.
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Expert Panel Comments
An Expert Panel comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-US Expert Panel page.
An interesting example of preservation and restoration engineering in a historic building. Churches dating back from centuries past exemplify what our forefathers were able to achieve with craftsmanship and long experience. In this case the core of the matter is the construction and condition of the bellframe. Bellframes are used to cope with large forces and are subject to dynamic loads from rotating bells coupled with mechanical wear of the moving parts.
The real heroes of this saga are the concerned staff and community members who had the wisdom to seek the services of a qualified structural engineer. Far too often, decisions related to building and bridge structures are made without the involvement of a structural engineer. This is particularly problematic with historic structures. The structural engineer should also be commended for their skepticism of the initial information that was provided, choosing instead to thoroughly investigate and establish the facts.
Information on stone towers can be found in literature such as The Stone Skeleton: Structural Engineering of Masonry Architecture by Jacques Heyman published online in 2014. Heyman identifies ‘ringing cracks’ and says: Indeed, wind forces can help to propagate the ‘ringing cracks’ attributed to the bells. Wind forces, although greater than the bell forces, are however, intermittent; 40,320 combinations of an eight-bell ring can be achieved in something over 3 hours, during which time the tenor bell will have sounded 5,040 times.
In this case there were indications in the report that the bell sounding method had been changed at some stage from full circle ringing to activation by a side-mounted hammer. If indeed the bells had originally been full circle mounted the above quotation indicates that there must have been an enormous number of cycles of soundings over the years. Nowadays, purists, might say that the original mechanism should have been restored instead of installing a different (electrically powered) system.
The fact that the ringing mechanism was changed may indicate that historic cracks were induced by the ringing and had started to form before the alteration which might have just slowed their development. More might have been learned by a study of the location and shape of the cracks. It is also interesting to note that there was an independent structure supporting the bellframe, so the original designer/builder had come across the problem before.
It is also an excellent example of why it is important to “consult your local structural engineer” for issues that appear to be structural in nature prior to making big decisions. For structural engineers this is also a reminder of the need to verify information presented as facts, particularly if they are simply presented by the users/owner. Taking the time to do due diligence and investigate all aspects/possibilities prior to drawing conclusions is important.
Finally, it is important to remember that the assessment and remediation of old stonework are specialist tasks and it is always well worth consulting a specialist before making irrevocable decisions about historically valuable buildings.