Tuesday, August 23, 2011 at 1:51 PM EDT, a magnitude 5.8Mw earthquake was recorded by the United States Geological Survey (USGS) approximately 84 miles southwest of Washington DC, near Mineral, Virginia. Due to the geology of the US’ eastern seaboard, even moderate earthquake events will usually be felt across a far wider region than an earthquake of equivalent magnitude in the west, shaking an inventory of buildings generally designed to resist far smaller earthquake forces. The USGS has reported that this was the most widely-felt earthquake in US history.
According to the Instrumental Intensity map available from the USGS, the Modified Mercalli Intensity (Imm) value estimated for Washington DC was about V, which correlates to a moderate level of perceived shaking and a very light potential for damage. However, the configuration and construction of the Washington National Cathedral is highly atypical due to its ornate Gothic architectural features and rendered it more vulnerable to earth quake ground shaking.
After an extensive damage assessment, the owner elected to undertake a phased restoration process with trial repairs that will minimize disruption to public visitation and use, as well as align with their expected funding stream. The phased approach also allows the in-house stone carvers to participate in the restoration as their schedule allows.
The Washington National Cathedral (Cathedral), officially named the Cathedral Church of Saint Peter and Saint Paul, is a cathedral of the Episcopal Church in Washington DC. It is the sixth-largest cathedral in the world, the second-largest in the US and the fourth-tallest structure in Washington DC. Construction spanned from 1910 to 1990. The Cathedral is a 100-year-old unreinforced stone masonry structure. More slender portions of the decorative elements are composed of solid limestone, while most other elements are constructed compositely with a brick masonry core and limestone facing units. Some of the larger solid stone pieces weigh as much as five tons.
The Cathedral is designed in the Gothic Revival style characterized by carved stone flying buttresses, cantilevered spires, pinnacles and its 330′ central tower. The main structure consists of a long, narrow rectangular mass formed of a nine-bay nave with wide side aisles and a five-bay chancel, inter sect ed by a six-bay transept. Above the crossing (intersection of the transepts, nave and sanctuary) is the central tower. At the north and south ends of the narthex are two smaller towers, approximately 200′ in height. Figure 1 illustrates the relationship between principle exterior building elements.
Structural Condition Assessment Approach
Wiss Janney Elstner was retained to evaluate the integrity of the structure and ornamentation following the Mineral earthquake. It was quickly determined that the complexity of the Cathedral with respect to dynamic response, made a whole building dynamic analysis impractical. Instead, locations of damage were carefully evaluated to identify the most vulnerable elements, then rationally analyzed to develop strengthening concepts to improve future performance.
A close-range survey and assessment of the vaulted ceiling, interior wall and column surfaces was performed at the side aisles of the nave, transept and choir/sanctuary, where the ceiling height is lower, to identify and remove any visibly loosened or other – wise potentially unstable joint mortar and limestone that existed prior to or as a result of the seismic event. Netting was used to protect the public at the inaccessible areas, such as the center of the nave.
A visual survey of all exterior building elements was performed from the ground, low roofs and towers. This effort enabled identification of unstable elements in need of in-situ stabilization, located critical over head protection areas, assessed the extent of damage and provided information necessary to prioritize future restoration efforts. A close-range visual condition assessment and sounding of the northwest tower and southwest tower exterior walls was performed to identify any hazardous or unstable elements along the tower shafts and allow re-opening of the Cathedral to the general public and provide safe access through the main entrances. This work was completed using rope-access techniques and a difficult access team (DAT) populated by architects, engineers and rope-access specialists.
Damage to the exterior resulting from the earthquake is widespread. The majority observed during the survey was at non-structural and largely ornamental sections of lime – stone. Damage ranged from a loss of entire courses of limestone from the grand pinnacles at the Central Tower to minor spalling and chipping at joints between stone masonry units. Mortar in numerous joints was cracked or debonded from the stone surfaces, despite its rich mix pro portions (1 part cement, 1/2 part lime putty and 2 parts sand).
Earthquake damage was sorted into three categories based on its severity and the extent of in-situ stabilization, repair, dismantling required. Figure 2 The first category is No Damage illustrating areas where no visible damage was observed and that will not require any scaffolding or similar means of access for repair. The second category is Minor Damage, which consists of limestone elements that contain visible damage at the exposed surfaces of the stone, but remain materially intact and fully engaged to the structure. Repair of these elements, which in several locations include surfaces and features that do not contribute significantly to the architecture of the Cathedral, could be under taken on a voluntary basis as funding becomes available. The final category, Major Damage, includes: missing or other – wise visibly unstable elements that remain a potential fall-hazard and damage considered structurally insignificant but in need of immediate repair to restore the originally intended architectural expression of the element. Areas identified as Major Damage require scaffolding and/or the use of mobile cranes that can facilitate the removal, replacement and resetting of large stone units.
Damage at the interior of the Cathedral was limited primarily to loss of joint mortar from wall and vaulted ceiling surfaces, with some small chips and spalling in ad joining sections of lime stone. During this survey, several cracked and partially disengaged sections of limestone (incipient spalls) were also removed to eliminate potential fall-hazards. It was apparent that the majority of interior damage predated the earthquake and was likely caused by more conventional building movements; how – ever, a significant number of fragments were further loosen ed or fell as a result of seismic activity.
Cathedral’s Response to the Earthquake
Pinnacles throughout the Cathedral vary in size, mass and detailing; however, they all are typically tall and slender. Some are present at the lower elevations such as those on the buttress piers; others are present at the towers and occupy the highest points of the Cathedral. Construction of the pinnacles spanned from 1915 (apse flying buttress pinnacles) to 1990 (west tower grand pinnacles). Earlier period pinnacles do not include any doweling between masonry elements, while those built more recently incorporate dowels and cramp anchors between stone masonry elements. Damage to the pinnacles consists of rotated finials, missing or damaged ornament, cracked or displaced stones and displaced portions. Figures 3 and 4
Turrets at the south transept are unique in that the major pinnacle is supported on an open colonnade just above the transept roof line. The entire pinnacle shifted laterally at the colonnade that acted as a soft story. Figures 5 and 6 North transept turrets do not have the open colonnade, but do incorporate slender tertiary pinnacles that are sup ported by small fliers that fractured, leaving the pinnacles unstable. Figure 7
Metal roofs and limestone parapets at the towers and throughout the lower portions of the Cathedral suffered only indirect damage from the earth quake. Damage observed here is largely due to impacts from stone fragments falling from the pinnacles at the towers and flying buttress es. Although several small sections of limestone were removed from the north west and south west towers during the damage assessment, remaining surfaces of each tower were found to be structurally intact and stable, with only isolated locations of cracks, spalls, missing mortar and debonded sealant. There was little evidence of new damage resulting from the earthquake.
Seismic Strengthening Considerations
When an event such as the Mineral earthquake occurs, the natural reaction for any owner is to consider whether the structure is adequate to resist seismic forces from a future event, or if the structure should be enhanced or upgraded to reduce or eliminate future damage. The answer is complicated, particularly for a monumental one-of-a-kind unreinforced masonry structure such as the Cathedral.
The most common objective of contemporary seismic engineering is not to eliminate all damage from an earthquake, but to prevent structural collapse that would endanger occupants or render the structure unusable. Except for those with special post-earth quake critical functions, the goal of preventing all damage to a building during an earthquake is normally considered overly conservative and wasteful given the rarity of such events. Keeping that measure in mind, and assuming that the Mineral earth – quake generated forces that met or exceeded the current design requirements, one can argue that the Cathedral met the commonly used seismic design objective: the overall building remained stable with no major structural failure, and damage sustained can be classified as non-critical and repairable, albeit at a substantial cost due to its architectural complexity since this is a unique architectural work rather than a simple structure.
There are no code requirements that necessitate upgrading the Cathedral structure for seismic forces. Without such a mandate, any seismic upgrades would be clearly voluntary as a hedge against the potential for future damage and loss of life. To make an informed decision about expending substantial costs to upgrade the structure, one should first carefully consider the risk of a future major seismic event. Secondly, one should have a reliable understanding of the physical properties and dynamic behavior of the structure to accurately predict the effects that a major earthquake might have on the structure and for what level of seismic event. Lastly, but no less importantly, one must understand the potential for injury or loss of life associated with the use of the structure. Since none of these factors is easily defined, it would be premature to embark on a plan for upgrading the Cathedral, including the signature gothic design elements, in its entirety. After appropriate study, it might be found that upgrading nothing, or selective upgrading only those components of the Cathedral that present the greatest risk, is the most reasonable course of action; wholesale upgrading may be wasteful or impossible to achieve without significantly damaging the extant historic fabric.
Given the uniqueness of the Cathedral as a treasured landmark, strengthening of all the elements that are known to be deficient by contemporary construction standards using conventional means and conservative assumptions of lateral forces based on the information available might, of course, be prudent. However, costs associated with modifying all of the elements potentially at risk could exceed the currently available financial resources and could damage the building fabric that the upgrade would be intended to preserve for future generations. The potential for costs outstripping available funds necessitates prioritization of repairs so that any upgrades that are performed will balance cost with value, and will balance any desire to prevent crippling damage during an earthquake against the potential architectural damage that major seismic retrofits can cause.
A simpler, more funda mental approach was adopted to address concerns about future seismic event similar to Mineral, or events that might be significantly different in character and intensity. Prioritization of the structural improvements is based on life safety, repair accessibility and vulnerability of particular elements to seismic forces.
The primary factor used in prioritizing the level of seismic strengthening is life safety, or protection of the public. Those elements at most risk of becoming a falling hazard or that are located directly above an area where pedestrians congregate are given a higher priority for structural upgrade. For elements requiring re-anchoring, upgrading will be accomplished with relatively small incremental cost if performed at the same time as other already necessary repairs.
There are damaged elements from the narthex to the apse, from the north transept to the south transept, and from the west to the central towers. Access to these slender elements with both labor and materials is the most formidable challenge and most costly components of the future repair work. Despite the breadth of damage, there are many areas and individual elements on the structure that are not damaged or experienced such minor distress that the cost of accessing these locations to repair and/or strengthen them is not justified economically. Therefore, access is a key factor in determining what elements are strengthened. In other words, structural modifications are limited to areas that are otherwise accessible as part of the overall restoration.
The geometry and locations of many design elements employ ed at the Cathedral make them uniquely vulnerable to future seismic loads. As observed during our survey, the tall and slender pinnacles and apse flying buttresses suffered the most damage. These elements were studied in more depth to avoid strengthening unnecessarily but also to avoid missing potentially serious safety issues.
Limited Strengthening of Cathedral Design Elements
There are a number of techniques for improving performance of unreinforced masonry structural elements exposed to seismically generated forces, most of which rely on the introduction of steel reinforcement to provide improved connectivity between masonry elements and greater ductility to absorb seismic energy with minimal instability. Some of these are integrated directly into the reconstruction of the Cathedral; for example, adding dowels or vertical reinforcement between stacked stone masonry units or adding lateral anchors to secure stones to each other or to a masonry core. Other techniques will be implemented without removal of the stone masonry and installed from the exterior surfaces of vulnerable masonry elements. Provided access is available, the cost of these techniques is relatively minor when compared to that of a global restoration. Any intervention undertaken should be based on reasonable structural assumptions to determine whether the modifications will adversely affect the structural behavior of the element or the structure as a whole, and to determine if certain potential modifications provide meaningful improvement.
Setting the Standard for Earthquake Restoration
Many fallen stone pieces were salvageable. Masons could make Dutchman repairs, selectively replacing the damaged portion of the stone element with new stone material to match as closely as possible, or re-carve an ornament that was too damaged to repair. The Cathedral is built of Select Buff Indiana limestone, which is in abundant supply and consistent even 100 years later, regardless of the pit, or even quarry, from which it was cut. Buff describes the color— light creamy to tan, while select describes the characteristics—fine to average grain with minimum distinguishable characteristics, such as fossils, iron spots, calcite streaks, etc.
Head Mason Joe Alonzo, Sean Callahan and Andy Uhl, were part of the crew who worked on the final phase of the new construction in 1990. They have provided continual maintenance and as needed repair on the structure ever since. Their intimate know ledge of the Cathedral and expertise, together under the leadership of Director of Preservation and Facilities Jim Shepherd, will set the standard for earthquake restoration.
Matthew C Farmer, PE, principal, joined the New Jersey office of Wiss, Janney, Elstner Associates in 1986 and moved to the Washington DC office in 1990. He has served as principal investigator on evaluations of clay, concrete, stone and cast stone masonry. He has concentrated his practice in the area of masonry building enclosure engineering, design, investigation, analysis and repair. Projects have included institutional, commercial and residential buildings as well as numerous historic landmarks. Farmer is a licensed engineer in Washington DC, Maryland and Virginia. He is a graduate of the University of Colorado and Cornell University. firstname.lastname@example.org|703.641.4601
Cortney L Fried, PE, is a senior associate with Wiss , Janney, Elstner Associates. Since joining the firm in 2001, Fried has been involved in the investigation and assessment , as well as analysis and repair design of various facades and structural systems. Her experience includes work in various materials: clay, stone and concrete masonry among them; Fried is a graduate of the Georgia Institute of Technology and Stanford University. email@example.com|703.641.4601