2 6 T H RC I I N T E R N A T I O N A L C O N V E N T I O N A N D T R A D E S H OW • A P R I L 7 – 1 2 , 2 0 1 1 L EWI S , S C H M I D T, A N D A N D R EWS • 1 4 5 REHABILITATION OF HISTORIC LIMESTONE FAÇADES BY JONATHAN E. LEWIS, SE; BLAKE M. ANDREWS, EIT; AND MARK K. SCHMIDT, SE WISS, JANNEY, ELSTNER ASSOCIATES, INC. 330 Pfingsten Road, Northbrook, IL 60062 P: 847-272-7400 • F: 847-291-9599 • E-mail: jlewis@wje.com; mschmidt@wje.com 1 4 6 • L EWI S , S C H M I D T, A N D A N D R EWS 2 6 T H RC I I N T E R N A T I O N A L C O N V E N T I O N A N D T R A D E S H OW • A P R I L 7 – 1 2 , 2 0 1 1 ABSTRACT The building boom of the 1920s produced numerous state-of-the-art high-rise buildings in large cities across the country. In Chicago, the façades of many of these new buildings were clad with brilliant white limestone quarried in nearby southern Indiana. Now, more than 80 years later, these façades may have lost some of their initial sheen and luster, but their significance as architectural masterpieces of a bygone era endures. This paper will explore various types of distress and appropriate remedial measures for several historic limestone façades, with an emphasis on choosing repairs that maintain the aesthetic fabric of the buildings. SPEAKER JONATHAN E. LEWIS, SE — WISS, JANNEY, ELSTNER ASSOCIATES, INC. Jonathan E. Lewis, SE, is a senior associate at Wiss, Janney, Elstner Associates, Inc. in Northbrook, Illinois. Mr. Lewis has been involved in a variety of structural investigations, condition assessments, repair designs, and peer reviews. He has performed structural analyses and assisted in the development of repairs for several notable structures across the country. MARK K. SCHMIDT, SE — WISS, JANNEY, ELSTNER ASSOCIATES, INC. Mark K. Schmidt, SE, is a principal and unit manager at Wiss, Janney, Elstner Associates. Mr. Schmidt has focused on the assessment, preservation, remedial design, and implementation of restoration programs for a variety of building envelopes. He has led investigations involving glass and aluminum curtain walls, architectural precast concrete panels, thin stone veneers, stone and brick masonry, terra cotta, door and window assemblies, skylights, composite panels, mosaic tile systems, and EIFS and stucco systems. 2 6 T H RC I I N T E R N A T I O N A L C O N V E N T I O N A N D T R A D E S H OW • A P R I L 7 – 1 2 , 2 0 1 1 L EWI S , S C H M I D T, A N D A N D R EWS • 1 4 7 ABSTRACT The building boom of the 1920s produced numerous state-of-the-art high-rise buildings in large cities across the country. In Chicago, the façades of many of these new buildings were clad with buffcolored limestone quarried in nearby southern Indiana. Now, more than 80 years later, these façades may have lost some of their initial sheen and luster, but their significance as architectural icons of a bygone era endures. The durability of limestone has been well known for many centuries, dating back at least to the construction of the pyramids in ancient Egypt. However, certain façade construction practices employed in the early part of the previous century continue to cause widespread distress and deterioration in historic limestone façades today—conditions that must be addressed, both to protect the public and preserve these signature components of our architectural heritage. Namely, the use of unprotected carbon steel backup framing and anchorage to support the limestone cladding, coupled with many decades of moisture exposure, has led to pervasive limestone cracking and spalling due to corrosion-related expansion of embedded steel. This paper explores various types of distress and appropriate remedial measures for historic limestone façades, with an emphasis on choosing repairs that maintain the aesthetic character of the buildings. INTRODUCTION A significant chapter in the architectural heritage of Chicago involves the building boom that began at the end of the First World War and reached its peak in the 1920s when the modern high-rise office tower became the desired address for any aspiring business. This construction spree continued until the early 1930s, when the credit used to finance these towers dried up, and the country was plunged into the Great Depression. Indeed, commercial building construction did not fully recover until the post-World-War expansion of the late 1940s and early 1950s, nearly 20 years later. The architectural styles of the structures built in the pre-Depression boom vary, but many share at least one common trait: they are clad with Indiana limestone. Several prime examples of these seminal limestone-clad high-rise buildings are discussed herein. Historic Limestone Buildings of Chicago Perhaps the most famous of Chicago’s limestone buildings is the Tribune Tower, which opened in 1925. The building was the result of a highly publicized design competition sponsored by the Chicago Tribune in its desire for a state-of-the-art office tower. Designed by John Mead Howells and Raymond Hood, the 462-ft-tall neoGothic tower consists of vertical bands of limestone panels with pronounced buttresses at the top (Figure 1). Its position of prominence on Michigan Avenue makes the tower a veritable cornerstone of the Magnificent Mile. In 1929, the Medinah Athletic Club building (Figure 1) opened immediately north of the Tribune Tower. The club, which was to serve as the home of the Shriners’ organization, was envisioned as a tower of the Orient with its many setbacks, minarets, and a distinctive gold-colored dome at the top. The façade of the 470-ft-tall building, which now houses the InterContinental Chicago Hotel, is clad with limestone panels and carved decorative limestone friezes, medallions, warriors, and gargoyles. Amid the deepening financial crisis at the onset of the Great Depression, two of the most famous art deco structures in Chicago were completed in 1930. Holabird and Root’s inimitable Board of Trade Building still serves as the anchor of the LaSalle Street financial district, and was for more than 20 years the city’s tallest building. The monolithic Merchandise Mart, a 4- REHABILITATION OF HISTORIC LIMESTONE FAÇADES Figure 1 – Tribune Tower (foreground) and Medinah Athletic Club. million-sq-ft facility commissioned by Marshall Field & Company to consolidate its warehouse operations, was the largest building in the world when it opened. The façades of both structures are primarily composed of Indiana limestone arranged in repetitive vertical bands that give each building the inherent sleekness that is often associated with the art deco genre. Both structures are shown in Figure 2. In 1931, the estate of Marshall Field announced plans for a new office tower in the LaSalle Street financial district to house its corporate offices. The Field Building, designed by Graham, Anderson, Probst, and White–the firm responsible for the Merchandise Mart and the Federal Reserve Bank of Chicago–was to be 45 stories tall and second in height only to the Board of Trade Building. Like the Merchandise Mart, the verticality of the limestone panels and pronounced setbacks at mid-height of the building give the structure its characteristic art deco look (Figure 3). Details of Historic Limestone Façade Construction Like most masonry façades, limestone-clad buildings typically utilize mild steel angles (commonly referred to as shelf angles) at each floor level to support the weight of the cladding components. These shelf angles are usually connected to steel spandrel beams or columns via riveted connections and are often located directly above the window head at each floor. A cross-section showing various features of limestone façade construction is provided in Figure 4. Limestone panels are usually four or five inches in thickness and measure several sq ft in elevation. However, Figure 3 – The Field Building. some decorative stones or parapet capstones can be much 1 4 8 • L EWI S , S C H M I D T, A N D A N D R EWS 2 6 T H RC I I N T E R N A T I O N A L C O N V E N T I O N A N D T R A D E S H OW • A P R I L 7 – 1 2 , 2 0 1 1 Figure 2 – Chicago Board of Trade Building (left) and Merchandise Mart (below). larger and thicker. In most façades, steel strap anchors engage small carved receptacles, or kerfs, concealed in the sides of the stone to provide lateral resistance against wind loading. Individual panels are often set in a full bed of mortar. Behind the limestone panels are typically several wythes of common brick backup, which serve as an anchorage point for the lateral straps and also provide significant absorptive capacity for any moisture that penetrates the limestone exterior. Buildings of this vintage were usually not provided with direct means (e.g., flashings and a drainage cavity) to control and direct to the exterior any water that permeates the exterior façade. COMMON FORMS OF LIMESTONE DISTRESS As this class of historic buildings has aged, several forms of recurrent distress have become commonplace in their limestone façades, necessitating routine periodic inspection and repair. Lack of preventative maintenance has in some cases also contributed to and accelerated façade deterioration. Common types of distress and their causes are discussed in the following paragraphs. Spalls and Cracking at Embedded Steel Components The most prevalent type of distress in limestone façades is cracking and spalling at the location of embedded steel components. Steel shelf angles and strap anchors were usually protected by only a lead-based primer, leaving them susceptible to corrosion if exposed to moisture. Limestone façades are not watertight, and become more porous with time as corrosion- or movement- related cracking in limestone units and joints provides additional avenues for moisture penetration. As steel corrodes, it expands due to the build-up of corrosion by-products, which occupy several times the volume of uncorroded steel (e.g., pack rust). This corrosion build-up results in expansive forces being exerted on the adjacent limestone and mortar comprising the façade and eventually leads to cracking and spalling similar to that shown in Figure 5. If left unaddressed, the expansive forces can eventually dislodge pieces of the façade, posing a significant falling hazard. Reduction in the load-carrying capacity of corroded steel components can also become a concern. Construction methods commonly employed in the 1920s and 1930s involved fully filling head and bed joints of limestone façades with mortar. While this approach initially helped prevent moisture infiltration, it now serves as a significant factor in ongoing cracking and spalling in limestone façades supported by unprotected steel framing. Since the mortar is packed tightly around shelf angles and strap anchors, there is no accommodation for the build-up of rust, resulting in cracking and spalling of the limestone. Bowing and Stability Issues at Rooftop Parapets Parapets are a specific component of limestone buildings that are prone to deterioration resulting from moisture intrusion in the façade. As with other masonry construction, limestone-clad parapets are vulnerable to water infiltration on the front and back surfaces, as well as through open joints in the copings. Poor flashing details at roof membrane terminations and deteriorated parapet joints often allow water to penetrate the limestone façade below, in turn causing additional deterioration. One form of distress associated with limestone parapets is the formation of a distinct inward bow, visible in Figure 6. Bowing is usually attributable to a phe- Figure 4 – Cross section of typical limestone façade. Figure 5 – Limestone cracking and spalling at corroding steel shelf angle. 2 6 T H RC I I N T E R N A T I O N A L C O N V E N T I O N A N D T R A D E S H OW • A P R I L 7 – 1 2 , 2 0 1 1 L EWI S , S C H M I D T, A N D A N D R EWS • 1 4 9 nomenon commonly known as corrosion jacking, which involves the build-up of rust atop the first shelf angle below the parapet. As rust accumulates on top of the shelf angle, it pushes up on the stones beneath the parapet, which in turn lift up on the parapet units. This upward displacement causes the parapet units, which are often one or 2 ft thick, to rotate inward away from the building edge. If the parapet units are indeed solid limestone, the inward rotation itself is mainly an aesthetic issue. However, the jacking phenomenon may also transfer load from the heavy parapet units and into less robust cladding components—such as thinner ashlar units and steel shelf angles— that were likely never intended to resist the self-weight of the heavy parapet units. Further, since the jacking results from corrosion of the shelf angle below the parapet, the shelf typically experiences at least some reduction in capacity. Coupled with the additional load channeled to the shelf angle by the jacking, this loss of capacity can, in certain circumstances, pose a significant structural concern. General Movement-Related Cracking Thermal and wind-induced movements of the building frame can result in cracking in the limestone façade, especially at the building corners. Cracks can extend through both the mortar joints and the limestone units, providing a direct path for water to penetrate the façade and accelerate corrosion of the embedded steel components, which can lead to additional distress. Movement-related cracking is often difficult to permanently remedy, since cracks repointed with mortar usually reopen soon after being repaired, and installation of elastomeric sealant is often aesthetically unpleasing. MAINTENANCE AND REPAIR OF LIMESTONE FAÇADES Although the distress mechanisms described above can in many cases pose a significant risk for building owners and result in costly repairs, a proactive approach towards inspection and maintenance can help mitigate both risk and longterm cost. In addition, effective repairs that are aesthetically sympathetic to the existing façade architecture help maintain and preserve the public face of these historic structures. Typical inspection and repair methods for historic limestone façades are discussed in the following paragraphs. Façade Inspections Like other large cities such as New York, Boston, and Philadelphia, the city of Chicago has a local ordinance that mandates the regular inspection and, where necessary, repair of building façades within the city limits. For most historic limestone façades in Chicago, the local ordinance requires visual inspection from afar (e.g., grade level, adjacent rooftops, setbacks) every two years. Close-up inspections are at the discretion of the professional performing the investigation. However, it is important to note that the local ordinance requires only the minimum level of inspection needed for compliance. For many older limestone, brick masonry, and terra cotta façades, a more rigorous inspection and repair program is necessary to keep up with ongoing distress and maintain the façade in a safe and aesthetically appropriate condition. Further, since the mechanism that drives the cracking and spalling cannot effectively be “turned off,” completion of one comprehensive façade repair program rarely means that an owner is then finished with façade maintenance and repair for a long period of time. Unless all embedded steel components are repaired and protected by durable coatings and flashing, or are replaced with noncorrodible metal such as stainless steel, continued corrosion will result in additional cracking and spalling that will necessitate future inspection and repair. It is therefore important to establish an appropriate interval for close-up inspections so that cracks and spalls can be identified and repaired before they deteriorate into an imminently hazardous condition. The inspection interval should be coordinated between the building owner and architect/ engineer, with due consideration of the façade construction, proximity to public ways, past repair history, and likelihood of recurring distress at previously inspected and repaired areas. In the authors’ experience with one particular limestone building, a maximum inspection interval of eight to ten years was found to be appropriate. For some buildings, a much shorter interval may be required. With each inspection, all instances of potentially hazardous cracking and spalling should be remediated with durable repairs, the characteristics of which are discussed in a subsequent section of this paper. The importance of close-up inspections in maintaining historic limestone façades cannot be underestimated. The manner in which many cracks and spalls form and worsen with time is such that they often cannot be readily spotted from below, either with the naked eye or with binoculars. Namely, many spalls rotate outward so that their presence is masked to an observer on street level. These spalls are much easier to spot from above or head-on. Unfortunately, for most buildings, such a vantage point can only be obtained from a roof setback, the roof of an adjacent building, or suspended scaffolding on the building exterior. 1 5 0 • L EWI S , S C H M I D T, A N D A N D R EWS 2 6 T H RC I I N T E R N A T I O N A L C O N V E N T I O N A N D T R A D E S H OW • A P R I L 7 – 1 2 , 2 0 1 1 Figure 6 – Inward bow of limestone parapet due to corrosion jacking. Localized Limestone Repairs Localized distress similar to the spalling shown in Figure 5 can be effectively addressed by removing and replacing a portion of the ashlar stone with matching stone, commonly referred to as a dutchman repair. A finished dutchman repair set in a mortar bed on an existing shelf angle is shown in Figure 7. When executed properly, dutchman repairs can be an efficient and durable repair that is also sympathetic to the characteristics of the existing façade. Compared to full stone removal and replacement, which can pose significant challenges for larger and heavier stones, dutchman repairs are usually much more feasible, both logistically and economically. In order to promote visual conformity, limestone units used for repairs should be of similar color and texture to the existing stone. It is also advisable to maintain existing joint patterns whenever possible to make the repair as discreet as possible. The most important step in ensuring maximum durability of dutchman repairs is proper preparation and remediation of the corroded embedded steel elements responsible for the distress. In most instances, proper surface preparation entails removing built-up rust from exposed portions of the steel with power tools, and either installing membrane flashing over the steel or coating it with epoxy paint. If epoxy paint is used, the manufacturer’s requirements for cleaning the steel should be followed closely. In some cases, cleaning with a special solution to remove deleterious contaminants may be required to prevent premature coating failure. Depending on the extent of corrosion, further repair or even replacement of the steel may be required. Where possible, it is desirable to provide a small horizontal gap between any embedded steel and the new limestone repair or the mortar bed in which it is set. For example, in Figure 8, the space between the toe of the steel shelf angle and the notched dutchman repair (noted by arrow) should be kept free of mortar and should be at least 1/4-in wide. This gap will accommodate a moderate amount of future corrosive build-up if failure of the epoxy coating occurs and corrosion of the shelf angle resumes. Sealant should generally be avoided when filling the perimeter joints of limestone repairs, both for aesthetic reasons and also because it tends to prevent the evacuation of water. Dutchman repairs are also commonly secured to adjacent original stone with stainless steel pins set in over-drilled holes that are filled with mortar or sealant (note the pin shown in Figure 8). These pins are usually 1/4 or 3/8 inch in diameter and provide a supplemental means of lateral support for the dutchman repair beyond the mortar used to fill the joints around the repair perimeter. Supplemental lateral support at corroded or ineffective strap anchors can be efficiently provided by helical wall ties or epoxy-grouted dowels connecting the limestone veneer to the brick or concrete backup. Isolating or Replacing Embedded Steel In some cases where embedded steel is subjected to prolonged exposure to moisture, corrosion-related section loss can advance to the point where it compromises the structural capacity of the element. If water becomes trapped at the level of steel shelf angles (by a previously installed sealant joint without adequate weeps, for example), corrosion can continue unabated for decades. When repairing related cracking and spalling, the condition of the shelf angle or lateral anchor should be evaluated to determine if it has adequate capacity remaining to support the anticipated weight and/or lateral loads. Strengthening or replacement is required to address deficient components. It is often helpful to give field personnel a general rule-of-thumb on when corrosion-related section loss becomes significant and requires review by an architect/ engineer. The authors typically request that stone masons notify the architect/ engineer when thickness loss of greater than 1/8 inch is encountered. However, there is really no substitute for regular observations of repairs in progress by the professional overseeing the repairs. Replacement of steel components often requires removal of limestone cladding Figure 7 – Completed limestone Dutchman repairs. Figure 8 – Completed repair adjacent to a repair in progress. 2 6 T H RC I I N T E R N A T I O N A L C O N V E N T I O N A N D T R A D E S H OW • A P R I L 7 – 1 2 , 2 0 1 1 L EWI S , S C H M I D T, A N D A N D R EWS • 1 5 1 beyond the areas that exhibit distress in order to access connections to backup framing and maintain existing transitions in framing geometry. In most cases, replacement of shelf angles should be contemplated on a floor-by-floor basis, rather than intermittently on a particular floor level. However, in some instances it can be more economical to replace the steel framing as opposed to cleaning and coating all the existing steel. New framing components should be provided with robust protection from corrosion, by galvanization or coating, and installation of proper flashing. Use of stainless steel can also be considered, but material costs are significantly higher than other options. Parapet Remediation Inward bowing of limestone parapets caused by corrosion jacking usually requires a multistep repair approach. First, the source of moisture causing the corrosion and resultant displacement should be mitigated. For example, any open joints in the parapet or deteriorated roof flashings on the back side of the parapet that serve as avenues for moisture infiltration should be repaired. Second, the corroded steel shelf angle should be cleaned to remove all corrosive by-products. This step may require extensive removal of limestone to access the shelf angle and may also necessitate temporary shoring of adjacent stones that remain, especially the heavy limestone parapet units. If the remaining thickness of the shelf angle is sufficient, then it can remain in service and should be properly flashed to prevent further corrosion. If advanced section loss is uncovered, the angle should be replaced as described above. After completion of steel remediation or replacement, the existing stone units should be reinstalled. If the units are severely deteriorated, they should be replaced with new units of similar color and texture. Figure 9 shows parapet remediation in progress. It is difficult to remediate the actual bowing of the parapet because of the interconnected nature of the individual stones. Intermittent repointing and normal thermal cycling tend to create a “binding-up” phenomenon that would make it difficult to level and plumb the individual units unless all the intermediate joints are ground out. In most cases, the bowing is merely a minor aesthetic issue that is not readily visible from the street. Thus it is often not necessary to expend significant effort to eliminate the bowing, as long as the causes of distress are addressed and the parapet is determined to be both stable and aesthetically acceptable in its bowed configuration. CONCLUSION Historic limestone façades form a significant part of the architectural history of Chicago as well as many other American cities. Although limestone itself is a very durable construction material, corrosion of embedded mild steel supports can lead to pervasive distress that must be addressed as a matter of public safety and historic preservation. Limestone façade repairs must provide long-term durability, but also be sympathetic to the existing aesthetic character of the building. It is important for professionals and building owners to realize that in most cases, where corrosion of embedded steel is the mechanism driving deterioration in limestone façades, this deterioration cannot be stopped unless all of the corrodible metal is addressed. As such, continued vigilance is required in maintaining these key components of our architectural heritage. 1 5 2 • L EWI S , S C H M I D T, A N D A N D R EWS 2 6 T H RC I I N T E R N A T I O N A L C O N V E N T I O N A N D T R A D E S H OW • A P R I L 7 – 1 2 , 2 0 1 1 Figure 9 – Shelf angle remediation under bowed limestone parapet.
