During the advent of the 1900's, the use of thin-stone facades such as that seen in Figure 1, as an architectural feature on buildings became popularized due advances in technology and more economical means to erect these systems. Although these systems bring about a pleasing aesthetic appearance to the buildings that they adorn, the understanding of the science and engineering behind these systems is still in its infancy. Due to the relative unknown behaviors of thin-stone facades, there have been many cases involving the failure of these facades. Historical cases have shown that thin-stones facades are sensitive to environmental conditions, often related to temperature. Along with their sensitivity to the environment, special considerations must be taken into account in constructing these systems.
Causes
The thought of using stone for construction leaves the lasting impression that these systems will have a long life due to the thickness of the material. However, the characteristics of heavy stone facades and thin facades differ greatly. All stone veneers are subjected to weathering and incur surface loss as a result. Thin-stone panels are subjected to the full effect of temperature changes, resulting to these systems experiencing varying stresses throughout their life-cycle. The way thin-stone facade systems are attached to the main structure is often an aspect of design that is overlooked, resulting in devastating consequences. Frequent failures associated with thin-stone systems are attributed to the lack of understanding about the behavior of thin-stone when considering environmental factors (Beasley 2010).
Hysteresis
All stone facades experience expansion and shrinkage due to cyclic temperature changes. This phenomenon is known as hysteresis and is prevalent in thin-stone systems, as shown in Figure 2. The grains that compose these stone systems will dislocate during thermal cycles and will not return to their original positions. In thin-stone panels, the dislocation of these grains at the exposed surfaces results in uneven expansion, creating bowing. The additional fractures due to this process lead to increased water absorption, deteriorating the strength of these systems. The main component in many thins-stones is calcite, which is an anisotropic material.This means that calcite will expand more in one direction than another. When exposed to several cycles of expansion and contraction due to thermal conditions, irreversible granular fracturing will occur, causing a loss of flexural strength.
As mentioned, the effects of hysteresis result into greater absorption of moisture into the thin-stone panel systems. Water that is absorbed by these wall systems can lead to corrosion of metal anchors which reduces the strength of the steel supports and can result in spalls and cracks in the façade. The moisture that is retained by the stone is subjected to the freeze-thaw cycle. When water freezes, it expands leading to the creation of cracks and rupturing of material. Problems also occur due to the evaporation of moisture. When water evaporates from the surface of these façades, minerals will crystallize creating efflorescence deposits. If this process occurs in the surface of the façade, the surface may experience what is known as sugaring, which are pits that develop on the surface.
Restricted Movement
Inherently, stone materials are of a brittle nature. They react poorly to internal movement and will often crack or break when they exceed their elastic limit. Thin-stone facades are not designed to be load-bearing. Because of this, these systems are sensitive when force is applied and movement in the panels is limited. The frames which these facades are attached to are often sheltered from the environment and do not experience movement. When stress is induced in the thin-stone facades, from either external forces or internal forces, they are unable to relieve the pressure and often will crack or spall. Cracking and spalling can also be contributed to either the design of undersized expansion joints that do not allow for movement between panels or poor anchorage details that are not flexible enough to permit any movement. Rigid anchorage connections connection the veneers to the supporting structure react poorly to differential movement. This induces stresses in the stone panels causing damage.
Case Studies
There has been many cases within the last century resulting from the failure of thin-stone systems. Many of the failures attributed to these systems is due to lack of understanding of behavior. Often is the case, whole facades must be torn down and replaced since the original thin-stone veneer will have incurred significant damage that they can not be salvaged. These failures tend to lead to a lengthy litigation process as well as severe damages in terms of cost.
Case Study 1: Amoco Tower
Figure 3: The Aon Tower (formerly The Amoco Tower) (Photo Credit: FreePhoto.com)
The most famous historical case associated with thin-stone façade failure is the Amoco Tower failure in Chicago, now known as the Aon Center, seen in Figure 3. This failure is associated with the thin-stone marble cladding that adorned the 80-story building which was subjected to hysteresis. The Amoco Tower was covered with 43,000 panels of Carrara marble with a thickness of 1-1/2” above the 43rd floor and 1-1/4” for the stories below. The tower’s thin-marble façade was subjected to extreme thermal gradients resulting in the warping and spalling of the panels. This resulted in some panels bowing out at far as 1” and drooping more than 1” (Arndt 1988). After investigation, it was found that the marble used in actual installation had less than half the flexural strength of the materials used in the original testing. Panels that were most affected occurred where the building received the most sun exposure. It was concluded that within 10 years, the marble would lose up to 70% of its original flexural strength. The tower was eventually recladded with white granite as a cost of $60 million, half of the cost of the original building.
Case Study 2: The Empire State Building
The Empire State Building’s façade did not require repairs for almost 60 years of service, which represented a respectable service life of the exterior wall system. However, when repairs had commenced, there were problems discovered in the anchorage system of the wall from years of corrosion. The original wall system consisted of a thin limestone facing of thickness in the inches that was anchored into a brick back-up masonry wall that was tied into structural steel columns. When repairs were being done, it was discovered that the iron strap “cramp” anchors which tied the brick back-up to the column experienced significant corrosion due to oxidation. Because the brick masonry had been built directly against the steel column, the expansive forces that were developed by corrosion displaced and cracked the masonry back-up. This in turn resulted in cracking on the limestone façade. These cracked allowed for further water penetration into the wall system accelerating steel corrosion and masonry displacement. Repairs were attempted in the 1950’s to contain movement in the limestone by installing steel straps around the outside corners, but this did little to deter the deterioration process (Nacheman 2005).
Case Study 3: Finladia Hall
Like the Amoco Tower in Chicago, the Finladia Hall in Helsinki Finland was cladded in thin panels of Carrara marble. The original design intended for the performing arts center to have the flavor of Mediterranean culture (Loughran 2002). The marble that cladded this building had a maximum panel dimension of 55" high by 1-1/4" thick. Pins were inserted 14" from the panel edges, connecting each panel to four adjacent ones. This thin marble façade was poorly suited for the extreme environmental condition in Finland. Within a few years of its completion in 1971, noticeable deformations in the marble panels were detected. Efforts took place in the late 1980s to renovate the marble veneer. Political reasons resulted in that the Carrara marble be reused for the new façade. The new system was designed to allow for flexural connections, reducing the internal stresses in the stone. Improved flexural strength of the stone was included in the redesign, although the original thickness of 1-1/4” was maintained in the new wall system. The cost of the renovations was over 3 million Euros for the replacement of 75,320 square feet of panels. After completion of renovation in 1999, it was observed that the new panels experience deformation (Hester et. al 2009).
Prevention and Considerations for Design
Special care and consideration needs to be taken into account when designing and implementing a thin-stone facade system. New research and advancing technology has allowed for these systems to be incorporated into modern building design with a greater understanding of the systems behavior. The design for these systems can be broken down into five main categories: anchorage, moisture protection, environment, and post-construction considerations.
Special care and consideration needs to be taken into account when designing and implementing a thin-stone facade system. New research and advancing technology has allowed for these systems to be incorporated into modern building design with a greater understanding of the systems behavior. The leading resource for proper thin-stone façade design is provided by the Marble Institute of America’s Dimension Stone Design Manual, first published in 1971. Other additional resources for design include the Indiana Limestone Handbook which details the design of limestone facades and The Whole Building Design Guide on Thin-Stone Walls written by Michael Scheffler.
Design Considerations
Although thin-stone veneers are not designed to be the main structural resisting element of lateral loads, careful considerations must be taken to ensure that they are properly designed for loading, especially for wind design. In order to properly design the anchorage system, the proper wind loads must be applied to the building component and cladding. The American Society of Civil Engineers (ASCE) 7 standard lays out the procedure to properly determine the wind loads acting on the façade system. In addition to using ASCE 7, wind tunneling analysis may be performed to provide a more accurate calculation of wind loading, at the expense of additional cost and resources (Kuriyama 2010).
Moisture Control
Sealants are generally provided to join stone paneling, although mortar is another alternative. If mortar is used in design and construction, its strength should be evaluated as well as its adhesive properties. To achieve optimal mortar adhesion, tooling profiles should create a pathway for moisture to drain. For sealant joints, considerations for bonding to the stone, cohesion to resist internal cracking, and flexibility must be taken into account. The seal should be designed to resist moisture passing through it and not be subjected to thermal erosion. Adhesion tests should be conducted in order to determine the proper selection of sealant material. Recommendations from the Dimension Stone Design Manual recommend the following for moisture control:
1/4" weep holes shall be provided at each story
Ventilation and proper drainage shall be provided at a maximum of 5' horizontal and at 20' vertical
Cavity walls also help provide moisture control in thin-stone veneer systems (Scheffler 2011). Through wall flashing should be provided at regular intervals to ensure that water enters the cavity and through the exterior. Rain screens are also typically used in conjunction with thin-stone systems. In these systems, the water resistant barrier is to be placed at the surface of the backup wall. All joints are to be left unsealed and the actual wall system acts as an additional barrier to prevent moisture from entering the backup wall.
Anchorage Design
The anchorage system is designed to transfer lateral loads from the thin-stone veneer to the supporting structure. It is vital that these systems perform up to serviceability and strength criteria to ensure that the envelope does not incur damage. Anchors should be designed to adjust to the change in shape of the stone as time goes on. Figure 4 depicts the planning of laying out where the anchors to a thin-stone panel should be place. They should allow for some flexible tolerance due to movement of the stone system. Steel anchors should be properly protected from moisture to prevent corrosion and loss of strength. Typically, the weight of the system, thickness, composition, and exposure will dictate the proper safety factors for detailing the anchorage.
Figure 4: Laying out of Veneer Anchors (Photo Credit: Hoffmann Architects)
The Section 1405.6 of IBC 2006 presents two specifications in which stone veneers should be anchored to masonry, concrete, or studs in construction:
For concrete or masonry backup, corrosion resistant anchor ties are required to be laid in mortar joints at every 12". A wire tie is required for every 2 square feet of area. The legs of each loop is to be laid in the backup mortar joint and stone veneer mortar joint. 1" thick cement grout is to be placed in between the backing and the stone veneer.
For wood studs, wire mesh shall be applied over the studs. Corrosion-resistant wire will be looped for every 2 square feet of mesh. The tie shall lie in the mortar joint and 1" thick grout shall be placed between the backing stud and stone veneer.
Movement Control
The thin-stone veneer system should incorporate some tolerances in the case of movement due to expansion. Expansions joints should be provided to allow for the expansion of material. Lintels and horizontal supports should limit deflections to one-five-hundredths of the overall height of the panel (Hester et. al 2009). Expansion joints should be provided at every 20 ft maximum between panels to allow for enough movement and prevent cracking.
Investigation
If a thin-stone façade failure is detected, there are proper methods in order to inspect the system as well as techniques to implement repairs. Periodic inspections should be performed in order to detect thin-stone façade failures. These inspections can also serve to determine the service life of the façade and its associated components. Visual condition surveys of building facades involve documenting observations by commenting on the building condition as well as taken photographs. These surveys should identify the locations where problems are detected as well the nature of the problem. These surveys can help in determining the need for additional studies and laboratory tests. They also provide clues into the underlying cause of failure. If the situation is critical, it may be necessary to remove panels to identify the problem. Although the removal of panel may be disruptive and aesthetically unpleasing, it can help in determining the systems’ physical condition and whether repairs are required (Beasely 2009). Below are a few non-destructive investigative tools and procedures that can be utilized to investigate the condition of an existing veneer system.
Borescope
The borescope is a fiberoptic device that enables an inspector to view a veneer system through an existing cavity. A borescope consists of fiberoptics or a micro-miniature video camera that is able to view behind a facade. It is an optical device made of a rigid or flexible tube with an eyepiece at one end, and an optic lens at the other which relays information. The camera's small tube (6-8mm) can be inserted through either a small hole or existing joint. This makes this tool extremely effective in assessing the condition of a wall because it involves very little if any alteration to the existing veneer system.
Pulse Velocity Method
The strength of the stone in veneers can be tested using a pulse velocity test conducted according to ASTM C597-02. This test consists of a waveform or velocity of a sonic pulse that is sent through the material of interest. A transmitting inducer creates an ultrasonic or sonic energy pulse through the stone and a transducer detects the signal. The incorporation of timing units allows for the measuring of elapsed time of transit. By dividing the velocity of the wave by the distance between transducers, the material strength and elastic modulus can be determined.
Repair
In the situation where repair is required as seen in Figure 5, several considerations must be taken into account. Safety, aesthetics, feasibility, cost, and serviceability are the five main areas to consider in repair work (Scheffler 2001).
Figure 5: Recladding of Marble Veneer (Photo Credit: Hoffmann Architects)
Safety - In the scenario where extensive repair work must be performed, temporary stabilization should be provided to protect the welfare of the public and workers. Proper repairs should be performed to provide a structurally adequate thin-stone veneer system. The design should incorporate new codes to meet minimum requirements. Considerations for the impact on the existing structure from the new system as well impact on support elements should also be considered.
Aesthetics - The structural repairs should be aesthetically acceptable. Anchoring should not be visible and mortar and sealant patch repairs should be acceptable. Not all defects need to be repaired, but judgment should be used to determine if are visually intrusive. Care must be taken into matching new material with the original material. If the original material is not available, the best efforts must be used to find a suitable replacement.
Feasibility - Repairs must have some reasonable chance to be implemented. Physical limitations and dimensional constraints must be considered when implementing repairs. The existing stone must be thick enough for repair anchors. The anchors must be able to be attached to the backup support. The backup support must be structurally sound and constructed of a material that is adequate enough for the veneer to be attached to. The causes of initial distress in the system must be remedied in repairs or else this can lead to repeat of failure.
Cost - Cost should be taken into account and determined whether or not is the primary consideration in repair. The minimum cost should be determined that will provide a safe façade. This will aid the owner in establishing budget for repairs. The cost of repair should be compared with the cost of removal and replacement. This can lead to a decision of whether the existing system should be salvaged or a new system be implemented. If a substitute material is used, its cost should be evaluated in order to determine if it is a viable solution.
Serviceability - The long term effects of repairs are important to consider. The repair should provide a long term solution so that the original problem does not reoccur.Temporary stabilization repairs should allow for ongoing inspections to occur. Implications of retaining temporary structures should also be taken into account.
Conclusion
Failures associated with the use of thin-stone veneers can be attributed to the lack of understanding on how to properly design and construct these systems. Problems with these systems will continue to persist if designers do not consider what conditions they plan on using these systems in. When not properly designed, large costs associated with repairing or replacing these systems can be incurred, as seen in the Aon Tower and Finladia Hall case study. Further research is currently being further developed to help develop a better understanding of how these systems act. Until enough information is available on thin-stone veneers, designers should consider historical failures as a means of design when planning to use these systems.
Annotated Bibliography
Arndt, Micahel (May 22, 2044). "Amoco Tower's Fate May Be Carved in Stone." Chicago Tribune. http://articles.chicagotribune.com/1988-05-22/business/8801010440_1_amoco-building-marble%20-tower Newspaper article detailing the most expensive thin stone facade failure, The Amoco Tower in Chicago. The article covers a brief history of the case and highlights the investigative teams involved in determining the cause of failure.
Beasley, Kimball J (2010). "Building Facades." Forensic Structural Engineering Handbook 2nd Edition http://www.accessengineeringlibrary.com/mghpdf/0071749594_ar015.pdf This is a piece written by Beasley from the Forensic Structural Engineering Handbook. Chapter 15 is dedicated to building facades. It contains pictures associated with thin stone facade failures and details the reasons behind the facades not performing as intended.
Hester, Daron et. al. (2009). "Thin Stone Marble Facades: History, Evaluation, and Maintenance" Building Envelope Technology Symposium. October 2009. This paper reviews the history of marble facades and descibes various design techniques. It also discusses the various deterioration mechanisms of thin-stone facades and offers maintenance recommendations.
Kuriyama, Juan (2010). "Thin Stone Marble Facade Systems." Hoffman Architects Journal Issue 4, Volume 27. October 12, 2011. http://www.hoffarch.com/images/userfiles/journal61.pdf This is another published online article, written specifically about thin marble usage. It is written from an archtitect's perspective, but highlights considerations when using using thin marble as the building facade. The article breaks down design and installation considerations, including anchorage, panel movement, and environmental factors. Many pictures are also presented to illustrate points.
Loughran, Patrick (2002). "Failed Stone." Birkhäuser Architecture. November 20, 2002
This book was used to expand upon the case study of Finladia Hall. Overall, the book dives into many failures associated with stones used as a construction material.
Nacheman, Robert J (2005). "The Empire State Building Facade: Evaluation and Repair of an Engineering Landmark." 2005 Structures and the 2005 Forensic Symnopsium In this paper, Robert Nacheman writes about restoring the thin stone facade that was originally on fhte Empire State Building. A key part of this article is how he details the anchorage system for the facade and how the original was not sufficient.
Scheffler, Michael J (2001). "Thin-Stone Veneer Building Facades: Evolution and Preservation." APT Bulletin, Vol. 32, No.1 http://www.jstor.org/stable/pdfplus/1504690.pdf?acceptTC=true This bulletin from the Association for Preservation Technology International details the evolution of thin-facade veneers in buildings. It then goes into detail about various failures associated with these facade systems. Included are many images.
Scheffler, Michael J(2010). "Thin Stone Wall Systems" Whole Building Design Guide. November 20, 2011 http://www.wbdg.org/design/env_wall_thin_stone.php This Whole Building Design Guide article on Thin Stone Wall Systems provides basic information on how to properly design these veneers. Additional resources are provided at the end of the publication to provide further detail for design.
Additional Resources
"Dimension Stone Design Manual" Version 7.2. 2011.
"Indiana Limestone Handbook" 22nd Edition. Bedford, IN. 2007.
Thin-Stone Facade Failure
Table of Contents
Keywords: thin-stone facade, marble, anchorage, The Amoco Tower, hysteresis, Finladia Hall, borescope
Introduction
During the advent of the 1900's, the use of thin-stone facades such as that seen in Figure 1, as an architectural feature on buildings became popularized due advances in technology and more economical means to erect these systems. Although these systems bring about a pleasing aesthetic appearance to the buildings that they adorn, the understanding of the science and engineering behind these systems is still in its infancy. Due to the relative unknown behaviors of thin-stone facades, there have been many cases involving the failure of these facades. Historical cases have shown that thin-stones facades are sensitive to environmental conditions, often related to temperature. Along with their sensitivity to the environment, special considerations must be taken into account in constructing these systems.
Causes
The thought of using stone for construction leaves the lasting impression that these systems will have a long life due to the thickness of the material. However, the characteristics of heavy stone facades and thin facades differ greatly. All stone veneers are subjected to weathering and incur surface loss as a result. Thin-stone panels are subjected to the full effect of temperature changes, resulting to these systems experiencing varying stresses throughout their life-cycle. The way thin-stone facade systems are attached to the main structure is often an aspect of design that is overlooked, resulting in devastating consequences. Frequent failures associated with thin-stone systems are attributed to the lack of understanding about the behavior of thin-stone when considering environmental factors (Beasley 2010).
Hysteresis
All stone facades experience expansion and shrinkage due to cyclic temperature changes. This phenomenon is known as hysteresis and is prevalent in thin-stone systems, as shown in Figure 2. The grains that compose these stone systems will dislocate during thermal cycles and will not return to their original positions. In thin-stone panels, the dislocation of these grains at the exposed surfaces results in uneven expansion, creating bowing. The additional fractures due to this process lead to increased water absorption, deteriorating the strength of these systems. The main component in many thins-stones is calcite, which is an anisotropic material.This means that calcite will expand more in one direction than another. When exposed to several cycles of expansion and contraction due to thermal conditions, irreversible granular fracturing will occur, causing a loss of flexural strength.
Moisture Intrusion
As mentioned, the effects of hysteresis result into greater absorption of moisture into the thin-stone panel systems. Water that is absorbed by these wall systems can lead to corrosion of metal anchors which reduces the strength of the steel supports and can result in spalls and cracks in the façade. The moisture that is retained by the stone is subjected to the freeze-thaw cycle. When water freezes, it expands leading to the creation of cracks and rupturing of material. Problems also occur due to the evaporation of moisture. When water evaporates from the surface of these façades, minerals will crystallize creating efflorescence deposits. If this process occurs in the surface of the façade, the surface may experience what is known as sugaring, which are pits that develop on the surface.
Restricted Movement
Inherently, stone materials are of a brittle nature. They react poorly to internal movement and will often crack or break when they exceed their elastic limit. Thin-stone facades are not designed to be load-bearing. Because of this, these systems are sensitive when force is applied and movement in the panels is limited. The frames which these facades are attached to are often sheltered from the environment and do not experience movement. When stress is induced in the thin-stone facades, from either external forces or internal forces, they are unable to relieve the pressure and often will crack or spall. Cracking and spalling can also be contributed to either the design of undersized expansion joints that do not allow for movement between panels or poor anchorage details that are not flexible enough to permit any movement. Rigid anchorage connections connection the veneers to the supporting structure react poorly to differential movement. This induces stresses in the stone panels causing damage.
Case Studies
There has been many cases within the last century resulting from the failure of thin-stone systems. Many of the failures attributed to these systems is due to lack of understanding of behavior. Often is the case, whole facades must be torn down and replaced since the original thin-stone veneer will have incurred significant damage that they can not be salvaged. These failures tend to lead to a lengthy litigation process as well as severe damages in terms of cost.
Case Study 1: Amoco Tower
The most famous historical case associated with thin-stone façade failure is the Amoco Tower failure in Chicago, now known as the Aon Center, seen in Figure 3. This failure is associated with the thin-stone marble cladding that adorned the 80-story building which was subjected to hysteresis. The Amoco Tower was covered with 43,000 panels of Carrara marble with a thickness of 1-1/2” above the 43rd floor and 1-1/4” for the stories below. The tower’s thin-marble façade was subjected to extreme thermal gradients resulting in the warping and spalling of the panels. This resulted in some panels bowing out at far as 1” and drooping more than 1” (Arndt 1988). After investigation, it was found that the marble used in actual installation had less than half the flexural strength of the materials used in the original testing. Panels that were most affected occurred where the building received the most sun exposure. It was concluded that within 10 years, the marble would lose up to 70% of its original flexural strength. The tower was eventually recladded with white granite as a cost of $60 million, half of the cost of the original building.
Case Study 2: The Empire State Building
The Empire State Building’s façade did not require repairs for almost 60 years of service, which represented a respectable service life of the exterior wall system. However, when repairs had commenced, there were problems discovered in the anchorage system of the wall from years of corrosion. The original wall system consisted of a thin limestone facing of thickness in the inches that was anchored into a brick back-up masonry wall that was tied into structural steel columns. When repairs were being done, it was discovered that the iron strap “cramp” anchors which tied the brick back-up to the column experienced significant corrosion due to oxidation. Because the brick masonry had been built directly against the steel column, the expansive forces that were developed by corrosion displaced and cracked the masonry back-up. This in turn resulted in cracking on the limestone façade. These cracked allowed for further water penetration into the wall system accelerating steel corrosion and masonry displacement. Repairs were attempted in the 1950’s to contain movement in the limestone by installing steel straps around the outside corners, but this did little to deter the deterioration process (Nacheman 2005).
Case Study 3: Finladia Hall
Like the Amoco Tower in Chicago, the Finladia Hall in Helsinki Finland was cladded in thin panels of Carrara marble. The original design intended for the performing arts center to have the flavor of Mediterranean culture (Loughran 2002). The marble that cladded this building had a maximum panel dimension of 55" high by 1-1/4" thick. Pins were inserted 14" from the panel edges, connecting each panel to four adjacent ones. This thin marble façade was poorly suited for the extreme environmental condition in Finland. Within a few years of its completion in 1971, noticeable deformations in the marble panels were detected. Efforts took place in the late 1980s to renovate the marble veneer. Political reasons resulted in that the Carrara marble be reused for the new façade. The new system was designed to allow for flexural connections, reducing the internal stresses in the stone. Improved flexural strength of the stone was included in the redesign, although the original thickness of 1-1/4” was maintained in the new wall system. The cost of the renovations was over 3 million Euros for the replacement of 75,320 square feet of panels. After completion of renovation in 1999, it was observed that the new panels experience deformation (Hester et. al 2009).
Prevention and Considerations for Design
Special care and consideration needs to be taken into account when designing and implementing a thin-stone facade system. New research and advancing technology has allowed for these systems to be incorporated into modern building design with a greater understanding of the systems behavior. The design for these systems can be broken down into five main categories: anchorage, moisture protection, environment, and post-construction considerations.
Special care and consideration needs to be taken into account when designing and implementing a thin-stone facade system. New research and advancing technology has allowed for these systems to be incorporated into modern building design with a greater understanding of the systems behavior. The leading resource for proper thin-stone façade design is provided by the Marble Institute of America’s Dimension Stone Design Manual, first published in 1971. Other additional resources for design include the Indiana Limestone Handbook which details the design of limestone facades and The Whole Building Design Guide on Thin-Stone Walls written by Michael Scheffler.
Design Considerations
Although thin-stone veneers are not designed to be the main structural resisting element of lateral loads, careful considerations must be taken to ensure that they are properly designed for loading, especially for wind design. In order to properly design the anchorage system, the proper wind loads must be applied to the building component and cladding. The American Society of Civil Engineers (ASCE) 7 standard lays out the procedure to properly determine the wind loads acting on the façade system. In addition to using ASCE 7, wind tunneling analysis may be performed to provide a more accurate calculation of wind loading, at the expense of additional cost and resources (Kuriyama 2010).
Moisture Control
Sealants are generally provided to join stone paneling, although mortar is another alternative. If mortar is used in design and construction, its strength should be evaluated as well as its adhesive properties. To achieve optimal mortar adhesion, tooling profiles should create a pathway for moisture to drain. For sealant joints, considerations for bonding to the stone, cohesion to resist internal cracking, and flexibility must be taken into account. The seal should be designed to resist moisture passing through it and not be subjected to thermal erosion. Adhesion tests should be conducted in order to determine the proper selection of sealant material. Recommendations from the Dimension Stone Design Manual recommend the following for moisture control:
Cavity walls also help provide moisture control in thin-stone veneer systems (Scheffler 2011). Through wall flashing should be provided at regular intervals to ensure that water enters the cavity and through the exterior. Rain screens are also typically used in conjunction with thin-stone systems. In these systems, the water resistant barrier is to be placed at the surface of the backup wall. All joints are to be left unsealed and the actual wall system acts as an additional barrier to prevent moisture from entering the backup wall.
Anchorage Design
The anchorage system is designed to transfer lateral loads from the thin-stone veneer to the supporting structure. It is vital that these systems perform up to serviceability and strength criteria to ensure that the envelope does not incur damage. Anchors should be designed to adjust to the change in shape of the stone as time goes on. Figure 4 depicts the planning of laying out where the anchors to a thin-stone panel should be place. They should allow for some flexible tolerance due to movement of the stone system. Steel anchors should be properly protected from moisture to prevent corrosion and loss of strength. Typically, the weight of the system, thickness, composition, and exposure will dictate the proper safety factors for detailing the anchorage.
The Section 1405.6 of IBC 2006 presents two specifications in which stone veneers should be anchored to masonry, concrete, or studs in construction:
Movement Control
The thin-stone veneer system should incorporate some tolerances in the case of movement due to expansion. Expansions joints should be provided to allow for the expansion of material. Lintels and horizontal supports should limit deflections to one-five-hundredths of the overall height of the panel (Hester et. al 2009). Expansion joints should be provided at every 20 ft maximum between panels to allow for enough movement and prevent cracking.
Investigation
If a thin-stone façade failure is detected, there are proper methods in order to inspect the system as well as techniques to implement repairs. Periodic inspections should be performed in order to detect thin-stone façade failures. These inspections can also serve to determine the service life of the façade and its associated components. Visual condition surveys of building facades involve documenting observations by commenting on the building condition as well as taken photographs. These surveys should identify the locations where problems are detected as well the nature of the problem. These surveys can help in determining the need for additional studies and laboratory tests. They also provide clues into the underlying cause of failure. If the situation is critical, it may be necessary to remove panels to identify the problem. Although the removal of panel may be disruptive and aesthetically unpleasing, it can help in determining the systems’ physical condition and whether repairs are required (Beasely 2009). Below are a few non-destructive investigative tools and procedures that can be utilized to investigate the condition of an existing veneer system.
Borescope
The borescope is a fiberoptic device that enables an inspector to view a veneer system through an existing cavity. A borescope consists of fiberoptics or a micro-miniature video camera that is able to view behind a facade. It is an optical device made of a rigid or flexible tube with an eyepiece at one end, and an optic lens at the other which relays information. The camera's small tube (6-8mm) can be inserted through either a small hole or existing joint. This makes this tool extremely effective in assessing the condition of a wall because it involves very little if any alteration to the existing veneer system.
Pulse Velocity Method
The strength of the stone in veneers can be tested using a pulse velocity test conducted according to ASTM C597-02. This test consists of a waveform or velocity of a sonic pulse that is sent through the material of interest. A transmitting inducer creates an ultrasonic or sonic energy pulse through the stone and a transducer detects the signal. The incorporation of timing units allows for the measuring of elapsed time of transit. By dividing the velocity of the wave by the distance between transducers, the material strength and elastic modulus can be determined.
Repair
In the situation where repair is required as seen in Figure 5, several considerations must be taken into account. Safety, aesthetics, feasibility, cost, and serviceability are the five main areas to consider in repair work (Scheffler 2001).
Safety - In the scenario where extensive repair work must be performed, temporary stabilization should be provided to protect the welfare of the public and workers. Proper repairs should be performed to provide a structurally adequate thin-stone veneer system. The design should incorporate new codes to meet minimum requirements. Considerations for the impact on the existing structure from the new system as well impact on support elements should also be considered.
Aesthetics - The structural repairs should be aesthetically acceptable. Anchoring should not be visible and mortar and sealant patch repairs should be acceptable. Not all defects need to be repaired, but judgment should be used to determine if are visually intrusive. Care must be taken into matching new material with the original material. If the original material is not available, the best efforts must be used to find a suitable replacement.
Feasibility - Repairs must have some reasonable chance to be implemented. Physical limitations and dimensional constraints must be considered when implementing repairs. The existing stone must be thick enough for repair anchors. The anchors must be able to be attached to the backup support. The backup support must be structurally sound and constructed of a material that is adequate enough for the veneer to be attached to. The causes of initial distress in the system must be remedied in repairs or else this can lead to repeat of failure.
Cost - Cost should be taken into account and determined whether or not is the primary consideration in repair. The minimum cost should be determined that will provide a safe façade. This will aid the owner in establishing budget for repairs. The cost of repair should be compared with the cost of removal and replacement. This can lead to a decision of whether the existing system should be salvaged or a new system be implemented. If a substitute material is used, its cost should be evaluated in order to determine if it is a viable solution.
Serviceability - The long term effects of repairs are important to consider. The repair should provide a long term solution so that the original problem does not reoccur.Temporary stabilization repairs should allow for ongoing inspections to occur. Implications of retaining temporary structures should also be taken into account.
Conclusion
Failures associated with the use of thin-stone veneers can be attributed to the lack of understanding on how to properly design and construct these systems. Problems with these systems will continue to persist if designers do not consider what conditions they plan on using these systems in. When not properly designed, large costs associated with repairing or replacing these systems can be incurred, as seen in the Aon Tower and Finladia Hall case study. Further research is currently being further developed to help develop a better understanding of how these systems act. Until enough information is available on thin-stone veneers, designers should consider historical failures as a means of design when planning to use these systems.
Annotated Bibliography
Arndt, Micahel (May 22, 2044). "Amoco Tower's Fate May Be Carved in Stone." Chicago Tribune.
http://articles.chicagotribune.com/1988-05-22/business/8801010440_1_amoco-building-marble%20-tower
Newspaper article detailing the most expensive thin stone facade failure, The Amoco Tower in Chicago. The article covers a brief history of the case and highlights the investigative teams involved in determining the cause of failure.
Beasley, Kimball J (2010). "Building Facades." Forensic Structural Engineering Handbook 2nd Edition
http://www.accessengineeringlibrary.com/mghpdf/0071749594_ar015.pdf
This is a piece written by Beasley from the Forensic Structural Engineering Handbook. Chapter 15 is dedicated to building facades. It contains pictures associated with thin stone facade failures and details the reasons behind the facades not performing as intended.
Beasley, Kimball J (2009). "Identification and Diagnosis of Building Facade Failures." Forensic Engineering 2009: pathology of the Built Environment. October 30, 2009.
http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=ASCECP000362041082000008000001&idtype=cvips&doi=10.1061/41082%28362%298&prog=normal
In this article written by Beasley, he dives into the investigative techniques used to diagnose facade failures. He puts forth various techniques to help investigate facade systems.
Hester, Daron et. al. (2009). "Thin Stone Marble Facades: History, Evaluation, and Maintenance" Building Envelope Technology Symposium. October 2009.
This paper reviews the history of marble facades and descibes various design techniques. It also discusses the various deterioration mechanisms of thin-stone facades and offers maintenance recommendations.
Kuriyama, Juan (2010). "Thin Stone Marble Facade Systems." Hoffman Architects Journal Issue 4, Volume 27. October 12, 2011.
http://www.hoffarch.com/images/userfiles/journal61.pdf
This is another published online article, written specifically about thin marble usage. It is written from an archtitect's perspective, but highlights considerations when using using thin marble as the building facade. The article breaks down design and installation considerations, including anchorage, panel movement, and environmental factors. Many pictures are also presented to illustrate points.
Loughran, Patrick (2002). "Failed Stone." Birkhäuser Architecture. November 20, 2002
This book was used to expand upon the case study of Finladia Hall. Overall, the book dives into many failures associated with stones used as a construction material.
Nacheman, Robert J (2005). "The Empire State Building Facade: Evaluation and Repair of an Engineering Landmark." 2005 Structures and the 2005 Forensic Symnopsium
In this paper, Robert Nacheman writes about restoring the thin stone facade that was originally on fhte Empire State Building. A key part of this article is how he details the anchorage system for the facade and how the original was not sufficient.
Scheffler, Michael J (2001). "Thin-Stone Veneer Building Facades: Evolution and Preservation." APT Bulletin, Vol. 32, No.1
http://www.jstor.org/stable/pdfplus/1504690.pdf?acceptTC=true
This bulletin from the Association for Preservation Technology International details the evolution of thin-facade veneers in buildings. It then goes into detail about various failures associated with these facade systems. Included are many images.
Scheffler, Michael J (2010). "Thin Stone Wall Systems" Whole Building Design Guide. November 20, 2011
http://www.wbdg.org/design/env_wall_thin_stone.php
This Whole Building Design Guide article on Thin Stone Wall Systems provides basic information on how to properly design these veneers. Additional resources are provided at the end of the publication to provide further detail for design.
Additional Resources
"Dimension Stone Design Manual" Version 7.2. 2011.
"Indiana Limestone Handbook" 22nd Edition. Bedford, IN. 2007.
"Testing and Assessment of Marble and Limestone"
http://www-v2.sp.se/building/team/default.htm