INTRODUCTION

Thin stone wall systems used for exterior building envelopes typically consist of stone panels ranging in thickness from 3/4 inches to 2 inches. Most panels are fabricated from granite, while marble; limestone, travertine, and sandstone are also used to a lesser extent. A common panel thickness is 1-3/16 inch (3 cm). Overall panel dimensions can vary significantly for different buildings, depending on the strength of the stone used and architectural affect desired. However, maximum panel dimensions are usually approximately 3 to 4 feet and usually not more than approximately 6 feet. Typically each panel is independently supported to the building structure or back up system using an assemblage of metal components and anchors. Joints at the perimeter of each panel are usually 3/8 inch in width and are filled with sealant. A drainage cavity is typically located behind the stone panels to collect and divert to the exterior water that penetrates through the joints.

For certain applications, such as at building entrances and near grade level installations of limited extent, the stone system may not incorporate a drainage cavity but instead be a barrier system. In this type of system, the stone is applied directly against and attached to solid masonry backup such as concrete masonry units (CMU) or concrete. In these localized applications, several panels may be stacked.

DESCRIPTION

Stone Types

Granite is the most commonly used stone type in thin stone wall systems. The commercial classification of granite usually refers to a stone that includes any visibly granular igneous rock consisting of mostly feldspar and quartz minerals. This commercial term encompasses a wide variety of geologic stone types rather than only the limited number that fall under the geologic classification of granite. Geologically, marble is a metamorphic rock resulting from the recrystallization of limestone. While less commonly used in this type of application today, marble is also sometimes used in thin stone wall systems. Commercially, the term marble refers to many rocks with a wide variety of geologic classifications. These can be true marbles of calcite and dolomite, as well as dense limestones which will polish, serpentine rocks, and travertine. Sedimentary rocks such as limestone and sandstone can also be used in thin stone wall systems. However, panels fabricated from these stone types are usually not less than 2 inches in thickness because of the lesser strengths of these stones relative to granite and marble. Commercially, limestone refers to rocks that are both limestone and dolomite. Sandstone belongs to the commercial "quartz-based" group which includes stones with high quartz and silica contents.

Support And Anchorage Systems

There are two primary types of stone installation. The first is the "hand-set" method, in which each stone is individually attached to the building's primary structural frame or onto a secondary wall framing system. The second is the panelized installation method, in which the stone panel or multiple panels are preinstalled onto a frame or attached to a precast concrete panel. The frames or panels are transported to the building, where the entire assembly is attached to the building's structural frame or secondary structural members or framing system.

In either installation system, anchors must be used to attach and support the stone panels to the building's primary or secondary framing system, or to the panelized system frame or element. Anchors that are in direct contact with stone are usually constructed of non-corroding metals such as Type 304 stainless steel or aluminum.

HAND-SET SYSTEMS

There are numerous types and styles of anchors used to support and anchor individual stone panels. Commonly used anchor types include:

• Kerf supported stone with stainless steel or aluminum angles
• Side supports, dowels, straps, and disks
• Undercut anchor
• Embedded Adhesive Pin Anchor

PANELIZED SYSTEMS

• Precast Systems
• Steel Truss Systems

JOINTS AND JOINT TREATMENTS

In most applications, joints and joint treatments in thin stone wall systems will include one or more of the following:

• Mortar
• Elastomeric Sealant
• Epoxy

Depending upon the overall design of the wall assembly and manner in which the stone is set, the appropriate use of these materials will vary from project to project. Joint mortar should be carefully evaluated relative to mix design and compressive strength, particularly with regard to load transfer (either intentional or inadvertent), bond intimacy (necessary for improved water penetration resistance) and the potential for moisture and/or thermally-induced degradation/spalling of the mortar and/or surrounding stone. Joint sealant should be carefully evaluated for elongation and movement capacity, adhesion, cohesion, and staining of stone substrates. Epoxies should be evaluated for adhesion and bond strength, as well as UV stability.

Note: In general, it is not advisable to rely exclusively on epoxies to bond two or more sections of stone together to form a single shape. Stainless steel threaded dowel pins or similar mechanical attachment, together with stone "liner blocks" as required, are typically recommended for these applications. Consult the appropriate industry standard for further guidance.

Joint profiles on exterior wall surfaces should also be designed with a positive slope to shed rainwater away from the building in a manner that will prevent "ponding" of water along the joint. When designing for joint sealant, overall joint widths should be designed to accommodate differential thermal movement between individual stone veneer panels without damage to the stone substrate or failure of the sealant. Joint configurations should also be designed to conform to the sealant manufacturer's guidelines and applicable industry standards for width-to-depth ratios, and minimum bond surfaces at joint substrates.

 

 

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COMMON BACKUP WALL ELEMENTS

• Insulation
• Air and moisture barrier
• Metal stud framing
• CMU
• Flashing

FUNDAMENTALS

Structural Aspects Of Design

Stone wall systems are traditionally constructed as a curtain wall or veneer, in which no building loads are transferred to the stone panels. Most typically, the stone wall system must resist lateral loads directly imparted on it, such as from wind and earthquake, as well as vertical loads resulting from the weight of the stone wall system. These loads must be transmitted through the stone wall system and secondary structural elements to the building's structure. Other loads related to impact, construction, and transportation must also be taken into account in the design.

Steel elements of a stone wall system are designed in accordance with the American Institute for Steel Construction (AISC) specifications for steel construction. Precast concrete elements are designed in accordance with American Concrete Institute (ACI) and Portland Cement Institute (PCI) specifications.

The stone portions of the wall system are usually designed in accordance with industry recommended safety factors, Allowable Stress Design (ASD), and physical/mechanical properties of the stone determined by testing of that particular stone. For granite, based on the recommendations of the National Building Granite Quarries Association (NBGQA) and the Dimension Stone Design Manual, a safety factor of 3 is usually used for granite panel stresses away from connections, while a safety factor of 4 is usually used for stresses in the granite at connections. For example the maximum mid-span bending or flexural stress determined by structural analysis for a panel is compared to and must not exceed the allowable stress, which is the average flexural strength of tested specimens divided by 3, the safety factor for granite away from connections. Based on the recommendations of the Dimension Stone Design Manual a safety factor of 5 is commonly used for marble panel stresses that result from wind loading. For travertine, limestone, and quartz-based stone, a factor of safety of 8 is recommended. The Indiana Limestone Institute Handbook recommends a safety factor of 6 be used for Indiana Limestone.

Joints between panels must be wide enough to accommodate thermal expansion and differential movements between panels; 3/8 inch wide joints are typically used. Joints between panels are most commonly sealed with sealant and are the primary line of protection against water penetration into the wall cavity. The wall cavity space and the back up wall, which is usually covered with a water resistant membrane, provide a secondary line of protection against water penetration into the building. Through-wall flashing is usually located throughout the height of the wall at regular intervals to divert water that enters the cavity back to the exterior.

Performance Issues

THERMAL PERFORMANCE

Thin stone wall systems derive their thermal performance characteristics primarily from the amount of insulation placed in the wall cavity or within the backup wall. The stone and supporting elements of the wall provide little insulating value.

MOISTURE PROTECTION

The most common moisture protection system used with stone wall systems is the wall cavity drainage system described above. Rain screen systems are also used with thin stone wall systems. In these systems, the primary water resistant barrier is located on the surface of the backup wall, joints are left unsealed, and the stone panels provide a rain screen that minimizes the amount of water that can reach the back up wall. Barrier systems are sometimes employed on certain stone wall systems where the stone panels are in direct contact with the backup wall.

FIRE SAFETY

Stone wall systems are not considered to provide any improvement in fire safety for the building exterior wall. In fact, for high-rise buildings stone wall systems can pose a serious safety hazard when a fire occurs that breeches the exterior envelope. Because stone exposed to intense heat from fire can crack and the cracked portions of stone can fall from the building, fire safety personnel may be in danger from falling stone.

ACOUSTICS

Because of their mass, stone wall systems may provide better sound insulation than lighter wall systems such as metal panels.

MATERIAL/FINISH DURABILITY

Stone used in stone wall systems can have several finishes: for granites and marbles, a polished, highly reflective finish is common. Thermal finish is a rough textured finish that is often employed with granite. Also, smooth honed finishes are commonly used on all stone types used in stone wall systems. Granites have had a long history of durable service. Certain marbles have a long history of successful use. However some marble types, particularly white marbles of pure calcite, have been found not to be durable materials because of their susceptibility to deterioration from heating and cooling cycles. Travertine, limestone, and sandstone have a good history of use as thick stone wall elements but their service history as thin stone wall elements is fairly limited, particularly in terms of durability. However, few notable material related failures have been encountered.

Most distress observed in stone wall systems can be attributed to anchors used to attach stone panels to the structure. Panel cracking, displacements, or other distress conditions can occur at locations where anchors are inadequately or improperly connected to the stone. Poor construction is often the result of poor quality control and out of tolerance fabrication or erection of the panels. Also damage from handling during construction can result in panel cracking, some of which may not become evident for several years.

Evaluation of future stone durability is performed in several ways. History of use of a stone can in certain circumstances provide useful though limited information. Evaluation and study of the historic performance of a particular stone type in service is the most commonly used approach, and is used on almost all projects where stone is being evaluated for use in an exterior environment. More important than reviewing past performance is review of the physical and mechanical properties of a particular stone. The stone's tested properties are often compared to minimum standards or to the physical and mechanical properties of other durable stones, or historic data for that stone. Petrographic evaluation is also commonly used to evaluate stone in an effort to identify the mineral composition and microstructure of a stone, and based on these observations and past knowledge of those characteristics, to predict future performance. Another method of evaluation is to expose samples of stone to an accelerated weathering procedure, and then evaluate the stones physical and mechanical properties for changes.

MAINTAINABILITY

When properly constructed, stone wall systems require relatively little maintenance as compared to other wall systems. Typically the only maintenance required is replacement of sealant in joints between panels; the time frame for this activity depends on the sealant used but usually ranges from every 7 to 20 years. However, it should also be noted that periodic review and evaluation of thin stone veneers may be desirable in order to determine if any evidence of structural distress exists in the panels due to strength loss (selected marbles) and/or accumulated stresses at anchor points. Similarly, periodic review and evaluation of exposed stone surfaces may also be desirable, depending upon the location and exposure of the building, to determine the nature and extent of any discoloration or staining that may exist in order to develop a cleaning program that is not overly aggressive and, as such, results in irreversible damage to the stone substrate.

APPLICATIONS

Stone wall systems have been employed to achieve a wide range of architectural styles, aesthetic affects, and appearances. Generally, thin stone wall systems are used in all environments. However certain stone types such as certain marbles may not be appropriate for environments with significant thermal cycling.

See Appendices for climate-specific guidance regarding building enclosure design.