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New Guidance for Design of Stone Anchoring Systems

In Chicago, the Amoco Building soars 1136 ft. (346 meters) out of sight. Mt. Airy granite is anchored to its steel frame, but granite wasn’t the original facade. Its original marble exterior began to buckle after the skyscraper was completed in 1972. About ten years later, 80 stories of marble had to be replaced with granite at a loss of millions of dollars.

This improper use of stone and anchoring raised an outcry from the building industry for testing and analysis of the materials and systems that failed. According to Mike Lewis, principle, THP Limited, Inc., Ohio, and adjunct professor of architecture at the University of Cincinnati, the “critical concern” was, “How could stone construction, recognized for its durability that lasted centuries, be adapted to the world’s largest buildings?”

ASTM Committee C18 on Dimension Stone has addressed stone construction since 1926. After nearly 10 years of careful consensus, the committee released a standard guide for stone anchoring in 1993. Its latest edition, C 1242, Standard Guide for Design, Selection, and Installation of Exterior Di-mension Stone Anchors and Anchoring Systems, was published in February.

Standard C 1242 is referenced on the Web site of the National Building Granite Quarries Association, whose member-companies produce most of the nation’s architectural granite. Both ASTM C 1242 and C 1201, Standard Test Method for Structural Performance of Exterior Dimension Stone Cladding Systems by Uniform Static Air Pressure Difference, are listed “for information about suitability and use of various anchoring systems and devices.”

Within Subcommittee C18.06 on Anchors and Anchoring Systems, a team of architects, stone manufacturers, installers, structural engineers, academics, testing agencies, and code officials updated C 1242. “We’re hoping that the use of stone will be easier and safer by following the guidelines that are the consensus of the group’s international expertise,” said Lewis, the subcommittee chairman. When followed, he said the guidelines can “improve the performance of those whole-building envelope systems.”

The team deliberated several years over the guide, which was last updated in 1996. “The scope of the ’96 document gave examples of different anchors and considerations for designing anchors,” he continued, “but wasn’t specific about how to interpret and apply existing standards and test methods to a project, or how to involve existing structures as exemplars in predesign.”

A significant Section 8 was added to describe “engineering process including scope and interpretation of tests, and review of exemplars (existing buildings) for pre-design,” he said. “The new section describes a procedure where users can study existing buildings and make preliminary evaluations, then proceed to a rational engineering testing and analysis process to produce an overall anchor and support system, using other test methods under the jurisdiction of ASTM Committee C18.”

As stated in the standard guide, ASTM C 1242 pertains to:

• The anchoring of stone panels directly to the building structure for support;
• The anchoring of stone panels to subframes or to curtainwall components after these support systems are attached to the building structure;
• The anchoring of stone panels to subframes or to curtainwall components with stone cladding preassembled before these support systems are attached to the building structure; and
• The supervision/inspection of fabrication/installation of the above.

“Parts of C 1242 will probably become part of future International Building Code updates, or will probably be referenced,” Lewis offered, saying that discussion is under way by various code bodies “to address stone cladding specifically but avoid overlapping other areas. Basically, we’re all interested in offering experience in this specific discipline to preserve public safety, and we just have to make our language versatile enough to fit different conditions and multiple viewpoints—the challenge of consensus.

“In [Committee] C18, our particular concentration is stone, exterior wall and cladding,” he explained. “You have the concrete group, the steel group, the sustainable building group—all are contributing to a body of rules intended to improve building performance and safety.

“We’ve recently met with the standards group in Europe [CEN] to understand their agenda,” Lewis said, explaining that Europeans are standardizing test methods, but haven’t developed standard guidance for interpretation or application as in C 1242. “European stone installers rely more on tradition than on technical standards in this particular field due to their construction culture,” he said.

As building methods have changed significantly since 1900, standard guidance such as C 1242, which is updated regularly, becomes key. Structures that are built differently “perform differently and have experienced problems that have taken time to understand and diagnose,” Lewis reasoned. “This is why standards like C 1242 take time to develop and evolve.

“Our committee is continuing to work on these particular tasks—safety factors, durability evaluation—to predict how long cladding system materials or assemblies will last,” he concluded.
For further reference consult ASTM Manual 21, Modern Stone Cladding, which Lewis authored, that provides “foreground, future direction, and commentary by example to the current state of stone cladding from a certain point of view.”

For further technical information, contact Michael Lewis, THP Limited, Inc., Cincinnati, Ohio (phone: 513/241-3222). Committee C18 plans to meet in October; for further details contact Staff Manager Jeff Adkins, ASTM (phone: 610/832-9738). //

Copyright 2001, ASTM