Another challenge we encountered was finding replacement granite that met both the aesthetic and strength requirements of the design. The contractor submitted proposed replacement stones along with testing data provided by the stone quarry for each stone type. Matches for the Baltic Brown, Impala Black and Azelea Pink stone types were approved with the first submittal. It was more difficult to find the Taivassalo stone, a red granite from Finland, or an acceptable substitute. The first two stones submitted as an alternate were Canada Rose and Oklahoma Red. Both were rejected because they did not match the color of the existing Taivassalo. Eagle Red, the third red granite submitted, was a good aesthetic match, and was approved pending results of required laboratory tests. We tested the Eagle Red granite in accordance with ASTM C880 for flexural strength in our laboratory and found that the sample of stone consistently fractured at less than 1,000 psi, which is less than the strength required by ASTM C880, which is 1,200 psi (Photo 6). The low strength of the stone made it unsuitable for larger panel sizes. Finally, Balmoral TLO granite was submitted and approved, as it was a good aesthetic match to replace the Taivassalo stone, and it passed the flexural strength test -- averaging a strength of 1,800 psi.
Key concepts to consider when designing a thin-stone veneer cladding include:
1. The architect's vision for the building, 2. anchorage of the panels to the supporting structure, 3. layout of the panels and anchors to accommodate geometric and strength limitations of the supporting structure, 4. flexibility in the design to accommodate unforeseen conditions, and 5. necessity to verify testing data to be sure the requirements of the design are met.
Each of these design considerations are further complicated when designing a cladding system for use on an existing building.
AUTHORS' BOX Katherine S. Wissink, LEED AP is a project engineer at national engineering firm Simpson Gumpertz & Heger Inc. (SGH). She is involved in the design, investigation, construction litigation support, construction administration and peer review of building envelopes. She has experience with a variety of wall and roofing systems, including dimensional stone cladding, multi-wythe masonry walls, brick veneer walls, stucco wall systems and slate roofing systems. Wissink can be reached at kswissink@sgh.com. Annemarie Rabazzi, P.E. LEED AP is a project engineer at SGH. She is involved in the design and investigation of building envelopes, construction administration of building envelope repairs and peer reviews. She has experience in a variety of wall and roofing systems, including brick and granite veneer walls, dimensional stone cladding, EIFS and stucco wall claddings, curtain walls, single-ply and built-up roofing systems, and shingle and slate-steep roofing systems. Rabazzi can be reached at alrabazzi@sgh.com.