Anatomy of a Wall When Malcolm Holzman, FAIA, first approached Architectural Engineers Collaborative with the desire to use load-bearing stone in a modern building, he rekindled ideas that we at AEC I had been discussing for many years. Like Holzman, we were enamored with the capacity of stone to provide robust and beautiful buildings that last for centuries. We were also dissatisfied by common stone veneer where the stone functions largely as a rain-screen or an ornamental part of the facade.
Why not build in load-bearing stone? The answer is not simple. Modem buildings need to resist moisture intrusion and provide thermal comfort, two areas where solid stone walls do not score well. In addition, from a structural engineering point of view, a wall of solid stone, while incredibly strong in compression, is limited by its inability to resist tension in the mortar bed joints. The tension comes from gravity, wind, or seismic loading that causes bending or shear stress. When tension capacity is exceeded, stone walls crack and can become unstable.
How could a modern stone bearing wall address both environmental and structural issues? The answers came from research and collaboration within a vested team of design and construction professionals, each offering expertise and talent that culminated in the design and construction of the curving wall at the Wylie Municipal Complex. The structural design of the wall is based on reinforced masonry as prescribed in the International Building Code. The wall is 18 inches thick, consisting of two 4-inch thick, 5000-psi limestone layers separated by a 10-inch cavity. Within the cavity, vertical-grouted and reinforced pilasters are typically placed at 10 feet on center, in alignment with the structural steel framing of the building floors and roof. The pilasters extend for the full height of the wall and parapet. In addition to the pilasters, grouted and reinforced beams are placed within the cavity above large openings and at the floor and roof lines. This internal frame of pilasters and beams addresses the structural weakness inherent in a solid stone wall. The frames allow the two stone wythes to remain in compression with tensile forces from bending and shear distributed to the reinforced frame. In addition, areas of high structural demand – such as high concentrated loads or long horizontal spans – are specifically designed with the stone, grout, and reinforcement acting compositely to resist the load. From a thermal perspective, the wall performs well. The dense stone on each wall face functions as thermal mass, effectively reducing the peak temperature differential between inside and outside air. Within the 10-inch wall cavity, Icynene insulation foam is applied. Moisture intrusion – from rain or moisture-laden air – is resisted by a continuous air/vapor barrier sprayed on the inside face of the exterior wythe in sequence with the wall construction.
THREE-STORY WALL SECTION 1 STEEL STRUCTURE 2 CONTINUOUS GROUTED BEAM AND PARAPET 3 GLAZED ALUMINUM ASSEMBLY 4 CONTINUOUS CROSS-WALL STAINLESS STEEL TIES 5 FOAMED-IN-PLACE INSULATION TO FILL ENTIRE CAVITY 6 AIR/VAPOR BARRIER 7 LIMESTONE 8 GALVANIZED STEEL ON EXTERIOR AT WINDOW
The limestone was quarried at TexaStone Quarries in Garden City, with wall construction by DMG Masonry under general contractor Byrne Construction Services. The masons quickly embraced the sequence of the wall construction incorporating insulation, waterproofing, and reinforcement within the masonry trade. AEC’s Josh Bedre, PE, served as the project’s structural engineer of record. The wall was completed on schedule and within budget. Chuck Naeve, PE
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The July/August 2011 edition explores the theme of “placemaking” through feature projects designed to foster a sense of a greater whole inst...