Austin’s Central Library is an iconic, 250,000 sq. ft. concrete landmark. It features a 6-story public atrium, capped by a skylight feature that came to be appropriately nicknamed, “The Cowboy Hat.” A 130-foot-long central skylight on the crown, flanked by a row of 10-foot-tall clerestory windows under the 70-foot-wide brim on either side, creates a voluminous column-free space--and a major design challenge.
Our engineers determined that the two side walls of the crown of the hat were perfect spaces to install 8-ft-deep trusses to span the 130-ft length of the space. The only problem was the need for bracing the top flanges of these trusses in compression. X-bracing across the skylight was not aesthetically acceptable. The only opportunity left for bracing the top flanges was to use a pony truss concept, whereby transverse members rigidly connect to the bottom of the vertical truss chord members effectively bracing the top chord. One innovation made this solution economical. We recognized that a counter-slope would be needed to drain the water that would collect in the brim of the hat. Thus, extra steel framing was added to form roof “crickets.” This framing acts like a knee brace and eliminates the need for costly welded moment connections. It further provides maximum stability at the midspan, where it is was most needed.
Situated above the north shore of Lady Bird Lake, the library needed a 2-story, below grade was challenged by the water level at elevation 429-feet, which is slightly lower than the water table. Our designers conceived a giant mat foundation for the building. A66-in.-thick mat was built above a system of drainage pipes connected to duplexed pumps. This economical approach, extremely rare in Austin, eliminated the threat of rising waters blowing out a slab-on-grade, as the mat resists the buoyant forces of the water and, with waterproofing, provides a water-tight base for the building.