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Austin Central Library



STRUCTURAL FRAMING SYSTEM

The primary criteria driving the selection of the structural system were constructability, overall economy, sustainability, future flexibility of the floor space, and anticipated long service life for the building. Framing the building above Level 1 utilized cast-in-place, wide-pan joists and flush bottom girders. The pan forms were 24” deep x 66” wide with some 30” wide pans in the center of the bays. The joists were 9 inches wide typically with 24 inch to 30 inch wide ribs at each column line for rigid frame action. The joists have a 5 inch thick slab for a total thickness of 29 inches. Flush bottom girders were 48 inches wide with 60 inch wide girders in the 45 foot western bays. Due to long 42 foot to  45 foot exterior bays, the joists and girders were post-tensioned with mild reinforcement. (Structural soffits were not exposed, so forming finish quality was reduced.)  Round columns were selected to meet the desired aesthetic. Concrete transfer girders were provided as needed to meet the large span requirements over larger assembly spaces.

Developing a foundation system at the new library site proved challenging for the design team. Clayey sand and poorly graded sand strata occupied the top 70 feet of in-situ soils, followed by a 30 feet thick partially-weathered shale stratum. Drilled piers would be the typical choice for a foundation system in the Austin area, but any piers at this site would have to extend over 100 feet to reach Georgetown Formation Limestone (15 feet embedment in limestone recommended for straight-sided shafts). In addition to poor soils, the site is adjacent to the Colorado River running through downtown Austin, putting the water table at a higher elevation than the building’s foundations and the lowest level of the below-ground parking. With such a high water table, waterproofing and drainage in the garage became critical; drilled shaft foundations were also undesirable as they would require special construction techniques foor dewatering and would penetrate the waterproofing barrier encasing the outside of the below-grade portions of the building. After considering the available options, the design team determined that a mat foundation system was the best solution: a mat slab would not penetrate the waterproofing membrane surrounding the building and could be supported by the weak sand strata.  The designed foundation system was a 5’-6” thick cast-in-place concrete mat slab reinforced top and bottom. The mat is cast over a 4 inch fiber-reinforced work slab over the existing soils. The structural designers worked closely with the geotechnical engineer throughout the design process to establish subgrade pressures and stiffnesses under the various areas of the mat slab and to check initial and long-term settlement.  For the mass concrete placement, the structural engineers required the contractor to develop a construction plan to monitor and control the heat of concrete hydration concrete placement and cure time. The mat slab was cast in one continuous pour which lasted 24 hours. Almost every concrete supplier in the region provided concrete for the mass pour or was on reserve to do so. At the time it was the largest concrete pour in Austin.

UNIQUE DESIGN FEATURES

The biggest project challenge, was simply the uniqueness and complexity of the building.  The design creates a dizzying array of unique spaces in order to allow for a wide variety of uses, users, and experiences for the occupants. It also creates a sense of wonder and inspiration for everyone who comes in.  This begins with the irregularly carved central atrium, with its exposed concrete. The atrium serves as a source of light into the center of the building, as a hub of activity, as a tool for orientation and wayfinding, as a meeting spot, as a place to work, and as a breathtaking introduction for the user.A driving factor in the overall design of the building was daylighting, openness, and sustainability through a flexible, long-life building.  Given reinforced concrete’s inherent durability, and its ability to economically support large loads which will change and move over time, the modular reinforced concrete beam and slab system was a clear choice for this building. It offers a durable, low maintenance structure which can accommodate large loads anywhere.  Further, the modularity of the layout increases the predictability and flexibility for space planning.

The structure is based on a 30 foot grid, which works well both with the functional planning of the library spaces, and the underground parking garage below. This allowed the building and garage columns to align up and down the building without transfer girders. Only two transfer girders were needed for the project, to create the 60 foot clear span event space.  Beams were spaced at 10 foot to module out with the 30 foot grid.

REASONS FOR CHOOSING REINFORCED CONCRETE

The ability of reinforced concrete to accommodate heavy loads with a minimal cost premium was an important factor.  Because the formwork costs for various depths of floor framing systems do not vary significantly, and because reinforcing tonnages for deeper systems with heavy loads do not increase significantly, reinforced concrete has the ability to handle heavy loads like a library without a big increase in cost.  The aesthetics of reinforced concrete in the atrium were also important to the architect.

Project Details

Location:

Austin, TX

Owner:

City of Austin

Architect:

Lake Flato Architects & Shepley Bulfinch

Engineer:

Datum Engineers

Construction Management:

Hensel Phelps

General Contractor:

Hensel Phelps

Total Project Cost:

$120M

Total Concrete Yards:

30,400 CY

Reinforcing Tons:

3,280 Tons