Application of Tilt-Up Construction to Public School Buildings in the Leander Independent School District
In recent years, particularly in central Texas, Tilt-Up wall construction has gained momentum in public school construction. Case in point is the Leander Independent School District, located northwest of Austin and in the southwest corner of Williamson County.
The district has become an innovative leader in the use of Tilt-Up for construction of new schools in its district. The district’s newest high school, which is currently under construction, the three most recently completed elementary schools and its sixth middle school have been constructed using a combination of exterior insulated and interior uninsulated Tilt-Up wall panels. Members of the Tilt-Up industry will benefit from learning more about the adoption of Tilt-Up concrete construction as the structural system of choice for new facilities in the Leander Independent School District.
HISTORY OF THE LEANDER SCHOOL DISTRICT
Typical of early rural Texas, schooling of youngsters in the Leander area was performed in homes. The first public school opened in Leander in 1893. Leander businessmen formed a high school association in 1899 to generate support for development of an educational system. By 1907, Leander provided education in first through tenth grades. Students were required to attend Liberty Hill or Georgetown schools to complete the requirements for graduation. A school was finally constructed and completed in November 1938 to serve the educational requirements of the Leander School District.
In nearby Cedar Park, a Baptist church was used as a public school from 1871 until 1919, when two new schools, Cedar Park and Block House, were begun. These two schools were combined to form the Whitestone School in 1923. The Whitestone School was consolidated with the Leander School District in 1952.
Both the Cedar Park and Leander communities remained small, rural towns through the 1960s. However, in the early 1970s, Cedar Park, followed shortly by Leander, began to experience a sustained period of growth due to the expansion of Austin. Austin’s urban expansion and development as well as associated increases in the cost of land and living attracted families to southwest Williamson County. Cedar Park incorporated in February 1973. Leander followed by incorporating in January 1978. The rapid development of housing subdivisions around both communities also led to explosive growth of the Leander School District. Construction of Leander High School was completed in 1984. Since that time, the district has built 17 elementary, six middle and three high schools, with another four elementary schools and a middle school planned. The fourth high school is currently under construction. Total student enrollment has grown from 300 in 1948 to approximately 2,500 in 1980 to more than 22,000 in 2005.
THE LISD APPROACH
Management of the explosive growth of the district presented a considerable challenge to the district’s administration. With the rapid growth of the district has come the need for rapid implementation of facilities. Despite the pace of construction and recent volatility of construction costs, the district strived to build high quality school facilities that provide good value, durability and efficiency, while also providing an outstanding educational environment. To that end, in 1996, the district adopted a partnering approach to facility development that involved architects, engineers, construction managers and the district’s staff. Communication, cooperation and genuine teamwork were fostered among all parties to help plan, develop and execute the construction of new school facilities to meet the district’s functional, operational and budgetary goals within the tight, aggressive schedules demanded by a rapidly growing population. The adoption of Tilt-Up structural systems for the construction of new school facilities was the result of this collaborative effort.
The Leander School District’s goals for new construction include the following:
2. High resistance to growth of mold;
3. Minimum durable life of 50 years;
4. Express style and stature comparable to existing facilities within the district;
5. Cost effectiveness.
Although many buildings are constructed across the state using exterior Tilt-Up wall panel construction, the Leander School District has pioneered the use of exterior insulated Tilt-Up wall panels in combination with interior Tilt-Up panels. Structural systems for new school buildings in the Leander School District consist of the following:
1. Roofs: At classrooms, sloped standing seam metal over polyisocyanurate insulation over steel roof decks. Decks are supported by open web steel joists and steel beams. At areas other than classrooms, roofs consist of built-up membranes over rigid board or concrete insulation over steel decks. Decks are supported by open web steel joists and beams.
2. Floors: Concrete slabs on metal deck supported by steel framing.
3. Foundations: Concrete ground supported slabs with grade beams and straight sided drilled shafts
Vertical support of roof and floor loads are provided primarily by exterior insulated Tilt-Up concrete wall panels and a combination of interior uninsulated Tilt-Up concrete walls and steel columns. Interior Tilt-Up panels are provided along corridors and at separations between major functional areas.
Tilt-Up construction was evaluated and determined to meet the goals of the district. Specifically, Tilt-Up construction was selected because of the following:
1. Insulated Tilt-Up wall panels eliminate the mold hazard by elimination of wall cavity and cellulose containing materials in the exterior wall system. The insulated exterior Tilt-Up wall system consists of 2 inches of extruded polystyrene (EPS) insulation board sandwiched between a 3-inch concrete exterior facing thickness and the structural concrete wall thickness. The insulation is bonded and tied to each layer of concrete by fiber reinforced ties placed at 16 inches on center in each direction. There are no voids in the wall and the EPS insulation is not a medium conducive to mold growth.
Uninsulated Tilt-Up walls also provide resistance to mold growth. Concrete walls are much more resistant to mold growth due to exposure to excess mop and cleaning water than are walls constructed of drywall and studs.
2. Exterior insulated Tilt-Up construction improves the integrity of the building envelope. Watertightness is improved relative to cavity wall construction because there are fewer joints and no cavities or voids within the wall system to collect or trap moisture. As such, there is also a reduced reliance on sealants and caulks for weatherproofing and waterproofing. Further, Tilt- Up construction generally performs better in severe weather events, such as high velocity wind driven rain, due to the reduced permeability relative to cavity wall construction. Increased resistance to moisture penetration also contributes to the system’s resistance to mold growth.
3. Insulated Tilt-Up exterior wall panels provide a higher thermal resistance factor, or R-factor, and thus increase building energy efficiency. Properly detailed and constructed insulated Tilt-Up wall systems can provide effective R-values of 18 to 22 or more, when accounting for the thermal mass affect of the concrete walls. The thermal mass affect of the concrete allows the concrete to slow, or buffer, the flow of heat from the exterior to the building interior during hot weather, or, in cold weather, slow the flow of heat in the opposite direction. The concrete will slowly absorb and hold heat during the day and release the heat at night. The thermal mass affect reduces extremes of temperature swings within the building. As a result, mechanical cooling or heating equipment does not have to work as hard or long to maintain a comfortable environment.
By comparison, a concrete wall with stud furring and batt insulation provides an R-value in the range of 10 to 14. A masonry veneer wall with light gage stud backup and batt insulation provides an R-value in the range of 14 to 18.
The image shown above detail that, for an air temperature of approximately 28 degrees and interior temperature of approximately 70 degrees, the temperature at the exterior face of the cavity wall is approximately 42 degrees, while the temperature of the face of the insulated Tilt-Up panel is about 30 degrees. The higher surface temperature at the cavity wall is due to its greater heat loss.
4. Tilt-Up construction provides a high level of structural integrity. At the building exterior, monolithic wall panel sections provide a higher level of resistance to out-of-plane loads than the usual masonry cavity wall construction, which depends on multiple components and connections for out-of-plane strength. Further, the combination of exterior and interior Tilt-Up wall construction inherently provides excellent resistance of the building to lateral loads without the need for braced or moment resistant frames. The Tilt-Up panels act as shear walls to resist lateral loads on the buildings.
5. Tilt-Up wall panels provide a high level of fire resistance. Tilt-Up panels of the thicknesses normally used in school construction, with single mats of reinforcement, provide fire resistance ratings of two or more hours with no further requirements for fireproofing. Tilt-Up walls are also effective sound barriers.
6. Speed and efficiency of construction. Tilt-Up construction increases the speed of construction in a number of ways. The materials required for the forming and casting of Tilt-Up wall panels are readily available so that the lead time to begin construction of panels is reduced. Tilt-Up panel shop drawings are prepared while mobilization, site preparation and placement of casting slabs, usually the building floor slab, take place.
Once panels have been cast and erected, wall construction is virtually complete. Except for windows and doors, there are minimal requirements for additional trades or the installation of additional materials to complete the wall systems. As such, Tilt-Up buildings can be dried-in faster than can those using conventional cavity wall construction. Further, Tilt-Up construction virtually eliminates exterior columns and greatly reduces the number of interior columns required for the building, which eliminates or reduces the number of pilasters or “bump outs” in finish walls that architects and space planners must work around. And, since the Tilt-Up panels provide long, continuous lines of structural support, floor and roof framing can be laid out at regular intervals to maximize deck efficiencies and repetition of framing members and connection details.
7. Tilt-Up construction is cost effective. The ability to construct a simpler, complete wall system at ground level, with easy access, minimal labor, fewer material and component requirements, fewer trades and no requirement for scaffolding, provides significant construction economies. Further, with the dramatic increases in the cost of structural steel, light gage studs and gypboard that have occurred in recent years, the reduction in requirements for these materials provides overall material and installation cost savings. As noted above, Tilt-Up construction permits more efficient layout of roof and floor framing systems, eliminates or reduces the need for braced or moment resistant frames and reduces the quantity of structural steel required.
8. Tilt-Up walls are durable. Concrete walls clearly provide dramatically higher levels of durability than drywall, or even concrete masonry. At interior walls, Tilt-Up panels provide solid, strong backup for applied finishes, or they may be painted with high durability paint. Although occasional repainting may be required, the basic wall systems will remain intact and serviceable well beyond 50 years. The durability of Tilt-Up walls is particularly important in high use areas, such as corridors, gymnasiums and vocational education areas.
9. An almost endless variety of exterior finishes may be used. Surface finishes may consist of paint, textured paint and coatings, exposed architectural concrete, brick, concrete masonry, natural stone, formed patterns and formed textures. The Leander School District has elected to primarily use a combination of thin brick and sand blasted architectural concrete as exterior finishes for new school buildings.
TILT-UP DESIGN AND CONSTRUCTION ISSUES
Although the application of Tilt-Up construction to schools in the Leander School District has been very successful, there are a few issues that have required special attention from the design and construction teams.
The design of the architectural building envelope must be integrated with the building structural design. Development of exterior elevations, including window, door and canopy layouts, must be closely and carefully coordinated between the architect and the structural engineer. For economy, panel dimensions and layouts must be as repetitive as practical.
Where thin brick will be used, architectural elevations must be carefully developed to account for panel joint locations and door and window details and to coordinate brick module dimensions.
Building design requires a high level of coordination between the architect and structural, mechanical, plumbing and electrical engineers. In particular, at interior Tilt-Up panels, openings for ductwork, piping and electrical conduits must be coordinated with the Tilt-Up panel layout and elevations.
A higher level of effort is required to prepare shop drawings for exterior insulated wall panels. Shop drawings must provide dimensions and details for the usual structural reinforcement, embeds and dimensions. Further, additional elevations and details are required for the architectural face of the panel to properly address the thin brick construction, reveal layouts and details for windows and doors. Shop drawings must also include layouts of electrical conduit, junction boxes, switches and receptacles to be embedded within the panels.
Construction of the panels requires a high level of field quality control and coordination by the contractor. The architectural details at the exterior wythe and proper placement of electrical embedded items, and overall coordination of these items with the structural wall panel, require additional time and attention from the contractor. Panels that are to receive an exposed concrete finish require that close inspection of the casting slab be performed to ensure that any blemishes or flaws in the surface of the casting bed are not reflected in the finished panel. Extreme care must be used to monitor and control concrete mix designs to ensure minimal variance in the color, texture or finish of exposed concrete panels.
The Leander School District recently commissioned a study by an independent consultant to evaluate the district’s performance with regard to construction of new facilities. The consultant’s report provided comparisons of the cost of recently completed district facilities to statewide median figures.
The current median cost of elementary school construction in Texas was reported by the consultant to be $133 per square foot. The district’s most recent elementary schools built using Tilt-Up have been constructed at an average inflation adjusted cost of $127 per square foot, or 4.5 percent below the state median cost.
For middle schools, the statewide median construction cost was reported to be $145 per square foot, with a lower quartile cost of $129 per square foot. The inflation adjusted cost of the district’s most recent middle school was $121 per square foot, or 17 percent below the state median construction cost.
It should be noted that the Leander School District’s results cannot be attributable to lower construction costs in the central Texas area. The consultant’s report indicated that Austin area construction costs are actually four percent higher than statewide averages. Also, the district has generally selected materials, systems and finishes for new facilities that actually increase the upfront construction costs, but return benefits in terms of lower maintenance and energy costs.
Chuck Fields, principal for Fields and Associates Architects, sees many benefits to the use of insulated Tilt- Up panel construction for educational facilities. According to Fields, “This approach reduces the amount of structural steel required, shortens the construction schedule and provides an excellent thermal envelope to reduce building operating costs. Also, the use of thin brick at the exterior panels provides the beauty and versatility of brick, but with a level of quality and durability that is superior to hand laid brick. The use of thin brick eliminates the need for painting and the long-term operational costs associated with the periodic reapplication of paint or coatings. Overall, the concrete Tilt-Up panel system provides a cost-effective, mold resistant construction method, lower long-term maintenance and operating costs and a durable, yet attractive, exterior wall system.”
The Leander School District facilities that have used the exterior insulated Tilt-Up walls in combination with the uninsulated interior walls have been constructed by American Constructors, L.P. In reviewing the completed construction, Martin Burger, Executive Vice President of the firm, noted “The use of Tilt-Up walls for building construction has evolved well beyond the big, ugly boxes of old. Many building owners, both public and private, are finding this building system to be highly desirable due to its low initial cost, energy efficiency, low maintenance cost and speed of construction. And, the system provides these advantages in a highly attractive building envelope.”
In summary, the innovative application of Tilt-Up construction to recent school projects in the Leander School District, implemented through a highly collaborative effort between the district and its architects, engineers and construction manager, has proven to be very successful in the attainment of the district’s goals for new school construction.
The author would like to thank Jimmy Disler, Executive Director for Capital Improvements for the Leander Independent School District; Martin Burger, Tim Cahalane and Tammy Isley, of American Contructors, LP; and Chuck Fields, of Fields and Associates, for their assistance and contributions in providing information for this article.