Cast-in-Place (CIP) technology for constructing building walls is more than 100 years old. Thomas Edison was one of the first to recognize its features and benefits. Over the years, new construction methods have developed including forming systems, the use of materials for enhanced insulation, and improved methods for removable forms, most commonly in single-family housing.
CIP produces a very strong wall and has an intrinsic thermal mass which provides, along with the appropriate insulation, an energy efficient building. It also permits the application of traditional finishes to both the interior and external faces allowing an appearance similar to frame construction, but with much thicker walls.
Cast-in-place concrete procedures are also used for large building construction. The twin towers of Marina City in Chicago at 588 ft. in height, the One Shell Plaza in Houston built in 1970, and Chicago’s Lake Point Tower built in 1968 are just a few examples.
Earthquake-resistant buildings are constructed with three basic structural elements: the foundation; the vertical framing elements; and the diaphragms. In a reinforced concrete building, moment-resisting frames or shear walls, make up the vertical elements and consist of the vertical load-resistant and the lateral (seismic and wind) resistant systems. The vertical load-resistant system is composed of the floor (horizontal framing) and the column and walls (vertical framing).
During an earthquake and ground shaking, a building shifts through multiple displacement cycles. The reinforced concrete structural walls are designed and proportioned to resist the resultant combination of shear, moment, and axial force.
The design requirements are specified by building codes which assure the construction of a wall capable of resisting strong earthquake trembling without unacceptable loss of strength or stiffness. Shear walls, moment resisting frames, braced frames, or a combination of these systems make up the lateral resistant system.
The Recommended Seismic Provisions specified in the National Earthquake Hazards Reduction Program (NEHRP) account for variations in building seismic risks despite the design and construction quality. The risk is determined by several factors including:
- The ground shaking intensity and other effects the structure may experience during an earthquake.
- The number of occupants affected by the structure’s failure.
- Use requirements after an earthquake.
Structures are evaluated and categorized according to seismic risk specified in the Seismic Design Category (SDC). Six SDCs categories range from A, for structures posing a minimal seismic risk to F, for those with the highest risk.
To guarantee that all buildings provide a tolerable public risk, the NEHRP Provisions require progressively more rigorous seismic design and construction as a structure’s potential seismic risk increases. Strength, detailing requirements, and the seismic resistance expense also increase.
When cast-in-place or precast walls are implemented to combat seismic forces in new buildings assigned to SDC D, E, or F, the IBC requires special structural walls.
The Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-10) (ASCE 2010) specifies the design force levels, the design proportions, and details as defined in the Building Code Requirements for Structural Concrete (ACI 318-11) and Commentary (ACI 2011).
Building code editions most commonly adopted by state and local jurisdictions undergo continuous revisions to introduce improvements in design and construction practices. The codes applicable to earthquake considerations for building construction are the 2018 International Building Codes (IBC), the 2016 edition of ASCE 7, and the 2017 edition of ACI 318.
The International Building Code is an indispensable tool to preserve safety and public health that provides safeguards from hazards related to the construction environment. All 50 states have adopted it.
The benefits of the IBC are numerous. Application of the codes results in efficient and flexible designs. They encourage the implementation of new and smarter technology. The IBC emphasizes strict engineered solutions and permits the use of verified traditional methods.
In addition to the emphasis on safety, the IBC embraces innovative new technology. Revised on a three-year cycle, the IBC is specifically linked to ICC codes.
The ACI Building Code administers concrete structures designed and built in the United States.
The design provisions of the Code specify the minimum strength requirements for safety and prescribe serviceability and resilience requirements. Factors including deflection, concrete cover, cracking and corrosion protection which influence the design of the structural system also impact the design of the exterior wall.
Sustainability and Energy
Heating and cooling energy savings make insulated cast-in-place walls an appealing method of building construction. Cast-in-place walls contain few joints and have 10 to 30 percent better air containment than similar framed walls providing more consistent interior temperatures for tenants.
Cast-in-place systems are also appropriate for recycled materials. Supplementary materials like a slag or fly ash can replace a portion of the cement. Crushed concrete (aggregate) can be recycled to reduce the need for new aggregate. Recycled reinforcement steel and some polystyrene made with recycled material contribute to energy savings. These techniques earn points toward green rating systems such as LEED®.
The NEHRP publishes a 40-page Guide for Practicing Engineers: Seismic Design Technical Brief No. 6: Seismic Design of Cast-in-Place Concrete Special Structural Walls and Coupling Beams. The guide provides detailed information for the design of cast-in-place concrete buildings to meet seismic requirements.