The market interest in sustainability has quickly progressed from fledgling to noticeable to overwhelming. Architects are finding that post-tensioned concrete offers many advantages in achieving green building targets for high-rise multifamily housing projects. However, tomorrow's version of this ubiquitous building type will have to reach a much higher bar of sustainability, and although the structures still will be post-tensioned concrete, they will look quite different from the buildings we see today.

For the past 10 to 15 years, architects and engineers on the West Coast have been busy with projects that repair and reinvigorate the urban fabric. Land prices and the cost for construction have been high and the demand for living close to employment, services, and recreation has been strong as well. This resulted in a lot of high-rise condominiums and apartments appearing in the urban core.

Of the dozens of major multifamily housing projects Ankrom Moisan Architects, Portland, has designed since 1995, all but three are post-tensioned concrete structures. These projects are typically mid-rise and high-rise construction. In aggregate they have contributed thousands of urban residential units in major population centers of the West Coast: Los Angeles, San Jose, Portland, and Seattle.

Advantages of post-tensioning

Ankrom Moisan Architects first used a post-tensioned concrete system for housing on a project developed in Portland's Pearl District during the mid-1990s. There was a lot of attractive features to a post-tensioned system.

  • High thermal-mass construction is great for building-wide heat retention and reduced sound transmission between floors.
  • The system is flexible and could easily cantilever the slab to form an exterior deck for each living unit.
  • Column spans coordinate easily with underground parking garage layouts.
  • The structure can be exposed as interior finish.
  • It is easy to express structural slabs on the exterior.

It is easy to forget that the decision to use post-tensioned concrete for this particular building type was a pretty ground-breaking move at the time. Over the years architects, engineers, and contractors have become very adept at building PT structures for this particular use because it helps deliver a very high-value asset to the client.

Green building friendly

During the same period that post-tensioned concrete became commonly used for high-rise multifamily housing projects, a tremendous shifting of focus to sustainable design was seen. The USGBC was founded in 1993 and Architecture 2030 was founded in 2003. The LEED requirement to achieve at least two energy points occurred in 2007; this is seen as a first step to more directly influence buildings to be more energy efficient, as a response to global warming.

The current advantages of post-tensioned construction for green building standards are the same as for cast-in-place concrete structures.

  • They can use recycled content in the concrete mix, usually fly ash (a byproduct of burning coal).
  • Concrete typically is manufactured within a 500-mile radius of the project site and sometimes the gravel is extracted nearby as well.
  • If necessary, the concrete structure could be demolished and recycled into something else.
  • Very little internal temperature fluctuation results due to the high mass structure.

The challenge

Building construction and operations are responsible for 48% of energy consumption in the United States and 76% of electricity consumption. This is where nearly all future sustainability issues will focus.

A major opportunity lies in the current U.S. building stock, comprised of 300 billion square feet. For the next 20 years, 52 billion square feet will be demolished, 150 billion square feet will be remodeled, and 150 billion square feet will be new construction. By the year 2035, three-quarters of the building stock will be new or renovated. This is where architects and contractors will focus on modifying post-tensioned concrete designs to meet the new paradigm of sustainability.

Energy use is huge in the design world now. All of the assumptions regarding systems, financials, and aesthetics must be reevaluated. It is difficult, but not impossible, for large buildings to achieve the 50% to 60% efficiency levels that Architecture 2030 is proposing. Ultimately the aim is to design net-zero buildings by 2030; buildings that produce all the energy they need on an annual basis.

True sustainability

Post-tensioned concrete will continue to serve the building community well when it comes to high-rise multifamily housing, but in the future we will need to use it differently in order to meet the increasingly high demand for energy efficiency in the 21st century.

This is an area where designers, engineers, and contractors are trying to improve post-tensioning. The energy liability in post-tensioned construction occurs when slab edges are extended through the exterior envelope, either to express the structure or to create a deck. The slab acts as a radiator to the outside, bleeding heat from the building. In a heating-dominated climate, this is not good. In a cooling-dominated climate, the energy model might show a benefit.

One of the biggest challenges for PT concrete in multifamily housing will be to figure out how to do a nice-looking, cost-effective, code-compliant bolt-on deck that does not let water infiltrate the building exterior. This sounds like a simple task, but it is a true balancing act and one very worthy of the creative attention of innovators in the design and construction industry. Solving this problem will pave the way for much more energy efficient PT structures and ultimately the kind of buildings our clients will demand.

Carolyn Forsyth, AIA, LEED AP, is a senior associate at Ankrom Moisan Architects.