Iano's backfill

Structural Innovations In Seattle Highrise

Banking On A Museum (ENR, July 17, 2006) describes the construction of Seattle's Washington Mutual Center-Seattle Art Museum Downtown highrise, a 42-story office tower/museum expansion with innovative structural design and construction, including:

The building is the tallest to date in the US to use performance-based methods for its seismic design. Rather than adhere to the prescriptive seismic design requirements of the building code, structural engineers Magnusson Klemencic Associates performed a more complex and lengthy analysis of the building structure to prove that their alternative design could perform to the same levels. The result is structure with fewer architecturally intrusive elements.

The building is also the first in the US to use buckling restrained braces (BRBs). In this case, 44 BRBs within the first thirteen levels of the structure link the relatively slender concrete core to a pair of outlying concrete-filled steel pipe columns. This linked composite structure increases the core's effective depth, thereby increasing its stiffness and reducing overturning forces.

Where the main structural core is located eccentrically relative to the lower, larger floor plates, additional braced and moment frame structures provide balanced lateral force resistance to these portions of the structure.

Steel reinforcing in the "ductile concrete" core was so densly placed that it was cast with self-consolidating 10,000 psi concrete. In addition, the concrete mat foundation, constructed 95 feet below grade, ranged from 7 to 14 feet thick.

Apart from the structural design, other interesting aspects of this project include unusual financial and development relationships between the building's two banking and museum tenants, provisions for structural changes in floor configurations as occupancy between these two tenants changes over time, and a variety of innovative techniques applied to the construction process.

This article is good case study in the real-world interaction of the many forces that shape buildings and the interesting designs that can result.

More Info
Tall, skinny ... stable: Using novel technology, S.F. tower should resist quakes, gales (SFGate.com, July 2, 2006) discusses another innovative tall building designed by engineers Magnusson Klemencic Associates, this one in San Francisco. The companion Back Story also links to video and a podcast, in which engineer Ron Klemencic discusses the building's design.

Really Fast-Built Houses

Record-Breaking Concrete--22-minute set (Concrete Construction, November 2005) reports on the 2 Hour House project in Tyler Texas. According to the article, the goal of the project was to construct a home from the ground up in less than two and one-half hours. Two  teams competed to complete 2160 sf homes including concrete slab on grade, three bedrooms, two baths, two-car garages, and full landscaping in the allotted time. Of particular note for readers of this particular publication, was the specially formulated concrete with a 22-minute set time.

Developments In Low-Cost Residential Construction

ICF Homes
Concrete Homes Continue to Rule The Roost in Lubbock, Concrete Homes Newsletter, May/June 2004, describes the City of Lubbock, Texas's ongoing efforts to replace substandard and deteriorating homes with new homes built with insulating concrete form (ICF) construction.

The replacement program is part of the city's Affordable Housing Reconstruction Program, which receives funding from federal, state, and private sources. According to the article, ICF constructed homes have proven so energy efficient and disaster resistant that the city no longer accepts bids for conventional wood-frame constructed homes.

NAHB Demonstration Homes
Houses M.A.D.E. for research, Fine Homebuilding, July 2004, describes the National Association of Home Builder's (NAHB) four demonstration houses constructed using materials and methods highlighted in NAHB's 1999 Builder's Guide to Marketable, Affordable, Durable, Entry-Level Homes to Last (MADE).

Innovations demonstrated in these projects include:

• precast concrete foundation walls
• advanced framing methods
• maintenance-free siding and roofing materials
• air-admittance valves (mechanical venting ports used in a plumbing system to eliminate conventional pipe venting)
• CPVC plastic water piping
• standing seam metal roofs
• high-efficiency floor plans

Prefabricated Homes
Factory-Built Houses, Fine Homebuilding, June/July 2004, compares pros, cons, and costs of three types of prefabricated home construction technologies: panelized, modular, and structurally insulated panels (SIPs).

For each technology, this full-length article describes the basic systems involved, describes the building process with first-hand accounts and annotated photographs, and provides brief lists of pros and cons. The challenges in attempting to create apples-to-apples cost comparisons between the three technologies are also discussed. Some bottom-line conclusions include:

• Panelized construction offers shortened construction time with few if any limitations on design flexibility.
• Modular construction offers the shortest construction time and greatest potential costs savings, but is more limited in design flexibility. (Except for connecting plumbing, electric, and laying carpet, a house can be erected in as little as one day.)
• SIPs construction offers high energy efficiency and high structural strength, but at higher costs than the other systems featured in this article.

More Info
_For additional articles on Lubbock's ICF home building program see Lubbock emerges as frontrunner in concrete home construction, and Rebuilding a Community with Strong, Energy Efficient, Affordable Homes.
_More information on MADE homes is available at Results from the Field: MADE Project Path Field Evaluation Site.
_For more articles on this site related to innovative construction technologies see this site's innovations in project design & delivery category. For another example of modular construction aspiring to quality design and environmental friendliness not previously mentioned on this site, see also The Glidehouse by mkarchitecture.

More On Insulating Concrete Forms

Flex Formation, Building Design & Construction, 02-02, discusses the increasing adoption of insulating concrete form (ICF) systems for nonresidential construction, where it competes with construction systems such as light-gauge metal frame, wood frame, masonry, and precaset concrete. Some noteworthy points raised in the article include:

• ICF construction consists of cast-in-place concrete walls, 4 to 10 inches thick, constructed with expanded polystyrene forms that, unlike conventional formwork, are not removed after the concrete pour but rather become part of the final construction.
• In addition to higher energy efficiency, ICF system manufacturers claim benefits such as design flexibility, acoustic and thermal control, jobsite safety, cost efficiency, and ease of installation.
• The insulating effect of the formwork allows concrete pours in lower ambient temperatures.
• The Insulating Concrete Form Association claims that nonresidential construction now represents 20% of the ICF market.

According to the article, ICF systems may be categorized as block, panel, or plank. Insulating units for block systems range from 8 x 16 inches to 16 x 48 inches. They are stacked in place and fit together with various types of interlocking edges. Panel systems are preassembled formwork sections ranging in size from 1 x 8 feet to 4 x 12 feet. Sections may be prefabricated off- or on-site prior to placing in final position. Plank systems range in size from 8 to 12 inches high by 4 to 8 feet wide and in contrast to plank systems are assembled in place.

More Information:
Insulating Concrete Forms
Rastra Insulating Concrete Formwork

Low-Cost Concrete Homes

Building Poured-Concrete Homes, Journal of Light Construction December 2003, is an interesting article about one contractor's experience constructing cast-in-place concrete houses. Built in Florida, these houses offer termite resistance and hurricane wind resistance appropriate to this area's conditions. Additionally, this system relies on relatively low-skilled workers, making it well suited to that region's building boom economy. A crew of fifty field personnel is able to complete four buildings per day.

A few interesting points:

• Walls are constructed on top of thickened-edge slabs on grade.
• Wall reinforcing is very simple. Vertical #5 bars are tied to reinforcing projecting from the slab at 6-foot intervals. A horizontal #5 bar also runs continuously at the top of the wall. Where door and window openings occur, prefabricated header reinforcing is inserted. Between the vertical bars, 6 foot by 8 foot sheets of welded wire mesh are tied.
• Walls are cast 6 inches thick. One and half to two hours are required to pump the concrete for a typical house. Plasticizers are added to the mix to eliminate the need for concrete vibration during the pour. Forms can be stripped the day after they are poured.
• Interior partitions are most commonly light gauge steel studs, for additional termite resistance.
• Windows and doors are fastened directly to the concrete openings.
• Roofs are conventionally constructed from prefabricated wood trusses.
• The exterior is finished with one coat of stucco (Portland cement plaster) applied directly to the concrete. The interior is finished with 1 inch of rigid foam insulations, strapping, and gypsum drywall.

Overall, the system illustrated is remarkably simple and economical. While these houses won't likely be showing up in the architectural magazines, they are a good study in how to build efficiently and economically.

More On Concrete Floor Flatness

SpecPress, the newsletter for subscribers to the ARCOM MasterSpec system, has more to say about the contradictions faced in the specification of flatness of concrete slabs as previously discussed on this site in articles Concrete Floor Flatness and Concrete Slab/Floor Covering Issues. In The Concrete Floor Tolerance/Floor Covering Conundrum (Volume 9, Issue No. 4, Fourth Quarter 2003), Bruce Suprenant identifies the following basic problems:

• Concrete placement in Division 3 relies on the F-number system to specify floor flatness. Finish floor coverings specified in Division 9 rely on the straightedge method. There is no direct correlation between the two methods.
• The criteria for measuring F-numbers specified in Division 3 do not satisfy the requirements for Division 9 finishes. For example, F-number measurements may not be taken across construction joints, they may not be taken within 2 feet of a slab penetration, and they must be taken within 72 hours of the initial slab pour. However, floor coverings typically are applied across construction joints, close to slab penetrations, and after the slab is fully cured.
• Different floor coverings applied to different sections of the same slab may require different slab finishes (such as hard trowel, broom, etc.), even though it is often not practical to specify different finishes across a single pour.

Suprenant recommends the following possible approaches to dealing with the contradictions:

The specifier can require an arbitrarily higher initial floor flatness hoping that the final, cured slab will meet minimum requirements. The difficulty with this approach is that there is no method to predict final flatness based on flatness measured within the intial 72-hour measurement window.

Additional reinforcing can be added to the slab in order to reduce curling that occurs during curing. Suprenant claims that with additional reinforcing, control joints are not effective, and thereby money can be saved by "not cutting and filling joints".

Assign responsibility to grind or patch the floor as required to the floor covering contractor(s). The advantage to this method is that responsibility for achieving the required floor flatness is put into the hands of the floor covering contractor whose product dictates the flatness requirement. Suprenant suggests specifying an allowance amount in the specification to be applied toward anticipated grinding and patching.

Pervious Concrete

Pervious Concrete Pavement: A Win-Win System, Concrete Technology Today, August 2003, a three times a year newsletter from the Portland Cement Association, discusses concrete pavements with high porosity. Also known as no-fines concrete or porous concrete, this 50-year old technology is gaining renewed attention for its ability to contribute to improved storm water management.

Unlike conventional concrete mixes, the aggregate in pervious concrete is narrowly graded. By omitting smaller sized aggregates, and limiting the amount of cement paste, interconnected voids may constitute 15% to 35% of the final concrete after curing. The interconnected void spaces allow water to pass through the concrete and percolate into the ground below. According to the article, percolation rates of pervious concrete may range from 2 to 18 gallons per minute per square foot of surface. The large percentage of void space also makes pervious concrete lighter and less strong than conventionally formulated concrete. Its compressive strength may range from 500 psi to 4000 psi, and its density may vary from 100 to 120 pounds per cubic foot.

This article provides more information about mix design for pervious concrete, its application to various site pavement applications, and its benefits related to federal clean water regulations.

Falling Water Restoration

Restoration wrapping up at Fallingwater, Fine Homebuilding January 2004, reports that the structural portion of the $11.5 million restoration started in 1996 is coming to a close.

VSL, the concrete restoration company hired to implement the repairs, provides additional details regarding the structural repairs on their web site, including:
--Explanation and diagrams of the posttensioning reinforcement of the main cantilevers, consisting of bundles of 13 1/2-inch steel cables
--Project background and description of structural steel channels and fiber reinforced polymer bars used to reinforce other parts of the structure.

Future restoration includes waterproofing, finishes, furnishings, and landscaping.

More information:
The textbook discusses posttension and other prestressing concepts on pages 493 - 499.
VSL Falling Water Media Coverage provides additional links to articles and media coverage of the Falling Water Repair project.
Western Pennsylvania Conservancy provides information about the building, and visiting Falling Water.
The Frank Lloyd Wright Building Conservancy provides information about conservation efforts related to all of Frank Lloyd Wright's projects.

Highrise Aspires to LEED Platinum Certification

In What Will The City Look Like in 2010: Productivity, Metropolis Magazine reports on Genzyme's new corporate headquarters in Cambridge, Massachusetts. The 12-story, 350,000 square foot building will be applying for LEED Platinum certification, the US Green Building Council's highest rating. If successful, it will become the first Platinum certified building of this size in the United States.

According to the article, sustainable features include:

• The concrete structural frame adds thermal mass that contributes to energy efficiency. The concrete filigree system makes efficient use of materials, relies on locally produced materials, and is reinforced with recycled reinforcing bars.
• Natural daylighting is achieved with a mixture of skylights, fixed and motorized mirrors, prixmatic louvers, interior reflective surfaces, and blinds.


• A double wall glazing system along with selective use of reflective glass creates an energy efficient building skin.
• Heat exchangers reduce heat loss in winter heating mode. Steam absorption chillers, running entirely on waste heat from a nearby power plant, provide summer cooling. The building's atrium allows natural ventilation. Microclimate control allows localized control of heating and cooling in as many as 50 separate locations per floor.
• A system of water sensors control a water saving, on demand irrigation system for site landscaping.
• Sustainable site strategies include brownfield site development, urban redevelopment, and public transportation access.
• The project makes extensive use of sustainable woods and recycled materials.

More Information:
What Makes Genzyme Center a Green Building? provides more information about this project's compliance with LEED goals.
Genzyme's Genzyme Center web pages provide more information about the project in general.
Atlantic City NJ Casino Collapse, on this site, provides more information on the filigree concrete framing system.

Atlantic City NJ Casino Collapse

Missing Connection to Wall May Have Weakened Garage, ENR November 17, 2003, reports on the ongoing investigation into the October 30 collapse of multiple concrete floor decks of an Atlantic City New Jersey casino under construction. Missing or inadequate connections between the decks and an adjacent bearing wall may be a significant factor contributing to the incident. Other possible factors that have been cited included inadequate shoring and inadequate curing time between deck concrete pours. According to the article, some workers reported seeing bowed or bent shoring columns prior to the collapse. Four workers were killed in the collapse.

This incident has also brought attention to the innovative construction system being used on this project, the filigree wideslab concrete construction system. The filigree concrete system is a composite system using both precast concrete formwork and cast-in-place concrete. It is one of a variety of permanent form systems, which aim to achieve higher economy through reduction in forming costs.

In the filigree system, formwork for slabs and beams are precast at a fabrication plant. They may be either prestressed or conventionally reinforced. Additional reinforcing in the form of shallow-height steel bar joists are cast into the precast units with the upper portion of these joists projecting out of the upper surface of the precast. When concrete is later poured on top of these units, the bar joists create a bond between the precast and the site-poured concrete. After delivery to the construction site, the precast units are lifted into place and supported on temporary shoring. Next additional reinforcing is placed above the formwork. Foam plastic blocks may also be added to the assembly. Where these blocks are placed, voids will be formed in the final pour where excess concrete is not needed, thereby lessening the overall quantity of concrete and reducing the weight of the structure. Finally, concrete is poured on site to complete the system.

In the completed assembly, the precast and sitecast concrete work as a single composite structural unit, bonded through the shallow-height joists described above. Bottom steel is contained within the precast units, and the remainder of the required reinforcing resides within the cast-in-place portions of the system. Variations on this system include flat plate construction over structural steel beams and girders, all-concrete flat plate construction, and concrete beam and slab construction (as illustrated above). Additionally, both one-way and two-way systems are feasible.

More information:
See the text for more about innovative formwork systems (pages 536 - 537), prestressing concepts (beginning on page 493), and one-way and two-way slab systems (pages 490 - 491).
Mid State Filigree Systems describes the advantages of the filigree concrete system, and provides diagrams illustrating variations on the system.
BOCA Research Report No. 96-14 describes the filigree system in additional technical detail.