Iano's backfill

Tunneling the New York City Subway

Engineering News Record is publishing a series of videos about tunneling work for the New York City Second Avenue Subway line. The tunneling work itself, as well as the complex planning and logistics required, are both interesting:

Video Part 1

Video Part 2

 

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.

Rammed Aggregate Piers and Soil Reinforcing

Ideal Support, Rammed aggregate piers ensure subgrade stability (Structural Engineer, May 2005) reports on a proprietary foundation system used to provide both foundation support and soils reinforcing. Rammed aggregate piers are constructed by drilling piers 24 to 36 inches in diameter, 7 to 30 feet in depth. Drilled holes are then filled with layers of compacted crushed rock or gravel. As each approximately 12-inch layer is placed, a vertical impact ram is used to densify the aggregate and compress it into the surrounding soil. When properly spaced, rammed aggregate piers can provide both direct structural support, as well as strengthening of the surrounding soil in which they are placed.

The diagram at left illustrates rammed aggregate piers directly supporting a shallow spread footing (left-most) as well as providing soils reinforcing below a slab on grade.

The diagram below shows rammed aggregate piers below a retaining wall, in this case designed to improve the underlying soil's resistance to shear failure during a seismic event.

More Info:

• Deep foundations, such as piers and piles, are  discussed on pages 43 through 52 of the textbook.
• Additional information on rammed aggregate piers is available at the manufacturer's web site www.geopier.com.

Emerging Technologies for Residential Construction

The Partnership for Advanced Technology in Housing (PATH) has announced its Top Ten Technologies for 2004, including:

• Frost Protected Shallow Foundations: Protect foundations against frost damage without the need for excavating below the frost line.
• Home Run Plumbing Systems: Feeds flexible piping directly to one fixture from the central manifold.
• Engineered Panelized Systems: Prefabricated panels form a structural envelope that reduces or eliminates the need for on-site framing and can be assembled swiftly.
• HVAC Optimization: To maximize efficiency and comfort, use properly sized equipment, and place HVAC equipment inside the conditioned space.
• Tankless Water Heaters: Provide hot water on demand, reducing or eliminating standby loss.
• Shared (Community) Waste Water treatment: A single drainfield/treatment area connected to each house's individual septic tank.
• Air Admittance Vents: Pressure-activated, one-way plumbing valves, eliminating the need for conventional pipe venting and roof penetrations.
• Low Impact Development: LID techniques can offer a cost-effective way to address stormwater management through site design and "Best Management Practices".
• Integrated Steel/Wood Combination Framing: Combinations of wood and steel framing within the overall building shell use the best attributes and cost benefits of each.
• Pre-cast Concrete Panels (Walls and Foundation): Made under quality-controlled factory conditions, pre-cast concrete panels are ready in a fraction of the time needed for a poured foundation.

Several of the Top 10 may be of particular interest to readers of Fundamentals of Building Construction. See the PATH page links below for additional technical background, information on regulatory acceptance, and links to additional resources and manufacturers related to these systems:

Engineered panelized systems discusses a variety of innovative insulated panel systems including composites of rigid foam/light gauge steel framing, rigid foam/concrete, and rigid foam/fiberglass/PVC. For a brief introduction to prefabricated panel systems, see also page 100 of the textbook.

Shallow frost-protected foundations are also discussed and illustrated on page 65 of the textbook. In these systems, rigid insulation installed below grade protects against soil freezing and allows shallow footings to be constructed above the normal frost line.

Combination steel/wood framing combines light gauge steel studs and joists with wood framing members such as sills, band joists, and plates. These systems offer the advantages of steel framing (light weight, recyclability, dimensional stability, resistance to moisture, etc.) while relying on traditional wood light framing connection methods and tools (nail guns, staples, etc.). Both loadbearing and nonloadbearing systems are feasible.

Pre-cast concrete foundation panels are insulated, loadbearing, reinforced concrete panels that according to PATH, allow erection of residential basements in less than one day. Panels come with concrete "studs" and metal or preservative-treated furring for attachment of finishes. [Author's note: Waterproofing of joints between panels appears to rely on conventional joint sealants, a potential long-term vulnerability.]

What is PATH?
PATH describes itself as

a voluntary partnership in which leaders of the homebuilding, product manufacturing, insurance, and financial industries join forces with representatives of Federal agencies concerned with housing. By working together, PATH partners improve the quality and affordability of today's new and existing homes, strengthen the technology infrastructure of the United States, and help create the next generation of American housing.

Its Top 10 Technologies are intended to represent "practical, easy-to-use building technologies that are on the edge of industry acceptance and are quickly gaining importance."

Electromagnetic Waterproofing

Building Safety Journal (September 2003) reports on electro-osmotic demoisturizing technology, a method for protecting foundations from ground moisture with low-frequency radio waves. The method works with sub-grade concrete and masonry foundations. The electromagnetic waves neutralize electrical charges in the foundation material that allow capillary action to occur, thereby stopping water intrusion. According to the article, the system consumes about as much energy as a 100-watt light bulb, is minimally invasive, and is not limited by building size or use.

Bentonite Foundation Waterproofing

The following are images, with accompanying explanation, taken during several site visits to a bentonite waterproofing installation. More background on this type of waterproofing can be found in the textbook, Chapter 2, Foundations, Waterproofing and Drainage, or on a manufacturer's web site.

[Click on images to enlarge.] The foundation is cast-in-place concrete. The geotechnical report indicates a high likelihood of groundwater around the foundation, especially during wetter times of the year. The interior of the basement includes fully finished spaces. Due to the severity of the groundwater conditions, and the sensitivity of the interior spaces, waterproofing and a drainage system are being provided both around the foundation walls and under the basement slab on grade. This article will look at the waterproofing applied to walls, with special attention to the treatment of penetrations in the wall.

In this view, one side of the concrete wall formwork has been erected. Steel reinforcing and form ties are in place, and a collection of PVC pipe sleeves have been installed. These sleeves will be cast into the wall. Later, utility lines of various types (water service, waste, gas, power, etc.) will be installed through the openings created by these sleeves.

The ends of the sleeves are taped off with duct tape to prevent concrete from leaking into the pipes during the pour. Around the middle of each pipe is a bentonite mastic waterstop. These will be cast into the concrete and create a waterproof seal between the concrete and the outside of the PVC pipe.

Here are the sleeves after the concrete has been poured and the forms stripped. Note the rough concrete edges around the outside of the PVC pipes. It's easy to imagine water seeping through this juncture if there wasn't a waterstop part-way in to stop it.

Here is another example of the use of the bentonite waterstop. On this project, waterstops are to be applied at all "cold" joints (where new concrete is cast directly against previously poured concrete) throughout the foundation work, to prevent leakage across these vulnerable locations. Note however that bentonite waterstops are not suitable for use at moving joints.

This view, from the inside of the foundation, shows a short length of utility pipe installed through the PVC sleeve. The outer sleeve can be seen, its face flush with the concrete wall. The inner pipe, which projects beyond the surface of the wall, is the actual utility service line (the continuation of the pipe will be installed later). In between is a mechanical gasket, which when tightened, forms a waterproof seal in the space between the sleeve and the utility pipe.

Here is another view of a partially installed utility line, this time taken from the outside of the foundation wall. Bentonite panels have been partially installed around the pipe penetration. Note the additional layers of waterproofing and mastic applied around this penetration to provide additional protection against leakage.

This image shows an interesting detail at the foot of the concrete wall where it meets the spread footing. Additional bentonite granules contained in a plastic "sausage" are inserted behind the panel. The plastic will dissolve when it first comes in contact with water, depositing a thick extra layer of bentonite at this vulnerable juncture.

This final images shows a section of wall with the waterproofing work almost complete--the top edge of the waterproofing is set to come within six inches or less of finished grade. Drainage panel installation is still in progress, with the upper half of waterproofing yet to be covered. In addition to relieving water pressure against the foundation wall, these panels will also provide physical protection to the waterproofing as the foundation is backfilled. Note the filter fabric facing on the drainage panels. This material prevents soil particles from being carried into and eventually clogging the panel. Note the fasteners used to attach the bentonite and drainage panels to the wall. These powder-actuated fasteners penetrate through both panels into the concrete. When the bentonite first gets wet and swells, it will form a tight seal around these penetrations. Also partially visible at the bottom right of this photo is the trench along the sides of the spread footing, ready to receive perimeter footing drains.
(c) Joe Iano 2003

02 - Foundations Links

This article contains external links to resources on the Web relevant to Chapter 2 Foundations.

Case Foundation Company

Information on various deep foundation technologies

Geopier Foundations

Rammed aggregate foundation piers

Layne GeoConstruction

Specialty foundations and ground stabilization technology

Pin Foundations Inc.

Proprietary "pin pile" foundation system

Schnabel Foundation Company

Earth retention, soil mixing systems, and specialized foundation systems

Simplified Design of Building Foundations

Understandable, authoritative foundation design guide for architects and builders.

Unified Soil Classification--Laboratory Determination

Soil types and criteria for their determination in the laboratory (State of Viriginia DOT)

Unified Soil Classification--Field Determination

Soil types and methods for their determination in the field (Department of Infrastructure, Planning and Natural Resources, New South Wales, Australia)

Voltex Bentonite Waterproofing

One manufacturer's bentonite waterproofing product.

WR Grace Bituthane

One manufacturer's self-adhered bituminous waterproofing.