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Andres Garcia Lachner
An angular roof spans across this white house designed by architecture firm Studio Saxe in a remote part of Costa Rica.

Santiago Hills Villa is perched on a hillside in the Santiago area outside of Santa Teresa, Costa Rica.

The 400-square-metre residence is formed by several white volumes formed by steel frames that are each oriented towards the ocean. Atop the diagonally arranged structures is an angular roof held up by steel I beams.

"Every space in the home has been angled to view the ocean, and this twist creates a geometric relationship between the roofline and the spaces that became the primary element of design," Studio Saxe said.

The studio has fronted the volumes with large windows and added open patios and pathways around the house and between the structures to connect each of its faces with the lush greenery.

"The primary objective was not only to provide dwellers with a connection with its jungle surroundings, but also generate an intimate relationship with an amazing view of the shoreline which brings its dwellers back to a more profound appreciation of the natural world," the studio added.

Studio Saxe has also implemented energy-conserving schemes, such as orienting the expansive overhang towards the predominant wind direction for cooling. The roof also protects the house from sun and rain and collects rainwater.

Santiago Hills Villa's remote location made construction on-site quite difficult, so the studio prefabricated the house's steel framing off-site.

At one end of the residence the main living area, occupied by a recreational room and small kitchen, opens up completely to the outdoors.

The structure's pointed roof shields the space from harsh weather and sunlight, and a glass partition wall separates the interiors from the outdoor deck and swimming pool. Three white lounge beds face the rectangular pool, outdoor shower and wood deck that extends from the house.

Light-coloured gravel, tropical plants and a series of white pavers landscape the property and its perimeters.

A wood dining table, sofa and bookshelf in the living room contrast against the white interiors.

Each bedroom is housed inside an individual volume fronted with sliding glass doors and blue curtains. Similar to the living room, the space is furnished with wood pieces, including a bed frame, vanity and sliding doors for accessing the adjoined bathrooms and paths that run between each of the bedroom volumes.

Founded by Benjamin García Saxe, Studio Saxe has completed several residential projects in Costa Rica, including a wood and steel framed Jungle House and a house with several cantilevering porches.
Ryan Peltier
Timber buildings are popping up around the world. Are they the solution to construction’s carbon problem?

Construction materials alone, including carbon and steel, contribute 11% of global carbon emissions (by comparison, air travel contributes about 2.5%). That’s why architects and development companies around the world are opting for a novel but not-so-new solution: wood. A study from the Potsdam Institute for Climate Impact Research, in Germany, found that with proper forest management, a global boom in wood buildings could sequester up to 700 million tons of carbon a year (wood naturally stores carbon, preventing it from being released into the atmosphere). The idea is catching on: Google’s Sidewalk Labs has proposed a 12-acre timber neighborhood in Toronto, while in February, France mandated that all public buildings after 2022 be constructed of at least 50% wood or other organic materials. The University of Arkansas completed the largest timber building in the U.S. last fall, a 202,000-square-foot dormitory. Architects (and governments) are embracing the material and finding innovative ways to use it.

Zaha Hadid Architects
Gloucestershire, England
5,000-seat football stadium

The Forest Green Rovers is “the greenest football club in the world,” according to FIFA. All food served to players and fans is vegan. Fittingly, Zaha Hadid Architects’ proposed new home for the team is also a carbon-neutral, all-timber stadium—the first of its kind in the world. The 5,000-seat park will be made primarily of sustainably sourced wood and will be daylit, thanks to a lightweight, transparent “roof membrane.” The stadium’s unique shape, says project director Jim Heverin, is also thanks to the material, which “enables the use of unique shapes with little increase to cost,” due to the fabrication process and timber’s light weight. While typical stadiums contribute between 790 pounds and 2,800 pounds of carbon per seat, ZHA estimates Eco Park Stadium will contribute about 440 pounds per seat.

Henning Larsen
Copenhagen, Denmark
7,000-person neighborhood

This development, being planned on a former junkyard in Copenhagen, is the city’s first new neighborhood made entirely of timber. It was designed in collaboration with biologists and environmental engineers; 40% of the 45-acre project site is being preserved as undeveloped habitat for local flora and fauna. Henning Larsen partner Signe Kongebro cites the statistic that 70% of the planet’s population is expected to live in cities by 2050: “It is more crucial than ever that the relationship between city and nature is ‘both/and,’ not ‘either/or.'” The firm plans to use prefabricated timber panels sourced from partners throughout Europe and to integrate nests for birds and bats (built directly into the facades), among other elements that foster nature.

[Illustration: Ryan Peltier]
Voll Arkitekter
Brumunddal, Norway
18-story tower

There are a number of timber-and-steel hybrid high-rises proposed around the world—but Mjøs­tårnet, completed last spring, is the tallest all-timber tower ever built. Debates over timber’s fire safety are ongoing: Architects in the U.K. are fighting to get it removed from a flammable-materials ban instituted after the Grenfell Tower tragedy (though that tower was not made of timber), and Norway had similar laws until 1997. The wood used in Mjøstårnet underwent extensive tests, including one in which large columns were burned for 90 minutes. They proved to be structurally sound, despite charring on the exterior. The material, as well as structural precautions Voll Arkitekter added to the building to prevent fire from spreading, passed Norway’s stringent building standards.

Voll Arkitekter
Brumunddal, Norway
18-story tower

There are a number of timber-and-steel hybrid high-rises proposed around the world—but Mjøs­tårnet, completed last spring, is the tallest all-timber tower ever built. Debates over timber’s fire safety are ongoing: Architects in the U.K. are fighting to get it removed from a flammable-materials ban instituted after the Grenfell Tower tragedy (though that tower was not made of timber), and Norway had similar laws until 1997. The wood used in Mjøstårnet underwent extensive tests, including one in which large columns were burned for 90 minutes. They proved to be structurally sound, despit
Dutch architecture practice UNStudio has recreated a typical Dutch townhouse's facade in bricks made from stainless steel and glass for Louis Vuitton's store on Amsterdam shopping street PC Hooftstraat.

Named The Brick Pixelation by UNStudio, the store for Louis Vuitton is on luxury shopping street PC Hooftstraat.

The store at 140-142 is directly next door to The Looking Glass, which was also designed by the architecture studio, and along the street from MVRDV's Crystal House.

On the lower floor of the building's facade, which contains the store, traditional bricks have been replaced with stainless steel bricks with glass inlays that create a pixelated effect.

"The tapered stainless steel bricks, while following the pattern and scale of the traditional brick facades that line this street, create a distinctive textured and detailed appearance that contrasts with other buildings," said UNStudio.

"The glass inlays give the facade a semi-transparent quality and make it possible for the facade to be illuminated from within the bricks."

The decision to create a facade using an alternative brick reflects both the traditional townhouses on the street and The Looking Glass next-door.

"The design reinterprets the traditional brick facades of the other townhouses that line the PC Hooftstraat through materiality, but especially, and intentionally, contrasts to The Looking Glass," explained UNStudio.

"Where The Looking Glass display windows protrude and flow over two floors, at 140-142 the windows slant inwards and are more traditionally scaled on the second floor."

"Overall scale is an important factor: in the Glass House both the concept and resulting design present a 'big detail', whereas in 140-142 the focus is on fine detailing," continued UNStudio.

"Both projects however, are inspired by fabric and couture clothing, involve a high degree of craftsmanship, and present different ways to approach 'standing out while fitting in'."

Above the store are three apartments on the first and second floors, which have a brickwork facade similar to that of a traditional Dutch townhouse. The apartments are entered from a door that is made from the same stainless steel bricks, but with an opaque glass inlay.

UNStudio is an Amsterdam-based architecture practice established by Ben van Berkel and Caroline Bos in 1988. It is currently creating the "smartest neighbourhood in the world" in the south of the Netherlands and is masterplanning a smart city in Bangalore.
Shao Feng/NBBJ Architects
NBBJ Architects designed the all-new Hangzhou Olympic Sports Center using far less steel than is standard in stadium design, meaning less carbon dioxide was emitted to create the structure

With dozens of massive sports arenas across the country, China is by no means a stranger to the world of impressive stadium design—take, for instance, Beijing’s iconic Bird’s Nest. But the newest stadium addition impresses not only with its inspired design but also with its approach to sustainable architecture.

In need of additional public space for sporting events, Hangzhou, the capital city of Zhejiang province, sought to build new facilities—the result is the Hangzhou Olympic Sports Center, designed by Los Angeles-based architecture firm NBBJ, which comprises an 80,000-seat arena, a 10,000-seat tennis court, swimming pools, and retail space.

“Hangzhou is one of the most scenic cities in China, and its West Lake is renowned for its beauty, elegance, and unique foliage,” says NBBJ partner Robert Mankin. “The stadium draws upon this beauty by using the indigenous water lily, or lotus, of the West Lake as its conceptual inspiration, interpreting the form into a series of modular ‘petal’ structures that gracefully surround the stadium.”

While steel is a necessary component in stadium architecture—and it does play a prominent role at the Hangzhou Olympic Sports Center—it’s also a major cause for concern from a sustainability perspective. In 2018, the production of each ton of steel resulted in the creation of two tons of carbon dioxide. So from the get-go, NBBJ was tasked with reducing the amount of steel used in the structure. Though the Hangzhou Olympic Stadium is roughly the same size as the Beijing National Stadium, the iconic Bird’s Nest from the 2008 Olympics, it uses 60% less steel—approximately 16,000 tons versus 40,000 tons.

“Hangzhou is one of the most scenic cities in China, and its West Lake is renowned for its beauty, elegance, and unique foliage,” says NBBJ partner Robert Mankin. “The stadium draws upon this beauty by using the indigenous water lily, or lotus, of the West Lake as its conceptual inspiration, interpreting the form into a series of modular ‘petal’ structures that gracefully surround the stadium.”

While steel is a necessary component in stadium architecture—and it does play a prominent role at the Hangzhou Olympic Sports Center—it’s also a major cause for concern from a sustainability perspective. In 2018, the production of each ton of steel resulted in the creation of two tons of carbon dioxide. So from the get-go, NBBJ was tasked with reducing the amount of steel used in the structure. Though the Hangzhou Olympic Stadium is roughly the same size as the Beijing National Stadium, the iconic Bird’s Nest from the 2008 Olympics, it uses 60% less steel—approximately 16,000 tons versus 40,000 tons.

There’s also the matter of the plaza surrounding the stadium, which NBBJ also aimed to keep as eco-friendly as possible. “The overall planning of the complex makes use of porous, light-colored surfaces and green park spaces with maximum vegetation,” says Mankin. “This approach will greatly reduce water runoff from the site, and will avoid the heat sink challenges that most other sports centers in China, including the Beijing Olympic Plaza, experience.”

The Hangzhou Olympic Sports Center is now open to the public and is expected to encourage new (and hopefully sustainable) development in the rapidly growing city. “The special role that the Hangzhou Olympic Park will play in creating a civic center for the surrounding community based upon sports and wellness is perhaps the most exciting and unique aspect of the development,” says Mankin.

Michael Moran
When a husband and wife purchased five acres of bluff top property overlooking the Peconic Bay in the Hamptons, they knew from the beginning that landscape preservation would be a major focus of their future home. To bring their vision of an environmentally sensitive residence to life, the couple turned to Mapos, a New York-based architectural studio that they had worked with previously. By treading lightly on the site, the architects crafted a modernist multigenerational family retreat—the Peconic House—that blends into its meadow setting with a lush green roof, Corten steel exterior and timber interior.

Designed in part as a reaction against the “insensitive residential development…and reputation for showing off” that has characterized recent real estate development in the Hamptons, the Peconic House is a callback to the modernist legacy of Long Island’s South Fork. Featuring simple and low-slung proportions, the rectangular 4,000-square-foot shuns ostentatious displays and instead uses a roof of native meadow grasses to camouflage its appearance and minimize its impact on the watershed. The residence also embraces indoor/outdoor living with a 2,000-square-foot terrace that faces the Peconic Bay and culminates in a 75-foot-long infinity-edge lap pool.

In positioning the building, the architects were careful to preserve the property’s existing vegetation—particularly a 70-foot-tall sycamore located at the center of the meadow. To relate the architecture to the old-growth forest, the architects relied on a predominately timber palette that includes cedar and reclaimed ipe wood that are complemented by concrete and Corten steel. All materials are left unfinished and will develop a natural patina over time.

Inside the open-plan living area “further abstracts the bluff-top landscape, with unfinished cedar and reclaimed white oak,” note the architects. The blurring of indoors and out are also achieved with 100-foot-long walls of glass that slide open and seamlessly unite the indoor living spaces with the outdoor terrace. The cantilevered roof helps block unwanted solar gain and supports a thriving green roof of native grasses that promote biodiversity.

Casey Dunn
Texas architecture studio Clayton & Little has built a barn from reclaimed oil field pipes and weathering steel panels, topped with solar panels to provide power to a vineyard in California.

The Saxum Vineyard Equipment Barn is on the James Berry Vineyard – part of the Saxum Vineyard group – in Paso Robles, a town in central California known for its olive groves, hillsides and wineries. It has been longlisted in the business building category of this year's Dezeen Awards.

Clayton & Little created the simple barn to provide covered storage for farming vehicles, implements and livestock supplies, while also being a structure to hold solar panels to power the nearby Saxum Winery.

The studio describes the structure, which is made reclaimed oil field pipes, as a modern version of a pole barn – a type of agricultural building that began being built in the USA in the 1930s.

The structure is mostly symmetrical with an open central storage area and enclosed rooms for storage on either side. The building is clad with perforated Corten panels that protect the machinery from the sunlight, while allowing the wind to pass through.

On top of the barn's long roof, which is supported by the reclaimed pipes, are a series of photovoltaic panels. These laminated glass solar modules act as the barn's roof, reducing cost as there was no need for a separate structure.

The solar panels offset the electrical demand of the winery, freeing it from the dependence of grid-tied power. Energy from the panels also supports irrigation well pumps at the vineyard.

"Designed to harnesses the local climate to maximise cross ventilation, daylight and solar energy, the recycled oilfield pipe structure holds a laminated glass photovoltaic roof system that produces a third more power than needed [at the winery]," Clayton & Little said.

The roof is also designed to collect rainfall for irrigating trees and adjacent grazing meadow, which is stored in cylinders nearby.

This is intended to dramatically reduce the vineyards dependence on mains water, and combined with the solar panels, effectively take it off the grid.

Saxum Vineyard Equipment Barn was awarded as one of this year's AIA Small Projects and joins BIG'S Klein A45 cabin in the Catskill Mountains and South 5th Residence in Austin by Alterstudio Architecture.

Founded in 2005, Clayton & Little is led by partners Paul Clayton, Brian Korte, Sam Manning and Nathan Quiring. The studio has two offices in Austin and San Antonio, Texas.

Other barns include an oak-clad structure in the Netherlands, Swallowfield Barn in British Columbia and a blackened wood structure by Worrell Yeung in Upstate New York.
The Imperial War Museum has a new archive in Cambridgeshire, England, called the IWM Paper Store, which is the most airtight building in the world.

The IWM Paper Store in Duxford was designed by sustainability specialist Architype to hold the museum's most sensitive collections and records.

Wrapped in a weathering-steel facade, the 1,238-square-metre facility preserves artworks, photographs, letters and diaries documenting warfare since 1914.

The building is designed to passivhaus standards, so it has an extremely low ecological footprint.

Passive building technologies – ensuring the building uses almost no energy for heating or cooling – help it achieve its high environmental standards. These technologies help to preserve the building's sensitive contents while also reducing its running costs.

The archive's simple box-like form is designed to look modern, while complementing the colour of the bricks of the existing buildings on the airfield site, many of which date from the first world war.

Its facade is made from full-height weathering-steel panels, with a single panel representing each year since 1914, when the Imperial War Museum's collection begins.

Perforations in the panels denote the volume and quantity of material in the collection from each year.

This creates a broad visual timeline, with the panels representing years with high levels of conflict much more heavily perforated than those of relative peace.

Architype chose the material as it aligns with the building's programme as an archive to record conflict.

"Weathered steel hardens and gets stronger with time, a concept that is congruent with an archive which likewise gets stronger with time through continued collection," explained Jessica Taylor head of business development at the studio.
Niles Bolton Associates
Using the Prescient design/build light-gauge steel structural system, a Florida-based developer is building a 336-unit apartment tower in downtown Atlanta that will rise to a height of 12 steel-framed stories above a five-level concrete parking structure. The system allowed an extra four floors of height over competing structural systems, says Nathan Kaplan, partner at Atlanta-based Kaplan Residential.

The project, Generation Atlanta, achieved a density of 217 units per acre (536 units per ha) using a technology that will reduce total development costs enough to make the downtown project feasible, enabling developers to comply with Atlanta’s initiative to provide more affordable housing and competitive market-rate high-rise units. Fifteen percent of the project’s units are intended to be affordable to residents with incomes at 80 percent of area median income (AMI). Completion is expected in 2020.

The Atlanta site is 1.6 acres (0.6 ha) with a considerable slope across its nearly 400-foot (122 m) length. That meant five levels of parking, containing 380 spaces for vehicles and 51 for bicycles, could most efficiently be placed in a concrete parking structure under the light-gauge steel structure, with only four parking levels exposed above grade being counted toward the height limit.

The lighter weight of the metal structure decreased the engineered load requirements versus traditional steel or concrete construction. Architect Ray Kimsey, president of Atlanta-based project designer Niles Bolton, says foundation costs were reduced about 15 percent due to a reduction in the number of pilings required by the significantly lighter structural loading.

Market Conditions

Kaplan notes that the site is attractive for housing. “We felt the downtown submarket was very job heavy and lacking apartments,” he says. “There are some 140,000 jobs in the downtown [central business district] and less than 2,000 new apartments delivered in the last 10 years, predominantly in higher-priced concrete buildings.”

Generation Atlanta units range in size from 459 to 1,512 square feet (43 to 140 sq m) with an average size of 832 square feet (77 sq m). The unit mix is about 25 percent studios at rents from $1,400 to $1,550 per month, 50 percent one-bedroom units at $1,650 to $1,850 per month, and about 25 percent two-bedroom units at $1,900 to $2,300 per month.

To meet the city’s affordability goals, Generation Atlanta will offer 15 percent of its units at rents from $1,300 to $1,500, depending on unit type, targeted to those with incomes at 80 percent of AMI. “We strongly believe the combination of a good land price and a cost-effective design will allow Generation Atlanta to be more competitive,” Kaplan says.

Though the site is at the Spring Street ramp of Interstate 85, it rates high on three 100-point scales—an 88 Walk Score, a 76 Transit Score, and a 75 Bike Score. Its 380 parking spaces supply a 1.13:1 parking ratio for its 336 units. “The location is very walkable to many job centers and also very close to Atlanta’s public transportation system,” the Metropolitan Atlanta Rapid Transit Authority (MARTA), Kaplan says.
Modus Studio
Heat-treated pine and steel were used to create this unique treehouse inside the Garvan Woodland Gardens in central Arkansas. The space, a 210-acre nature park with botanical gardens owned by the University of Arkansas, borders the Quachita River. The structure itself resides inside a children’s garden full of native trees such as oak and pine, and serves as an interactive experience for children.

The treatment process to create the “thermalised” southern yellow pine uses heat and steam to ensure longer-lasting durability of the wood. It also makes the wood less susceptible to weather and the elements.

The slatted design provides a strong, safe way for the children to feel more connected with nature. The curves were intentional, influenced by “dendrology,” or the study of trees, exhibited by the way the house changes shape while walking through.

Using six pairs of skinny steel columns, the treehouse is lifted 13 to 25 feet into the air. This ensures that the natural ground below wouldn’t have to be manipulated in order to install the structure— an important consideration for the protected nature park. It’s a win-win situation, as the elevated location allows visitors to feel suspended into the air among the trees without a dangerous climb.

The treehouse is multi-storied and has a raised walkway leading into the entrance in the center. When it comes to the actual structural design, the designers built a centralized spine made of steel that runs along the entire treehouse. Connected to the spine are 113 ribs (10 made of steel and the rest out of pine) that act as a sort-of skeleton, as well as features floor plates and six pairs of columns.
Nadine M. Post for ENR
The repair of two fractured girders spanning Fremont Street and the reinforcement of twin girders spanning First Street are complete at the beleaguered Salesforce Transit Center in San Francisco.

To date, an independent panel of the Metropolitan Transportation Commission has found no other issues affecting the transit center, according to the public owner, the Transbay Joint Powers Authority. TJPA will announce a reopening date after the panel concludes its review.

TJPA shuttered the facility in late September, some six weeks after it had opened to bus traffic. This was shortly after a ceiling panel installer had inadvertently discovered the most serious of the two brittle fractures in the bottom flange of one of the twin built-up plate girders spanning Fremont Street.

Shoring Removed

To date, crews have removed all shoring from both sets of third-floor girders that span 87 ft and support both a public rooftop park above and hang the second floor bus level. All traffic lanes are now open during the day. Night street closures will continue throughout May to restore lights, MUNI overhead lines and to reinstall ceiling panels.

Recommissioning of the 4.5-block-long facility will continue through the end of this month. The quality assurance process includes retesting and re-inspecting fire and life safety systems throughout the facility and retesting the building’s mechanical and electrical systems, reports TJPA.

Since the closure, transit agencies have been providing bus service out of a temporary terminal at Howard and Main streets. The mostly outdoor facility served as the depot during construction of the new facility.
Tom Ferguson
Transported in a single shipping container and raised in two weeks, this clam-shaped cabin nestles against a rocky outcrop outside Sydney.

The idea of a prefabricated cabin, with components light enough to carry and assemble without heavy machinery, had long had a siren call for owner and architect Mark Fullagar. After many nights sketching and dreaming of the perfect solution, Fullagar came up with the Fabshack—a 540-square-foot cabin with 90 percent of its structure built off-site.

Designed and built by Fullagar, the compact, comfortable cabin has a queen-size Murphy bed, seating area, storage, open kitchen and dining room, bathroom, laundry room, and deck from which to enjoy mountain views on his 26-acre property in Cattai, a suburb of Sydney, Australia.

The cabin is constructed of cost-effective, non-combustible materials in 8-foot-wide bays. Each bay consists of hollow-insulated plywood panels nestled within a steel frame, elevated on piers that lift the structure above the ground. Insulated plywood forms the continuous ceiling and curved wall of the cabin, creating a soft interior while facilitating necessary rainwater drainage on the outside.

Corrugated steel, aluminum window frames, and strengthened glass create an industrial exterior. By contrast, the plywood-wrapped interior adds warmth and texture to complement the natural setting. Green linoleum countertops and black millwork enliven the muted palette. Large windows face west to take in the mountain views, with sliding shutters that protect against the afternoon sun.

Ninety percent of the work was completed in a workshop, meaning minimal labor time was spent on site. All components were packed in one shipping container, and each component was carried by one or two people to the site.

The Fabshack demonstrates the flexibility and adaptability of prefab construction. Since the cabin is comprised of modular bays, it can be modified for different purposes or sites, its components easily dismantled and recycled, and its materials upgraded depending on preferences and budget.

Attendees of a recent presentation on the earthquake-resistant structure of San Francisco’s Salesforce Transit Center—intended to provide a safe haven when the Big One hits—lauded the engineering of the 4.5-block-long hollow tube that supports the 1.2-million-sq-ft “groundscraper.” But there also was much talk of the project’s black eye, as a consequence of brittle fractures of the bottom flanges of two bridge-like built-up plate girders that span 87 ft over Fremont Street.

“It’s such a shame about the girders; it’s such a beautiful structure overall,” said one engineer, who declined to be named, after Bruce Gibbons, a managing principal of the hub’s engineer-of-record, Thornton Tomasetti (TT), presented at the American Institute of Steel Construction’s 2019 NASCC: The Steel Conference. The April 3-5 event drew more than 5,100 registrants to St. Louis.

A brouhaha over the blame for the girder fractures that shuttered the three-story transit hub last Sept. 25, six weeks after it opened, is just one dispute marring the project. There is another. Last Oct. 16, the general contractor, the Webcor/Obayashi Joint Venture, filed suit in the Superior Court of California against the public owner, the Transbay Joint Powers Authority, alleging breach of contract and seeking more than $150 million in extras to its $994,517,600 contract (ENR 10/29/18 p. 11). On Feb. 22, TJPA filed a cross-complaint. And on March 15, WO filed a complaint against seven subcontractors, including the steel sub Skanska USA Civil West California District and the girder fabricator, the Herrick Corp.

TJPA alleges WO breached its preconstruction services obligations by failing to provide accurate estimates, ensure competitive bidding and rebid packages at no cost to TJPA. The authority also claims WO breached its construction services obligations by failing to enforce Buy America requirements; misusing the request for information (RFI) process; submitting late, inaccurate and incomplete submittals; installing nonconforming work; staffing the project with inexperienced personnel; and failing to perform its close-out obligations in a timely manner.

The breaches resulted in “substantial additional costs” to TJPA and delayed the substantial and final completion of the project, says the complaint.

The complaints do not specifically relate to the girder fractures, but a future claim is likely over that responsibility. The third-floor girders support a rooftop park and hang the second-floor bus level, via a tension plate that thickens the web’s midspan and slots through the bottom flange.

Though the girders met all specifications and passed all inspections, TJPA’s investigator, forensic metallurgist LPI, blamed the fabricator for the fractures.

The root cause is failure to grind and polish 2 in. x 4 in. cuts where hanger plates slot through the bottom flanges, said LPI. The failure to grind, in combination with very brittle plate material at the mid-depth of the 4-in.-thick flange and service loads, created the fractures.

Thermal cutting caused expected microcracks. Shrinkage stresses from butt welds of the flange plate ends at the midspan enlarged the cracks. Stress from service loads turned the cracks into fractures.

The unraveling appears to have begun with a TT note for a “missing weld access hole” in the web. TT added the WAH to an approved-as-noted shop drawing. In an RFI, Skanska’s detailer questioned the WAH in the web and suggested holes in the bottom flange, which TT approved as WAHs. Revised shop drawings show cuts, not WAHs, in the bottom flanges.

TT declined to comment until the TJPA investigation is complete. Herrick and Skanska deny blame (ENR 4/1-8 p. 14). “Engineers know the quality of material is not consistent through the depth of the plate,” said Bob Hazleton, Herrick’s president. And he maintains the investigation is not focusing enough on the hanger slot, which is not a defined hole in the code.

The girder failures have rekindled discussions about potential pitfalls of connection design and detailing practices, especially for more complicated structures. Engineers are pressed for time, which can result in incomplete drawings. Shop drawing review is typically relegated to junior engineers. “It might be beneficial to flag certain conditions for special review,” said another engineer at the conferenc
Anti Hamar
Mass timber construction is on the rise, with advocates saying it could revolutionize the building industry and be part of a climate change solution. But some are questioning whether the logging and manufacturing required to produce the new material outweigh any benefits.

The eight-story Carbon 12 building in Portland, Oregon is the tallest commercial structure in the United States to be built from something called mass timber.

If the many fervent boosters of this new construction material are right, however, it is only one of the first mass timber buildings among many, the beginning of a construction revolution. “The design community in Portland is enthralled with the material,” said Emily Dawson, an architect at Kaiser + Path, the locally-based firm that designed Carbon 12.

The move to mass timber is even farther along in Europe. That’s because mass timber – large structural panels, posts, and beams glued under pressure or nailed together in layers, with the wood’s grain stacked perpendicular for extra strength – is not only prized as an innovative building material, superior to concrete and steel in many ways, it is also hoped it will come into its own as a significant part of a climate change solution.

Among architects, manufacturers, and environmentalists, many want nothing less than to turn the coming decades of global commercial construction from a giant source of carbon emissions into a giant carbon sink by replacing concrete and steel construction with mass timber. That, they say, would avoid the CO2 generated in the production of those building materials and sequester massive amounts of carbon by tying up the wood in buildings for decades or even longer, perhaps in perpetuity.

There are new mass timber buildings in London, Atlanta, and Minneapolis, and an 80-story high-rise is proposed for Chicago.

“Say the typical steel and concrete building has an emissions profile of 2,000 metric tons of CO2,” said Andrew Ruff, of Connecticut-based Gray Organschi Architecture, a leading proponent of the laminated wood revolution. “With mass timber you can easily invert so you are sequestering 2,000 tons of CO2. Instead of adding to climate change you are mitigating climate change. That’s the goal.”

And it is taking off. Mass timber has a two-decade track record in Europe. The 18-story Mjösa Tower just opened last month in Norway. An 18-story mass timber building was recently built in Vancouver as well, and an 80-story high-rise is proposed for Chicago. There are new commercial mass timber buildings in London, Atlanta, and Minneapolis. Some 21 timber buildings over 50 meters (164 feet) tall will be completed in Europe by the end of the year, according to one report.

But there are big questions being asked about just how sustainable the new building material is –especially about how forests that produce mass timber are managed, and how much CO2 would be emitted in the logging, manufacture, and transport of the wood products used in the construction. So far, critics say, there aren’t good answers to these questions.
Santiago Mejia / The Chronicle
Four levels of inspections and plans failed to prevent or uncover a construction flaw that resulted in the cracked girders that forced the closure of the Transbay Transit Center 5½ months ago, the agency’s executive director said Thursday.

Mark Zabaneh, head of the Transbay Joint Powers Authority, said three teams of quality-control inspectors working for contractors didn’t discover that a necessary grinding process, used to eliminate small cracks associated with holes that were cut into the girders, was not performed and that the authority’s own spot inspections also missed the oversight.

Those micro-cracks developed into larger cracks and, finally, a large fissure that ripped through two large steel girders that support the building. Similar girders over First Street have not cracked.

“The execution was not done properly, and that is something we are looking into,” Zabaneh said Thursday after an authority meeting. “It’s an area of great concern for us.”

The failure of the quality-control and quality-assurance processes to detect the fracture-inducing error prompted the authority to order engineers and architects to pore over tens of thousands of drawings and inspection documents to make sure no other major potential flaws were overlooked.

A pair of outside engineers told the Transbay authority’s board Thursday that the two girders that support the three-block-long transit hub where it crosses Fremont Street fractured because of an unusual confluence of factors, including the relative strength of the steel, the design of the structure and the fabrication process.

Robert Vecchio, of LPI, a New York laboratory that analyzed metal samples from the failed girders, pointed toward the cutting of holes, known as welding access holes, in the beams as the source of the problem. The holes over Fremont Street were cut in the beams before they were welded. The welding caused tiny cracks to grow and then fracture. Holes in the girders over First Street were welded before cutting and did not crack.

Grinding the edges of the holes smooth before welding can eliminate the micro-cracking, Vechhio said, and would have prevented the Fremont Street girders from fracturing, he said.

Connie Zhou/OTTO
Designed by ZGF Architects with Arup, the striking 48-story tower in Seattle features an innovative diagonal mega-brace system.

Between Amazon’s expanding vertical campus and the in-progress replacement of the Alaskan Way Viaduct with a promising waterfront park, Seattle continues to undergo a rapid transformation. Among downtown’s newest jewels is the Mark, a 48-story hotel and office tower designed by the local office of ZGF Architects that rises from a quadrant of a city block.

The 750,000-square-foot faceted structure rises between two centenarian neighbors, the Beaux-Arts sanctuary of the First United Methodist Church and the Jacobean-style Rainier Club, cantilevering over the former by up to 20 feet. As a part of the project—developed by Kevin Daniels, president of Daniels Real Estate and a member of the National Trust for Historic Preservation board of trustees—the sanctuary was restored for use as an event space.

Constrained to a footprint of just 15,000 square feet, the tower's floor plates had to be extended to achieve the desired square footage. Daniels also tasked ZGF with creating an iconic structure to reflect Seattle’s aspirational spirit.

ZGF used paper models and the classic proportions of the human anatomy to explore dozens of designs that satisfied the constraints. Early concepts featured more “rudimentary” cantilevers or “heavier, more geometric" forms, says ZGF partner Allyn Stellmacher, AIA. “Ultimately, we came back to a disposition of the parts of the building in a way that we thought was more artful, but that also was melded with a more effective [line] for the bracing.”

The final form is an asymmetrical obelisk, with exposed diagonal steel braces that zigzag up each elevation, emphasizing the tower’s verticality. The architects used the long, clean lines of the steel members to differentiate each facet of the façade, whose subtle shifts are enhanced by the reflective glazing. Unlike some exoskeletons, the bracing’s stainless steel cladding recedes 11 inches into the Mark, as if the zigzags are etched into its skin.

The diagonal mega-brace system—among the first of its kind in a seismic zone—derives from ZGF’s close collaboration with Arup, the project’s structural engineer. The tower structure consists of a central concrete core with steel-framed, concrete-infilled floor plates supported by beams spanning up to 50 feet, and four steel columns slightly inset from each building corner on each elevation, leaving the interiors and the corners of the tower column-free.

The perimeter bracing system “acts like a closed tube that engages the axial stiffness and strength of the perimeter steel columns,” according to text supplied by Arup. As a result, it transfers wind and seismic load requirements from the concrete core to the building perimeter, where the diagonal members transfer the loads to the columns. Arup estimates this system uses 10 percent, or 750 tons, less steel than alternative designs.

The 200- to 325-foot-long diagonal braces (inboard of the cladding) consist of approximately 30-foot-long wide flange beams with a depth of 2 feet and flange thickness exceeding 4 inches. At building corners, the X-shaped intersections created where the diagonal braces meet—dubbed “knuckles” by the design team—were among the hardest to detail.

Each shop-fabricated knuckle is uniquely made to accommodate the various incoming angles of the four intersecting diagonal braces, in addition to three floor beams. The brace members were initially bolted to the knuckle during fit up and erection, and then all connections were made permanent via full penetration welds.
Investigators scrutinize hanger connection detail as well as design, material and fabrication

SPECIAL REPORT The underlying causes of the trouble at San Francisco’s 4.5-block-long Salesforce Transit Center are coming into focus. A combination of low fracture toughness deep inside thick steel plates, cracks present as a consequence of normal steel fabrication and stress levels from loads, which are a function of design, apparently caused brittle fractures in the bottom flanges of the center's twin built-up plate girders that span 80 ft across Fremont Street.

All three ingredients were needed for the brittle fractures. “Take away any one and there is no brittle fracture because the other two compensate,” says Michael D. Engelhardt, a professor of engineering at the University of Texas, Austin, and the chair of the independent peer review panel formed in October by the area’s Metropolitan Transportation Commission.

One ingredient is a material’s low fracture toughness, which indicates how intrinsically resistant the steel is to brittle fracture, says Engelhardt. Another is an initiating discontinuity, known informally as cracking, often present as a consequence of normal steel fabrication. A crack, as in a pane of glass, causes a stress concentration, explains Engelhardt. It is benign if the other ingredients are not present.

The third ingredient is a high-enough level of stress, which is a function of design. Service loads, in this case live loads such as buses, generate the stress.

The troubles at Saleforce Transit Center open a window on some of the process, quality-control and responsibility challenges that occur when designing, detailing, fabricating and building long-span steel structures with large or very thick sections. Among the questions raised are whether codes and standards should be changed.

The MTC panel, in addition to its charge of technical review and guarding public safety, expects to make recommendations for change in standard practice, and possibly codes and procedures.

“There are a lot of things we can learn from this,” says Engelhardt. “People who write codes and standards should take this seriously,” he adds.