art by design
ART BY DESIGN The University of Iowa’s Visual Arts Building opened last fall on a campus that is still recovering from flooding in 2008 that caused nearly $800 million in damage. The structure houses the university’s studio arts education program and is a physical manifestation of the theories that will be taught within its walls. Boasting offset floor plates and perimeter cutouts, the building itself is a piece of art composed of both volumes and voids.
HW OA T ON EBDA © IPIW A NC RB AI TA AGN NO E S ©
HERE
The rainscreen gives the building an ethereal, diaphanous appearance at dusk, when light within the building escapes through the punched windows in the exterior concrete walls to backlight the screen.
[48] C i v i l E n g i n e e r i n g M A Y 2 0 1 7
0885-7024/14-0009-00??/$30.00 PER ARTICLE
PHOTOCREDIT GOES HERE
By Derrick Roorda, p.e., s.e., leed ap, and G. Kelley Gipple, p.e.
M AY 2017
C i v i l E n g i n e e r i n g [49]
N ortheast E levation of V isual A rts B uilding and A rt B uilding W est
L
[50] C i v i l E n g i n e e r i n g M A Y 2 0 1 7
0885-7024/17-0005-0048/$30.00 PER ARTICLE
BUROHAPPOLD ENGINEERING
the mission of the university’s School of Art and Art HisArts Building opened to widespread acclaim, tory. Designed for today’s integrated and collaborative apmarking a new chapter in the 170-year-old cam- proaches to art education and practice, the building houses pus’s distinguished legacy of arts education. De- the school’s studio arts programs, which includes ceramics, signed by Steven Holl Architects, of New York sculpture, metals, photography, printmaking, and 3-D mulCity, with bnim, of Kansas City, Mis- timedia works. The structure also offers practice studios, ofsouri, serving as executive architect, fices, and gallery space. the new building utilizes volumes and The building itself is of five levels and has a 200 by 180 ft voids to create a visually striking structure. rectilinear footprint. Located on a sloping site, the building has The University of Iowa entrances on the north side at campus encompasses 1,700 the first level and on the south acres and stretches westward side at the second level. The from downtown Iowa City, stacked floor plates are offset which is in the eastern part from one another to form balof the state. The Iowa Rivconies along the exterior edges er flows through the campus, of the building’s 68 ft height. dividing the grounds into Seven vertical cutouts, six of east and west sides. A celthem along the perimeter of ebrated natural feature, the the building and one inside it, river can indeed be formidaturn what would have been an ble. In the summer of 2008, almost square footprint into devastating floods caused one that resembles a complex nearly $800 million in dampuzzle piece. The overall viage to the campus alone. sual impression given by the Set in buildings along the building suggests a vertically river’s west bank, the univerporous series of stacked volsity’s arts program was hit umes. Internally, these volhard by the flood. But with umes offer loftlike common funding from the Federspaces for informal gatherings al Emergency Management and discussions, as well as Agency, the catastrophe beclassroom and studio spaces. came an opportunity for transformation. Mul- The rainscreen features a Located next to the horizontally porous and tiple arts-related buildings were renovated, stainless steel scrim with planar composition of Art Building West, the Virepaired, or replaced because of the flood, in- customized perforations. sual Arts Building reflects a foundational premcluding the new Visual Arts Building (its predeise of the school: the merging of art history and cessor now shuttered). Hancher Auditorium, a state-of-the- studio practice into a single program. This approach was deart performing arts venue, opened last fall on higher ground, veloped by the university in the 1920s and is now commonly and plans for a new art museum are unfolding. Art Building referred to within the field as the Iowa idea. The two buildings West, the award-winning edifice completed by Steven Holl frame a common green, striking a delicate balance with one anArchitects in 2006, also was repaired and returned to active other. At nearly twice the size of the earlier building, the new use by the school’s art history division. Visual Arts Building expresses the school’s vision for the future In planning and design, the $77-million, 126,000 sq ft of art practice while forming a dialogue with its smaller comVisual Arts Building works to strengthen arts education and panion. The new building’s higher elevation places its ground
© S T E V E N H O L L A R C H I T E C T S , A B O V E ; © C H R I S M C V O Y, B E L O W
ast fall the University of Iowa’s new Visual
floor at least 2 ft above the 500-year flood level, greatly reducThe architects wanted this main stairwell to have the same ing the risk of damage from future floods. heavy materiality as the building while also appearing to float Steven Holl Architects won the project through a design within the space. This goal, however, was complicated by the competition in 2010, and its entry was submitted after it placement of the stair within the building’s center, where the had considered more than 30 designs. A structural engineer- floor plates are the least supported. Monocoque construction, a ing team from the San Francisco office of the international concept taken from aircraft design, consolidated the stair’s risfirm BuroHappold Engineering worked with Steven Holl er, tread, and step elements into an efficient U-shaped compoArchitects to develop the winning proposal. The engineer nent of extruded concrete that could be treated as a unit. This of record—Structural Engineering Associates, of Kansas reduced the weight of the stair and allowed the entire stair asCity, Missouri—completed sembly to participate as part the construction documents of the structure, in contrast to and administered the project. a heavy appendage that would Designing a new neighhave required the support of bor to a celebrated work of arthe slabs. Intermediate conchitecture can be a daunting nections between the stair and undertaking. For Steven Holl the building are carefully loArchitects, which designed cated and detailed to miniArt Building West, the new mize their visibility. Above, design would also have to steel framing supports the complement its own work. atrium skylight, the only part The design of the Visuof the building that is not of al Arts Building reflects the concrete. blurring of boundaries beThe six cutouts on the petween disciplines and the colrimeter of the building are laborative approaches to learnenclosed in 20 ft tall channel ing that have become the glass walls so that the horihallmark of contemporary arts zontally shifting floor plates education. Interconnectivity create a series of outdoor baland the potential for multidisconies that can be used for ciplinary interaction are cenmeetings, work, or socializtral to the building’s design. ing. Punched windows in the safe, developed by Computers As part of this, the seven vertical openings that exterior concrete walls, many of them opercut through the floor plates create multiple & Structures, Inc., was utilized able for natural ventilation, add light and air to model the long-term deadcenters of light within the building, becoming to the building. even more complex given the offset nature of load deflection contours of each Structural Engineering Associates modfloor, including the fourth. the floor plates. eled the shape and location of each window The largest cutout is located entirely withindividually to produce accurate lateral disin the structure and creates a central atrium that allows views placement numbers. These square, irregularly placed winacross and between floors. The cutout is wrapped in a series of dows of various sizes are located in such a way that they can long stairs that are shaped to encourage interaction and dis- transfer loads while accommodating the building’s integral, cussion. In this way the atrium itself forms a vertical social heavily reinforced columns or base beams. heart for the building. The ground level of the atrium serves The entire building is clad in a rainscreen produced by the as a place for gatherings and events. international firm rheinzinc that is designed to weather to M AY 2017
C i v i l E n g i n e e r i n g [51]
T he core challenge for B uro H appold ’ s
a blue color. The rainscreen features a stainless steel scrim with customized perforations that wraps the southeast and southwest facades, treatments that also serve to give the building a striking ethereal presence. The core challenge for BuroHappold’s structural engineers was realizing in concrete and glass the dramatic form envisioned by the architects while retaining the desired open interior volumes. The variation of volumes and voids that gives the building its sculptural beauty hindered the efficient transfer of gravity forces to the ground. Complicating matters, solid concrete slabs could not span the distances required for expansive interior spaces without bowing. These issues were resolved by using a castin-place concrete design that utilizes concrete columns supporting the five concrete slabs. The column spacing varies from 15 ft to 34 ft, with most of the columns continuous from the building’s top level to the foundation, although four concrete transfer beams 18 in. wide and 36 to 54 in. deep are utilized in the design. The interior concrete walls provide the reinforcement needed for the structure’s lateral stability. The structural system of the build[52] C i v i l E n g i n e e r i n g M A Y 2 0 1 7
© IWAN BAAN
ing is supported by concrete spread footings and drilled piers bearing on the subterranean limestone formation below the building. The open floor plates and the loftlike interior volumes were made possible by voided slab construction, which is described below. An integrated thermally active radiant heating and cooling system in the slabs eliminated the need for visible ductwork and makes it possible for each concrete slab to function both as a pristine floor and as a ceiling. The lower-level walls are of reinforced concrete at the northwest edge and portions of the southwest and northeast sides, where they are subterranean. As the walls were designed to require lateral support at the top The floors in the Visual Arts Build- by the second-level slab, it was necessary to ing are composed of voided slabs. provide shoring on the outside of the walls In this method, recycled plasuntil the second-level slab was constructed tic disks held in steel cages disand had achieved its full design strength. place a portion of the concrete. The building was positioned 52 in. from the Polyethylene tubing used for ranorthwestern and northeastern property diant heating and cooling also lines to allow room for this temporary is contained within the slabs. shoring system.
S T RU C T U R A L E N G I N E E R I N G A S S O C I AT E S , A B OV E ; © S T E V E N H O L L A R C H I T E C T S , B E L O W
structural engineers was realizing in concrete and glass the dramatic form envisioned by the architects while retaining the desired open interior volumes .
The building was designed for the Offset floor plates and six vertical 100 psf to offer flexibility in the future wind, snow, and live loads specified in perimeter cutouts give the building its use of the building. the 2011 edition of Iowa’s State Building stacked appearance and form balconies The vertical system of reinforced-conCode, which is based on the 2009 edition along the exterior edges. The balconies crete columns and concrete walls carries are enclosed in channel glass walls. of the International Building Code (Washgravity loads to the foundation. The conington, D.C.: International Code Councrete columns are 18 in. square in cross cil). Based on the seismic and wind parameters in Iowa section throughout the building and 20 in. square where the City, wind load forces, rather than seismic forces, governed loads are higher, as is the case under portions of discontinuous the lateral loads and controlled the design. All floors were walls. To provide large open volumes, especially around the designed for a live load of central stair, concrete walls 10 in. thick are used at isolated interior locations and along the perimeter of the building to S ection of V isual provide enclosure and lateral stability. A rts B uilding Three-dimensional analysis was critical to the design process and for ensuring constructability. The structure was modeled using the 3-D finite-element modeling software etabs (v9.7.2), developed by Computers & Structures, Inc., which has offices in Walnut Creek, California, and New York City. etabs was used to model the governing lateral load and wind loading against the ordinary reinforced shear wall lateral system. The slabs were modeled using safe (v12.3.1), also developed by Computers & Structures. Each floor was modeled individually, with gravity loads added to the model to accurately design the floor slab. Both software packages allow a direct transfer of building geometry into Revit, developed by Autodesk, Inc., and Revit was used for documentation and to effect the extensive coordination that the project required. The columns were designed using spColumn—developed by structurepoint (formerly the Engineering Software Group of the M AY 2017
C i v i l E n g i n e e r i n g [53]
What is more, because the weight of the concrete itself accounts for CHALLENGING CONDITIONS BELOW two-thirds of the load in a building, he site of the University of Iowa’s Visual Arts Building is underlain even when considering occupant live by bedrock. This moderately weathered limestone is overlain by fill in the loads, the lighter slabs make it possiform of lean to silty clays. The upper fill material was found unsuitable for ble to use smaller and lighter beams, supporting the five-level building’s foundations, so all supporting elements columns, and foundations. This ripextend into the bedrock. ple effect lowers the overall cost and The limestone bedrock varies in depth, being closer to the surface under has the added benefit of reducing the the northwestern portion of the building and deeper under the southeastern amount of concrete and cement needportion. For this reason, two types of foundation elements were used. Spread ed, thereby significantly reducing the footings were placed directly on the bedrock where it was shallow (approxicarbon footprint of the construction. mately 2 ft below the surface), and drilled piers circular in cross section were Slab voids were omitted locally at extended down to the bedrock where it was deep (approximately 48 ft below column or wall locations, around slab the surface). openings, and along the slab perimeThe project’s geotechnical engineer, Terracon, set an allowable bearter to increase shear strength. Where ing pressure of 15,000 psf for sizing spread footings and 80,000 psf for the punching shear demands still exceeddrilled piers, both to be supported by the limestone bedrock. The higher valed the capacity of the full-depth conue was achieved for the piers by drilling them 3 ft into the rock. crete slab, the design incorporated The spread footings were square and ranged in size from 3 to 8 ft on a side headed shear stud plates made into a and in thickness from 18 to 33 in. The drilled piers ranged in diameter from cross pattern, in contrast to introducing the more traditional drop panels 2.5 to 5.5 ft and in depth from 4.5 to 36 ft. with thickened slabs. Finite-element analysis allowed the design team to determine visually where The University of Iowa’s Visual Arts necessary to meet the deflection critedeflection exceeded tolerable limits and to Building, top, plays with the idea of ria. While the need for such beams was quickly assess potential solutions. The strat- vertical volumes and voids, creating greatly reduced by using voided slabs, in a dialogue with the horizontally egies implemented included modifying span some cases the columns remained too far porous and planar composition lengths, thickening the slabs in certain arapart and beams were needed. of Art Building West, left. eas, and adding dropped beams only where This analysis (Continued on Page 80)
[54] C i v i l E n g i n e e r i n g M A Y 2 0 1 7
PHOTOCREDIT GOES HERE
Portland Cement Association), headquarThe building includes a large ment, and the magnitude of sustained loads. internal vertical cutout that tered in Skokie, Illinois—and internally The estimated average values of the ultimate creates a central atrium. This generated spreadsheets. creep coefficient and the ultimate shrinkage The design process involved multi- vertical space is edged in a series strain were taken from the American Conof long stairs that were created ple iterations of slab thickness and column crete Institute’s report Control of Deflection in using monocoque construction, shifts to keep deflections within the limConcrete Structures (435R-95) and used for the which allows the entire stair its set by the architects in order to prevent nonlinear cracked-section analysis. assembly to participate as part damage to the finish materials. This inControlling deflections in flat slabs of of the structure rather than cluded the evaluation of long-term deflecconcrete can lead to a circular and somebeing an appendage requiring tions, which required careful consideration times spiraling design process. To decrease the support of the slabs. of the concrete mix that would be used and deflections the thickness must be increased, of the extent to which the concrete would but that thickness adds weight and further undergo creep when placed under stress for prolonged pe- increases deflections. Lightweight concrete can be specified riods. BuroHappold provided color contour maps of slab to alleviate this issue somewhat, but the same principle apdeflections based on a 3-D finite-element analysis of each plies. Lightweight aggregate mixes generally have a lower floor to Steven Holl Architects so that each option could be modulus of elasticity, meaning they are not as stiff and decarefully reviewed. flect more for a given thickness. Cracked-section analysis was used to limit slab deflections To resolve this conundrum, BuroHappold suggested an to 1 in. for long-term deflections under dead load. Calculat- alternative proposal: introduce voids into the slabs to reduce ing concrete deflections is complicated because of the inher- their weight. Referred to as voided slab or bubble slab conent nonlinear behavior of the material, for when a concrete struction, this approach uses hollow balls or disks made of slab or beam cracks, it deflects two or three times as much as recycled plastic and held in place by steel reinforcing bars to it would if it remained uncracked. Long-term deflection is in- displace a portion of the concrete. Balls were used only for creased by the shrinkage and creep that occur in response to thicker slabs, while disks of lower profile were used more exsustained loads. These deflections are higher than the elastic tensively. Overall the system eliminated as much as a third deflections that occur when loads are initially placed on the of the concrete weight while maintaining more than 90 perstructure. The shrinkage and creep contributions are influ- cent of the stiffness, thereby enabling a given slab thickness enced by temperature, humidity, curing conditions, age at to span much farther and deflect less than would otherwise the time of loading, the quantity of compression reinforce- have been possible.
© IWAN BAAN, BOTH
T
M AY 2017
C i v i l E n g i n e e r i n g [55]
Art by Design (Continued from Page 55) also proved critical to elegantly integrating the channel glass walls, especially at balconies and doors. Understanding how the building would move above and below each of the fragile elements enabled the team to adjust the form or strengthen the concrete. In this way heavy details within the glass walls were avoided, preserving their transparency. While voided slab construction is relatively rare in the United States, BuroHappold had experience with it in Europe. At the time the project was under design, only two U.S. contractors were offering voided systems, both of them proprietary but essentially the same in terms of engineering function. BuroHappold’s experience helped convince the client and the construction team that this was the correct approach for the Visual Arts Building. However, cost remained the principal consideration. BuroHappold completed preliminary designs using both thicker slabs of solid concrete and thinner slabs with voids with input from the contractors. The voided slab solution was found to be cheaper, and Cobiax usa, Inc., of Dedham, Massachusetts, was ultimately selected as the subcontractor to provide the voids for the project because of its bid and its experience with such construction. Being a pioneer comes with its challenges. With the Visual Arts Building, implementing voided slab construction for such a complex structure made for a steep learning curve, but the general contractor—Miron Construction Co., Inc., of Neenah, Wisconsin—worked with Structural Engineering Associates to devise a strategy for coordinating building systems and constructing the slabs. Incorporating the thermally active slab radiant heating and cooling system with the voided slab construction required painstaking coordination on the part of Structural Engineering Associates, Miron Construction, and the mechanical engineering consultants. Miles of cross-linked polyethylene (“pex”) tubing 5/8 in. in diameter wind through the slabs and exit in precise locations to connect with the cast-in-place walls. Coordination for the pex loops and void cages that encase them began during design. Design Engineers, of Cedar Rapids, Iowa, served as the mechanical, electrical, and plumbing engineers for the project. However, as this firm worked with its subcontractors to define the construction proRoorda cess, it became clear that the team should consider off-site assembly of the pex tubing grid. The concrete slab thickness was increased to 12.5 in. and number 6 reinforcing bars were used as spacers to give the pex its own layer. A 3 in. top layer of concrete was added to refine the slab finish. This layer was polished and left exposed. The concrete placement process also proved complex. Wire cages holding multiple disks would arrive on-site by the truckload so that the construction crew could install large areas at one time. A first placement of about 3 in. of concrete was allowed to set just enough to hold the void cages and prevent them from floating up during the second concrete placement. Because the original mock-up occurred in winter, [80] C i v i l E n g i n e e r i n g M A Y 2 0 1 7
when concrete set times were longer, the timing of the second placement needed to be adjusted on-site on the basis of temperature and humidity. Personnel from Structural Engineering Associates attended several of the initial placements and worked with Miron Construction to determine the timing for the second concrete placement. While the voided slabs provide the needed structural performance, they required more care when it came to drilling. A few slabs were damaged when holes drilled for formwork braces filled the voids with water and froze during the winter. Furthermore, fasteners used in construction were limited to a depth of 3/4 in. Despite these drawbacks and the complexity of working with the voided slab construction, the effort was worthwhile because the thermally active slab heating and cooling system, along with the structure’s thermal mass, makes the building very comfortable and efficient to heat and cool. In an era defined by increasing collaboration by architects, engineers, consultants, and contractors, the Visual Arts Building offers a reminder of the importance of a bold architectural vision. Here the success of the collaboration can be measured in the purity of the final building, as visitors encounter an arresting sculptural form and minimally detailed, flowing open spaces. The design has won several awards, including a design award bestowed this year by the American Institute of Architects’ New York chapter. Acclaim aside, the building’s legacy will depend upon the extent to which it furthers the mission of the School of Art and Art History. The central atrium, the glass-enclosed studios, and the abundance of indoor and outdoor gathering places promote the connection and interaction underpinning the art school’s pedagogical vision. Through advanced modeling tools, a willingness to embrace unconventional construction systems, and careful coordination, the design and construction team has delivered a setting that will enable the Visual Arts Center to realize its full promise. CE
Gipple
Derrick Roorda, p.e., s.e., leed ap, is the director of BuroHappold Engineering’s California structural engineering group, which has offices in Los Angeles and San Francisco. G. Kelley Gipple, p.e., is a principal and the director of design for Structural Engineering Associates in Kansas City, Missouri.
University of Iowa Architect: Steven Holl Architects, New York City Associate architects: bnim, Kansas City, Missouri Structural engineers: BuroHappold Engineering, San Francisco office, and Structural Engineering Associates, Kansas City, Missouri General contractor: Miron Construction Co., Inc., Neenah, Wisconsin Geotechnical consultant: Terracon, Cedar Rapids, Iowa, office Civil engineer: Shive-Hattery, Cedar Rapids, Iowa, office Mechanical engineer: Design Engineers, Cedar Rapids, Iowa Curtain wall consultant: W.J. Higgins & Associates, Inc., Weston, Wisconsin Landscape architect: bnim, Kansas City, Missouri P R O J E C T C R E D I T S Owner:
HW OA T ON EBDA © IPIW A NC RB AI TA AGN NO E S ©
HERE
The rainscreen gives the building an ethereal, diaphanous appearance at dusk, when light within the building escapes through the punched windows in the exterior concrete walls to backlight the screen.
[48] C i v i l E n g i n e e r i n g M A Y 2 0 1 7
0885-7024/14-0009-00??/$30.00 PER ARTICLE
PHOTOCREDIT GOES HERE
By Derrick Roorda, p.e., s.e., leed ap, and G. Kelley Gipple, p.e.
M AY 2017
C i v i l E n g i n e e r i n g [49]
N ortheast E levation of V isual A rts B uilding and A rt B uilding W est
L
[50] C i v i l E n g i n e e r i n g M A Y 2 0 1 7
0885-7024/17-0005-0048/$30.00 PER ARTICLE
BUROHAPPOLD ENGINEERING
the mission of the university’s School of Art and Art HisArts Building opened to widespread acclaim, tory. Designed for today’s integrated and collaborative apmarking a new chapter in the 170-year-old cam- proaches to art education and practice, the building houses pus’s distinguished legacy of arts education. De- the school’s studio arts programs, which includes ceramics, signed by Steven Holl Architects, of New York sculpture, metals, photography, printmaking, and 3-D mulCity, with bnim, of Kansas City, Mis- timedia works. The structure also offers practice studios, ofsouri, serving as executive architect, fices, and gallery space. the new building utilizes volumes and The building itself is of five levels and has a 200 by 180 ft voids to create a visually striking structure. rectilinear footprint. Located on a sloping site, the building has The University of Iowa entrances on the north side at campus encompasses 1,700 the first level and on the south acres and stretches westward side at the second level. The from downtown Iowa City, stacked floor plates are offset which is in the eastern part from one another to form balof the state. The Iowa Rivconies along the exterior edges er flows through the campus, of the building’s 68 ft height. dividing the grounds into Seven vertical cutouts, six of east and west sides. A celthem along the perimeter of ebrated natural feature, the the building and one inside it, river can indeed be formidaturn what would have been an ble. In the summer of 2008, almost square footprint into devastating floods caused one that resembles a complex nearly $800 million in dampuzzle piece. The overall viage to the campus alone. sual impression given by the Set in buildings along the building suggests a vertically river’s west bank, the univerporous series of stacked volsity’s arts program was hit umes. Internally, these volhard by the flood. But with umes offer loftlike common funding from the Federspaces for informal gatherings al Emergency Management and discussions, as well as Agency, the catastrophe beclassroom and studio spaces. came an opportunity for transformation. Mul- The rainscreen features a Located next to the horizontally porous and tiple arts-related buildings were renovated, stainless steel scrim with planar composition of Art Building West, the Virepaired, or replaced because of the flood, in- customized perforations. sual Arts Building reflects a foundational premcluding the new Visual Arts Building (its predeise of the school: the merging of art history and cessor now shuttered). Hancher Auditorium, a state-of-the- studio practice into a single program. This approach was deart performing arts venue, opened last fall on higher ground, veloped by the university in the 1920s and is now commonly and plans for a new art museum are unfolding. Art Building referred to within the field as the Iowa idea. The two buildings West, the award-winning edifice completed by Steven Holl frame a common green, striking a delicate balance with one anArchitects in 2006, also was repaired and returned to active other. At nearly twice the size of the earlier building, the new use by the school’s art history division. Visual Arts Building expresses the school’s vision for the future In planning and design, the $77-million, 126,000 sq ft of art practice while forming a dialogue with its smaller comVisual Arts Building works to strengthen arts education and panion. The new building’s higher elevation places its ground
© S T E V E N H O L L A R C H I T E C T S , A B O V E ; © C H R I S M C V O Y, B E L O W
ast fall the University of Iowa’s new Visual
floor at least 2 ft above the 500-year flood level, greatly reducThe architects wanted this main stairwell to have the same ing the risk of damage from future floods. heavy materiality as the building while also appearing to float Steven Holl Architects won the project through a design within the space. This goal, however, was complicated by the competition in 2010, and its entry was submitted after it placement of the stair within the building’s center, where the had considered more than 30 designs. A structural engineer- floor plates are the least supported. Monocoque construction, a ing team from the San Francisco office of the international concept taken from aircraft design, consolidated the stair’s risfirm BuroHappold Engineering worked with Steven Holl er, tread, and step elements into an efficient U-shaped compoArchitects to develop the winning proposal. The engineer nent of extruded concrete that could be treated as a unit. This of record—Structural Engineering Associates, of Kansas reduced the weight of the stair and allowed the entire stair asCity, Missouri—completed sembly to participate as part the construction documents of the structure, in contrast to and administered the project. a heavy appendage that would Designing a new neighhave required the support of bor to a celebrated work of arthe slabs. Intermediate conchitecture can be a daunting nections between the stair and undertaking. For Steven Holl the building are carefully loArchitects, which designed cated and detailed to miniArt Building West, the new mize their visibility. Above, design would also have to steel framing supports the complement its own work. atrium skylight, the only part The design of the Visuof the building that is not of al Arts Building reflects the concrete. blurring of boundaries beThe six cutouts on the petween disciplines and the colrimeter of the building are laborative approaches to learnenclosed in 20 ft tall channel ing that have become the glass walls so that the horihallmark of contemporary arts zontally shifting floor plates education. Interconnectivity create a series of outdoor baland the potential for multidisconies that can be used for ciplinary interaction are cenmeetings, work, or socializtral to the building’s design. ing. Punched windows in the safe, developed by Computers As part of this, the seven vertical openings that exterior concrete walls, many of them opercut through the floor plates create multiple & Structures, Inc., was utilized able for natural ventilation, add light and air to model the long-term deadcenters of light within the building, becoming to the building. even more complex given the offset nature of load deflection contours of each Structural Engineering Associates modfloor, including the fourth. the floor plates. eled the shape and location of each window The largest cutout is located entirely withindividually to produce accurate lateral disin the structure and creates a central atrium that allows views placement numbers. These square, irregularly placed winacross and between floors. The cutout is wrapped in a series of dows of various sizes are located in such a way that they can long stairs that are shaped to encourage interaction and dis- transfer loads while accommodating the building’s integral, cussion. In this way the atrium itself forms a vertical social heavily reinforced columns or base beams. heart for the building. The ground level of the atrium serves The entire building is clad in a rainscreen produced by the as a place for gatherings and events. international firm rheinzinc that is designed to weather to M AY 2017
C i v i l E n g i n e e r i n g [51]
T he core challenge for B uro H appold ’ s
a blue color. The rainscreen features a stainless steel scrim with customized perforations that wraps the southeast and southwest facades, treatments that also serve to give the building a striking ethereal presence. The core challenge for BuroHappold’s structural engineers was realizing in concrete and glass the dramatic form envisioned by the architects while retaining the desired open interior volumes. The variation of volumes and voids that gives the building its sculptural beauty hindered the efficient transfer of gravity forces to the ground. Complicating matters, solid concrete slabs could not span the distances required for expansive interior spaces without bowing. These issues were resolved by using a castin-place concrete design that utilizes concrete columns supporting the five concrete slabs. The column spacing varies from 15 ft to 34 ft, with most of the columns continuous from the building’s top level to the foundation, although four concrete transfer beams 18 in. wide and 36 to 54 in. deep are utilized in the design. The interior concrete walls provide the reinforcement needed for the structure’s lateral stability. The structural system of the build[52] C i v i l E n g i n e e r i n g M A Y 2 0 1 7
© IWAN BAAN
ing is supported by concrete spread footings and drilled piers bearing on the subterranean limestone formation below the building. The open floor plates and the loftlike interior volumes were made possible by voided slab construction, which is described below. An integrated thermally active radiant heating and cooling system in the slabs eliminated the need for visible ductwork and makes it possible for each concrete slab to function both as a pristine floor and as a ceiling. The lower-level walls are of reinforced concrete at the northwest edge and portions of the southwest and northeast sides, where they are subterranean. As the walls were designed to require lateral support at the top The floors in the Visual Arts Build- by the second-level slab, it was necessary to ing are composed of voided slabs. provide shoring on the outside of the walls In this method, recycled plasuntil the second-level slab was constructed tic disks held in steel cages disand had achieved its full design strength. place a portion of the concrete. The building was positioned 52 in. from the Polyethylene tubing used for ranorthwestern and northeastern property diant heating and cooling also lines to allow room for this temporary is contained within the slabs. shoring system.
S T RU C T U R A L E N G I N E E R I N G A S S O C I AT E S , A B OV E ; © S T E V E N H O L L A R C H I T E C T S , B E L O W
structural engineers was realizing in concrete and glass the dramatic form envisioned by the architects while retaining the desired open interior volumes .
The building was designed for the Offset floor plates and six vertical 100 psf to offer flexibility in the future wind, snow, and live loads specified in perimeter cutouts give the building its use of the building. the 2011 edition of Iowa’s State Building stacked appearance and form balconies The vertical system of reinforced-conCode, which is based on the 2009 edition along the exterior edges. The balconies crete columns and concrete walls carries are enclosed in channel glass walls. of the International Building Code (Washgravity loads to the foundation. The conington, D.C.: International Code Councrete columns are 18 in. square in cross cil). Based on the seismic and wind parameters in Iowa section throughout the building and 20 in. square where the City, wind load forces, rather than seismic forces, governed loads are higher, as is the case under portions of discontinuous the lateral loads and controlled the design. All floors were walls. To provide large open volumes, especially around the designed for a live load of central stair, concrete walls 10 in. thick are used at isolated interior locations and along the perimeter of the building to S ection of V isual provide enclosure and lateral stability. A rts B uilding Three-dimensional analysis was critical to the design process and for ensuring constructability. The structure was modeled using the 3-D finite-element modeling software etabs (v9.7.2), developed by Computers & Structures, Inc., which has offices in Walnut Creek, California, and New York City. etabs was used to model the governing lateral load and wind loading against the ordinary reinforced shear wall lateral system. The slabs were modeled using safe (v12.3.1), also developed by Computers & Structures. Each floor was modeled individually, with gravity loads added to the model to accurately design the floor slab. Both software packages allow a direct transfer of building geometry into Revit, developed by Autodesk, Inc., and Revit was used for documentation and to effect the extensive coordination that the project required. The columns were designed using spColumn—developed by structurepoint (formerly the Engineering Software Group of the M AY 2017
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What is more, because the weight of the concrete itself accounts for CHALLENGING CONDITIONS BELOW two-thirds of the load in a building, he site of the University of Iowa’s Visual Arts Building is underlain even when considering occupant live by bedrock. This moderately weathered limestone is overlain by fill in the loads, the lighter slabs make it possiform of lean to silty clays. The upper fill material was found unsuitable for ble to use smaller and lighter beams, supporting the five-level building’s foundations, so all supporting elements columns, and foundations. This ripextend into the bedrock. ple effect lowers the overall cost and The limestone bedrock varies in depth, being closer to the surface under has the added benefit of reducing the the northwestern portion of the building and deeper under the southeastern amount of concrete and cement needportion. For this reason, two types of foundation elements were used. Spread ed, thereby significantly reducing the footings were placed directly on the bedrock where it was shallow (approxicarbon footprint of the construction. mately 2 ft below the surface), and drilled piers circular in cross section were Slab voids were omitted locally at extended down to the bedrock where it was deep (approximately 48 ft below column or wall locations, around slab the surface). openings, and along the slab perimeThe project’s geotechnical engineer, Terracon, set an allowable bearter to increase shear strength. Where ing pressure of 15,000 psf for sizing spread footings and 80,000 psf for the punching shear demands still exceeddrilled piers, both to be supported by the limestone bedrock. The higher valed the capacity of the full-depth conue was achieved for the piers by drilling them 3 ft into the rock. crete slab, the design incorporated The spread footings were square and ranged in size from 3 to 8 ft on a side headed shear stud plates made into a and in thickness from 18 to 33 in. The drilled piers ranged in diameter from cross pattern, in contrast to introducing the more traditional drop panels 2.5 to 5.5 ft and in depth from 4.5 to 36 ft. with thickened slabs. Finite-element analysis allowed the design team to determine visually where The University of Iowa’s Visual Arts necessary to meet the deflection critedeflection exceeded tolerable limits and to Building, top, plays with the idea of ria. While the need for such beams was quickly assess potential solutions. The strat- vertical volumes and voids, creating greatly reduced by using voided slabs, in a dialogue with the horizontally egies implemented included modifying span some cases the columns remained too far porous and planar composition lengths, thickening the slabs in certain arapart and beams were needed. of Art Building West, left. eas, and adding dropped beams only where This analysis (Continued on Page 80)
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Portland Cement Association), headquarThe building includes a large ment, and the magnitude of sustained loads. internal vertical cutout that tered in Skokie, Illinois—and internally The estimated average values of the ultimate creates a central atrium. This generated spreadsheets. creep coefficient and the ultimate shrinkage The design process involved multi- vertical space is edged in a series strain were taken from the American Conof long stairs that were created ple iterations of slab thickness and column crete Institute’s report Control of Deflection in using monocoque construction, shifts to keep deflections within the limConcrete Structures (435R-95) and used for the which allows the entire stair its set by the architects in order to prevent nonlinear cracked-section analysis. assembly to participate as part damage to the finish materials. This inControlling deflections in flat slabs of of the structure rather than cluded the evaluation of long-term deflecconcrete can lead to a circular and somebeing an appendage requiring tions, which required careful consideration times spiraling design process. To decrease the support of the slabs. of the concrete mix that would be used and deflections the thickness must be increased, of the extent to which the concrete would but that thickness adds weight and further undergo creep when placed under stress for prolonged pe- increases deflections. Lightweight concrete can be specified riods. BuroHappold provided color contour maps of slab to alleviate this issue somewhat, but the same principle apdeflections based on a 3-D finite-element analysis of each plies. Lightweight aggregate mixes generally have a lower floor to Steven Holl Architects so that each option could be modulus of elasticity, meaning they are not as stiff and decarefully reviewed. flect more for a given thickness. Cracked-section analysis was used to limit slab deflections To resolve this conundrum, BuroHappold suggested an to 1 in. for long-term deflections under dead load. Calculat- alternative proposal: introduce voids into the slabs to reduce ing concrete deflections is complicated because of the inher- their weight. Referred to as voided slab or bubble slab conent nonlinear behavior of the material, for when a concrete struction, this approach uses hollow balls or disks made of slab or beam cracks, it deflects two or three times as much as recycled plastic and held in place by steel reinforcing bars to it would if it remained uncracked. Long-term deflection is in- displace a portion of the concrete. Balls were used only for creased by the shrinkage and creep that occur in response to thicker slabs, while disks of lower profile were used more exsustained loads. These deflections are higher than the elastic tensively. Overall the system eliminated as much as a third deflections that occur when loads are initially placed on the of the concrete weight while maintaining more than 90 perstructure. The shrinkage and creep contributions are influ- cent of the stiffness, thereby enabling a given slab thickness enced by temperature, humidity, curing conditions, age at to span much farther and deflect less than would otherwise the time of loading, the quantity of compression reinforce- have been possible.
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Art by Design (Continued from Page 55) also proved critical to elegantly integrating the channel glass walls, especially at balconies and doors. Understanding how the building would move above and below each of the fragile elements enabled the team to adjust the form or strengthen the concrete. In this way heavy details within the glass walls were avoided, preserving their transparency. While voided slab construction is relatively rare in the United States, BuroHappold had experience with it in Europe. At the time the project was under design, only two U.S. contractors were offering voided systems, both of them proprietary but essentially the same in terms of engineering function. BuroHappold’s experience helped convince the client and the construction team that this was the correct approach for the Visual Arts Building. However, cost remained the principal consideration. BuroHappold completed preliminary designs using both thicker slabs of solid concrete and thinner slabs with voids with input from the contractors. The voided slab solution was found to be cheaper, and Cobiax usa, Inc., of Dedham, Massachusetts, was ultimately selected as the subcontractor to provide the voids for the project because of its bid and its experience with such construction. Being a pioneer comes with its challenges. With the Visual Arts Building, implementing voided slab construction for such a complex structure made for a steep learning curve, but the general contractor—Miron Construction Co., Inc., of Neenah, Wisconsin—worked with Structural Engineering Associates to devise a strategy for coordinating building systems and constructing the slabs. Incorporating the thermally active slab radiant heating and cooling system with the voided slab construction required painstaking coordination on the part of Structural Engineering Associates, Miron Construction, and the mechanical engineering consultants. Miles of cross-linked polyethylene (“pex”) tubing 5/8 in. in diameter wind through the slabs and exit in precise locations to connect with the cast-in-place walls. Coordination for the pex loops and void cages that encase them began during design. Design Engineers, of Cedar Rapids, Iowa, served as the mechanical, electrical, and plumbing engineers for the project. However, as this firm worked with its subcontractors to define the construction proRoorda cess, it became clear that the team should consider off-site assembly of the pex tubing grid. The concrete slab thickness was increased to 12.5 in. and number 6 reinforcing bars were used as spacers to give the pex its own layer. A 3 in. top layer of concrete was added to refine the slab finish. This layer was polished and left exposed. The concrete placement process also proved complex. Wire cages holding multiple disks would arrive on-site by the truckload so that the construction crew could install large areas at one time. A first placement of about 3 in. of concrete was allowed to set just enough to hold the void cages and prevent them from floating up during the second concrete placement. Because the original mock-up occurred in winter, [80] C i v i l E n g i n e e r i n g M A Y 2 0 1 7
when concrete set times were longer, the timing of the second placement needed to be adjusted on-site on the basis of temperature and humidity. Personnel from Structural Engineering Associates attended several of the initial placements and worked with Miron Construction to determine the timing for the second concrete placement. While the voided slabs provide the needed structural performance, they required more care when it came to drilling. A few slabs were damaged when holes drilled for formwork braces filled the voids with water and froze during the winter. Furthermore, fasteners used in construction were limited to a depth of 3/4 in. Despite these drawbacks and the complexity of working with the voided slab construction, the effort was worthwhile because the thermally active slab heating and cooling system, along with the structure’s thermal mass, makes the building very comfortable and efficient to heat and cool. In an era defined by increasing collaboration by architects, engineers, consultants, and contractors, the Visual Arts Building offers a reminder of the importance of a bold architectural vision. Here the success of the collaboration can be measured in the purity of the final building, as visitors encounter an arresting sculptural form and minimally detailed, flowing open spaces. The design has won several awards, including a design award bestowed this year by the American Institute of Architects’ New York chapter. Acclaim aside, the building’s legacy will depend upon the extent to which it furthers the mission of the School of Art and Art History. The central atrium, the glass-enclosed studios, and the abundance of indoor and outdoor gathering places promote the connection and interaction underpinning the art school’s pedagogical vision. Through advanced modeling tools, a willingness to embrace unconventional construction systems, and careful coordination, the design and construction team has delivered a setting that will enable the Visual Arts Center to realize its full promise. CE
Gipple
Derrick Roorda, p.e., s.e., leed ap, is the director of BuroHappold Engineering’s California structural engineering group, which has offices in Los Angeles and San Francisco. G. Kelley Gipple, p.e., is a principal and the director of design for Structural Engineering Associates in Kansas City, Missouri.
University of Iowa Architect: Steven Holl Architects, New York City Associate architects: bnim, Kansas City, Missouri Structural engineers: BuroHappold Engineering, San Francisco office, and Structural Engineering Associates, Kansas City, Missouri General contractor: Miron Construction Co., Inc., Neenah, Wisconsin Geotechnical consultant: Terracon, Cedar Rapids, Iowa, office Civil engineer: Shive-Hattery, Cedar Rapids, Iowa, office Mechanical engineer: Design Engineers, Cedar Rapids, Iowa Curtain wall consultant: W.J. Higgins & Associates, Inc., Weston, Wisconsin Landscape architect: bnim, Kansas City, Missouri P R O J E C T C R E D I T S Owner:
