By EDDY ROBERTS, P.E., AND YUE ZHAO, P.E.
The Charles Library at Temple University, in Philadelphia, sets high expectations for what a modern library should be. The soaring and dynamic spaces made possible by the building's innovative steel structure allow for study and collaboration spaces that fit modern students' needs. Despite major design changes at the start of construction, the school's vision was achieved: a multiuse building that serves as the focal point of an architecturally evolving campus.
The new Charles Library on the campus of Philadelphia's Temple University is a departure from the traditional college library and its rows of bookshelves, low ceilings, and sometimes dark and cramped spaces. What was imagined for the Charles Library was quite the opposite: a building that incorporates open volumes, lots of light, and grand staircases, among other unique design features—including virtual-reality rooms and a workshop with 3D printers.
The new building comprises approximately 220,000 sq ft of usable space and an additional 40,000 sq ft of green roof. The overall building is approximately 420 ft long, 160 ft wide, and 70 ft tall with four stories aboveground. Its features are well balanced, owing in part to the innovative structural systems supporting the building. Much of the interior space is fluidly shaped, with multistory openings that visually interconnect and enable visitors to easily navigate the library. The center of the library revolves around a vast three-story atrium that has a large oculus that opens to the fourth story. Monumental staircases lead to the fourth floor, where the open-perusal bookshelves are located.
In order to programmatically afford these large spaces, the majority of the new library's collection has been relegated to a three-story book vault that begins at the basement level, at the northwest side of the building. The books are stored in high-density shelving and are retrieved via automatic service retrieval-system robots, nicknamed BookBots. The ASRS enables a new means for perusing. Users read a book's description online, select the book with a click of the mouse, and the book is delivered to them within minutes.
The fourth floor is surrounded on all four sides by floor-to-ceiling glass, which provides uninterrupted views of the campus. From outside, this floor appears as a glass prism floating on a granite block. Doors from the fourth floor lead to a large terrace that cantilevers 45 ft beyond the building's footprint. Starting from the terrace, the sloping roof wraps upward to the roof of the fourth floor, which is covered in plants.
The green roof, one of the largest in Pennsylvania, was designed with sustainability in mind, and it also minimizes stormwater runoff. Additionally, large water-retaining basins are depressed below the building's two primary entrances. These efforts to lessen the building's environmental impact are part of the university's plan to attain a gold-level certification in the U.S. Green Building Council's Leadership in Energy and Environmental Design rating system.
Placing the building at the heart of the campus was a logistical challenge involving a puzzle of ever-shifting pieces. The first was the demolition of Barton Hall, the existing building on the site. Below the hall is a utilities tunnel that houses campus steam lines and cuts a swath across the southern part of the site. Originally, the design team pursued a strategy to relocate the tunnel's utilities to run to the south of the site and then demolish the tunnel. However, about halfway through the design process, at a time when much of the structural system had been determined, the design team found out that the tunnel could not be moved nor could it be impacted by the new structure. Some of the largest structural elements—three concrete arches used to span across and create the large atrium space—happened to land where the utilities tunnel is located. This problem was resolved by redesigning the arches to span farther and clear the utilities tunnel.
Later in the construction, the general contractor encountered another challenge. While digging for a new water-retention basin, the team encountered foundations from an even older building that were not part of the known surveys. Another piece of the puzzle was relocating the existing collection to the new building, a process that had to occur over the summer when demand for the collection was low. However, these challenges turned out to be minor compared with the developments the team subsequently encountered: After the construction documents were complete and sent out to bid, the team was tasked with redesigning the building's structural system from primarily concrete to primarily steel.
From the outset of the concept design, the design team pushed boundaries. The design architect, Snøhetta, realized early on that several structural systems would need to be unconventional. LERA, a firm known for its history of designing complex cultural buildings using cutting-edge structural systems, was brought on to serve as the structural engineer. The design team proposed two concepts: a building with a large gateway arch that spanned 200 ft and a building with a 75 ft cantilever at the fourth floor. Structural schemes were developed for each option. The gateway option included a ribbed structure, and the latter option included one-story-tall cantilevered trusses. While the team anticipated that the realities of the university's budget would result in a simplified version of one of the concepts, the architect pursued a design that effectively combined both options. That is the design today—albeit with scaled-down spans from the original concepts.
The site has an undulating rock profile from approximately 10 ft below the region of the book vault at the northern end of the site to approximately 60 ft below the basement at the southern end. The team used caissons as the foundation system because of the nature of the rock profile, the high concentration of loads at the landing points of the long-span trusses, the stringent deflection criteria of the ASRS, and the need to minimize differential settlements. The caissons at the foundation have diameters of 36, 48, and 60 in. and were designed by the geotechnical engineers, Pennoni and McClymont & Rak Engineers Inc.
Below the basement, a new concrete tunnel spans approximately 200 ft and supplies air through the building to the mechanical air-handling equipment in the basement. The groundwater level of the site is approximately 6 ft above the basement level. Furthermore, there are several water mains in close proximity to the basement, so it was designed for the possibility of a major water main break.
To stop any water infiltration, waterproof membranes were placed below and around all sides of the basement, effectively acting as a bathtub. Because of the large hydrostatic pressures, the basement has pressure slabs that are typically 14 in. thick and that taper to a thicker depth of 30 in. at each caisson and at the perimeter basement walls. At the ASRS area, the book racks required post-installed anchors such that the fasteners would be freely located, depending on the fit-out of the racks. To avoid the possibility of any of the post-installed anchors penetrating the waterproofing membrane below the pressure slab, a 5 in. thick topping slab was placed on top of the 14 in. pressure slab.
Philadelphia has relatively modest snow loads compared with many regions of the United States. However, because of the complex geometry of the roof and the possibility for snowdrifts across long roof distances, heavier snow loads (greater than the design ground snow load of 22 psf) were considered in the design. The terraces above the cantilevered canopies are designed to accommodate snowdrifts ranging from approximately 60 to 130 psf. And although Philadelphia is not known for its seismicity, much of the controlling loads in the lateral load-resisting system are due to seismic considerations. Some of the lateral load-resisting systems include vertical transfers from one system to another, so to comply with the ASCE 7 code, these systems follow the requirements for structural irregularities, and overstrength factors are applied to the design of the affected structural components.
Overall, the structural layout follows two sets of grids, which are slightly skewed in relation to each other. The northern side of the library follows a regular column grid of approximately 30 by 30.5 ft. The fourth floor and the southern side of the building follow a column grid of approximately 30 by 28 ft. One of the challenges was to devise structural systems that would transfer loads from one set of grids to another; these column transfers were made possible predominantly by transfer beams or sloping columns.
An example of this can be seen in the three-story atrium lobby that is covered by a wood-clad dome. The structural columns in this area, which follow the regular column grid at the fourth floor, slope through the atrium, puncturing the wooden dome and creating a large open space lined by leaning columns. Out of approximately 120 column locations throughout the building, 50 are transferred either via deep transfer girders, the sloping columns, or long-span trusses. With more than 40 percent of the columns transferred in the building, one of the more significant structural hurdles was to account for the various resultant forces while engineering appropriate details and delivering a cost-effective structure.
The many leaning columns and trusses required special consideration, not only with regard to the final building but also for each stage of erection. The design team reviewed the load paths of the finished state of the composite concrete and steel building, while the steel erector studied the erection sequence of the steel, which was not self-supporting.
Some of the unique structural aspects of the library correlate to the shape of the building. The architectural volume of the building is carved away at the primary entrances at the southwest corner and at the middle of the eastern side, leaving soaring canopies at these locations. Under the canopies, the building entrances are deeply set back—up to 45 ft—at each location. The entrances feature multistory glass facades, and part of the architectural concept was to visually connect these entrances. This was magnificently done with a series of multistory wood domes that line the underside of the canopies, carrying beyond the glass entrances and into the interior, where the wood domes carve out and form the multistory atrium and lobby spaces.
To span the domed atrium and lobby spaces, a system of long-spanning trusses—as long as 100 ft—were used. The depth of the trusses was limited by the available space below the third floor and above the wood arch. At the peak of the arch, the available space for the truss is relatively small, so deep girders were used in lieu of truss members. The largest of the steel girders is a built-up steel I section that is 60 in.deep with 3 in. thick flanges.
The domed canopies at the main building entrances have cantilevered trusses that extend approximately 40 to 45 ft beyond the column support at the lower floors.
At the eastern canopy, a series of one-story high trusses extend into the building. These trusses were used to balance the cantilevered spans. The back span of each cantilevered truss is tied down with a sloping column, resulting in additional loads that must transfer through the adjacent floor diaphragm; load paths are directed to nearby shear walls.
At the southwestern canopy, the structure comprises a 40 ft cantilevered truss at the south elevation and a 100 ft cantilevered truss along the west elevation. To provide stiff back spans for these trusses, there is a steel-braced frame at the north end of the east truss and a concrete shear wall at the back span of the south truss. A temporary erection column at the southwest corner supported the trusses, remaining in place until the system was self-supporting—after the completion of the concrete shear walls and slabs up to the fourth floor.
The cantilevered trusses are designed for the possibility of disproportionate collapse, so that the loss of any one member would not result in an overall structural failure. Each cantilevered truss is interconnected to the other trusses to meet these criteria. Bridging trusses were used at the eastern canopy, and the two perimeter trusses at the southwest corner are interconnected and will carry each other's structural weight if one fails.
While the entrances of the building are light and welcoming, with multistory glass facades, the majority of the new library is clad in vertical striations of heavy stone, which was selected because of its similar look and feel to the more established buildings on campus. The stone is backed by concrete masonry unit walls that extend from the floor to the underside of the next floor. The cladding system of stone and CMU is considerably heavier than the glass facade, weighing approximately 120 psf. Additionally, the structural spandrel beams supporting the stone and CMU required stringent deflection criteria.
The green-roof system has a substrate of 4 in. or 9 in. of soil over a waterproofing membrane. The depth of the substrate was determined by the type of planting. The weight of the green-roof system is 60 psf for the 4 in.soil substrate and 100 psf for the 9 in. soil substrate.
Much of the building is a composite system of steel floor framing, slab-on-metal deck, and concrete shear walls. The exception is at the northern portion of the building, which is all concrete. This is the location of the large book vault, which measures about 130 ft long, 80 ft wide, and 50 ft tall.
The book vault is located partially below ground level: 20 ft below ground at the southern end and approximately 34 ft below ground at the northern end of the gently sloping site. Because the book vault is so far below ground level, there are large lateral earth and water pressures acting on the perimeter walls. These pressures are carried into a series of 50 ft tall concrete "fin" walls that span from the basement to the third floor. They are spaced around the perimeter at approximately 30 ft intervals. These fin walls, as well as the concrete columns inside the book vault, are sized to displace as few book bins in the system as possible. The interior concrete columns are 27 in. sq and 50 ft tall and were designed as slender columns with an L/R ratio of 77.
The book vault has a design load of 2,700 psf acting on the basement slab. The portion of the library's collection that is available for perusal occupies more traditional library shelving on the fourth floor. At this floor, the open library stacks have a design live load of 160 psf.
The building was originally envisioned and designed primarily as a concrete structure with steel trusses and steel framing only at the cantilevered canopies at the two entrances. Concrete shear walls were selected as the lateral system.
The structural engineer sought to guide the architect early in the project by establishing a column grid and providing the architect and the university options for concrete-floor framing versus steel-floor framing. Early on, cost estimators found that the steel and concrete-floor framing options were similar in cost. Because the matter was less driven by differences in cost, the architect considered the aesthetics of the structural system and opted for concrete-floor framing because the underside of the floors could be exposed to view, which the architect preferred. The original design included concrete columns, slabs, and arched walls that spanned over the three-story atrium space. To meet the architect's desire that a large portion of the concrete slab soffit be exposed to view, a voided-slab system by COBIAX was considered as the floor system in lieu of a more typical flat-slab system with drop panels.
When the design of the building was complete and the project documents sent out to bid in January 2016, the Philadelphia construction market was very active, and concrete contractors were in high demand. The bids received from the concrete contractors were well above the allocated budget and far exceeded the predictions of the cost estimators.
By the end of that summer, when decisions were made to proceed, there remained approximately three months to reissue contract documents for bid and still meet the scheduled opening date of fall 2019. This was a substantial undertaking. What resulted was a comprehensive redesign from a mostly concrete building to a building that was mostly steel.
In the new design, the lateral shear walls, steel truss framing over the two entrances, and the concrete structure that encloses the book vault remained largely unchanged. The floor framing was redesigned to 4.5 in. normal-weight concrete slab on 3 in. metal deck with W18 or W21 steel beams and W14 steel columns. The concrete arches over the atrium were redesigned as long-span trusses with steel plate girders.
The redesign of the building from mostly concrete to predominantly steel was a herculean task, but the time constraints and logistics were even more complicated. The caissons that support the building's foundation were being installed when the decision was made to redesign the structure. Caissons are engineered for a building's weight. The basement elevation was determined to be the optimal location whereby the weight of the original concrete building would counteract the uplift produced by hydrostatic water pressures. The steel building was more economical, but it was also lighter, resulting in portions of the foundation (the caissons and the pressure slabs) being in net uplift.
So a concerted effort was made to review the changes to the caissons while the caisson installation progressed. To expedite the caisson design process, LERA worked closely with the geotechnical engineer and construction manager to develop a plan to divide the site's caissons into four staged regions. The caisson loads were reviewed based on the sequencing of their installation, and a total reassessment of their loads was accomplished in three weeks, sometimes by extrapolating the predicted weight of the building before completing the redesign. In some cases, this left the design team with one day of lead time before the next installation of caissons.
The structure of the Charles Library was effectively designed twice—once as a concrete building and again as a steel building. There were signiflcant obstacles, but the design team was able to nimbly adapt and overcome them in a timely and effective manner.
The library opened as scheduled in September 2019 to many positive reviews from the Philadelphia and national design communities. More importantly, the students immediately embraced the new building, using it as a study space, admiring the workings of the BookBots, and sprawling across the many unique architectural spaces that enable new kinds of collaboration. It is an example of what the library of the future will resemble.
As a testament to the signiflcant impact the new library has made, it has been awarded the 2020 American Library Association/International Interior Design Association Library Interior Design Award, the 2020 Excellence in Structural Engineering Award from the National Council of Structural Engineers Associations, and the 2020 Excellence in Structural Engineering Award from the Structural Engineers Association of New York.
Eddy Roberts, P.E., is a senior associate and Yue Zhao, P.E., is an associate for LERA Consulting Structural Engineers in New York City.
PROJECT CREDITS Lead architectural design and landscape architect Snøhetta, New York City office Local architect and mechanical engineer Stantec Architecture and Engineering, Philadelphia office General contractor Keating Co., Narberth, Pennsylvania Structural engineer LERA Consulting Structural Engineers, New York City office Steel fabricator/erector Owen Steel Co., Columbia, South Carolina Building envelope and curtain wall consultant Heintges, New York City office Civil engineer Hunt Engineering Co., Malvern, Pennsylvania Geotechnical engineers Pennoni, Philadelphia, and McClymont & Rak Engineers Inc., Philadelphia office Automatic service retrieval system and BookBot designer Dematic, Atlanta
Civil Engineering, October 2020, © American Society Of Civil Engineers. All Rights Reserved