Description

The structural approach to libraries is tightly intertwined with the overarching programmatic and aesthetic building notion. The quintessence of any structural analysis is the consideration of all static and dynamic loads acting on a building and the comprehension of the resulting reactive forces within the structural members. Optimization of the frame considering actually occurring load conditions and load combinations, limiting excessive deflections and respecting material characteristics to resist the acting forces, and to consequently transfer the loads into the bearing soil is the objective of the structural design. Extending the contemporary structural systems beyond the traditional frame, for example by assigning structural tasks to envelope components, paired with the availability of complex analytical tools, and an integrated project delivery process where interdisciplinary knowledge and risks are shared among team members transforms the historic typological parameters of libraries. Freed from load-bearing walls and limited spans necessitating a compartmental design approach, contemporary concepts dissolve the boundaries between interior and exterior, and create fluid space configurations within envelopes of exceptional complexity and the ability to communicate the building’s purpose.

Acknowledging that there is no standard formula for structuring a library building, this chapter will focus as a point of departure on the engineering aspects framed by the mathematical constraints of loads, building code regulations, material proper­ties and construction systems leading to the design of the load-bearing skeleton expressing the highest degree of structural economy. Considering the typical book as a three-dimensional unit of a particular width, height and depth, associated with a particular weight, the organization of library material can be numerically comprehended and translated into spatially and dimensionally anticipated arrangements. A direct link therefore exists between the module book, the shelving unit, the array of shelving units, and the structural grid of the building. At the same time the number of volumes per linear foot corresponds to the loads the structural frame is required to support.

Designing for flexibility and further expansion of a collection or archive, often a programmatic prerequisite, constitutes a challenge to the structural project economy and requires a careful analysis regarding the number of volumes, their anticipated location and organization within the given envelope. In particular the concept of transforming parts of an open-access shelving system to high-density, compact shelving to accommodate collection growth requires an initial design response regarding floor load capacity and is usually not retroactively achievable. Similarly, future relocation or expansion of a collection to a different part of the building demands consideration early in the planning process.

From a structural perspective libraries are designed for their various components – reading areas, study spaces and administrative functions have to accommodate far smaller life loads than stack areas or spaces equipped with compact shelving. However, not only the structural integrity of the building’s superstructure is relevant, shelving units and storage systems themselves require resisting gravitational and lateral loads and, depending on the library’s location, withstanding code-regulated seismic loads.

The following paragraphs will explore in greater depth the relationship between collection layout and structural grid, applicable load requirements, and the selection of structural systems with respect to material choices and constructability. Advantages and disadvantages pertaining to plan flexibility, building quality and performance characteristics, as well as the interface with mechanical systems will be discussed on a theoretical level. Various case studies will be referenced to illustrate actual solutions and to document their particular structural concepts in the context of a holistic design approach

Structural Analysis in the International Building Code

It is beyond the scope of this publication to provide inclusive information on standards, codes and regulations applicable to particular jurisdictions. We will use the International Building Code, IBC, regulating construction in the USA,[1] as one base reference. The IBC was first published in 1997 and establishes quite detailed and user-friendly minimum requirements for new construction. It emphasizes safety, especially in terms of fire protection. Constantly integrating new research and experience, it is updated every three years. It takes various climate zones and seismic conditions into account and is therefore also a useful reference tool in other parts of the world. Within this context, we will also use imperial dimensions. Determining occupancy classification and the relevant construction type defined by materiality, a stringent parameter of a library’s permissible height and area, are a prerequisite to any structural analysis and hence also applicable elsewhere.

Occupancy

Chapter 3 of the IBC classifies structures with respect to use and occupancy. Libraries are listed in Assembly Group A-3, pertaining to spaces for the gathering of persons for civic purposes.[2] If a program includes occupancies that differ from the assembly use, for example a library as integral part of a shopping mall or an educational facility, further analysis as to construction type, area and height limitations is required to determine the need and rating of fire separation between occupancies.[3]

General Building Heights and Area

The IBC regulates height and area of a library in correlation to the type of construction. It defines five construction types, Type I through Type V, with decreasing fire resistance rating of structural frame, bearing walls, floor assembly and roof construction.

Representing the highest fire resistance rating, Type I and Type II construction require the use of non-combustible materials; reinforced concrete and steel respectively. Type III and Type IV both require exterior walls constructed of non-combustible materials. Any code-compliant material, including combustible materials, is permitted for interior building elements of Type III construction, while interior building elements of Type IV, also referred to as heavy timber construction, are of solid or laminated wood without concealed spaces. Type V, the most vulnerable type of construction, permits structural elements, exterior and interior walls composed of combustible materials. If exposed to fire, a structural member manufactured of a non-combustible material such as steel would lose structural integrity through deformation, increasing the potential for the collapse of the larger system. Vice versa, a combustible structural member constructed of wood, for example, could be protected against the destructive forces of a fire by gypsum wallboard encasement. Consequently, the IBC introduces for each construction type (except for Type IV) a subcategory A and B prescribing the fire resistance rating measured in hours of withstanding exposure to fire of the exterior walls, the structural frame, the floor/ceiling assembly and the roof construction.

A library constructed according to Type I A construction with a fire resistance rating of three hours for the structural frame, the exterior and interior bearing walls, a two-hour rated floor construction and a 1 ½ hour rated roof construction could be of unlimited height, number of stories and floor area while a Type II A construction with one-hour fire-rated building elements would limit the height to 65 ft (19.8 m), the total number of stories to three and the area to 15,500 sq ft (1,440 m²) per floor (fig. 1). Type V A with a one-hour protection would limit the allowable height to 50 ft (15.2 m), the number of stories to two and the plate area to 11,500 sq ft (1,068 m²), while a Type V B construction would limit the height to 40 ft (12.2 m), the number of stories to one and the floor area to 6,000 sq ft (557 m²). If the building is equipped with an approved automatic sprinkler system, the code grants a height increase by 20 ft (6.1 m), an increase of the maximum permitted stories by one story and an area increase of 200 % for multi-story buildings and even 300 % for single-story structures.

1 Allowable building heights and areas for librariesBased on table 503, Allowable Building Heights and Areas, IBC, chapter 5.

2 Minimum uniformly distributed and minimum concentrated live loads for library floors

For obvious reasons new library construction is generally executed as Type I and II construction, allowing large floor plates with high load capacities on multiple stories. The adaptive reuse of a multi-story building of a construction type with a higher content of combustible materials and a lower fire rating can prove difficult with regard to the allowable size of the floor plate and the location within the building – for example in a Type III B construction with non-combustible two-hour rated exterior walls and interior building elements of any material assuming zero-hour rating, a library floor could not be established above the second floor (first floor in Europe) since the code-regulated story limitations are shown as stories above grade plane.

Structural Load Assumptions

The use and occupancy of a building produces live loads encompassing people, collected materials and furnishings. Location-specific environmental loads include snow, rain, wind and seismic loads. Dead loads are comprised of the self-weight of the structure including all building-integral components.

Books, periodicals, archival documents, microfiche and audio-visual collections establish within a library building the most critical load conditions closely interrelated to their organization, display and storage. The uniformly distributed live loads for library stack rooms and compact shelving are based on the average book weight of 65 lbs/cu ft (10.2 KN/m³) and are listed in fig. 2.[5]

Centered on a typical U.S. double-faced shelving unit organized in ranges spaced 3 ft (0.91 m) apart, the following computations regarding weight of books and number of volumes for the purpose of load calculation and collection layout can be performed.[6]

Load and Volume Calculation for a Typical Library Collection

To give a calculation example, we assume a double-faced shelving unit (DFS) 2 ft (0.61 m) deep with a shelf depth of 12 in (0.3 m), 3 ft (0.91 m) wide, and 90 in (2.29 m) high, on a 4 in (10 cm) base and with six adjustable shelves spaced 12 in (0.3 m) on center for maximum book storage capacity on seven tiers between base and canopy. If we further assume a 100 % working capacity,[7] one DFS would store about 35 cu ft (1 m³) of books weighing a total of 2,275 lbs (10.12 KN) based on the code-regulated book weight of 65 lbs/cu ft (10.2 KN/m³). Converted to a uniformly distributed load, the actual floor load based on linear rows of shelving, separated by 3 ft (0.91 m) aisles, thus allocating 15 sq ft (1.39 m²) of space per DFS, would compute as 150 psf (7.2 KN/m²) – equivalent to the minimum uniformly distributed live load specified by the building code for library stacks.

High-density or compact shelving of books and archival material as well as the storage of microfiche cabinets require floors of far higher load capacity. Typically, library compact shelving reduces open aisle space by half or even a third depending on the size of the installation, demanding a minimum of 250 psf (12.0 KN/m²) floor capacity while microfiche usually constitutes a concentrated load covered by the listed 1,000 lbs (4.45 KN).

The average shelf capacity based on volumes per linear foot is in agreement with the code-specified minimum uniformly distributed live load for stack areas. Legal volumes with a spine width of 2 in (5 cm) and bound periodicals with a spine width of 1¾ in (4 cm) usually constitute the highest shelving loads due to their large format, uniform size and density. The same DFS unit used for the calculation of the load-­accepting area would accommodate 252 volumes, equivalent of about 30 cu ft (0.84 m³), and thus staying well below 150 psf (7.2 KN/m²). Eight volumes per linear foot (0.3 m) for textbooks, fiction or classics and a working capacity of 80 % resulting in 336 volumes[8] per DFS are assumed standard for common stack areas. Despite the high number of volumes, typical variations in book formats and weight assure that the permissible live load is within limits.

Both the forward high reach of the average user and the advantage of organizing oversized volumes within the running order of the collection (requiring greater spacing of the shelves) typically limits library ranges for public access to a maximum of six shelves. Maintaining sight lines in collection areas may reduce the height of shelving even further. Due to the reduced number of tiers paired with the decreased book volume density per DFS shelf to provide for organizational flexibility and future growth of the collection, the load capacity thresholds stipulated by code are usually not reached.

Structural Grid

The structural bay size is a direct function of the stack layout and overall structural economy. Again based on a conventional linear shelving concept, a 2 ft (0.61) wide double-faced shelving range and a 3 ft (0.91 m) wide aisle would generate a grid dimension perpendicular to the rows of shelving derived from multiples of 5 ft (1.52 m). The preferred aisle width referenced in the Americans with Disabilities Act (ADA)[9] is 3 ft 6 in (1.07 m),[10] the minimum required width is 3 ft (0.91 m), requiring a 40 in (1.02 m) clear perimeter aisle, and often aisle widths of 5 ft (1.52 m), equivalent to the turning radius of a wheelchair, are requested for circulation and browsing. The grid dimension parallel to the rows is a multitude of the 3 ft (0.91 m) shelf unit plus the space required for the column. Economical structural grid dimensions vary between 25 ft (7.62 m) and 30 ft (9.14 m). A grid of 30 ft (9.14 m) is preferred since it allows the organization of five 2 ft (0.61 m) wide double-faced shelving ranges separated by 4 ft (1.22 m) wide aisles or six double-faced shelving ranges separated by 3 ft (0.91 m) aisles respectively.

3 Calculation of load accepting area per DFS for typical library stack area based on 2 ft (0.61 m) deep DFS units and 3 ft (0.91 m) wide aisles. The DFS load of 2,275 lbs (10.12 kN) is uniformly distributed on 15 sq ft (1.39 m²)

Structural Economy

Accommodating the book collection will always remain the primary aspect of library construction. Total number of volumes, book formats, collection organization, anti­cipated growth and overall flexibility as well as floor area efficiency and the density, width and height of stacks – wider aisles encourage browsing while lower shelves increase overview and consequently heighten safety within the collection area – constitute some of the parameters impacting the selection of the structural concept.

4 Typical grid layouts in the imperial system: 25 × 24 ft and 30 × 30 ft column grid based on 3 ft aisle and 2 ft deep DFS units accommodating five or six DFS ranges respectively (left); 30 × 30 ft column grid based on 4 ft aisle and 2 ft deep DFS units accommodating five DFS ranges (right)

5 Typical grid layouts in the metric system: 7.2 × 7.2 m and 9 × 9 m column grid based on 1.2 m aisle and 60 cm deep DFS units accommodating four or five ranges respectively (left); 9 × 9 m column grid based on 0.9 m aisle and 60 cm deep DFS units accommodating six ranges. The shelving grid provides the possibility of a 12 m span accommodating 8 ranges. Generally, the column spacing along the centerline of the ranges is more flexible.

Various strategies to achieve structural economy and spatial quality exist. The Jacob-und-Wilhelm-Grimm-Zentrum (fig. 6) in Berlin (Max Dudler, 2009) submits to an orthogonal grid and celebrates the book as the basic construction module. DFS units, 0.6 m wide and separated by 0.9 m aisles, arranged perpendicular to the perimeter of a typical library floor plate, generate the 1.5 m wide sub-grid defining the placement of structural columns, load-bearing walls and shear walls, the rhythm of the building envelope, the sequence of the intermediate non-load-bearing interior pilasters, the structural grid of the skylight above the central reading room and the width of the custom-made worktables.

6 Jacob-und-Wilhelm-Grimm-Zentrum Berlin, Max Dudler, 2009. The sub-grid of 1.5 m is reflected in the rhythm of the envelope fluctuating between window openings aligned with the stack aisles and sandstone wall panels. Along the perimeter load-bearing columns are spaced 6 m apart. The organization of the DFS units on a 1.5 m wide sub-grid generates the library’s design concept. Depending on load requirements column spans alternate between 12 m, 6 m and 4.5 m in the stack bays.

Toyo Ito’s 2007 Tama Art University Library in Tokyo (fig. 7) pursues a very different approach: it is a library building with a seemingly random grid composed of intersecting elliptical arches with varying spans. The sloped entry floor plate and the tilted roof plane require the steady increase of the rise of the arches. Constructed of concrete-clad steel plates, the spring points of the arches are placed at the intersections or rhamphoid cusps of alternating concave and convex curves. Low, three shelves high, seamlessly constructed, meandering double-faced stacks are interwoven with the archways conveying the curvature of the structure into the horizontality of the floor plate.

7 Tama Art University Library, Tokyo, Toyo Ito, 2007. The non-linear structural grid is formed by the confluence of elliptical arches.

Structural Systems

Large spans, open uninterrupted floor plates, multiple stories and the requirement for high load capacities promote steel and concrete or a combination as the most likely construction materials for a library. Inherent to the various systems are advantages and disadvantages regarding floor assembly depth, structural spans, lateral stability, layout flexibility, vibration control, system integration, fire protection and aesthetics. Furthermore, striving for structural economy demands attention to constructability, construction schedule and market parameters like the availability of technical know-how, labor force and lead times.

08 Overview of selected gravity load systems and their properties (all dimensions are approximations)

Both structural systems, i.e. steel and concrete, as well as their amalgamation allow for highly innovative solutions. fig. 9 will provide an overview of selected structural systems. Maximum span requirements dictate the selection of the structural system, and consequently determine the depth of the floor assembly and the achievable floor-to-ceiling height as a function of the interface between building systems and the superstructure.

09 Advantages and disadvantages of frequently used structural systems

Structural Innovations

Interdisciplinary dialogue, building information modeling, system computational analysis and the employment of an integrated project delivery concept with the objective to share knowledge and responsibilities throughout all stages of the design and construction process allow the realization of structures outside the conventional realm of library construction. The structural framework becomes an intrinsic part of the building’s spatial qualities and material expression as documented in Toyo Ito’s Sendai Mediatheque, the Seattle Central Library by OMA and the Rolex Center by SANAA. All three libraries challenge the common typology by radically reinterpreting the structural super-frame.

Sendai Mediatheque: Tubular Towers

The multi-story Sendai Mediatheque (Toyo Ito, structural design Mutsuro Sasaki, 2001)is of interest in the structural context (fig. 10) because of its ingenious substitution of a conventional column grid for a complex array of circular towers constructed of a lattice of round tubes. The towers organize the volume in a non-hierarchical fashion both horizontally and vertically without curtailing program flexibility or transparency.

10 Sendai Mediatheque, Toyo Ito, 2001. Ground floor plan (left); The axonometric shows the engagement of the tubular towers with the floor structure (right)

Thirteen round towers varying between 2 and 9 m in diameter placed on a sinuous grid inside a 50 m square footprint constitute the vertical superstructure of the Sendai Mediatheque. Constructed of a multitude of inclined, round steel tubes ranging from 12 to 25 cm in diameter, encompassed at floor levels and between plates by horizontal circular rings of steel, each tower generates a uniquely configured “hyper shell” supporting the gravitational loads of the seven-story library. From plate to plate the circular floor penetrations both shift eccentrically and vary in diameter, forcing the individual steel tubes to directional deviations generating scissor-like juxtapositions and a complex three-dimensional spatial experience. At the same time the meandering pattern of the floor plan is conveyed into the verticality of the structure. Only the four triangulated corner towers are designed to absorb the lateral forces transported in the floor diaphragms. Below grade the truss structure of these towers changes to a ductile rigid frame providing the energy dissipation capacity required by seismic design standards.

A composite floor sandwich constructed of a network of narrowly spaced steel ribs changing in depth and orientation depending on the prevailing load conditions and a 20 cm thick concrete topping slab spans between the ring beams of the tubular towers. The skin of the building is suspended from the slab edges cantilevered beyond the perimeter towers, thus greatly enhancing the transparency and dematerialization of the building by blending the inside with the outside. The net-like structural concept of vertical tubes interconnected with the steel grid of the composite floors eliminates the need for vertical shear walls enabling the realization of flexible program space throughout the media library.

Seattle Central Library: Exoskeleton

The organizational concept of the Seattle Central Library (OMA and LMN Architects, structural design Arup Structural Engineers and Magnusson Klemencic, 2004), comprised of three distinct rectilinear cuboids forming programmatic clusters (termed Assembly, Book Spiral, Headquarter) floating within a multi-faceted transparent skin above a two-story concrete base, demanded a highly complex engineering approach (fig. 11). The basic structural objective to resist gravitational, lateral and seismic forces is partially met through the unconventional employment of building components functioning in both the architectural and structural realm, such as the glass skin engulfing the stacked program elements.

11 Seattle Central Library, OMA, 2004. A diamond-shaped exoskeleton and mullion system support the glass envelope. A 60 m tall rectangular concrete stair tower and mechanical shaft contribute to the support of the floating cuboids.

The multi-story Book Spiral is centered between the cuboids. The dislocation in space of the three rectangular cuboids generates extensive cantilevers. A system of skewed columns along their perimeter counterbalances the cuboids and transfers proportionate gravity loads into the in-situ concrete base levels where the resulting thrust forces are channeled into the floor diaphragms and accepted by concrete columns. On the interior of the rectangular volumes an orthogonal column grid supports the stacked floor plates and connects the three cuboids. A vertical, 60 m tall rectangular concrete core contributes to the support of the floating cuboids and their structural stabilization. Perimeter cross-braced trusses located in the sidewalls of the rectangular volumes resist lateral forces and carry gravitational loads.

The cuboids are woven into a structural skin, an inclined exoskeleton, deviating between 20 and 45 from the horizontal, and extending along the street elevations over the entire height of the building. The diamond grid formed of thousands of individual rhombi constructed of 30.5 cm wide-flange steel members with 1.2 m wide and 2.1 m high diagonals is continuous within the individual facets, bending at horizontal seams. The steel diagrid of the envelope opposes exclusively wind and seismic loads (but not gravity forces) and for this reason was permitted by code to remain unprotected against fire. Structurally the diagrid and the cuboids are connected with slip joints, allowing the skin to stabilize the platforms against lateral loads without absorbing gravitational forces.

At the slanted envelope planes, the lattice-like seismic steel frame directly supports a shallow glass mullion system. Contrary, at the vertical facades due to the lack of such a seismic framework, deep floor-to-floor-spanning mullions sized to withstand lateral curtain wall loads, not only resemble the geometry of the diamond grid but also emulate the profile of the seismic I-beams, thus creating a coherent appearance.

Rolex Learning Center: Ultrathin Free-form Shells

Realizing the Rolex Learning Center in Lausanne (SANAA, structural design SAPS and Bollinger + Grohmann Ingenieure with Walther Mory Maier, general contractor Losinger Marazzi, Lausanne, 2010) as topographic architecture with the intent to create space for a highly experimental learning atmosphere demanded exceptional structural expertise and creativity. (fig. 12) The floor structure of the single-story undulating volume, pierced by 14 curvilinear openings or patios, is comprised of two shallow, irregularly shaped concrete shells of different size and varying curvature; both shells are structurally tied into the roof plate of the basement, congruent in shape with the rectangular 162.5 × 121.5 m perimeter of the Learning Center above.

12 Rolex Learning Center, SANAA, Lausanne, 2010. The large shell under construction. To stabilize the shell slabs, seven arches were embedded into the large shell and four arches into the small shell.

The slender shells, exposed to significant bending moments in addition to the in-plane forces, are strengthened by highly reinforced concrete arches located along the perimeter and in the zones between patios. Seven arches, spanning between 55 and 90 m, are embedded into the large shell, and four arches, with spans between 30 and 40 m, into the small shell. Horizontal outward thrust forces are resisted by post-tensioned cables in the roof slab of the basement. Coplanar with the underside of the shell surfaces, the arches and patio edge beams, designed with greater structural depth than the adjacent slabs, project up to 20 cm above the top of the shell slabs.

Pouring the sloped shell slabs, even in zones up to 15 %, without the need of counter formwork was achieved by adding plastic fibers into the concrete mix, providing the necessary viscosity for the oblique installation while facilitating pumping. The reinforcement steel in the arches, reaching a concentration of up to 470 kg/m3 for the transfer of stresses induced by the bending moments, necessitated the use of bars of up to 50 mm in diameter, to assure space between bars for correct installation and consolidation of the performance concrete. The reinforcement also reduced long-term deflections due to creeping and shrinkage, thus minimizing the tolerances required for the execution of the glazing system. The roof plane follows the fluidity of the floor slab. Executed as a membrane roof over wood decking and supported by a 9 × 9 m grid of steel beams with laminated wood filler joists on slender steel columns, it provided sufficient lateral stiffness.

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Originally published in: Nolan Lushington, Wolfgang Rudorf, Liliane Wong, Libraries: A Design Manual, Birkhäuser, 2016.