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Unique structural sculptures in public spaces fulfill a demand for art, city beautification, memorials, etc., as well as to meet the desire for more geometric forms and materials.
Bringing creativity to life, however, requires a design approach that is more complex than the classic load-bearing capacity of a structure. It expands into issues including load application, structural response, and performance expectations. However, specific guidance for the structural design of sculptures is lacking.
Examples include new materials or systems with little to no prior data in structural use, methods of applying loads to non-standard geometries, how humans might interact with the sculpture (whether intentional to the art or not) with defining limit state design approaches that are applicable, and approaches of communicating expectations with Clients for the behavior of unique non-standard structural sculptures.
Most importantly, if the structural engineer is to contribute to the success of the artwork, they must understand the artist’s creative intentions to create a material object in space that has not existed before (and may never again). This is the antithesis of classic engineering education and examination based on complete provided information and a singular correct answer. You are in the land of the unknown, assigned to make it real!
As every artwork is unique, there cannot be a singular checklist that will suffice, but sculpture projects in general face some or all the issues discussed here. These guidelines are a starting place for future engineers in this creative realm—providing the flexibility required to truly encapsulate the very artistic spirit you are working to protect.
When Sculptures Need Structural Engineers
A common question for structural engineers about sculpture design is, “You’re designing the plinth for the sculpture, right?” While the authors have certainly done this for both the standard monolithic concrete base and for “smarter” plinths such as an isolation table for slender sculptures in high seismic zones (Fig. 1), the focus of this article is the design of the sculpture itself, the heart of the artist’s expression.
Another common question: “Is there a minimum size when you need a structural engineer for a sculpture?” The answer is “no.” A large sculpture may be “typical” whereas a small sculpture may be unique and of higher risk to the public.
In general, a structural engineer is required when:
- Someone has explicitly required a structural engineer such as: the owner asks based on where it’s being installed; a code requirement triggers the need due to the scale of the installation or local loads; or a building official says the installation needs an engineer.
- Life safety: When the location and/or premise of the particular installation could present a risk to human life such as in a large and/or very public place. For example, the “Heart of Hearts” sculpture, which was located in Times Square NYC over Valentine’s Day in 2015, a busy location where a lot of people interacted with the installation (Fig. 2).
- Something is unique about the installation. This category can cover a large variety of situations, and some examples include: 1) The geometry is unique so understanding the structural behavior and necessary sizes and connections is not intuitive; 2) the material(s) intended for the sculpture may either be new to the artist, new to the application, or generally have limited industry guidance for the intended application. These limitations often lead to the need for laboratory testing and more careful performance considerations; and 3)The artist might be looking for (or looking to avoid) a particular type of behavior that requires some analysis or structural consideration.
Materials
The Ashby plot in Figure 3 shows the range of material types plotted as Youngs Modulus vs. Strength. It is a reminder of the broad world of material options when based only on their material properties. Given the size and specificity of unique sculpture projects, the full range of material options is available and, when considered with combinations of complementary materials and fabrication processes, opens a world of possibilities.
The authors have worked on projects using robotically milled stone with and without GRP rod reinforcement, 3D-printed glass, 3D-printed titanium, 3D-printed concrete (post-stressed with steel tendons), bamboo with steel rods, double-curved corian surfaces, glass and UV-cured adhesive, rigid foam, inflatable membranes etc.
As engineering design is based on data, new materials (or new combinations of materials) require physical testing. Establishing the extent of testing, acceptable standard deviation in the results, and an appropriate safety factor requires significant engineering judgment given the specific use case of the sculpture while managing budget constraints given the data is required only for a single project.
The authors have established the following approach, with the help of an in-house materials testing lab and materials specialists on staff:
- Given the specifics of the sculpture, define the highest risks associated with the least known material information.
- Determine the appropriate tests to address the above and with what test sample size(s).
- Consult appropriate ASTM standards but also allow for flexibility in the approach to testing from the standards as needed by the specific installation.
- Determine a number of tests to perform either based on the referenced ASTM standards or based on past experience with the material type and test.
- Determine what constitutes “passing” and “failing” the test. This is often dictated by the referenced ASTM standards but, when none are applicable, we reference the International Building Code (IBC) section on Preconstruction Load Tests, which recommends a Factor of Safety of 2.5 (Section 1709 in IBC 2021). The resulting magnitude of the load for this testing can vary greatly, from a couple hundred pounds for a unique but small wood connection to multiple tons for a compression test.
- Observe the testing in person, specifically the failure mechanism, as this can produce a reassessment of the defined process.
Inviting the artist and/or client to witness the physical tests or sharing video of the results afterwards becomes a critical moment to manage expectations as, while any material can be tested to failure, many of the people involved have never seen actual material failure. They can become concerned (or better educated) watching the moment of failure in the technical process that is providing the data required for a safe design.
Two concerns related to material testing data for projects are:
- The testing and data are specific to a singular use and hence there is no mechanism for sharing this with the wider design community who may be using the same or similar materials in a future design.
- Engineers may lack access to material specialists and/or the range of appropriate physical testing equipment such that critical tests are not identified or not completed to the extent sufficient for a safe design.
Sculpture's Behavior and Performance Design Loads
Typically, the primary environmental loading conditions for an outdoor sculpture are snow and wind loads. While the geometry can make load application and determining wind/snow forces in accordance with ASCE difficult, ASCE continues to evolve with additional guidance and research around some of these more unique non-building forms.
A frequent consideration introduced with sculptures is the potential for the installation to be “temporary” with seasonal exhibits, often over the summer months. While this often allows for ignoring snow load or reducing the wind load in accordance with the ASCE guidance, it is common for artwork to be available for purchase, so a temporary summer-only sculpture can become a permanent year-round installation in a different location. Not only is the reduced snow and/or wind load no longer applicable for the permanent installation, but the load requirements of the new site can be far greater than the original location, such as higher wind loads for a sculpture moving to the Florida coast or higher snow loads for a sculpture moving to the White Mountains of New Hampshire. Hence, discussion on design loads and expectations for future use becomes critical to prevent code-required changes the to the structure of the sculpture in the future that might adversely affect the artistic intent.
The loading conditions for an interior sculpture are typically less clear. Situations where the sculpture may have human interaction loads is discussed below, but when sculptures will not be occupied, there is limited code guidance. The authors generally utilize the code suggested partition load of 5psf. While IBC directs this for use in partitions of 6 feet in height, it is a reasonable way to confirm lateral stability of the installation. This also gives some sense of the magnitude of the sculpture’s movement under lateral loads from doors opening and mechanical equipment for discussion with the owner.
Human Interaction
Unique among structural design with sculptures is the range of human interaction which can be fundamental to the performance of the piece or extend to acts of vandalism.
While ASCE addresses considerations for some non-building structures, the bulk of the code provides guidance on objects of the scale of buildings, where forces from a few human occupants are minor and not usually an explicit consideration beyond the consideration of vibrations and the well-known guardrail loads. When considering human interaction with artwork, the artist’s intent and even the fundamental idea of art exemplifies how this interaction element cannot always be defined.
When artwork is accessible at grade, analyze two lateral loads:
- A 30-pound lateral load at 42 inches above grade to know the movement achieved by someone standing adjacent to the sculpture and “giving it a shove.” If there’s enough movement, the person might continue to interact more, which can be discussed with the artist whether this is desirable.
- The equivalent of the single guardrail type point load of 200 pounds to confirm the structure is stable should someone (or a group) run into it. Although the job of a guardrail is to contain people from a fall which may not be applicable to the sculpture at grade, we do this both to meet some aspect of code safety and, as noted, because no other guidance exists.
In one example, a client wanted a vertical cantilever (30-feet tall tower of glass bricks connected with UV-cured adhesive) positioned in a public city park “for two guys coming from the bars on a Saturday night who try to shake it” only for the client to add “wait they might have friends, so you should design it for four guys shaking it.” While ASCE 7 is obviously silent on this particular load case, the concern behind the request is clear: make sure this sculpture is safe from a circumstance we can all imagine happening. This is the essence of the recommended design approach and discussion: given the particular artwork and what the artist wants, what we can all imagine is likely to happen, and what is the Client/Owner willing to accept in terms of performance that should be designed for?
The most common theme related to the above is that of human interaction with the sculpture and expected movement. The “Untitled Sounding Sculpture” by Harry Bertoia in Chicago 1975 is an ethereal piece of vertical steel rods sufficiently slender and closely spaced that a strong breeze causes them to sway and hit each other creating a beautiful sound. This level of slenderness and intended movement has related consequences. Over time, many are no longer perfectly vertical, some seem to be permanently curved and out of plane (therefore not just visually misaligned but less effective at sounding), and any or all of this piece can be easily vandalized and bent.
Seaport by Skylar Tibbits (2020) is similar to Bertoia’s piece but with the intent of sounding only when people move the vertical rods as opposed to a breeze. Initial design and prototyping of the preferred size for movement by children resulted in the ability of an adult being able to permanently damage the rod by leaning back and progressively moving their hands further up the bent rod (a creative P-delta effect). The end result shown in Figure 6 allowed for successful movement and sounding by energetic children with vigorous shaking although it did not protect one of the rods from later being damaged and bent from an accidental impact of a snowplow.
Putting a “Do Not Climb” or equivalent warning sign adjacent to certain sculptures is often discussed.
This addresses the legal “attractive nuisance” issue where someone may climb an interesting sculpture, fall, and file a claim. However, the authors are also conscious of the reality of both installing and maintaining the sculptures and the human desire for a photo regardless of warning signs. Hence in addition to the 200-pound guardrail load, the authors typically design sculptures for a minimum of one person point load of 250 pounds (representative of OSHA’s ladder loads) located anywhere on the sculpture. For larger sculptures, the number of point loads may be increased accordingly, especially when it will take multiple workers to erect and/or maintain the structure. These load considerations are not meant to capture all instances of possible use but build a defined level of safety and stability into the sculpture that might not otherwise be captured.
Communication on Expectations for Movement
As a result of form, material, and exposure to elements or people, sculptures can noticeably move; however, there are no guidelines on what is or is not acceptable or, more relevantly, what the right approach is to discuss what is acceptable with various parties.
Sculptures do not have to meet typical deflection limits for buildings if their form, performance, and materials are different. AISC Design Guide 3 implies that if a structure is only occupied for maintenance, the allowable drift can be h/240 with a maximum of 2 inches under those maintenance lateral forces or the ten-year wind should be considered for the maintenance person to feel “safe.”
However, some sculptures are meant to move in the wind or easily with human interaction. The important discussion here is if others will consider it dangerous when they observe movement.
Proactive approaches during the design phase include:
- Relate wind speed discussions to the Beaufort Scale. The empirical and descriptive nature of the scale keeps the discussion focused on human experience rather than technical aspects. The reference provided at https://www.weather.gov/mfl/beaufort is helpful.
- Use videos taken of items such as suspended lights or window blinds moving inside tall buildings on windy days showing movement is perceptible (and to counteract assumptions that something must “not move”).
- Use videos of everyday outdoor objects vibrating e.g. long cantilever traffic lights.
Use videos of recorded high wind speeds. Showing a client a video of 120mph winds (various exist on YouTube) reminds them that people are not outside looking at a moving sculpture in such conditions.
These approaches were used in discussions with various parties for the carbon-fiber MIT Kendall Pavilion placed on slender cantilever columns and above an active metro station.
Note, in some cases, an alternate path can reduce the concern about movement under large loads, especially predictable loads like snow and wind. Some Owners are willing to modify the artwork by either removing large portions of the structure or by adding stabilizing systems when a large snow or windstorm is expected. However, if the artwork is permanent, it is not guaranteed this requirement will always be adhered to, so consider this method only for temporary installations.
Communication of Responsibility
While the idea of multiple structural engineers on the same installation is not unique (many documents address delegated design responsibilities), the unique nature of design and procurement of various sculptures can also leave communication and responsibility gaps. If the sculpture is interacting with another structure, whether it be a foundation designed by someone else or the sculpture being hung from an existing building, it is important to define early on where the sculpture engineer’s responsibility ends and the supporting structure’s engineer’s responsibility begins, making sure there is a continuous load and responsibility path all the way through. The authors typically physically draw out what this means, color coding who is responsible for what and providing “red dashed lines” where the line shows demarcation of responsibility and the “dashed” nature noting information needs to be exchanged between the associated parties for coordination purposes.
For example, Figure 10 represents a suspended artwork designed by Engineer 1, who gives load information to Engineer 2 to design the supporting hangers, and who gives load information to Engineer 3, the designer of the base building. Also, as per IBC, a registered design professional must be responsible for both the overall structural system behavior and coordination of the parties involved. Some of the roles shown in the diagram may be the same engineer (e.g. suspended hangers and the artwork) or multiple parties. The more parties involved, the higher the need for a diagram to prevent confusion during design. Also, if there is the possibility of interaction between the structural elements then that also must be communicated and addressed.
While this approach is standard on building projects with multiple engineers, if some parties are not experienced with sculptures or non-standard materials, they may push back on engaging as required. Guidelines for the structural design of sculptures would provide a reference document for this process as well.
Conclusion
Helping to realize the artistic vision from a structural and material engineering design approach for sculptures, installations, pavilions, canopies etc., is a joy. These projects hold many unique aspects that extend beyond the typical experience of a structural engineer to ensure both the built sculpture and all parties involved are successful. The majority address human understanding and emotions while providing a technical service. Given how unique sculptures can be and the importance of the structural engineer’s role in their success, this article is a starting point for engineers considering appropriate guidelines for the successful creation of more beautiful art in the public realm. ■
About the Authors
Rebecca H. Lubrano, PE, is a Senior Project Manager with Simpson Gumpertz & Heger in Waltham, MA (rhlubrano@sgh.com).
Paul E. Kassabian, PE, P.Eng., C.Eng., is a Principal with Simpson Gumpertz & Heger in Waltham, MA (pekassabian@sgh.com).