May 19, 2020
Authored by: Howard D. Hopper, FPE, Global Regulatory Services Manager
3D printing of physical objects is no longer a futuristic concept. This technique is being used extensively in homes, businesses and industrial applications. 3D printing is also being used to construct building elements and structures in locations around the world. If you have not seen how buildings can be constructed using 3D printing, simply search the internet for 3D printed building videos.
To fully embrace this new technology, stakeholders – including code authorities – need confidence that 3D printed structures are safe, code compliant, durable and can withstand the elements for their anticipated lifetime. In addition, variations in 3D printing materials and fabrication processes that can significantly impact a structure’s physical characteristics must be addressed.
UL takes the lead in addressing challenges
UL has been researching safety considerations of 3D printing for more than five years. We determined that, unlike traditional manufacturing techniques, the 3D printing process introduces variability that significantly impacts properties and performance based on how products are printed. This research, which initially focused on plastic materials, led to the development of the UL Blue Card program in 2016.
In 2017, we began examining safety, durability and code compliance factors associated with 3D printed building construction. This research formed the basis for a 3D printed building construction evaluation methodology documented in UL 3401, Outline of Investigation for 3D Printed Building Construction. This methodology determines that a fabricator’s 3D printing equipment, additive manufacturing material (AMM) and fabrication process will consistently produce building elements with properties that don’t vary from the samples initially tested.
During the development of UL 3401, UL worked with building authorities to obtain their input on the scope of the evaluation program. Additionally, the program was discussed at two International Code Council (ICC) Major Jurisdiction Committee meetings to make sure building authority concerns were addressed.
Code compliance and acceptance challenges
Builders need to demonstrate that the 3D printed structures comply with applicable building or residential codes to gain building code authority approval for 3D printed construction in jurisdictions. Code compliance presents a challenge for both a builder and code authority because building and residential codes currently lack prescriptive requirements for 3D printed construction. Even code requirements for concrete construction are not directly applicable for cementitious-based 3D printed construction, since mortar and cement-based fabrication, printed in a layer-upon-layer fashion without forming members, are not specifically covered by the concrete standards referenced in the model codes.
Since there are no prescriptive code requirements for 3D printed construction, code authorities must consider each project under the alternate materials and methods provisions in the code for their evaluation and approval. This approach allows them to approve 3D printed building constructions, provided they are shown to comply with the intent of the code provisions and provide the code prescribed quality, strength, effectiveness, fire resistance, durability and level of safety. Using testing and evaluation data from standards such as UL 3401 is one recognized method whereby a code authority can determine equivalent code compliance for a 3D printed building.
UL 3401 evaluation fills in the gaps
UL 3401 covers the evaluation of building structures and assemblies such as panels, walls, partitions, floor-ceilings, roofs, columns and beams fabricated using an additive manufacturing or 3D printing process. The UL 3401 evaluation produces the technical information report needed to determine if a 3D printed building element complies with a given building code. The evaluation will also document compliance with performance (test) standards referenced in the code.
A UL 3401 evaluation determines that the key production elements adequately and consistently produce structures with properties equivalent to the 3D printed samples initially tested. These elements include:
- 3D printing equipment
- Fabrication process
- Additive manufacturing materials (AMM)
- Quality control procedures
- Production records
The evaluation covers properties such as:
- Mechanical properties
- Fire performance
- Vapor, air and water barriers
- Thermal insulation
- Indoor air quality
- Durability, integrity, and performance before and after environmental exposure conditions
The 3D printing production process is documented in a Fabrication Process Description report and referenced in the Report of Findings.
Testing of 3D printed samples is required to determine compliance with referenced standards in building code, such as UL 723 (surface burning characteristics, UL 263 (fire resistance), ASTM E331 (water barrier), ASTM C1363 (thermal performance) and other standards.
In addition to testing required by the building code, UL 3401 includes requirements for material property and performance testing, both before and after environmental conditioning, to provide technical data on the durability of the 3D printed construction. Environmental conditioning includes UV exposure, water immersion and freeze-thaw cycling.
Because test performance can vary depending on several production factors, test samples are printed using the documented 3D printing fabrication process and associated 3D printing material (AMM).
Report of Findings
The UL 3401 evaluation produces a Report of Findings intended for use by designers and code authorities. This report describes the building element construction covered by the report, and identifies the fabrication process, 3D printing equipment and AMM used to produce the printed structure. It also documents any ratings, material properties, and material performance characteristics established by tests. The Report of Findings is provided to the sponsor of the evaluation, who can include it with required plan review construction documents necessary for the permitting process.
Code authority recognition
Thanks to public testimony provided by several building officials at ICC code development hearings, the 2021 edition of the International Residential Code (IRC) includes an adoptable appendix on 3D printed building construction. It requires buildings and structures fabricated in whole or in part using 3D printed construction techniques to be designed, constructed and inspected in accordance with UL 3401. Having these requirements documented in an nationally recognized code provides builders and code authorities with a sound technical basis for designing, fabricating and approving 3D printed construction.