Detailing & Engineering Of Complex Geometry Copper Panels

This paper will review the key steps for the successful use of copper in both a wide format, a complex geometry, and being built to hold to dimensions while the material was being fabricated and finished off site.

Context and Envelope Design:

^{Figure #1: Ledger Building. "Photo courtesy of Michel Rojkind and Callaghan Horiuchi + Marlon Blackwell Architects}”

The Ledger is a 6-Story office building in Bentonville, Arkansas. What makes it unique is a façade comprised of a continuous copper ribbon that integrates the levels of the building with a ramp that starts at ground level and continues all the way to the top of the building. The ramp was designed wide enough for both pedestrians and bicyclists to make their journey up the building with plazas and planters along its path. The ramp is bordered with a composition of wide format copper panels that also separates the floors of the building. The bikeable building is now a landmark for Bentonville, Arkansas that considers itself the mountain biking capital of the world.

^{Figure #2: Building Massing “Rendering courtesy of Michel Rojkind and Callaghan Horiuchi + Marlon Blackwell Architects”}

Ledger was a joint design effort between Marlon Blackwell Architects, Michel Rojkind, and Callaghan Horiuchi Architects. The project's initial design started in 2019. The building envelope went through a number of design iterations from a series of vertical fins as solar shading elements to alternating solid concrete panels with interspersed transparent glass panels. Ultimately, the final design proposed using only glass integrated with a dark purple-black metal horizontal ribbon. The overall intent was for the building to have a “disappearing” effect. The façade had to blend with the surrounding local architecture of Bentonville, famous for its dark red brick buildings.

^{Figure #3: Design iterations: Vertical wood fins and Concrete Panels “Rendering courtesy of Michel Rojkind and Callaghan Horiuchi + Marlon Blackwell Architects”}

At first, with the idea to bring the pedestrian or bicyclist from the street all the way to the 6th floor, the architects introduced an exterior ramp with continuous planters along the guardrail. Deep wedge-shaped planters were intended to be clad with weathering steel in a dialogue with the typical red brick facades of the city.

^{Figure #4: Design iterations – Corten with continuous planters “Rendering courtesy of Michel Rojkind and Callaghan Horiuchi + Marlon Blackwell Architects”}

As the design developed, the deep wedged planters were limited to only the south and north elevations of the building. Additionally, zoning setbacks along the east and west elevations, resulted in the tapering geometry of what would become metal panels along the ramp guardrail and the horizontal ribbons along the west elevation.

^{Figure #5: Final design: Corten with planters at North and South Elevations only. Panels are tapered along East and West Elevations. “Rendering courtesy of Michel Rojkind and Callaghan Horiuchi + Marlon Blackwell Architects”}

Material Selection:

Marlon Blackwell Architects sought out a living material that would evolve and age with time. Copper was ultimately selected. The dry environment in Bentonville was ideal for transforming the initial brilliant and orange finish of copper as it patinaed to a purple-black luster, complementing the local vernacular of dark red brick of the surrounding buildings.

Rather than using raw copper and waiting for time and atmosphere to patina the finish, a natural finish developed by DMD, a Seattle-based fabricator specializing in natural metal cladding, was selected. This initial finish protected the panels during installation and allowed for a copper with a natural finish already in place for the building’s opening day.

To generate this natural finish, the formed copper metal panels are cycled through a continuous series of mechanical abrasion, chemical etching / patination, and timed weathering cycles. This is performed cyclically until all of the surfaces develop the specified finish, a golden espresso brown copper shade for this project. At this point, the panels are buffed, and surfaces are blocked and protected for crating and shipping.

As part of the overall design, the architect anticipated the aging of the copper and the change of its color when designing the adjacent materials. Soffits, handrails, and aluminum mullions were specified with darker colors to blend better with the final look of the metal panels.

^{Figure #6: Initial patina color of the copper panels just after installation. “Photo courtesy of Modern Steel Magazine, Issue of October 2023”}

^{Figure #7: Mature patina color. “Photo courtesy of Modern Steel Magazine, Issue of October 2023”}

Understanding the Geometry / the Faceted Façade:

The entire building was designed based on a 5’x10’ module dictated by an optimal glazing unit size. The metal panels would end up using a 2’-6” module. This size worked well as it is within a typical copper coil width, thus both avoiding waste and working within material limits which aided the project budget. The joints of the copper were set to align with the mullions, and with the concrete joints.

^{Figure #8: The 5’ module alignment among concrete, mullions, guardrails, and copper panels. “Photo courtesy of Modern Steel Magazine, Issue of October 2023”}

The architect’s design created a continuous ribbon of panels that varied from a deep wedge at the integral planter to a vertical plane at the other end of the façade, and vice versa. In addition, the top and bottom of the ribbon had horizontal returns to engage with either a curtain wall, guardrail, or soffit. To add a third level of complexity, the system's depth varied along its width creating tapered fascias. Therefore, the geometry of the continuous ribbon varied in depth, shape, and height from floor to floor and from one elevation of the building to the next. In cross section, the shapes had three to five planes intersecting. To better understand the transforming geometry of the façade the panels were color-coded based on shapes: vertical fascia, sloped fascia, and tapered fascia.

^{Figure #9: Copper panels’ geometries. “Rendering by Neme Design Studio”}

Developing the attachment System

The initial approach for the copper façade was to use a typical flat lock system. While developing the design, the system was adapted from a traditional flatlock system in order to accommodate the wide format and lengths of the copper panels.

A typical flat lock panel system consists of thin sheet metal panels with hemmed edges that integrate by hooking onto a series of clips. The clips connect the panels to the substructure. The overlap of one panel over the next with the clip integrated results in a system with concealed attachments. To accomplish this, the clips are installed on one side of the panel, holding the panel in place. The next panel in sequence hooks over both the clips and the previously installed panel. This process continues along the façade and sets up a sequence of installation. The clips can be oriented vertically or horizontally based on the system's orientation. The image below is a plan view of a vertically oriented system. (See Figure #)

Though it is called a flatlock system, the panels once installed, are not flat. One panel hooking over the next causes an overlap and places each panel at a slight angle. When viewing an installed flat lock system, the final result is a series of overlapping panels in sequential order akin to fish scales. For example, a typical rectangular panel has two sides folded inward for the hidden connection and two sides folded outward to receive the clips and then to be covered by the successive panel. This overlap is both visible to the discerning eye and made more pronounced with sunlight.

^{Figure #10: Iso view and enlarged section detail of a typical flat lock attachment. “Rendering by Neme Design Studio”}

A flatlock system is usually designed using thin material, typically 22 GA. This provides flexibility in shaping and bending the material, though, on the other hand, the panels could have an oil canning visual effect. Oil Canning is when panels appear to have a visible wavy distortion.

^{Figure #11: Peter Das Museum by Snohetta and Pritzker Pavilion by Frank Gehry}

Flat lock systems typically use a combination of narrow panels and matte finishes to help reduce visible distortions inherent to a thin sheet metal material. Moreover, in a typical installation the use of narrow panels minimizes the load on each panel which allows for the use of light gauge clips to attach the system to its supporting structure. A typical system utilizes common off-the-shelf clips such as a Bermuda clip that is both affordable and readily accessible. The Bermuda Clip has the integral shape to hook onto one panel and allow for the installation of the successive panel for a sequential installation.

^{Figure #12: Standard Bermuda Clip and hemmed metal panel. “Photo and Rendering courtesy of AMSI Supply”}

Panel attachment detailing and engineering:

The panels for the Ledger project are 2’-6” wide with varying heights starting at 3’-0” along the east and west elevations below the glazing, and as large as 5’-6” along the north and south elevations at the planters.

^{Figure #13: Iso view of different types of panels with varying heights. “Rendering by Neme Design Studio”}

The nonconventional large panel sizes resulted in higher loads at the connection points. The first step was to determine the basis of the design to attach the panels. Based on the project engineering design criteria, the wind suction force was the greatest and was used as the basis for the calculations. - 29.86 PSF. The maximum deflection for the panel under wind pressure and suction was calculated to be less than L/150.

To be conservative, the engineering approach assumed that only the panel's vertical edges are pinned using the clips to attach them to rigid supports. This approach considers the panel flexible, similar to a cable/chain, with a horizontal span of 2’-6”, equal to the span between the two clips attaching the panel along its vertical edges.

^{Figure #14: Illustration of engineering analysis of wind impact and panel attachment concept. “Graphics by Neme Design Studio”}

To resist the forces of the wind, the clips installed along the panel's vertical edges are under both tension and flexural due to pressure and suction forces from the wind loads. These forces will try to bend the clip open. The 20 gauge industry-standard Bermuda clip could not meet the engineering criteria to resist these forces. The calculations led to the use of a custom clip in stainless steel. The main difference was in gauge and length from the standard clip. The project-specific clip was to be 7” long and 18 gauge versus the typical Bermuda clip that is 1” long and 20 gauge.

^{Figure #15: (Left) 20 ga Bermuda Clip, “Photo Courtesy of AMSI Supply” vs (Right) project Custom Clip “Illustration by Neme Design Studio”}

Addition to clip sizing, the spacing of the clips needed to be 12” everywhere and 9” in zone 5, the corner zones, where the wind forces were the highest.

^{Figure #16: Section Detail at Typical Panel showing clip spacing. “Illustrations by Neme Design Studio”}

# 10 screw with a pancake head was selected to allow minimum intrusion into the panel's surface. 2 screws fasten the custom clip to light gauge steel members. These members attach to thermally broken horizontal clips that transfer the loads to the building structure.

Adapting the panel geometry:

After the basic engineering of the panels was accomplished, the next step was to study the panel geometry for installation. The vertical copper panel at the curtain wall had 5 bent edges for example and the curb panel had 4 edges.

^{Figure #17: Initial panels with 4 to 5 edges. “Illustration By Neme Design Studio”}

Tabletop mockups demonstrated to the team that having more than two bends along one edge prevented the panels from interlocking correctly.

^{Figure #18: Photo of Curb Panel Tabletop Mock Up showing the issue with interlocking panels that have more than 2 bent edges. “Photo By Neme Design Studio”}

Therefore, the panels, which sometimes had up to four bends, were broken down to be limited to two bends along any given edge.

^{Figure #19: At the top, panels before mock up showing multiple bends. At the bottom, same panels after being redesigned to have only two bends each “Illustrations by Neme Design Studio”}

The new panel assemblies would consist of the following:

• A narrow soffit piece, 5 feet long, that gets fastened to the light gauge metal behind.
• A horizontal 20 inch stainless steel cleat, that the copper fascia panel hooks on along the bottom edge.
• The copper fascia panel, that would be fastened to the light gauge metal with a concealed fastener when a coping covers it.
• Or, it would bend and get tucked under a 5-foot-long “receiver” mounted along the curtain wall sill.
• The backer rod and silicone joint between the receiver and the glass is for aesthetics only. The water drainage plane is located behind and below.

The result is a panel that is hooked on a cleat along the bottom edge and a custom clip along the vertical edge. The top edge is either fastened with a concealed screw or tucked under a receiver.

^{Figure #20: To the left, J soffit, cleat, and bottom of fascia panel detail. To the right, Top of panel and receiver detail at glazing “Illustrations by Neme Design Studio”}

Some of the more complex panels have a tapering effect and are located along the east and west long elevations. These had to be split into two panels each. A continuous reveal and a cleat along the tapered edge allowed the fabrication and installation of these panels.

^{Figure #21: Different sizes of tapered panels varying in depth and in taper location along to extreme ends of the west elevation “Illustrations by Neme Design Studio”}

The corner panels were particularly complex to build. The corners were assembled from multiple parts to overcome the limitations of coil width and the flatlock attachment system. These were the only parts that would be fabricated based on field measurements. Additionally, this allowed the team a level of tolerance to accommodate for field conditions as two sides came together to a complex geometry.

^{Figure #22: Corner panel assembly “Illustrations by Neme Design Studio”}

Although breaking up the panels to reduce the number of bent edges per panel increased the total number of parts, this approach helped the project fabrication and installation. It simplified the fabrication of each part and provided the installer with more flexibility in assembling the parts on site.

Physical Mock Up

A full size mock up was built before the full approval of the shop drawings. It allowed the team to test the fabrication and installation of different panels, coordinate the alignment with the concrete and the glass, and let the architect comment on the assembly details. The most important lesson learned on the mock up was locating the clips and determining the size of the hemmed edges to provide flexibility during installation and reduce the build up of material at areas of overlap.

Although the panel was set to align with the 5’ module of the windows, the actual panels had to overlap. The folded edge was the datum point that needed to center. Therefore, the panels, clips and metal framing had to be located to account for the offset between finished edge and point of connection.

^{Figure #23: “Photos of physical Mock Up courtesy of Harness Roofing”}

^{Figure #24: panel and clip attachment to secondary framing “Illustrations by Neme Design Studio”}

Developing the 3d Model for shop drawings and fabrication layouts:

As in all complicated façade projects, there needed to be a high degree of sophistication for exact panel shapes and alignments built into the system based on the use of a 3D model. Revit was tested as a tool to generate not only the 3D model for the design phase and for the shop drawings panel scope documentation, but for fabrication drawings production as well.

Revit is a proven tool to document an architectural project and to create shop drawings. However, the software is not set up to create fabrication layouts. A fabrication drawing requires each component to be assigned its own sheet with corresponding views of the component to instruct the machine operator on the final product. Additionally, a complimentary digital file is utilized to direct the CNC machine to perform its operation based on a flattened layout representation of the panel. To create these, the industry standard is to export the model and the drawings from Revit to another platform such as Autocad, Inventor, or Solidworks and generate the necessary views using those files. For this project, Revit was tested as the single tool to create the fabrication drawings as well. The goal was to reduce the amount of time spent exporting models between different platforms which is usually time consuming and results in the loss of data associated with the models.

^{Figure #25: Typical fabrication drawings layout showing all views necessary for the fabrication of a single panel. Layout generated in Revit. “Illustrations by Neme Design Studio”}

As a result of this effort a single Revit family was developed that could generate every type of panel assembly for the project. Through the use of Revit families all panels were generated with dimensions and quantities available to be extracted for fabrication. A panel assembly could consist of a combination, and sometimes all, of the following project specific components starting from the bottom of the system to the top:

• J shaped soffit
• Fascia, with or without top horizontal return
• Coping
• Top horizontal receiver

The fascia and coping components were the most complicated components. Almost each panel had unique dimensions with hems and folded returns to allow them to interlock as part of the flat lock system. While, the J-shaped soffit and top receiver had no hemmed edges and were all identical.

In the end, the final Revit family required 130 parameters to cover all the different conditions. These parameters were split into 3 main categories:

• Panel Shapes and Assemblies: This allowed for control of the components of each panel assembly to include the J soffit, a coping, or a receive as needed.
• Graphic parameters: to generate a graphic representation of the panel to be used in creating fabrication layouts: Front, Top, Section, and Isometric Views. It showed the hems' direction and sizes.
• Dimension parameters: These use complex formulas to generate the overall dimensions of each panel in a flat layout and its formed layout. This information was then extracted into an Excel spreadsheet that was used for nesting, the process of laying out cutting patterns to minimize the raw material waste. The dimensions were used for the CNC machining and later to QC the final sizes of the fabricated panels.

^{Figure #26: Two very different panels generated with the same Revit family: Panel to the left has receiver, a vertical fascia with horizontal return, and a 90 degree J soffit. Panel to the right has a receiver, a coping, a sloped fascia, and a sloped J soffit. “Illustrations by Neme Design Studio”}

^{Figure #27: Extract from the spreadsheet generated using parameters in the Revit family. It identifies panel quantities and unrolled dimensions, which will be used for QC and for nesting the panels on the copper coil. “Illustrations by Neme Design Studio”}

Using Revit to develop the properties of the panels for the façade had two other advantages: tagging the assembly and each component. This allowed the panels to be identified within the installation set elevations and exported with their quantities in an Excel spreadsheet. Additionally, the Revit family that was used to generate the panels had key parameters that allowed the panels to be assigned specific colors. This function was found to be effective in a few different ways that were an added benefit. The installation drawings used colors to identify panels per type and helped the installer to identify their location on the building. Assembly and shipping coordination documentation used colors to identify the sequencing of production, crating, and shipping. All of which was inherent to the Revit family which made it easy to set up, track and verify throughout the process.

^{Figure #28: To the left, panels are identified for four shipping sequences. To the right, panels are identified by type for installation. “Illustrations by Neme Design Studio”}

^{Figure #29: Extract from installation set showing panel type, assembly components and tags. “Illustrations by Neme Design Studio”}

To generate fabrication drawings, Dynamo scripts were developed to orient the 5000 panels in the correct order in the model space, generate sheets, create views of each panel, and place them on the sheets. Two manual steps still needed to be performed: adding dimensions to the panels in all views and creating flat layouts for DXF export. A rigorous quality control process was still required to make sure the panel flat layout dimensions were correct.

The installation:

The overall assembly of the building for the copper panels was designed as a rainscreen system mounted over a watertight surface behind a secondary attachment system consisting of thermally broken clips and cold-formed metal framing. As the building became watertight with mullions, glazing, and weather resistive barrier over sheathing in place, the framing and panels could follow the sequence of construction in quick succession. Designing the secondary structure using cold formed metal parts provided much-needed flexibility during installation. Both secondary framing and copper panels were installed using boom lifts.

^{Figure #30: Flat lock attachments – Exploded Axon – Exterior. “Illustration courtesy of Studio NYL”}

^{Figure #31: Flat lock attachments – Exploded Axon – Interior. “Illustration courtesy of Studio NYL”}

To locate the framing, the elevations along with section details with reference dimensions points were used to coordinate between the glazing and the copper panels installers. The sections drawings included all variables of top and bottom conditions, be it mullions, soffits or guardrails, as well as the transitional panels that could start either with a sloped soffit panel transitioning to a vertical fascia panel, or vertical soffit panel transitioning to a sloped fascia panel.

^{Figure #32: Dimension points clarifying the relationship with adjacent systems. “Illustrations by Neme Design Studio”}

The corner panels were the only panels not fully prefabricated and shipped on-site. The fabricator provided prefinished sheets of copper and CAD files of flattened layouts based on dimensions from the model. As these panels were the last to be installed for each level, the installer was able to update their dimensions based on field conditions to make up for installation tolerance, then cut the sheet metal and fold it into shape.

In the end, the installer reported a high success rate of panels that were prefabricated with hold-to dimensions. Only a minimal amount of panels needed to be made on-site. This count would be in the single digits.

Conclusion

The Ledger project demonstrates that a standard type of installation can be modified to allow for complex shapes and sizes. The use of 3D software to model the forms is complemented by the use of physical mock ups to test the ability of shapes to overlap and to coordinate the interface with adjacent trades. The table top mock up and full size mock up complimented each other as a method to inform and receive feedback from the entire project team for a one off design and a custom installation.

Furthermore, it is possible to build large size projects based on hold to dimensions allowing panels to be fabricated early in construction. To do this requires a higher level of involvement from all stakeholders including the architect, the GC, the installer, and the fabricator. The essential aspects for the overall success of the project was to have shop drawings and installation drawings having clear dimension points that adjacent trades can refer from to check and confirm dimensions along with a built in flexibility in the system to accommodate for installation tolerances. In this project, using light gauge parts for sub framing installed in unison with the panels, increasing the hem for the flat lock system, and allowing for the on site fabrication of certain panels combined to allow the flexibility required for a successful outcome.

Special thanks to Ethan Kaplan, Marlon Blackwell Architects, James Russel, Harness roofing, Greg Kimm, Dissimilar metal design, Mahmoud Zadeh, RJC engineers, and Nicholette Ward, Neme Design Studio, for their contribution to this paper.

We also recognize the contributions of Marco Broccardo for the insight on detailing and engineering of the panels' attachment system and Elan Lipson for the development of the revit model and families. Their efforts were crucial to the success of this project.

Project Team involved with exterior enclosure scope:

Design Architects: Marlon Blackwell Architects, Michel Rojkind, and Callaghan Horiuchi Architects

Design Team façade consultant: Studio NYL

General Contractor: Nabohlz Construction

Installation Team: Harness Roofing / JAMA engineers

Fabrication Team: Dissimilar Metal Design / Neme Design Studio, RJC engineers