Appendix I - Stone Attachment Methods

Adjustable Stainless Steel Anchors

The OPC specifications call for, ”stone panels, individually supported by adjustable stainless steel anchors, attached to stainless steel support framing and intermittent relieving angles…”.

These individual clips have typically been used for thin and light stone panels that are attached close to the substrate. This method of attachment has been problematic since the start of its use. It relies on two intermittent support points at the bottom edge of the stone. This pressure frequently causes stone fractures at the points where the clips are supporting the stone. The two biggest factors that lead to stone failure using this type of attachment are the concentration of load bearing forces at a limited number of points along the bottom edge of the stone and the stresses that the clip type system transfers back to the stone. If we look at the Obama Presidential Center, a 3,000 pound stone that is 54” wide would transfer a 55.5 pounds per inch load on a full width support rail. This same stone mounted on two 6” wide support clips would have a 250 pounds per inch load and each clip would magically be supporting 1,500 pounds without deflecting. A clip thick enough to support that load with safety factors and wind loads would likely be so thick that the kerf cut in the bottom of the stone would significantly weaken the stone at the point where it needs to be strongest. A thick clip design also introduces the need to trim the back edge of the stone to hide the clip and maintain the joint width.

Lippage is a visible consideration with stone panel construction. The stone attachment system has to provide a way to absorb the irregular and varying dimensions of the building structure so that the individual and adjacent stone panels are perfectly aligned and form the desired impression of the architect's vision. Controlling lippage between stone panels using discrete adjustable clips requires adjusting each individual panel at both the top and bottom horizontal edge. This requirement is exacerbated with the extreme dimensions of the stone panels.

Bent Stainless Steel Anchors

Traditional formed stainless steel has micro cracks around bend areas that cause the bend area to be weaker than the surrounding metal. Due to this a thicker gauge of stainless must be used to compensate for the loss of strength. Micro cracks also lead to embrittlement failure over time. All of these issues are significant but the biggest problem with formed stainless parts are the very large radius bends that do not interface properly with the stone. The extra thickness of the stainless steel and the required extra steps in stone fabrication make this approach extremely expensive for a less than desirable system.

Gridworx Extreme SS Continuous Rail System

One of the guiding design principles for all Gridworx systems is stress free attachment of the stone panels. Continuous rail cladding systems maintain uniform dead load support for the entire stone panel. The Gridworx Extreme SS system delivers enhanced protection from damage attributable to wind loads, gravity, and seismic loads while providing enhanced mitigation of thermal transfer. It further minimizes the transfer of forces from the structure back to the stone. This is accomplished at several layers in our designs and one of the reasons our systems for quarried stone panels provide a continuous support anchor to carry the panel dead load and other forces across the full panel width instead of point loading those forces into small areas. This design also allows for better performance in seismic events and a much easier and safer installation of the stone onto the wall. The Obama Presidential Center presented a challenge to create a system that met our core design values made entirely from 316 stainless steel with exceptional strength for very large stone panels and Chicago conditions. We created the Gridworx Extreme SS System to meet the unique requirements of this project from installation to future decades of trouble free performance. The all 316 stainless steel construction provides longevity and ensures that corrosion will not be an issue. The non sequential installation feature allows the installer to work around any areas that need to be left open for tiebacks or to simply remove and replace any stone in the facade without disturbing surrounding stones if needed. The subframe system created for this system will meet the large cavity depth requirement and eliminate single point failure issues. Scalability of components allows the system to accommodate blast mitigation systems in some areas but not oversize components in other areas. Above all, this system was designed to make the stone attachment safe for the installation team and secure for the life of the system.

Appendix II - Gridworx Adjustable Sub-Frame System

The stone attachment system has to provide a way to absorb the irregular and varying dimensions of the building structure for the stone panels to be aligned and form the desired impression of the architect's vision. There are two ways to get the stone panels in alignment with no uneven edges between panels; adjusting each individual panel using adjustable discrete clips on the top and bottom of the panel or alternately using an adjustable subframe system to absorb the building irregularities and provide flat attachment points for the stone.

Using adjustable clips for each stone creates an installation nightmare for large heavy stones. Picture trying to get a large panel that weighs thousands of pounds into the correct position on a flat wall let alone one that tilts out as on the tower design. You now have four points to lock down while blowing in the wind; 225 feet in the air. You finish one panel, set several more and realize you need to go back and readjust the first because when taken as a group the face of your first stone is slightly off. This miscalculation extrapolates, causing all the surrounding stones to multiply the mistake.

Alternatively, the subframe system’s adjustable brackets are attached directly to the building structure. These brackets secure continuous vertical mullions. This system allows the adjustment points to easily be altered for plumb adjustments. Horizontal cladding channels are then installed on the mullions. This allows the installers to set up a perfectly flat frame system on the wall that can be easily measured and adjusted over a large area; before a single stone is lifted into the air. The structure of the subframe provides an ideal way to place and retain exterior insulation with fewer penetrations. The subframe approach spreads the load of the stone façade evenly over multiple points and has the added benefit of eliminating the chance of a single point failure causing issues.

Appendix III - Laser Fused Stainless Steel

Stainless steel first came into use with the top of the Chrysler building in 1930. Stainless steel was mostly a novelty material for anything outside of the aerospace industry until the 1960’s. The Gateway Arch in St. Louis was completed in 1965 and designed to last for a thousand years with a fully stainless steel exterior...at that time the cost was about 15 times more expensive than regular steel. Demand for stainless steel as a commodity material was low due to high prices. In the 1970’s new manufacturing processes cut the cost of stainless steel in half while making it even stronger. Welding processes and fabrication methods were further refined opening the door to broader use.

The next evolution was laser fusion. This revolutionary technique uses a very focused powerful laser to weld two pieces of stainless steel in a way that creates a weld that is stronger than the parent material. It further requires no filler material.

Over the past 30 years this patented process has been refined, perfected and is now scaled for large production runs. Complex parts are possible by welding together multiple shapes. The end result is a 316 stainless steel part that has sharp corners, very tight tolerances, and no weak spots. Since this type of fabrication does not confine the application to use a one size fits all approach we can optimize the attachment system to fit the project. We can even design the system to support blast mitigation systems as needed without affecting stone attachment or cavity depth. All components of the Gridworx Extreme SS system are sized exactly to the requirements of the project so there are no compromises from design to engineering to installation.

Appendix IV - Risk Mitigation

The Obama Presidential Center project is designated a risk category III according to the initially released specifications. The location of the project is in an area that receives up to ¾” of ice coating on the exterior in winter storms and in an active seismic area; having had seven earthquakes in the last 35 years. These and other definable forces make the very secure attachment of the large stone panels a top consideration.

The unique shape of the tower makes any stone failure a likely “stone release event”, unless the stone is continuously supported. The angle of the wall having the stones tilted out will not allow the dead load to rest on top of the stone below should a fracture occur. The size of the stones would lead to a very large and destructive impact if one were to release. In light of this very real factor it is clear that only the most secure attachment method should be utilized.

System longevity not only has to do with system strength but also the way it minimizes stresses on the stone panels. The Gridworx Extreme SS system provides a continuous support rail at both the top and bottom of the stone to eliminate point loading of the stone. The laser fused Gridworx Extreme SS system delivers greater structural incorruptibility than other attachment methods. The Extreme system allows for a thinner metal profile to interface with the stone. Keeping the kerf cuts in the panel as thin as possible maintains the strength of the panel in the area where it is most critical.

When required, a “center of stone” continuous full width connection (lateral support) can be incorporated to further enhance the attachment and greatly increase the modulus of rupture of the panel. The design of the system allows for building movement and thermal expansion and contraction without hard loading any of these forces back to the stone panels. Should an inspection show a stone that has been damaged in some way, the Gridworx Extreme SS system allows a single stone to be cleanly removed from the wall and replaced without having to remove any other stones in the process. This greatly reduces the time needed for the repair and risks of damage to other parts of the facade.