Target Field
The Minnesota Twins' future ballpark, Target Field, is scheduled for completion in 2010. This architecturally stunning new park promises to be one of the most unique and spectacular parks in all of Major League Baseball. It's most impressive architectural feature is the sunshade canopy, which boasts to be one of the largest in baseball. The canopy soffit is attached to the primary structural steel with the use of the Adjustable Clamp
"We were looking for a user friendly connection system that eliminated the touch-up hassles. We analyzed it and picked LNA Solutions," said Mike McGrath, Vice President of Operations at M.G. McGrath Architectural Sheet Metal. McGrath was looking for a simple, universal connecting system that did not require a lot of disparity in components, since the beam flanges varied from 8" to 12" wide and 1/2" to 3/4" thick. The beam thickness variations were addressed by utilizing the Adjustable Clamp in a 3/4" bolt size. This clamp is self-adjusting from 1/8" to 3/4" in thickness without the need of spacers or packings. Flange width variations were managed with the use of slotted location plates. The location plates sit between the connecting structural steel and serve to keep the bolts at the intersection of the steel as well as providing a seat for the clamp, to create the necessary clamping force. By oversizing and slotting them, they become universal for the full range of beam flange widths.
In addition, McGrath did not want to perform full corrosion protection touch up on the over 1200 connection points to the primary structural steel, which would be required if he drilled or welded the connections. Girder clamping these points does not breach the protective coating of the primary steel. More importantly, the structural integrity of the primary steel was maintained, not weakened through the introduction of intense heat or the removal of material caused by drilling.
The sunshade canopy posed a challenging merger of design and material. Radius Track Corporation was able to provide the expertise, 3D modeling capabilities, and box beam production necessary to actualize the architect's concept. Structural drawings were combined with the metal decking profiles to create the finished surface shape. Once the beams had been engineered, Radius Track was able to determine the length, quantity and shape of the curved and straight box beams; finally incorporating the location of the box beams to the structural frame of the stadium utilizing LNA Solutions connectors. Connecting the roof canopy to the structural steel presented some interesting design challenges due to the curvature of the canopy; the box beams are at slightly different angles to the supporting beams throughout. "We needed a connection scheme with a lot of flexibility because we were attaching to in-place structural steel with our steel studs, and the stud framing plane (particularly as it curves) is not necessarily parallel to the plane of the structural steel," stated Kelsey F. Brown, PE, SE Principal at Van Sickle, Allen & Associates. The slotting of the plates not only addressed the flange width variations, it also allowed for alignment of the studs on the location plate when connecting to the primary steel since the plates could be skewed to accommodate these differences. Due to the nature of the project, it was widely accepted that small field adjustments would be necessary. This would normally be accomplished by tack welding then grinding off and repeating the process until the final position was realized. With the Girder Clamp, the canopy risers can be moved and adjusted recurrently. Furthermore, since the canopy connection project encompasses elevated and overhead work, eliminating welding and grinding contributed greatly to the overall safety of the project. The connection design criteria required accommodation of 5 kips of wind load that could go in either direction, (tension or compression.) Though the 3/4" Adjustable Clamp was chosen primarily for its adjustability range, it is rated at a safe working load of 3300 pounds per bolt at a safety factor of five to one. Each connection to existing structural steel physically necessitated the use of four clamps and each riser was connected at three points for a total load rating that far exceeds the load requirements. The girder clamp components are admittedly more expensive than structural nuts and bolts or welding consumables. However, the savings in field costs and flexibility that was provided for the addition of the Target Field canopy far exceeded the negligible additional investment in material.
