By Roy Diez
Hanson Structural harmonizes new, existing precast concrete
When adding on to an older, existing structure, whether a building or a parking deck, often the biggest challenge is to blend the new and old together seamlessly—both structurally and architecturally.
For the new One Southwest Crossing Parking Ramp in Eden Prairie, Minn., the design-build team had to stack a new precast concrete parking deck next to and on top of part of an old cast-in-place deck. The existing parking deck serviced a 25-year-old, Class A office building. A major building tenant, Cigna, wanted more parking as a requirement of extending its lease, explains Scott Thompson, senior project manager for the project contractor, Welsh Companies, Minnetonka, Minn. The goal was to add 200 additional parking spaces to the original 975 stalls.
“Our sister company, Colliers International, asked us for a design-build number to expand the existing garage,” says Thompson. “Working with Hanson [Structural Precast] we came up with a guaranteed-maximum price and they hired us.”
With five stories, the One Southwest Crossing office building features a dramatic curving façade and pinkish or rose-colored exterior panels with white borders. Built into a hillside, the matching, existing parking ramp is terraced. This ramp’s 60-ft. outside bay has one covered level with ground floor and rooftop parking. The rest of the garage steps up to provide two covered levels of parking. The plan was to add a new, precast two-bay deck next to and on top of the old cast-in-place ramp. The first new precast bay was to be only one level set on top of the single-story portion of the existing bay, the rest of the new ramp was to have two supported levels.
The new ramp measures 123 ft. by 359 ft. and connects to the existing garage along its long axis. It is constructed utilizing precast concrete beams, columns, double tees and spandrels from Hanson Structural Precast, Maple Grove, Minn. Included are 30 precast beams (871 lineal ft.), 38 columns (1,064 lineal ft.), 93 pretopped double tees (63,701 sq. ft.), and 35 spandrel panels (1,383 lineal ft.). The spandrels measure 7 ft. tall, 36 ft. long, and vary from 10 in. to 12 in. thick. The pretopped double tees are 12 ft. wide, 27 in. thick and have 5-in. flanges and 22-in. stems. They span 60 feet. The new deck has a basic linear foundation with spread footings.
Matching the concrete mix
The challenge was to add the new deck to the old ramp and make the finished structure look uniform. This meant that Hanson had to match the color and design of the new parking deck’s spandrels to those on the existing structures.
A drainage plan for the new ramp entailed conveying runoff to east to a retention tank, draining toward a retention pond at the south end of the property.
“Trouble is, the existing ramp was so old no one could recall who built it or provided the precast cladding for it,” says Mike Keller, Hanson project manager. “We tried to locate the original precaster so we could duplicate the concrete mix. We called everyone in a five-state area but couldn’t locate the original firm. It may no longer exist. We had no idea where the original rock and sand came from or if it was still available. Even if it was available the source would have changed after so many years and the colors would be different. So, we had to start from scratch and develop our own mix.”
Hanson ended up trying two or three different rocks and two different sands to match the color, preparing numerous test samples. The finalized concrete mix for the pinkish color section of the new spandrels consists of grey and white cement; two coarse aggregates, Colonial Red and Arrowwood, ½ in.; two sands, Gilmore City and GW sand; and #20 Dynamic Color Solutions liquid pigment. The mix for the white section of the spandrels consists of gray and white cement; two coarse aggregates, Sheily #89 and Arrowood, ½ in.; and two sands, Gilmore City and GW sand, no color.
To complete the match, Hanson had to lightly sandblast the surface of the concrete spandrels and then do a heavy acid etch to bring out the color of the rock and sand. “The real tricky part,” says Keller, “was that the spandrels have a white border all the way around. We had to pay close attention not to stain the lighter border when we did the acid wash. The timing of the different surface treatments was critical.” It took three months to get it right.
Sliding in concrete columns
Building on top of a decades-old garage also took careful planning. The old cast-in-place deck was only mildly reinforced, lacked post-tensioning, and was built into a hillside. “For the new bay added on top of the single-story portion of the old parking deck, we did not want to put any additional load on the existing structure,” says Gary Pooley, Hanson sales manager. “We actually had to punch new columns through the old ramp.”
Notes Mike Desutter, president of Ericksen Roed Associates, “Holes were cut through the one level existing bay so that we could slide our new columns through and erect the second level above it. We had to find key locations to cut through the deck so as not to impair that structural capacity or undermine the foundation of the existing ramp but still make it work for the new structure. It was like a puzzle. Because of the sloped site, we also had to pull out an existing retaining wall and its foundation.”
The existing retaining wall was right where the new columns were to be positioned, adds Keller. As for cutting holes through the existing cast-in-place deck slab, “that was easy,” he says. “We just cut the holes and dropped our columns in. The 24-in. by 24-in. columns are spaced 36 feet apart. We installed an expansion joint where the new deck slab meets the existing deck slab.” A 350-ton hydro crane was used for the precast erection, spanning a mere two weeks, he adds.
Getting the slope right
Neither the new nor the old parking deck feature vehicle ramps for circulation. Since the structure is built into the side of a hill, vehicles can enter the grade level section from one side, the middle level section from a different side, and the top level from yet another entry. The new deck levels are aligned with those of the old deck so users can drive directly from one to the other. But working off the existing ramp meant that the elevations were fixed, leaving limited headroom. This created a drainage issue.
“With the existing ramp everything flowed to the south into a drainage pond,” says Hanson’s Gary Pooley. “In a typical parking structure the exterior spandrels are flat and the drainage happens at the interior bays. We couldn’t do that here. We had to come up with a plan for the new structure where everything drained to the exterior on the east away from the existing ramp and then would run to the south.”
This was accomplished by slopping the exterior spandrels on the new deck 1/8-in. per foot. Along the east elevation the spandrels are not flat or level, they actually slope up and down to accommodate that drainage to the exterior of the new ramp. “If you look at the side of the new structure, the spandrels are not at one level elevation,” describes Welsh’s Scott Thompson. “They go up and down and the new garage drains to those low points. Storm water then runs into an on-site retention tank buried in the hill and then, once it settles, it goes out to an area retention pond.”
Mating different drainage systems, coordinating structural elements, and matching a new concrete mix to the old, the One Southwest Crossing ramp addition is a testament to the fact that adding-on can often be more complicated than new construction.
ONE SOUTHWEST CROSSING RAMP
Eden Prairie, Minnesota
Owner Welsh Companies, Minnetonka, Minn.
Manager Colliers International, Minnetonka, Minn.
Architect Genesis Architecture, Minnetonka, Minn.
Engineer Ericksen Roed Assoc., St. Paul, Minn.
Contractor Welsh Companies, Minnetonka, Minn.
Precaster Hanson Structural Precast Inc., Midwest
Region, Maple Grove, Minn.