Waterfront Gathering Place

Two iconic steel-framed buildings anchor services at Tulsa, Okla.’s riverfront park.

By Falgun Surani, P.E.

At $465 million, Gathering Place in Tulsa, Okla., is the largest private gift to a public park in U.S. history. As the name suggests, this 66-acre park is designed to transform Tulsa’s iconic waterfront along the scenic Arkansas River into a dynamic, interactive environment where members of Tulsa’s diverse communities can come together to interact, explore, learn, and play.

The park, designed by Michael Van Valkenburgh Associates, Inc. (MVVA) contains two buildings that anchor the services available. Mack Scogin Merrill Elam (MSME) architects designed both buildings at Gathering Place. Structural Engineering Associates, Inc. (SEA), based in Kansas City, Mo., are structural engineer of record. Crossland Construction Company are the general contractor and Unique Metal Fabrications, Inc. (UMFI), based in Pittsburg, Kan., are the steel fabricator and supplier. Each building embraces the powerful park landscape in a combination of traditional modernist strategies that incorporate the rich, natural material palate of Oklahoma.

Williams Lodge

Williams Lodge, one of two iconic buildings in Gathering Place, offers restrooms, a cafe, a two-story fireplace, indoor lounge spaces, and educational activity rooms. Primarily made of stone floors, wood ceilings, and glass windows to provide an unobstructed view of the park. The underlying design theme for Williams Lodge was to provide an enclosed space that appears to be part of the landscape and provide shelter for cold and rainy weather.

The client’s wish for an unobstructed view and glass surrounding meant the traditional hollow structural section (HSS) or wide-flange steel columns at the perimeters were not feasible. Closely spaced solid vertical steel plates (2-1/2 inches by 8 inches) that also work as window mullions are utilized to be structural columns supporting steel floor and roof beams. Floor framing is traditional composite floor beams that support 3-inch-thick stone floor panels on top of 7-1/2-inch composite floor. Roof framing is traditional steel roof beams with metal deck, which support an elegant wood ceiling that depicts a large tree.

Nearly half of the plate columns are two stories tall and about 40 feet high. With concerns about losing plumbness during fabrication, shipping, and erection, steel fabricators proposed splicing the plate columns at floor level. However, the exposed nature of these columns as window mullions demanded a clean finish and the splice was not acceptable to the design team. Plate columns were erected first and temporary braces were provided during construction. It’s plumbness was repeatedly checked during construction and the steel floor and roof beams framing in the plate columns and the glass windows were fabricated per final field dimensions.

The centerpiece of the Williams Lodge is a massive, two-story stone façade fireplace. It’s large enough for a group of people to walk in and around it. The intricate shape of this structure and stone façade meant that the skin of the fireplace will have to be supported by a curved steel structure because cast-in-place concrete forming was not feasible due to the complexity of the shape. A uniform oval shape concrete shaft was utilized as a shear wall as well as the core structure supporting the two-way steel grid that contains curved wide flange members as verticals and curved HSS members as horizontals, both spaced roughly 4 feet on center.

An SAP 2000 3D design model was created to analyze and design this complex steel structure for out-of-plane wind loads and weight and moment coming from shelf angle supporting the stone façade eccentrically. There are three protruding arms of this fireplace — first is serving the flue for the fireplace at level 1, second is supporting the skylight in the center of the fireplace, and the third is serving the flue for the fireplace at level 2.

The third arm is also hanging from the face of the oval concrete shaft. It was nearly impossible to produce construction drawings for this steel structure that had varying radius in every direction. Therefore, a 3D Rhino centerline model was used as construction documents with 2D details and sections on the structural drawings. Steel fabricators (UMFI) extruded this 3D center line model to make a Tekla 3D model, which was then used to produce shop drawings.

The traditional way of rolling steel was not feasible since the steel grid structure changed shapes with non-uniform radiuses throughout its height and width. Additionally, shipping and erection of a roughly 60-foot-tall by 30-foot-wide steel structure was challenging. UMFI and Crossland proposed to make roughly 12-foot-tall ring grids, fabricated and shipped in one-quarter segments to be field welded to make the final shape of the structure. Each vertical and horizontal member was made by cutting 1/4-inch or 3/8-inch plates to its radiused shape from the 3D Tekla model and then welding it in shop to achieve the correct structural shape. All the shapes made with plates were stich welded in shop alternatively on both sides to avoid any local distortion or warping effect from the weld heat.

Original design bases were to have the continuous wide flange verticals supporting the HSS horizontals at every 4 feet. This was no longer feasible due to shipping and erection limitations mentioned above. Instead, the horizontal HSS were made to run continuous and vertical wide flanges were made discontinuous at every intersection with horizontals. Since the 3D Tekla model was extruded from a centerline Rhino model, this meant that all or a portion of the flanges of wide flange verticals were not getting a continuous weld on HSS below due to the local twist in the member because of the overall shape. This issue was resolved by introducing a larger base plate at every vertical wide flange top and bottom and then these base plates were welded to the HSS horizontals top and bottom to achieve continuity.

MSME and SEA coordinated locations of several floor and roof beams in the 3D model that were going through the steel grid structure of the fireplace and were framing into the oval concrete shaft. Shelf angles supporting the stone façade at the fireplace were also made by welding two plates together to achieve the shape of the fireplace. Fero brackets were used to connect shelf angles with the steel grid frame to provide a cavity for insulation.

Additionally, three stairs were designed and fabricated with steel plates. The grand stair had glass handrails and glass walls and was designed to cantilever between two floors. Two other steel plate stairs had stone flooring. The south stair was supported on a radiused channel stringer on one end and the other end was supported on a concrete foundation wall where plate trades were notched around the plate column base plates to avoid conflicts.

ONEOK Boathouse

The second iconic park building is ONEOK Boathouse. The three-story ONEOK Boathouse houses a restaurant that offers an elegant dining experience on the pavilion deck that offers beautiful views of the Arkansas river and downtown Tulsa, as well as the outdoor terrace where guests can dine as they take in the scenery.

Aside from amazing meals with views, a diverse range of ages and interests are drawn to the multi-generational programming offered within the middle level of the Boathouse. The Cabinet of Wonder resides in this area of ONEOK Boathouse, acting as an educational space for all ages as well as a social gathering space. It also lets visitors check out paddle boats, kayaks, and canoes from the lower level of the building to paddle along Peggy’s Pond and enjoy a new view of the park.

Typical floor framing is traditional composite floor beams that support 3-inch-thick stone floor panels on top of 7-1/2-inch composite floor. Roof framing of the restaurant is traditional steel roof beams with metal deck.

The centerpiece of the ONEOK Boathouse is the fiberglass canopy supported on steel pipe columns. Another unique feature of this building is the steel stair enclosure at the pavilion level that has a slate façade. The fiberglass canopy serves as a roof for the uppermost pavilion level, which provides open seating with views of the park, river, and downtown Tulsa. The pavilion structure is cast-in-place reinforced concrete with large cantilevers on all four sides. Serving the pavilion deck is a long cast-in-place stair with a nearly 20-foot cantilever overlooking Peggy’s Pond. The stair is supported on a single concrete column.

Stair enclosure covers the center floating cast-in-place concrete stair serving all floors and provides roof and skylight to the stair. Steel enclosure is made with all HSS members placed in 4-foot by 4-foot grids horizontally and vertically. This steel structure was supported on composite steel beams on level 3, which in some cases cantilevered. This structure changes shape with varying radius in every direction.

It was difficult to produce construction drawings for this structure. Hence, a 3D Rhino model was used as construction documents with 2D details and sections on structural drawings. Steel fabricators (UMFI) extruded this 3D centerline model to make a Tekla 3D model, which was then used to produce shop drawings. The traditional way of rolling steel was not feasible since the steel grid structure changed shapes with non-uniform radiuses throughout its height and width. Each vertical and horizontal member was made by cutting 1/4-inch or 3/8-inch plates to its radiused shape from the 3D Tekla model and then welding it in shop to achieve the correct structural shape. All the shapes made with plates were stich welded in shop alternatively on both sides to avoid any local distortion or warping effect from the weld heat. Metal studs are used as infill between the steel grid structure to support the sheathing that clads slate.

The perimeter of the building is made with cast-in-place concrete walls that support the large stone façade. One side of the building is retaining a three-story soil fill and the lateral pressure from the soil was a concern for the building. Therefore, the building is isolated from the soil by using mechanically stabilized earth (MSE) in three sides of the building with a 2-foot void gap between the building and MSE. Terracon consultants provided design for the MSE wall around the perimeter.

Level 2 Mezzanine is made with composite steel beam and steel deck with glass walls and it is hanging from the level 3 floor beams with four steel HSS hangers. Level 3 beams also support the entire roof structure for the restaurant, which also includes stone donor walls and steel screen walls on roof, stair enclosure for the center stairs, and a steel HSS hanger to support a floating concrete stair landing.

Falgun Surani, P.E., is a project manager and associate with Structural Engineering Associates, Inc. (www.seassociates.com) in Kansas City, Mo.