With nearly 50 feet of elevation change from front to back, the buildings essentially would have to be nestled into the existing hill, but the subsurface investigation revealed multiple challenges.

As a premier technical school founded in 1968, Hocking College has more than 40 associate degree programs available for its diverse student population. Located in Nelsonville, Ohio, the college is situated on 2,300 acres in the state’s Hocking Hills region. To increase on-campus housing options for its more than 5,500 students, the college planned two new dormitory buildings that would add a total of 91,000 square feet and 403 beds to the existing housing stock.

The selected site, centrally located in close proximity to both classroom buildings and existing residence halls, immediately presented several design challenges. With nearly 50 feet of elevation change from front to back, the buildings essentially would have to be nestled into the existing hill. Because preserving the natural habitat on this wooded site was paramount, extensive preplanning was necessary. The architect determined that both buildings would be seven stories tall and would step up the hill. This resulted in lower floors not being able to extend the full length of the buildings as they were cut off by the hill. One of the buildings is two stories at the rear, while the other is three stories at the rear.

A subsurface investigation revealed multiple challenges. The first was an abandoned underground coal mine. The primary challenge with the mine was its stability and preventing subsidence of the mine in the future. The second issue was the stability of the hillside, which was a concern primarily during construction when vertical cuts would need to be made to allow for the building to step up the hill. The investigation also revealed that the soil above the mine was not suitable to support a building. However, competent sandstone was present directly below the bottom of the mine. Determined not to let this discovery derail or relocate the project, the design team and college representatives held several meetings and consulted with specialists to find the best option for dealing with the mine. The geotechnical team — which consisted of two soils engineers, a mining engineer, and a geologist — and the structural engineering team explored foundation solutions. Since the elevation of the bottom of the mine was above the first floor elevation, both shallow and deep foundation options were explored. The deep foundation options were driven steel H-piles, caissons, and auger cast piles.

Ultimately, the design team decided to fill the mine void with a cementitious grout material in the area under the project limits. This solution was not only cost effective, but also mitigated the risk of mine subsidence, which would have resulted in a large sinkhole. The deep foundation system was caissons extending down through the grouted mine into the sandstone. The grouted mine eliminated the need for cased caissons. Since the first floor was below the elevation of the bottom of the mine and was cut into the sandstone, a shallow foundation system was used at the lower, front areas of the buildings. The team specified a grout material that had sufficient strength to support the soil above the mine, but also was soft enough to allow the drilling of caissons through the material.

To address topography challenges and reduce costs, the team wanted to minimize the number of foundation elements required to support the building. Floor framing options that were considered included conventional composite steel beams, post-tensioned concrete, and a girder-slab system. The framing option that was selected was a staggered steel truss system consisting of wide flange steel columns and story-deep trusses that span from one side of the building to the other and support 8-inch precast concrete floors. Because the trusses clear span the width of the building, columns are only required at the exterior walls, minimizing the number of foundations needed for support. The staggered truss system also provided an efficient structural system for the modular layout of the residence rooms. The exterior foundation walls were designed as deep beams that span between caissons and don’t require footings, which also reduced the need for foundation elements. Large notches in the walls allow the buildings to step their way down the slope. Lateral loads were resisted by the trusses across the width of the building and by bolted moment frames at each exterior wall along the length of the building.

The concrete floors were left exposed on the underside to become the finished ceiling surface for the floor below, a decision that reduced floor-to-floor depths and eliminated an additional layer of ceiling framing. Because the precast planks bear on both the top and bottom chord of the trusses, a staggered steel truss system results in trusses that are only placed on every other floor at each column location. The precast planks provide the overall stability for the truss system. However, temporary bracing of the steel frame was critical during erection. As such, an engineered erection sequence was specified.

The building’s exterior cladding — 8-inch-thick autoclaved aerated concrete (AAC) panels — provides another unique design element. This system was chosen over precast concrete wall panels because of cost. AAC and precast concrete were the only wall options considered because of the short construction schedule that was dictated by the start of the fall semester. An extremely durable material, AAC provides the owner a cost savings over the life-cycle of the structure in maintenance costs, as well as reduced energy bills from the exceptional thermal insulating qualities. Provided by manufacturer Xella AAC Texas, Inc., AAC is a lightweight cementitious material that possesses excellent thermal efficiency, structural integrity, mold resistance, and fire resistance. These properties allowed the material to serve as both the interior and exterior wall surfaces, while also meeting the owner’s vision to include sustainable design elements. However, AAC is a product that is not widely used in this part of the country. In addition to using a new material, the application on such a large scale is uncommon. Typically used with horizontal wall connections, the AAC was used vertically with connections that were quite different from standard applications. A great deal of communication took place between the AAC representatives and the design team to ensure that connection details were developed to allow the AAC to be attached to the staggered truss frame. In most cases, the panels extended down to the foundation wall, but at the front of each building, details were developed to allow the AAC panels to be suspended on the structural frame. This permitted the use of large openings in the common spaces at the first floor.

One of the key lessons learned on this project is the importance of having the structural system complement not only the design of the building, but also the site and subsurface constraints.

Although it faced many challenges on this project, the team addressed each one in a timely manner, and the final design was the result of a collaborative effort. The structural engineering solutions worked in harmony with the architectural design of the buildings, allowing them to blend in perfectly with the existing landscape. These unique engineering solutions saved the project from being abandoned due to budget concerns and created two impressive structures that will benefit not only the students of Hocking College, but also the entire community.

The building’s unique exterior cladding — 8-inch-thick autoclaved aerated concrete (AAC) panels — was attached to the staggered structural steel truss frame and suspended from the frame in some locations.

Stephen Metz, P.E., is principal at Shelley Metz Baumann & Hawk and can be reached at 614-481-9800.