Early involvement of a structural engineer can make a significant difference in reining in the cost of a school project. Structural engineers involved in the initial design phase—and even before—may help ensure project success.
In the beginning
An effective structural engineer starts by meeting with an owner or facility manager and architect in the initial design stage of a project. The structural engineer wants to understand not only the building itself, but also the site and expectations. This includes the building’s location, site geography, adjacencies to existing buildings, the building’s use, budget, and what is anticipated in terms of a basement and the number of floors.
Site and soil conditions are key. The conditions help determine the most appropriate structure. For any building structure, the owner also must secure required technical tests during construction, such as concrete-strength tests. Because even the most knowledgeable owner may not be aware of all the tests and requirements, the structural engineer makes recommendations along the way to ensure that the structure meets the necessary requirements. A structural engineer also will help an owner interpret the testing, and help the design and construction team determine how the testing may affect the design of the structure.
After initial preconstruction soil test borings are made, a structural engineer will counsel an owner on the site class designated by the test and whether any additional testing would change the designation. Soil borings are geologic engineering tests to determine the capability of the soil and the underlying rock to support proposed improvements.
These tests are necessary to determine the conditions of the soil and rock; they may affect the foundation design. The geotechnical firm conducts the tests and interacts with the education institution and the structural engineer.
Soil site classes range from A to F; A is the class that results in the lowest seismic loads to a building. It is found by measuring the type of soil and the depth and type of rock. The depth and type of rock affects how deep the seismic waves can propagate through the soil.
The structural engineer also works with a geotechnical engineer to determine the best foundation system for the building. This is based on soil and site conditions, the desired performance of the foundation, the structural system and loading of the building and the cost.
Case Study: Ohio State University, South High-Rise Project
With a goal of providing housing for all freshmen and sophomores, Ohio State University, Columbus, is building a 24/7 living-learning community. This project consists of two 11-story towers connecting the existing Stradley to Park Hall and Smith to Steeb Hall. When completed, the towers will provide community-oriented student housing in a contemporary setting.
In the early design phase, cast-in-place (CIP) concrete and staggered steel truss (SST) were considered as structural systems. CIP provides benefits, but posed disadvantages for this project: large columns; not fast enough to meet the schedule; not cost-effective; and incompatibility with the building design.
A staggered steel truss system can be erected quickly. It is the only system that fit the university’s construction schedule. It also saves space with smaller columns and similar floor thickness to CIP and the existing construction. The construction manager found it cost-effective, and the lead times worked well with the design and construction schedule. The SST system also allowed for the innovative design that the aesthetic of the building demanded.
The main construction challenges to solve: the two-story space at the first-floor lobby; the cantilevered student lounges at floors 3 to 10 with a 19-foot cantilever; and the transfer level at floor 3.
The architect wanted the first-floor entry to be a two-story well-lighted space. The typical configuration of a staggered truss system would have the trusses terminate at the second floor with bracing at the first floor. The two-story-high, open space would not allow for this. This meant that the lateral load-resisting system at the lower two levels had to become a moment frame.
To accomplish this, trusses from floors three to four were placed at every column line. The bottom chord of the trusses became part of the moment frame. Additionally, a 42-inch upturned plate girder that was 42 inches deep with ½-inch web was used.
The structural solutions for the student lounges were found by using a vertical frame composed of the column that stops at the third floor and the girders at each floor. This enables the 19-foot-long cantilever to be structured with shallow framing.
Two student lounges are at opposing corners of the addition at floors three through nine. The lounges extend 19 feet from the face of the main building and are about 15 feet wide. The lounge extensions could not be supported by columns that extend to the ground.
Interior space constraints also would not allow for the trusses to cantilever. So a single steel girder cantilevers from the column and supports the entire floor framing in the lounges. To counter the eccentricity of the unbalanced loading on the floor, the framing connects to the existing building columns for uplift support.
For the transfer level at the third floor, the columns at floors one and two needed to be offset 2 feet, 2½ inches toward the interior from the upper-level columns. This was done so that the lower-level columns could be inset from the exterior curtain wall system and the upper columns would be within the exterior wall system. This presented two challenges. Both the gravity and lateral loads had to be transferred back to the lower-level columns.
The gravity loads were transferred by using the trusses at floors three to four. The lateral loads were transferred by using a horizontal truss system at the third-floor level.
With a goal of providing housing for all freshmen and sophomores, Ohio State University, Columbus, is building a 24/7 living-learning community. This project consists of two 11-story towers connecting the existing Stradley to Park Hall and Smith to Steeb Hall. When completed, the towers will provide community-oriented student housing in a contemporary setting.
In the early design phase, cast-in-place (CIP) concrete and staggered steel truss (SST) were considered as structural systems. CIP provides benefits, but posed disadvantages for this project: large columns; not fast enough to meet the schedule; not cost-effective; and incompatibility with the building design.
A staggered steel truss system can be erected quickly. It is the only system that fit the university’s construction schedule. It also saves space with smaller columns and similar floor thickness to CIP and the existing construction. The construction manager found it cost-effective, and the lead times worked well with the design and construction schedule. The SST system also allowed for the innovative design that the aesthetic of the building demanded.
The main construction challenges to solve: the two-story space at the first-floor lobby; the cantilevered student lounges at floors 3 to 10 with a 19-foot cantilever; and the transfer level at floor 3.
The architect wanted the first-floor entry to be a two-story well-lighted space. The typical configuration of a staggered truss system would have the trusses terminate at the second floor with bracing at the first floor. The two-story-high, open space would not allow for this. This meant that the lateral load-resisting system at the lower two levels had to become a moment frame.
To accomplish this, trusses from floors three to four were placed at every column line. The bottom chord of the trusses became part of the moment frame. Additionally, a 42-inch upturned plate girder that was 42 inches deep with ½-inch web was used.
The structural solutions for the student lounges were found by using a vertical frame composed of the column that stops at the third floor and the girders at each floor. This enables the 19-foot-long cantilever to be structured with shallow framing.
Two student lounges are at opposing corners of the addition at floors three through nine. The lounges extend 19 feet from the face of the main building and are about 15 feet wide. The lounge extensions could not be supported by columns that extend to the ground.
Interior space constraints also would not allow for the trusses to cantilever. So a single steel girder cantilevers from the column and supports the entire floor framing in the lounges. To counter the eccentricity of the unbalanced loading on the floor, the framing connects to the existing building columns for uplift support.
For the transfer level at the third floor, the columns at floors one and two needed to be offset 2 feet, 2½ inches toward the interior from the upper-level columns. This was done so that the lower-level columns could be inset from the exterior curtain wall system and the upper columns would be within the exterior wall system. This presented two challenges. Both the gravity and lateral loads had to be transferred back to the lower-level columns.
The gravity loads were transferred by using the trusses at floors three to four. The lateral loads were transferred by using a horizontal truss system at the third-floor level.
Metz, P.E., LEED AP, is a principal at Shelley Metz Baumann Hawk (SMBH), a full-service structural engineering firm in Columbus, Ohio. He can be reached at [email protected].
