By Alex Berlin, PE
“I’m a Structural Engineer and I make sure buildings don’t fall down”. This is what I said at my daughters preschool on bring your parent to work day. In a nutshell, it’s what structural engineers do. But in practice, the execution of that is quite a bit more involved.
Structural Engineers typically work in close coordination with owners, architects, other engineering disciplined professionals (civil/geotechnical), and specialized tradespeople. The owner is the one with the idea and desire to fund a construction venture and get something built. This can be a local utility company looking to build a new office building, an investment firm looking to build a new housing development, or an NFL owner looking to build a new football stadium. The owner will work with an architect to create a rendering or detailed set of drawings of the building and its features. The structural engineer and the architect work closely to take the building that has been drawn and analyze the forces that will act on in and design every element of the building.
Using the governing jurisdictional design codes, a structural engineer will calculate and assume all of the expected forces that could possibly act on the structure. This includes: gravity loads, weight of the structure itself, live loads from occupancy, snow loads, lateral wind and seismic loads and any other potential load. These are combined into different loading scenarios and applied to the theoretical structure. Using tributary area methods and structural analysis, the distribution of forces is calculated for every element of the building. The load path is followed from the roof and exterior walls, to the floors of each level, to the connections to columns all the way down to the foundation, and into the ground. This is done either by hand or using computer models, depending on the complexity of the structure and loading. Once each member has a demand load both from the vertical and lateral loads applied, each member can be selected or designed based on the desired size and material specified by the owner and architect. Capacities for different common material types and shapes can be found in the respective code books. Every beam, column, bolt, weld, nail, concrete slab and piece of rebar are checked for sufficient capacity. For example, a residential wood framed, two-story building may have an open concept lower level. There are no columns or room dividing walls that can take the load of the second story. Large and deep timber beams may work to carry these loads, however, they will not fit within the thickness of the second floor so a more economical steel beam can be selected. Another example is the recently constructed Los Angeles football stadium was designed with a solid roof structure that could let natural light in. Specialized glass and a complex high tension cable system were used to create a safe roofing system that met the owner and architects vision.
On a design and construction project, the Structural Engineer can either be the prime consultant, who works directly with the owner, or the secondary consultant, who works for an architect that works directly for the owner. The responsibility and the function of the Structural Engineer is the same regardless of the contractual role and profit margin. Problems can by typical and easily solved or are extremely complex and require revolutionary building techniques with exotic material. The Structural Engineer at the end of the design phase is ethically responsible for ensuring that the anticipated loads have all been applied to the building and that each member is sufficiently designed so its capacity is larger than its demand. This ethical responsibility, coupled with the contractual obligation to make the vision of the architect and owner a reality are the main functions of a Structural Engineer.