Designing with Code
A Crash Course
Example of Final Product
The final product of a building code design is a series of drawings and specifications that show the final composition of the structure, including member sizes, materials, dimensions, and details, as well as the acceptable means and methods of construction. These member sizes and materials have been calculated as a design that has Code-calculated capacity that exceeds Code-calculated demand.
Detailed Process
Gravity Loads
The first step in a code-based design is to get an idea of the magnitude of gravity loads that the building must support. In general, a load on a building can be categorized as either a gravity load or a lateral load, as shown below. Gravity loads mostly consist of the dead load and the live load. Dead load is the weight of the building itself and anything permanently attached to the building (such as the façade, flooring, partition walls, etc.). Live load is the weight of the people who will use the building, as well as any and all accoutrements they will need, such as movable furniture, equipment, storage, etc. To summarize, dead loads are anything permanent, live loads are anything temporary or moveable. Gravity loads also consist of the loads applied to the roof, including rain, snow, and ice.
After the gravity loads have been established, the system must be designed to resist these loads. Since the example building is designed using steel, the prevailing technical code is the Specification for Structural Steel Buildings, released by the American Institute of Steel Construction (AISC), referred to as AISC 360. The design of the gravity system is a creative process, and there is no single “right answer” or perfect design.
Gravity systems in steel consist of four main components: the floor slab, the beams, the columns, and the foundations. Each member transmits load to the next member, except for the foundation that transmits load to the soil. This is called a load path. Each member of the load path must be designed individually.
Other gravity system elements include connections between beams, girders, and columns, as well as the floor slab and the foundation. Floor slabs and foundations are both concrete members, calculated in accordance with provisions in Building Code Requirements for Structural Concrete as published by the American Concrete Institute (ACI). This document is referred to as ACI 314.
Seismic
Seismic loading depends on several factors, including the seismicity of the site. The first factor is the intensity of ground motion for the region, referred to as the risk-targeted maximum considered earthquake, or MCER. Intensity of seismic events varies widely by location: the intensity of a design earthquake in Western United States will be much greater than the intensity of a design earthquake in the Midwest. The second factor is soil, since softer soils transmit seismic waves at a slower speed, resulting in amplification of ground motion intensity. There are several methods of structural analysis for determining the seismic demands on the lateral force resisting system. It is, at this point, critical to note that seismic design does not follow the traditional design paradigm. For gravity loads and wind loads, there is an external agent, either the wind or the weight of the building and all it holds, that applies a force to the building. A successful building under this scenario will be one that has a capacity to resist force that is greater than or equal to the force induced by these external agents. However, in seismic design, it is not quite so straightforward because the actual magnitude of the seismic load varies with the actual weight and stiffness of the building. This seismic demand is more difficult to calculate than demand caused by direct external force. As such, for the sake of convenience, the first option available to engineers is a method that approximates the earthquake load as a series of lateral forces applied at the level of each story in a structure. This is called the Equivalent Lateral Force method, or ELF. ELF simply states that the force due to earthquake motion at a level is equal to a percentage of the weight of the building at that level.
