Structural design is at the heart of many building projects. From the Burj Khalifia in Dubai to the Empire State building in New York City, all these projects were brought to life through a structural design. Structural design entails a number of processes aimed at achieving safe, durable and cost-effective specifications for a building’s components or structure. It is the typically the work of a structural engineer to carry out a structural design for a building. Structural engineers have to select materials, technologies and sizes for structural element that can adequately support the loads expected during the design life of the building.
The typical structural design process involves three stages; preliminary design, detailed design and drafting. Through these stages, ideas and information about building structural system are established to aid in its construction.
In this stage, designer has to review the architectural drawings and plan for the building. The designer then selects the position and location of the columns for the building based on the architectural plan. Initial sizes of the different members such as slabs, beams and columns are also made based on design code recommendations.
A review of the available documentation regarding the building’s location, such as geotechnic reports, seismic zone and wind loads, has also to be done. The results of this review can provide insights on the nature of analysis required in the detailed design and type of the loads to be considered as well as the kind of the structural system to use for the building.
During the preliminary design, the designer also has to plan for the loading in different areas of the building as recommended by codes of practice. For example, the loadings for bed rooms, kitchen, balconies and terraces may vary as specified by the design codes. The loadings on a building to be considered normally include dead loads, live loads, wind loads and seismic loads. The dead loads comprise of the self-weight of the building components, while live loads consist of movable loads such as loading to people in the building.
The designer also has to make a choice on the type of materials to be used in different areas. For example, the designer may choice solid concrete slabs for wet areas such as; bath room areas, and hollow or ribbed slabs for other spaces.
Upon completion of the preliminary design, the designer can then carry out a detailed design which includes a system design as well as iterative design for the different structural elements. The goal of the detailed design is to provide specifications which can be turned into structural drawings during the drafting process.
For the system design, the designer has to consider the path of the loads on the building such as lateral loads, gravity loads and uplift loads. At this stage, the designer also has to give consideration to any components of the building that may be load bearing. For example, the designer has to identify load bearing and non-load bearing walls with in the building.
During the detailed design, the design engineer also needs to make a choice on the design theory to use. The basic design theories include; the limit state design, plastic design and permissible stress design.
- For permissible stress design, the stresses in the structure at working loads are limited by a certain proportion of the yield stress of the construction material.
- In plastic design, load factors are used to account the behaviour of a structure when the yield point is reached. With the help of the load factors, the working loads on the structure can then be determined.
- The limit state method combines both the plastic and permissible stress methods. The limit state design aims at ensuring that there is an acceptable possibility that the structure will not become unfit for its intended use during its design life, and that the limit state is not exceed. The limit state design is also the most widely used in structural codes of practice.
Once the structural system for the building is defined, the design engineer has to prepare a structural model for the building consisting of the building elements such as slabs, beams and columns. Depending on the complexity of the project, the structural model may be either 2D or 3D, and a number of the software solutions are available to aid in the prepare of these models such as STAAD Pro, Prota Structure, Robot Structural Analysis and Midas Gen.
When the structural model is ready with; correct loading, accurate dimensions and material allocations for the different structural elements, the design engineer can then carry out can analysis on the model to determine the axial force, shear forces, bending forces and combined stress with in the elements. These results from the analysis can be used to size the different elements based on the design codes.
A structural system consists the components and mechanisms that a building requires to safely receive and transfer loadings to its base support. The structural system also consists of the superstructure and substructure. The superstructure is the part of the building that built above the ground level, while the substructure consists components below the ground level such as foundations.
The superstructure is supposed to be designed to safely transfer the building loads to the substructure. The superstructure made of; the floor system (slabs and decks), horizontal support members(beams) and vertical support members (columns and walls). The horizontal support members normally support the floor system, and transfer their loads to vertical support members. The horizontal support members include beams, lintels, stringers and purlins. The loads from the floor system and horizontal supports are transmitted to the vertical supports such as columns and load bearing walls. A much-detailed discussion of each of these structural elements will be given in the subsequent parts of this series.
Once the loads from the superstructure reach the foundation system, they have to be safely transmitted to the ground such that the final settlement of the building within an acceptable range. The type of foundation that a building has could vary depending on the soil condition in the building locale. The different types of the foundations include; raft footings, pad footings and strip footings.
Due to the need for the standards for the building to ensure safety, codes of practice are in place to ensure that designers use proper methods and principles in design. These codes of practice differ from the country to country, and hence it important for structural engineers to get familiar with the code requirements in their country of practice. Some of the codes of practice used include; Euro Codes, British Standards and American Codes.
Drafting is final stage of the design process which involves preparation of the structural drawings to be used during the construction process. The drawings and information provided at this stage need to be detailed and clear to ensure that the construction process moves smoothly.
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