At the core of any building is a set of structural elements that are designed to withstand and transmit loads through the structure without compromising on design or safety. There are several key elements that comprise any structural design that any builder should be aware of. The major design elements from the top-down are the roof deck (trusses), roof and floor slabs, beams, columns, load-bearing walls, and foundations (could be strip, isolated, raft, etc.)
Having a good understanding of these different elements and others such as shear walls will enable you to create safe and cost-effective structures that can live up to their service expectations. Without a solid understanding of these structural elements though, you might end up paying a premium for your structure when you could have enjoyed cost savings. Let’s dive into each element individually and explore how you can reduce the overall cost while maintaining your structure’s integrity.
1. The Roof Deck
The roof is an important component of a structure that provides cover from rain and moisture, and also shelters the building’s occupants from external elements. The roof is potentially one of the most expensive structural elements of a building. The complexity of the roof truss system, the roof plan area, and the type of roofing materials used will heavily influence the cost of the building project.
Cost-Saving Tip: Ensure that all roof members are designed to the correct sizes. Use a lightweight material on your truss, such as asphalt shingle or coated roofing sheets instead of stone or clay tiles. This will not only reduce the actual material costs but will also reduce the roof loads, subsequently reducing the sizes and costs of other members, particularly the columns and foundation depth. Use a simple roof plan design if budget is a major issue. Flat roofs are a cost-saving alternative for buildings with a small plan area.
2. Slabs
Slabs are key structural members that carry floor loads and are submitted to bending moments. Floor loading includes the weight of the slab itself, plus any machinery, equipment, furniture, and other types of live loading. Two-way slabs are slabs that are reinforced in both short and long-span directions, while one-way slabs are only reinforced in a single primary direction. Slab loads are transmitted to beams which then transit these loads to columns.
Cost-Saving Tip: Ensure to adhere to the design provided by your structural engineer since this will always be the most cost-effective. Usually, domestic builders seek the services of an architect who can create a great pictorial impression of the project but not necessarily the correct sizes. Consider using pre-fabricated lightweight slabs since these are quicker to install and also offer reduced costs, and better-quality slabs.
3. Beams
Like slabs, beams are major flexural members that deform due to bending. Thicker beams provide better resistance to flexural loading, but they are also more expensive due to the added need for extra reinforcement and concrete. This also affects the sizing of other structural members such as columns and foundation footings.
Cost-Saving Tip: All beams should be sized by the structural engineer and not an architect. The beam should also be well-designed to avoid using too much reinforcement. Pre-cast concrete beams offer a great alternative for cost and time savings, although not hugely popular in the country, especially for residential dwellings.
4. Columns
Columns are structural members that take axial compression loads, meaning that compressive forces act through the centroid of the column. Naturally, columns at the top of a building transmit fewer loads and should thus be provided with smaller thicknesses than columns in the lower parts of a building. It is best practice to provide columns for multistorey houses with a minimum of 12 mm diameter longitudinal (axial) reinforcement.
Cost-Saving Tip: Size all columns correctly about their position in the structure. Higher columns should be smaller in dimensions than lower columns. Columns should be sized by an engineer to avoid oversizing.
5. Load-bearing Walls
Load bearing can either act singly in a system (for example, retaining walls) or they can be connected to the rest of the column-beam framing system (for example, shear walls for elevator shafts, or shear walls in multi-story and high-rise buildings designed to counter the effect of wind loading.) They require longitudinal and transverse reinforcement, therefore, these structures are quite expensive.
Cost-Saving Tip: Don’t use load-bearing walls unless it is critical to the stability of your structure, for example, to counteract the effect of retained soil on a bottom floor, or in the case of a lift shaft.
6. Footings and Foundation
The foundation carries and dissipates the entire loading from a building to the subgrade below it. Therefore, it is best in design practice to ensure that the subgrade is stable first before building a foundation. The foundation should wholly be designed after geotechnical surveys (or rudimentary borehole surveys) have been carried out to establish the bearing capacity of the soil.
Footings (for example, isolated or strip footings) carry the entire load of the structure transmitted by the columns and therefore experience substantial pressure. Thus, footings experience both bending and compression forces and should be designed for such.
Cost-Saving Tip: All the building loadings should be calculated first before designing the foundation. Likewise, the subgrade bearing capacity should be determined before designing a foundation. Strip footings work best for simple structures with a smaller plan area or those with low loading, while isolated footings and raft footings are preferable for larger structures.
Conclusion
Every structural design including member sizing should be carried out by an experienced structural engineer. They are best placed to optimize your building costs, while an architect should be relied upon for the overall planning and aesthetic of your project. Never rely solely on your judgment when making building plans as you may overlook something important.
