The construction of multistory buildings requires the integration of various structural systems that ensure the building’s stability, strength, and durability, particularly against forces like earthquakes. These structural systems are designed to provide resistance to both vertical loads (such as the weight of the building) and lateral forces (such as those caused by wind or seismic activity). As cities grow taller, the complexity and necessity of choosing the right structural system for a building’s specific needs become paramount. This article will explore several key structural systems used in multistory buildings, emphasizing their strengths, weaknesses, and suitability for earthquake-resistant designs.
1. Moment Resisting Frames
A moment resisting frame is one of the most common systems used for multistory buildings, especially those up to 20 stories. In this system, the structure consists of plane frames arranged in two perpendicular directions, forming a rigid framework that resists lateral loads (such as those from earthquakes). This configuration provides high redundancy and torsional rigidity, making it capable of withstanding horizontal forces from any direction.
The key feature of moment resisting frames is their ability to resist bending moments, shear forces, and axial forces through their column and beam sections. However, these members must be enlarged at lower stories due to the increased magnitude of these forces as the height of the building increases. While the system is effective for structures of up to 20 stories, one critical point of failure lies at the joints between columns and beams. These joints must be properly detailed to avoid structural weaknesses, as improperly designed joints can lead to the occurrence of a “soft storey,” which can compromise the building’s integrity during an earthquake. Therefore, moment resisting frames should only be used if the design, particularly the joint detailing, is executed to a high standard.
2. Shear Wall Systems
A shear wall system is another common approach for multistory buildings. In this system, vertical shear walls are used to resist lateral forces, taking on a significant portion of the shear base when used alongside columns in a building. Shear walls are preferred because they offer superior stiffness compared to columns, resulting in reduced lateral deflection under earthquake loads. In addition, shear walls have a better capacity to handle bending moments than columns.
Typically suitable for buildings with up to 20 stories, shear wall systems have proven effective in seismic areas. However, one downside is their lower redundancy compared to framed systems, which means they are less flexible in terms of accommodating unexpected loads. Despite this, shear wall systems have been shown to perform well in earthquakes, making them a reliable choice in regions prone to seismic activity.
3. Frame-Wall or Dual Systems
A frame-wall or dual system combines the benefits of both shear walls and moment resisting frames, offering a more robust solution for tall buildings. This system typically places shear walls at the building’s core (often around staircases and elevators) and frames around the perimeter. The combination of these two systems provides enhanced strength and resistance to lateral forces.
One of the advantages of dual systems is that they eliminate the drawbacks of using either frames or shear walls alone. For instance, frames placed around the perimeter of the building and shear walls placed at the center help balance both the gravity load-carrying capacity and the stiffness of the structure. This dual configuration significantly reduces displacement during earthquakes, making it suitable for buildings up to 50 stories tall.
The dual system provides two lines of defense against seismic forces: the shear walls form plastic hinges at the base, and once these hinges are formed, the moment resisting frames take over, helping to control further movement. This redundancy makes the frame-wall system highly earthquake-resistant.
4. Flat Slabs Combined with Shear Walls and Frames
In some cases, a flat slab system combined with shear walls and frames is used, particularly in buildings up to 10 stories. In this system, two-way flat slabs rest on columns, with shear walls placed around the perimeter or core of the building. Moment-resisting frames are often located at the perimeter to resist lateral forces.
While this system is relatively simple and cost-effective, its main limitation is the low ductility of the flat slab-column joints. As such, this system is best suited for gravity loads rather than lateral forces from wind or earthquakes. Despite this, the flat slab system is an attractive option for structures that prioritize simplicity and clear storey height, especially for lower-rise buildings.
5. Tube Systems
The tube system is a more advanced version of the moment resisting frame. In a tube system, the structure is designed as a three-dimensional rigid frame that can withstand lateral loads, similar to a cantilever placed perpendicular to the ground. The system gets its name from the “tube” formed by the perimeter columns and beams, which provide the building with structural rigidity and the ability to resist lateral forces.
The strength of a tube system can be enhanced by reducing the spacing between the perimeter columns, increasing the depth of spandrel beams, and even combining multiple tubes within the structure. This makes the tube system an efficient and effective solution for taller buildings, capable of resisting both lateral earthquake forces and wind loads.
6. Mega Core Systems
For extremely tall buildings (50 stories and beyond), the mega core system is often employed. This system uses reinforced concrete or composite shear walls with much larger cross-sections than traditional shear walls, extending continuously throughout the full height of the structure. The mega core serves as the main vertical support for the building, providing stability against both lateral and vertical loads, including those caused by wind and seismic activity.
The mega core system is particularly useful for skyscrapers, where the core serves as the backbone of the structure, helping distribute both gravity loads and lateral forces evenly. An example of a building that uses a mega core system is the Shenton Way structure in Singapore, which is designed to withstand the immense forces exerted by earthquakes and high winds.
7. Conclusion
The design and construction of multistory buildings require careful consideration of the structural systems used. Each system discussed—whether it’s moment resisting frames, shear wall systems, dual systems, flat slabs, tube systems, or mega core systems—has its unique strengths and is suitable for different building heights, load requirements, and earthquake resistance needs.
Ultimately, the selection of the right structural system depends on the specific requirements of the building, including its height, location, and the anticipated seismic activity. As building technologies evolve, these systems continue to improve, offering safer and more efficient solutions for taller and more complex buildings. Understanding the advantages and limitations of each system is essential for engineers and architects in creating buildings that are both resilient and sustainable.