Seismic resistance is a critical factor in ensuring the safety and durability of buildings during earthquakes. Architectural features such as the size, shape, and layout of a structure can significantly impact how a building responds to seismic forces. As Henry Degenkolb, a noted expert in structural engineering, once said, “If we have a poor configuration to start with, all the engineer can do is to provide a band-aid — improve a basically poor solution as best as he can.” This statement emphasizes that the foundation of seismic resistance is laid in the planning phase, where architects and structural engineers must collaborate to ensure that the building is designed with seismic resilience in mind. In this article, we will explore how various architectural features can influence the seismic resistance of structures.
I. Size of Buildings
The size of a building plays a crucial role in its performance during an earthquake. Structures with dimensions that are disproportionate in terms of height, width, or length can face severe seismic challenges. Here are some ways in which the size of a building can impact its seismic resistance:
1. Tall Buildings
Tall buildings, particularly those with a large height-to-base ratio, experience significant horizontal movement during ground shaking. This can destabilize the structure, increasing the likelihood of overturning. Tall buildings may also face difficulties in transferring the seismic forces to the ground efficiently, which leads to poor seismic performance. When the height of a building far exceeds its width, the risk of toppling during an earthquake increases.
2. Short and Long Buildings
Short buildings that are extremely long tend to suffer from many damaging effects during earthquakes. These buildings can experience excessive horizontal seismic forces, which may be difficult to carry by the structural elements such as columns and walls. Consequently, these buildings may face significant deformations, or even failure, during seismic events.
3. Large Plan Area Buildings
Buildings with large plan areas, such as warehouses, are particularly susceptible to seismic forces. These structures often face challenges in distributing seismic forces evenly across their columns and walls, making it difficult to carry the load without significant stress or failure. The larger the plan area, the more likely the building will face performance issues during an earthquake.
4. General Rule
Buildings whose dimensions are not well-proportioned — one dimension significantly larger or smaller than the others — generally perform poorly in earthquakes. Such buildings tend to suffer from instability and excessive seismic forces in one direction, leading to poor seismic resistance.
II. Horizontal Layout of Buildings
The horizontal layout of a building — its shape in plan view — is another architectural feature that can dramatically influence its seismic behavior. Buildings with simple, symmetric shapes tend to perform better during earthquakes, while more complex geometries may be prone to higher levels of damage.
1. Simple Geometry
Buildings with simple, regular geometries such as rectangular or square plans generally perform well during strong earthquakes. Their uniform shape helps distribute seismic forces evenly, reducing the chances of torsional effects or instability. These structures have a better ability to resist seismic forces in all directions.
2. Re-entrant Corners
Buildings with re-entrant corners, such as those shaped like a U, V, H, or + in plan, are more vulnerable to earthquake damage. The interior corners in these shapes cause the building to twist during seismic shaking, which increases the risk of structural failure. These shapes can create localized stress concentrations that amplify the impact of seismic forces.
3. Breaking Up Complex Shapes
One strategy for mitigating the negative effects of complex building shapes is to break them up into simpler, more manageable components. For example, an L-shaped building can be separated into two rectangular sections with a joint at the junction. This technique helps to prevent torsional movements that would otherwise lead to significant damage.
4. Column/Wall Distribution
Even buildings with simple geometric shapes can suffer significant seismic damage if the distribution of columns or walls is uneven. If columns and walls are not symmetrically placed or fail to provide adequate support in certain areas, the building is more likely to twist and experience structural instability during an earthquake.
III. Vertical Layout of Buildings
In addition to horizontal layout, the vertical configuration of a building is essential for ensuring its seismic performance. The way earthquake forces are transmitted from the upper floors to the foundation must be continuous and without deviation. Any discontinuities in this load transfer path can significantly weaken the building’s seismic resistance.
1. Load Transfer Path
The earthquake forces generated at different levels of a building need to be efficiently transferred down to the ground. This transfer should follow the shortest and most direct path. If there are deviations or disruptions in the load transfer path, such as discontinuities between floors or structural elements, the building’s ability to withstand seismic forces is compromised.
2. Vertical Setbacks
Buildings with vertical setbacks, where a higher story is wider than the floors below (often seen in hotels), introduce sudden changes in the earthquake force distribution. These changes create discontinuities in the load transfer path, which can lead to increased stresses at the level of the setback and result in poor seismic performance.
3. Soft Stories
A soft story refers to a floor in a building that has fewer columns or walls than the floors above or below it, or has unusually tall stories. These soft stories are vulnerable to damage during earthquakes, as the lack of structural support at those levels can cause a collapse or failure to propagate upward from the soft level.
4. Unequal Column Heights on Slopes
Buildings constructed on sloped ground often experience unequal column heights due to the uneven terrain. These buildings are prone to twisting during earthquakes, as the shorter columns are subjected to more intense forces compared to the taller ones. This uneven distribution of forces can lead to significant damage, especially in buildings with long, sloping sections.
5. Hanging or Floating Columns
Buildings with hanging or floating columns — those that do not extend all the way to the foundation and are instead supported by beams at intermediate levels — experience a disruption in the load transfer path. These discontinuities significantly reduce the building’s ability to resist seismic forces, making such buildings highly vulnerable to damage or collapse during an earthquake.
IV. Adjacency of Buildings
The proximity of buildings to each other can also impact their seismic performance. When buildings are located too close together, they may collide or “pound” against each other during strong seismic shaking, exacerbating the structural damage.
1. Pounding Between Buildings
When two buildings are too close, the lateral movement caused by an earthquake can cause them to collide, leading to significant damage. The closer the buildings, the greater the risk of pounding. The problem becomes more pronounced as building height increases, as taller buildings may impart more force onto shorter buildings in close proximity.
2. Mismatch in Building Heights
Buildings of unequal height, especially when located next to each other, can lead to dangerous collisions during earthquakes. For example, the roof of a shorter building may collide with the mid-height of a taller building, causing localized damage to both structures. The height mismatch exacerbates the forces exerted during an earthquake, resulting in more significant damage than if the buildings were of similar size.
Conclusion
Architectural features such as size, shape, layout, and vertical configuration play a critical role in determining the seismic resistance of a building. A well-designed building that takes into account seismic forces from the outset can perform better during an earthquake and prevent catastrophic damage. Therefore, it is essential for architects and structural engineers to collaborate closely during the planning and design phases to ensure that the building is configured in a way that minimizes the risks associated with seismic events. By addressing these architectural features, we can create buildings that not only meet functional and aesthetic goals but also provide safety and durability during earthquakes.