In structural engineering, understanding the behavior of frames under different loading conditions is crucial for the safety and stability of buildings. A frame can be classified as either a sway frame (unbraced) or a non-sway frame (braced), depending on how it responds to lateral loads, such as wind or seismic forces. Properly identifying the type of frame helps engineers determine the appropriate analysis methods, design considerations, and material usage. This article explores the differences between sway and non-sway frames, their classification according to ACI 318-19, and factors influencing sway in frames.
Characteristics of Sway and Non-Sway Frames
Non-Sway (Braced) Frames
Non-sway frames are characterized by their ability to resist lateral forces through sufficient bracing elements, such as structural walls, shear walls, elevator shafts, and stairwells. These elements provide lateral support, preventing the frame from experiencing significant lateral displacement under applied loads. In these frames, the overall lateral stiffness is enough to ensure minimal deformation. As a result, the frame maintains its stability, and lateral displacements (or “sway”) are limited or negligible.
Sway (Unbraced) Frames
In contrast, sway frames do not have adequate lateral bracing. Instead, they rely primarily on the columns to resist lateral forces. This means that sway frames are more susceptible to lateral movement, especially under lateral loads like wind or seismic forces. When these forces are applied, the columns experience lateral displacement, which may cause the structure to sway. Because of this lateral movement, sway frames often require more detailed analysis, including second-order effects, to account for the additional forces and moments induced by the displacements.
ACI 318-19 Guidelines for Distinguishing Between Sway and Non-Sway Frames
According to ACI 318-19 (Section 6.6.4.3), a non-sway frame can be identified if one of the following conditions is satisfied:
- End Moments from Second-Order Effects: If the increase in the end moments of columns due to the second-order effects (lateral displacements) is not greater than 5% of the first-order end moments, the frame is considered non-sway.
- Stability Index (Q): If the stability index (Q) is less than or equal to 0.05, the frame qualifies as non-sway. The stability index (Q) is calculated using a specific equation based on the frame’s geometry and load conditions.
If neither of these conditions is met, the frame is classified as a sway frame. This classification indicates that the frame’s lateral resistance is insufficient, and it is prone to significant lateral displacement under horizontal loads.
Factors Contributing to Sway in Frames
Several factors can lead to sway in frames, making them behave like unbraced structures under certain conditions. These factors include:
- Eccentric Loading: Unsymmetrical or eccentric loading (where the load is not applied at the center of mass) can cause lateral displacement of the frame. This eccentricity results in a twisting or tilting effect, which increases the likelihood of sway.
- Non-Uniform Sections of Members: Variations in the size or material properties of the frame’s members (such as columns and beams) can create irregularities in the structure’s stiffness, leading to uneven resistance to lateral forces and, therefore, sway.
- Horizontal Loading on Columns: Horizontal forces, such as wind or earthquake loads, that act on the frame’s columns can cause these columns to displace laterally. This contributes to the overall sway of the frame.
- Different End Conditions of Columns: If the columns have different boundary conditions, such as fixed or pinned supports at their ends, the frame’s response to lateral loads becomes asymmetrical, potentially leading to sway.
- Settlement of Supports: Over time, the settlement of a building’s foundations or supports can lead to differential movement, which may cause the frame to sway. Uneven settlement can shift the alignment of the frame, making it more vulnerable to lateral displacements.
Key Differences Between Sway and Non-Sway Frames
The differences between sway and non-sway frames are crucial when designing a structure. Here is a summary of the key distinctions:
| Feature | Sway (Unbraced) Frames | Non-Sway (Braced) Frames |
|---|---|---|
| Stiffness | Lacks sufficient stiffness to resist horizontal forces, requiring additional forces/moments to be accounted for in analysis | Sufficient stiffness to resist lateral forces with minimal additional effects |
| Symmetry | Asymmetric loading or structure, leading to in-plane displacements | Symmetric structure and loading, leading to minimal lateral movement |
| Displacement | In-plane displacements due to unsymmetrical loading, varying supports, or different section properties | No in-plane displacements; only rotational freedom exists |
| Joint Movement | Joints move or change positions due to deformation | Joints remain in their original positions as the frame does not deform laterally |
| Analysis Requirement | Requires second-order analysis to account for additional forces and moments from displacement | Can be analyzed using first-order analysis without considering additional displacements |
Conclusion
In structural design, the classification of a frame as sway or non-sway is essential for determining how the structure will respond to lateral forces. While non-sway frames are adequately braced with lateral elements to prevent significant displacement, sway frames are more susceptible to lateral movement due to insufficient bracing. Understanding the guidelines outlined in ACI 318-19 helps engineers decide whether a frame will behave as a sway or non-sway structure. Factors such as eccentric loading, non-uniform member sections, and differential settlement all contribute to sway in frames and should be carefully considered during the design process. By understanding the distinctions between these two types of frames, engineers can ensure the stability and safety of the structures they design.
Frequently Asked Questions (FAQs)
What is a sway and non-sway frame?
A non-sway frame is a structure that is adequately braced with lateral elements like shear walls, preventing significant lateral displacement. A sway frame lacks sufficient bracing, and lateral resistance is only provided by the columns, leading to potential sway under horizontal loads.
What are the reasons for sway in frames?
Reasons for sway include eccentric loading, non-uniform sections of members, horizontal loading on columns, different end conditions of columns, and settlement of supports.
What is side sway?
Side sway, also known as lateral drift, refers to significant lateral movement of the top of a vertical member relative to its bottom due to lateral forces.