A shear frame, often also called a shear building, is an idealized structural model used in dynamic structural analysis to simplify calculations, particularly when dealing with dynamic loads like earthquakes or wind.
Here's a breakdown of what makes it a shear frame:
Key Characteristics of a Shear Frame
The core concept of a shear frame lies in its assumptions about stiffness:
- Floor Elements are Rigid: A shear frame assumes that the floor elements (beams and columns that make up the floors) possess extremely high stiffness compared to vertical elements (columns that run from floor to floor).
- This implies that the floors themselves don't deform or bend. Instead, they move as rigid bodies.
- Vertical Elements Provide Lateral Resistance: The shear frame relies heavily on the stiffness of its vertical elements (columns) for lateral (sideways) load resistance. This stiffness prevents the building from collapsing or significantly deforming horizontally.
- Shear Deformation Focus: The primary focus of the analysis is on the "shear" deformation or displacement of the frame between different floor levels. Hence the name "shear frame".
Understanding the Idealization
Imagine a multi-story building. In a real building, beams and columns bend under load. However, for a shear frame:
- The floors are treated as infinitely stiff diaphragms. Therefore, each floor moves horizontally without any internal bending.
- All the lateral displacement happens because of deformation in the columns between the floors. The columns bend under lateral forces, and this is the only deformation considered.
- The behavior is simplified to a stack of rigid floor slabs supported by flexible vertical columns.
Why Use a Shear Frame Model?
- Simplified Analysis: Using the shear frame approach allows for easier and faster calculations when dealing with complex structural dynamics.
- Focus on Overall Response: It provides an efficient means to focus on the overall dynamic response (like modal frequencies, mode shapes) of the structure without being bogged down by the detailed complexities of internal member behavior.
- Good Approximation for Some Buildings: For many regular buildings, particularly those with strong, deep floor slabs, the shear frame provides a reasonably accurate simplification.
Practical Insights
- Not for all Structures: It's important to remember that this model is an idealization. It will not be accurate for buildings with irregular layouts, very tall and slender structures, or when the floor slabs are not stiff.
Example
Consider a simple 3-story building:
- In a shear frame model, all three floors are considered rigid.
- When an earthquake strikes, the base columns deflect at their base. The base floor slab moves sideways.
- The second story moves even further, with deflection occurring at the columns between the first and second floor.
- The top story moves the most due to the deflection of the columns supporting it from the second floor.
This simplified movement and deformation analysis of a structure under dynamic forces helps in understanding a building's behavior using a shear frame model.
