A drag strut beam, also known simply as a drag strut or collector, is a structural component essential for transferring forces within a building's lateral-force-resisting system.
Understanding Drag Struts in Structural Design
In structural engineering, especially when designing for lateral loads like wind or seismic forces, buildings are designed with systems to resist these horizontal pressures. A key component of this system is often the diaphragm – typically the floor or roof structure – which acts like a large, rigid plate. The diaphragm collects the lateral forces applied to the building's exterior walls and distributes them to the vertical elements of the lateral-force-resisting system, such as shear walls or braced frames.
This is where the drag strut comes in. Based on code definitions, a drag strut is a diaphragm boundary element parallel to the applied load that collects and transfers diaphragm shear forces to the vertical elements of the lateral-force-resisting system or distributes forces within the diaphragm.
Think of the diaphragm as a large net catching forces. The drag struts are the ropes along the edges of the net, gathering the tension or compression forces and directing them to the main anchors (the vertical resisting elements). When referred to as a "drag strut beam," it simply emphasizes that this element is constructed as a beam, designed to carry these axial (tension/compression) and potentially some bending forces.
Function and Role
The primary functions of a drag strut beam include:
- Collecting Diaphragm Shear: It gathers the shear forces developed within the diaphragm boundary.
- Transferring Forces: It acts as a link, transferring these collected forces to the appropriate vertical load-resisting elements (shear walls, frames).
- Distributing Forces: In some cases, it helps distribute forces evenly within the diaphragm itself.
- Completing the Load Path: It ensures a continuous path for lateral forces from where they enter the building (e.g., exterior walls) through the diaphragm and into the vertical resisting elements, ultimately to the foundation.
Location and Examples
Drag strut beams are typically located:
- Along the edges or boundaries of diaphragms.
- Where changes in the diaphragm stiffness or geometry occur.
- Adjacent to openings in the diaphragm.
- Connecting wall elements or different parts of the lateral system.
Practical Examples:
- A beam running parallel to seismic force direction, connecting an exterior wall (which is acting as a shear wall) to an interior shear wall. The floor diaphragm collects load from walls perpendicular to the force; this load causes shear in the diaphragm, and the drag strut collects this shear force at the diaphragm boundary and "drags" or "pushes" it to the interior shear wall.
- A collector beam around a large opening in a floor slab, gathering forces that detour around the opening.
Why are Drag Struts Important?
Without properly designed drag struts, the forces collected by the diaphragm might not reach the vertical lateral-force-resisting elements effectively. This could lead to:
- Overstressing of diaphragm elements.
- Concentration of forces at connections.
- Potential failure of the load path, compromising the building's stability during seismic events or high winds.
Comparison with Other Elements
| Element | Primary Role in LFRS | Typical Force Carried | Location Example |
|---|---|---|---|
| Drag Strut Beam | Collects diaphragm shear and transfers to vertical LFRS | Axial (Tension/Compression) | Along diaphragm edges, connecting to shear walls |
| Shear Wall / Frame | Resists lateral forces in its plane, supports gravity loads | Shear, Bending, Axial (Vertical) | Vertical elements |
| Diaphragm | Collects lateral forces from walls, distributes to LFRS | Shear, Bending | Floors, Roofs |
In essence, a drag strut beam is a critical connector element in the lateral load path, ensuring that the forces gathered by the floor or roof diaphragm are effectively and safely delivered to the main vertical structural elements designed to resist them.
