A fully restrained (FR) connection, also known as a moment-resisting connection, is a type of structural connection used in steel construction. Unlike a partially restrained (PR) connection, a fully restrained connection exhibits high rotational stiffness and provides full resistance to bending moments. It is designed to eliminate or minimize any rotational movement at the joint, ensuring the transfer of moments between the connected members. FR connections are commonly used in steel frames to provide structural stability and resist lateral loads.
Here is a step-by-step procedure for making a fully restrained (FR) connection:
- Design and Preparation:
- Determine the required level of rotational stiffness and moment resistance for the connection based on the structural requirements.
- Select the appropriate connection components such as bolts, plates, and welds based on the design specifications, considering factors such as material strength, size, and corrosion resistance.
- Ensure that the beam and column surfaces are clean, smooth, and free from any debris, rust, or paint.
- Alignment and Positioning:
- Position the beam and column in their designated locations, ensuring proper alignment and fit.
- Use temporary clamps or tack welds to hold the beam and column in place, preventing movement during the connection process.
- Marking and Drilling:
- Mark the positions for the bolt holes on the beam and column, following the specified spacing and pattern.
- Use a drill press or portable drilling machine to drill holes through the marked positions on both the beam and column. Ensure that the holes are aligned and perpendicular to the surface.
- Deburring and Cleaning:
- Remove any burrs or sharp edges around the drilled holes using a deburring tool or file.
- Clean the holes and surfaces of the beam and column to remove any debris, oil, or grease that may affect the connection’s integrity.
- Attaching the Connection Components:
- Position the connection plates or angles on the beam and column, aligning the bolt holes with the drilled holes.
- Insert bolts through the holes, ensuring a proper fit.
- Place washers and nuts onto the exposed ends of the bolts.
- Tightening the Bolts:
- Begin tightening the bolts using an appropriate wrench or torque wrench.
- Tighten the bolts gradually and evenly, following the recommended torque specifications provided by the bolt manufacturer or the design engineer.
- Welding:
- Fully restrained connections typically require additional welding to enhance the connection’s strength and rigidity.
- Welds are applied between the connection plates or angles and the beam, as well as between the connection plates or angles and the column.
- The welding process should be carried out by qualified welders, following the applicable welding codes and standards.
- Final Inspection:
- Inspect the connection to ensure that the components are securely attached to the beam and column, and the bolts and nuts are properly seated and tightened.
- Check for any gaps, misalignment, or visible signs of damage or distortion.
- Verify that the connection exhibits the desired high rotational stiffness and provides full resistance to bending moments.
Example and Application: An example of a fully restrained (FR) connection is the connection between a steel beam and a steel column in a building frame. The connection is designed to eliminate rotational movement and provide full resistance to bending moments, ensuring the structural stability and load-bearing capacity of the frame.
Applications of fully restrained (FR) connections include:
- High-rise buildings, where FR connections are commonly used to connect beams and columns, providing robustness, stiffness, and resistance to lateral loads such as wind and seismic forces.
- Bridge structures, where FR connections are employed to resist bending moments and ensure the integrity of the bridge under varying loads and dynamic conditions.
- Industrial facilities and heavy-duty structures, where FR connections are utilized to support large loads and withstand dynamic forces generated by equipment or machinery.
Fully restrained (FR) connections offer excellent moment resistance, structural stability, and load-bearing capacity. They are extensively used in steel structures where strong, rigid connections are crucial for overall structural integrity and safety.
