Basic Types of Steel Frame Construction Connections
In frame structures, detailing of beam-to-column connections incorporates a range of connections to account for actions. In general, beam to column connections are made by bolting or welding or by using fixed I/J connections. Selection of the connections depends upon the requirements of the structure. For example, an un-stiffened frame, which is subjected to bending moments, may be fully welded I/J connections. For a mobile structure, connections made by Grade 8.8 bolts may provide a restraint, like seat belts. These connections are easy to remove and replace. Similar design approach may be taken to incorporate connections to account for potential movements, which may be by pin connections. For movements in bridge structures, pin connections may be used. Fixed connections in a frame are incorporated in mega frames and skyscrapers to account for resistance to lateral loads. From an economic point of view, other frames may incorporate less detailing. From an aesthetic point of view, mega frames include a high level of detailing. For big diameter members, welded connections satisfy more provisions of EN 1993-1-8 than bolted connections.
The use of welds in steel beam to column connections increases the overall frame stiffness and helps distribute loads across the frame. For full penetration welds, the joint is considered to be solid. Loads are applied to the joint over a long period of time and are sustained for a long time. Therefore, the use of full penetration welds is preferred over the use of a number of bolts. The phrase, "The provisions in EN 1993-1-8 support the engagement of competent persons", means that the involvement of such persons is commendable. It's the responsibility of such competent persons to determine and verify whether a weld is performed in accordance with the defined standards. A shop may perform ultrasonic tests to identify possible defects in a weld. Such welds eliminate the problems associated with the use of bolts. However, from a responsibility and stress management perspective, employees have to be extra cautious due to the effects of the surrounding environment and weather conditions, especially the variations that occur during the welding process.
Connections: End Plates, Seated, Fin Plate Systems
In the assembly of steel frames, bolted connections provide numerous benefits. The main systems in use today are:
- End plate connections, which facilitate moment transfer from the lower to the upper tension or lower to the upper compression flange.
- Seated connections, which provide vertical support with the aid of a lower angle cleat .
- The use of Fin plate connections has gained some popularity. These types of connections are said to reduce the field time required for connection of members.
As stated above, the main purpose of all these connection systems is to reduce the requirements for design and construction tolerances as affected by various modifications. Relatively recently conducted tests on Extended End Plates (EEP) showed an improvement in resistance of moments of up to 15% in comparison to a design of zero thickness. Variability in field changes is greater than that normally allowed in structural design.
Connection Factor Welded Bolted
Speed of Installation Fast (pre-fabricated) Slow (site welding)
Flexibility of Modification Limited (welding) Flexible (pre-fabricated)
Complexity of Inspection Requires NDT Visual Check (pre-fabricated)
Superior Resistance to Vibration Requires Use of Lock-nuts
Various Design Considerations for Efficient and Economic Steel Connections (pre-fabricated)
Transfer of Moments, Ductility and Joint Efficiency
In moment frames, ductility is needed to control the amount of lateral frame deformation. To control the deflection of moment connections, the ductility of the connection must be sufficient to accommodate the frame deformation. The connections should be designed to achieve the required level of ductility. During a seizmic event, if connections are designed in a stiff manner, then the connection may fail brittlely and prematurely. This does not mean that an extremely flexible connection is recommended. An ideal compromise is necessary.
H-shaped Section Design and Assembly
In field assembly, the protocol is to bolt high strength fasteners to approximately 70% of the full torque, with alignment of beams to within a limit of L/500. Until 75% of the connections reach full strength, structures require temporary bracing. This helps in minimizing field headaches. Consistent load distribution helps improve performance of the structure.
Frequently Asked Questions
What are the different types of connections in a steel frame structure?
Different types of connections in steel frame structures include, but are not limited to, welding, bolting, riveting and pin connections.
What are the advantages of using a welded joint in a steel frame structure?
The use of a welded joint results in a joint with much greater stiffness, and as such play a pivotal role in the performance of the frame structure. Also, a frame joint allows the frame structure to carry the load evenly which is distributed over the joint area.
What are the advantages of using bolted connections in a steel frame structure?
The main advantage of using a bolted connection is that the frame structure can be easily assembled, dismantled and transported. Bolted connections of frame members can be transported easily if the structure is large.
What are the frame connections for a steel structure, and how are they designed?
Moments of load acting on a particular frame are transferred to adjacent members. Joints in steel structures are designed to provide ease and economy in construction of structures.