There are three different methods of steel structure design, namely simple design, continuous design and semi-continuous steel design.
The joints in the structure are assumed to be fixed or rigid to make design calculations easy to manage.
In a simple design, the joint is idealized as a perfect pin. The continuous design assumes that the joint is rigid and the connected members will not rotate relative to each other regardless of the applied torque.
The vast majority of designs carried out today have adopted one of these two assumptions, but a more realistic alternative is now available, namely semi-continuous design.
The following is the method of structural steel design:
1. Simple design of steel structure
Simple design is the most traditional method and is still commonly used.
It is assumed that no moment is transferred from one connected member to another, except for the nominal moment due to joint eccentricity.
The resistance of the structure to lateral loads and sway is usually ensured by providing support or through concrete cores in certain multi-story buildings.
It is important for the designer to recognize the assumptions about joint response and ensure that the details of the connection are such that there are no moments that would adversely affect the structural performance.
Years of experience has shown that, to meet the standard type of detail, the designer should refer to the standard connection of the joint in a simple structure.
2. Continuous design of steel structure
In continuous design, it is assumed that the joint is rigid and transfers moments between members.
The stability of the frame against sloshing depends on the action of the frame (ie, by bending beams and columns).
Continuous design is more complicated than simple design, so software is usually used to analyze the framework.
When designing a continuous frame, the actual combination of pattern loading must be considered.
The connection between the components must have different characteristics, depending on whether the design method of the frame is elastic or plastic.
In elastic design, the joint must have sufficient rotational stiffness to ensure that the distribution of forces and moments around the frame does not differ significantly from the calculated distribution of forces and moments.
The joint must be able to withstand the moments, forces and shear forces generated by the frame analysis. In plastic design, the strength (not stiffness) of the joint is critical when determining the ultimate load capacity.
The strength of the joint will determine whether the plastic hinge appears in the joint or component, and will have a significant impact on the collapse mechanism.
If the hinge is designed to appear in the joint, the joint must be designed in detail so that it has sufficient ductility to accommodate the final rotation. When calculating beam deflection, yaw deflection and yaw stability, the stiffness of the joint will be very important.
3. Semi-continuous design of steel structure
A true semi-continuous design is much more complicated than a simple or continuous design because the real joint response can be represented more realistically.
Analysis routines that closely follow true connection behavior are very involved and not suitable for routine design because they require the use of complex computer programs.
However, there are indeed two simplified procedures for supporting and unsupported frames; these will be briefly described below.
A supporting frame is a frame that provides resistance to lateral loads through a supporting system or core; in an unsupported frame, this resistance is generated by the bending moments of the columns and beams.
The simplified process is:
(I) The wind moment method is suitable for unsupported frames. In this process, when considering the gravity load, it is assumed that the beam/column node has been pinned. However, they are assumed to be rigid under wind loads, which means that the lateral loads are borne by the frame. A complete description of the method can be found in the reference.
(Ii) Semi-continuous design of supporting frame. In this process, the actual joint behavior is considered to reduce the bending moment applied to the beam and reduce the deflection. The details of this method can be found in the reference.