Joints in structures have been assumed to behave as either pinned or rigid to render design calculations manageable. In ‘simple design’ the joints are idealised as perfect pins. ‘Continuous design’ assumes that joints are rigid and that no relative rotation of connected members occurs whatever the applied moment. The vast majority of designs carried out today make one of these two assumptions, but a more realistic alternative is now possible, which is known as semi-continuous design.

Following are the methods of structural steel design:

1. Simple design

Simple design is the most traditional approach and is still commonly used. It is assumed that no moment is transferred from one connected member to another, except for the nominal moments which arise as a result of eccentricity at joints.

The resistance of the structure to lateral loads and sway is usually ensured by the provision of bracing or, in some multi-storey buildings, by concrete cores. It is important that the designer recognises the assumptions regarding joint response and ensures that the detailing of the connections is such that no moments develop that can adversely affect the performance of the structure. Many years of experience have demonstrated the types of details that satisfy this criterion and the designer should refer to the standard connections on joints in simple construction.

2.  Continuous design

In continuous design, it is assumed that joints are rigid and transfer moment between members. The stability of the frame against sway is by frame action (i.e. by bending of beams and columns). Continuous design is more complex than simple design therefore software is commonly used to analyse the frame. Realistic combinations of pattern loading must be considered when designing continuous frames. The connections between members must have different characteristics depending on whether the design method for the frame is elastic or plastic.

In elastic design, the joints must possess sufficient rotational stiffness to ensure that the distribution of forces and moments around the frame are not significantly different to those calculated. The joint must be able to carry the moments, forces and shears arising from the frame analysis.

In plastic design, in determining the ultimate load capacity, the strength (not stiffness) of the joint is of prime importance. The strength of the joint will determine whether plastic hinges occur in the joints or in the members, and will have a significant effect on the collapse mechanism. If hinges are designed to occur in the joints, the joint must be detailed with sufficient ductility to accommodate the resulting rotations. The stiffness  of the joints will be important when calculating beam deflections, sway deflections and sway stability.

3.  Semi-continuous design

True semi-continuous design is more complex than either simple or continuous design as the real joint response is more realistically represented. Analytical routines to follow the true connection behaviour closely are highly involved and unsuitable for routine design, as they require the use of sophisticated computer programs. However, two simplified procedures do exist for both braced and unbraced frames; these are briefly referred to below. Braced frames are those where the resistance to lateral loads is provided by a bracing system or a core; in unbraced frames this resistance is generated by bending moments in the
columns and beams.

(i)  The wind moment method, for unbraced frames. In this procedure, the beam/column joints are assumed to be pinned when considering gravity loads. However, under wind loading they are assumed to
be rigid, which means that lateral loads are carried by frame action. A fuller description of the method can be found in reference.

(ii)  Semi-continuous design of braced frames. In this procedure, account of the real joint behaviour is taken to reduce the bending moments applied to the beams and to reduce the deflections. Details of the method can be found in reference.