Terminology and Concepts
The MasterKey: Moment and Simple Connections module is based on the design methodology employed in the Steel Construction Institute (SCI) Green Book publications. Steelwork Connections are classified in the design codes by the joint stiffness as rigid, semi-rigid or nominally pinned, or by strength as full strength, partial strength of nominally pinned. The SCI Green Book publications cover the design of simple (nominally pinned and moment resisting (rigid, full strength continuous) connections. UK design guidance does not cover the design of semi-rigid connections, where the rotational stiffness of the connection needs to be assessed and explicitly incorporated into the frame analysis to take express account of the rotational capacity and deformation of the end plate. The UK National Annex states that:
"Until experience is gained with the numerical method of calculating rotational stiffness given in BS EN 1993 1 8:2005, 6.3 and the classification by stiffness method given in BS EN 1993 1 8:2005, 5.2.2, semi-continuous elastic design should only be used where either it is supported by test evidence according to BS EN 1993 1 8:2005, 5.2.2.1(2) or where it is based on satisfactorily performance in a similar situation."
Connections are classified as:
Simple - the joint is assumed to transmit no bending moments
Continuous - the behaviour of the joint has no impact on the analysis of the frame or connected members
Semi-continuous - the behaviour of the connection will affect the structure behaviour and needs to be explicitly taken account of in the analysis.
Simple connections are considered to be pinned connections which transmit shear. The connection, particularly in beam to column connections, must be sufficiently flexible so as to avoid the development of significant end moments. Simple connections must also be able to transmit axial loading so as to satisfy the tying requirements. A simple connection is required to be sufficiently rotationally flexible such that the joint does not develop significant end moments.
Continuous connections are considered to transmit bending, shear and axial forces, in addition to resisting the required tying forces. The connection must be sufficiently stiff rotationally that it can develop the required end moments, with sufficient stiffness to allow the joint to be considered as rigid in the analysis. Continuous connections can also be classified based on their strength. A full strength connection is such that the moment capacity of the connection is greater than the moment capacity of the connected elements. Where the moment capacity of the connection is less than the moment capacity of the connected elements, the connection is classified as partial-strength.
Connections designed in accordance with the SCI Green Books for Simple Joints may be considered to be nominally pinned. For full depth end plates, the thickness of the end plate is limited to ensure that the moment resistance is less that 25% of a full strength joint, hence allowing the connection to be considered as nominally pinned.
Connections designed in accordance with the SCI Green Books for moment resisting connections can be assumed to be rigid joints in braced frames and single-storey portal frames.
For multi-storey unbraced frames, the rotational stiffness of the joint is fundamental to the stability of the frame. In this case, the stiffness of the joint should be evaluated explicitly in accordance with BS EN 1993-1-8. If the analysis assumes that the connection is rigid, then the joint classification must match this. Furthermore, in multi-storey unbraced frames, the sensitivity to second order effects depends on the stiffness of the beams, columns and the stiffness of the joints. Therefore, if the joints have been considered rigid when determining the sensitivity of the frame to second order effects, the joint design must satisfy this rigid classification.
MasterKey:Moment and Simple connections is suitable for the design of nominally pinned simple connections and continuous rigid moment connections. It is not suitable for the design of semi-rigid connections.
Within Masterframe, it is possible to define member partial fixities such that the rotational stiffness of the end of the member is defined as a percentage of the rotational capacity of the member the partial fixity is applied to. This, in effect, allows the model to assume semi-continuous connections which have an effect on the behaviour of the structure. However, if the partial fixity is assumed to to occur due to the rotational capacity of the connection, then the underlying assumption is the that connection behaves like a rotational spring, such that the connection is not free to rotate like a simple connection, but has sufficient ductility to allow some degree of rotation relative to a fully rigid connection. Therefore, where a partial fixity is used, the design of the connection as a moment connection is not valid, since the moment connection design is based on a rigid connection design. For a rigid connection, the end plate is considered to be stiff and so does not rotate, whereas in a semi-rigid connection, the ductility of the end plate allow partial rotation. If the end plate does not permit rotation, the connection will tend to attract a higher moment than is generated in the semi-rigid analysis, which will result in higher forces in the connection components than has been designed for. This may lead to a non-conservative connection design.
In portal frame design, the partial fixities on column to base plate connections are only assumed in the serviceability cases. At ultimate limit state, the column to base plate is assumed to be pinned and is designed accordingly.
Simple connections are defined as connections which transmit end shear only. This definition underlines both the connection design and the analysis of the overall structure, with beams being designed as simply supported and columns being designed for axial load and nominal moments which results from the end reactions of the beams considered with a nominal minimum eccentricity relative to the column. To accommodate the potential rotation of the joint, end plates, where used, need to be thin. In beam to column connections, to prevent the bottom flange of the beam bearing directly against the column when under going rotation, a minimum end gap is specified.
The connection design is based on a component based model, where each part of the connection is considered as an individual component, with each component checked for the load transfer through the connection elements including welds, plates, bolts and section webs or flanges. The capacity of each component is calculated. For each component to be considered to be satisfactory, the calculated capacity of the component must be greater than the applied force. The SCI Green Book checks each connection component in a series of design procedure checks. These Checks are used in the MasterKey:Simple connections module, as noted in the design output for each connection.
The Green Book design methodology covers only vertical shear design, where the orientation of the member is such that the vertical shear results from loading the beam about its major axis. Shear due to loading about the minor axis is not covered. Lateral loading, say due to wind load, is assumed to be resisted by other means, such as transfer of the load into a floor diaphragm. Torsional resistance is also not considered as part of the Green Book methodology.
The Green Book SCI P358 provides a design method for the following connection types to EC3:
Partial depth end plates, beam-to-column and beam-to-beam connections
Fin plate connections, beam-to-column and beam-to-beam
Column Splices
Column Bases
Bracing connections
The design of double-angle cleats was excluded form SCI P358, but these connection types were included in the earlier Green Book P212. MasterKey:Simple Connections has retained the double angle cleats in the EC3 design, by adapting the methods for the British Standard design in line with the requirements of BS EN 1993-1-8. The SCI Steel Designer's Manual also includes a method for the design of double-angel web cleats.
When designing tying forces in simple connections, the check for tying resistance is entirely separate to the check for vertical shear capacity and the tow forces are not considered to occur simultaneously. The tying force design is based on the ultimate material strengths, with an appropriate material factor. When calculating the tying resistance, only the ultimate strength is considered and significant permanent deformations are anticipated. For these reasons, the tying force calculation is not appropriate for the design of simply supported members with axial forces.
Moment connections are defined as connections which are required to resist bending moments in addition to shear forces, and may also include for the effects of compression or tension axial forces. Connections designed in accordance with the SCI Green Books are considered to be rigid, both in the classification of the connection itself, but also in terms of the analytic model. Moment connection generally tend to be bolted connections with a flush or extended end-plate, though fully welded connections can be considered. To produce a sufficient lever arm to resist the moment, a haunch is often required to enhance the section depth local to the connection.
The design of a moment connection is based on a component model of the connection, with each component or part of the model considered individually and it's capacity calculated to determine the potential resistance of the connection. For a connection to be satisfactory, the capacities of the individual components are complied into a joint model from which a design resistance is calculated, the design resistance required to be greater than the applied forces on the joint. The SCI Green Book checks each connection component in a series of design procedure checks. These Checks are used in the MasterKey:Moment Connections module, as noted in the design output for each connection.
The component model splits a moment connection into four zones. These zones are the tension zone, the compression zone, the horizontal shear zone and the vertical shear zone. Each zone has a series of component checks associated with it. The checks associated with each zone are:
Tension Zone:
bolt tension
end-plate bending
column flange bending
beam web in tension
column web in tension
Compression zone:
column web in bearing
column web in buckling
beam flange in compression
Horizontal shear:
column web panel shear
Vertical shear:
bolt shear
web to end plate weld
bolt bearing
The Green Book methodology applies to connections between open I and H-section members bending about their major axes only, with only vertical shear forces and in combination with axial forces. The method does not cover minor axis bending, horizontal shear or torsion. BS EN 1993-1-8 does not preclude the design of such connections, but the SCI guidance does not cover these. the MasterKey: Moment Connections modules is based on the UK best practice guidance as presented in the SCI Green Books.
The component based model of the SCI Green Books requires that each connection be split into one of the four zones outlines above. That is, each component requires a tension zone, a compression zone, a horizontal zone and a vertical shear zone. This, then, places certain constraints on the type of connections that can be designed using the SCI Green Book methodology. For example, for a connection with no applied bending moments, the method still attempts to determine a tension and compression zone. The presence of an axial force can exacerbate this issue, since in the component model the axial force in the member is taken to act at the level of the compression zone. To overcome this potential limitation, MasterKey:Moment Connections used a modified method, based on the SCI Green Book, to accommodate connections with zero moment in the presence of axial forces. This uses the compression or tension zone calculations, as appropriate, in place of a tension and compression zone.
Bolt row capacities
The design moment resistance of a bolted end-plate connection is determined by the summation of the moments of resistance of each bolt row multiplied by it's lever arm as measured from the assumed centre of compression. The effective design tension capacity of each bot row is determined from the smallest tensile capacity of the bolt in tension, column web in tension, column flange in bending, the end-plate bending or the tension capacity of the beam web. If the connection fails in compression, a reduction in the tensile capacity is then made to maintain equilibrium. This is achieved by a reduction in the tensile capacity of the bolt rows. The procedure involves first determining the potential resistance of each row, such that each row is first checked in isolation, then in combination with successive rows above it, since the capacity of combined bolt rows may be less than the capacity of a single row due to the failure pattern of the end plate. This approach requires sufficient ductility in the connection to allow the connection to develop the design strength of the lower bolts. To achieve this, limits are placed on the thickness of the column flange or end-plate relative to the strength of the bolts. If this ductility criterion is not achieved, the force distribution in the bolts is limited to an elastic triangular distribution.
Where MasterKey:Connections includes a design with insufficient moment to develop a compression and tension zone, a modified bolt force distribution is calculated to account for the fact there is only a compression (or only tension) zones, with the axial force being split equally between the zones.
Shear capacity
The vertical shear force is first allocated to those bolts in the compression zone. These bolts, since they are not also resisting tension forces, are able to resist shear forces up to their full shear resistance, limited by the lower capacity of the bolt shear capacity or the bearing of the bolt on the end plate or column flange. The remaining shear force is then resisted by the bolts in the tension zone, where the shear capacity of the bolts is reduced by an interaction criteria. Where a bolt is resisting a tensile force equal to it's tensile capacity, it's shear resistance is reduced to 28% of its shear capacity.
A similar distinction is made when designing welds, with a zone around the bolts required to act in tension taken as the tension zone and the rest of the weld length being taken as a shear zone.
The Green Book methodology is set out for dealing with major axis moments only. No account is taken of minor axis moments. The underlying assumption in terms of calculating the resulting capacities of the component comprising the connection is that the forces in each bolt row are even split between the bolts in that row. In the case of a connection with minor axis moments, such an assumption is clearly not valid. The presence of a minor axis moment would affect the distribution of forces in the bolts, and require consideration of non-uniform stresses in the compression zone, differential bending in the end plate and column flange and out of plane bending in the column web, as well as consideration of non-standard yield line patterns in the end plate.
The case of members with major axis bending connecting into the minor axis of another member is also not covered by the Green Book connection model. A typical example of this type of connection would be a moment connection of a beam or beams to the minor axis of a column. In the case of a single sided connection or a double-sided connection where there is an out of balance in the moments, the bending moment would have to be transferred to the column through the column web. For a large number of sections, the thickness of the web itself would not be enough to consider this as a non-flexible end plate and so the web, even if considered as an end plate would not be satisfactory. Even in cases where the thickness of the end plate was satisfactory, the yield line patterns given in the Green Book would not be applicable due to the geometry of the connection, nor would the compression zone of the Green Book method be applicable to a column web.
In the MasterKey: Moment Connections module, minor axis moments are not considered. In a stand-alone connection, no input is provided for minor axis moments. In the case of a connection design linked to a Masterframe model. where minor axis moments are detected at the joints, the minor axis moment will be ignored; an on-screen warning message will be displayed.
The connection model utilised in the SCI Green Books is based on the shear being applied due to vertical load and the member being oriented such that the web is vertical. Shear due to horizontal load, or loading about the minor axis of a member, is not considered.
In stand-alone connections, in both the MasterKey Moment and Simple connection modules, all inputs for shear refer to the shear due to vertically applied loading as per the Green Book connection models. In the case of connections linked to a Masterframe model, where minor axis shear is detected, the resultant of the shear forces is taken and applied in the vertical direction only. Thus the connection is designed for the resultant shear force. When this happens, an on-screen message is displayed in the connection module. Thus, in the case where horizontal shear is present, the shear design is based on the shear capacity in the vertical direction. Thus the edge distances and bolt spacing are those for the vertical direction. Often, this is satisfactory, but in certain cases this may not be appropriate. It is the responsibility of the design engineer to determine whether or not such an approach is appropriate.
Within the MasterKey: Moment and Simple Connections, each design for any particular member is referred to a brief. Each brief is a particular set of design checks, particular to the selected design code, carried out on a specific joint within the frame, based on the analysis results derived from the MasterFrame model, or, in the case standalone connections, applied to each user input joint.
For connection designs on a linked Masterframe model. design briefs can be assigned manually to each joint or connection by the user, by selecting the required joint type and then selecting the member. Once a brief is applied to a joint, it can be copied to other connections in the model, or the brief can be removed from a connection and applied to another another in the model.
When the MasterKey:Simple Connections module is used to design the connections of beam and/or columns which also incorporate bracing elements, the main connections and the bracing connections are designed as separate entities, but the additional forces due to the bracing can be accounted for by linking the required connection design briefs.
Linked connections are those connections based an analytic model in Masterframe. Where connections are linked, both the connection geometry and forces are based on the Masterframe model. The analysis loads are automatically imported from the Masterframe analysis.
The linked design is a one-way connection, in that changes in the Masterframe model, either in loads, sections or loadcase, will automatically be updated once the connection module is entered. While in the connection module, it is possible to make modifications to the section sizes and to the connection forces, as well as adding haunches to the connection. However, these changes are not made in the Masterframe model and the original geometry and forces will be loaded from the analysis model once the connection brief is re-loaded. Therefore, to make changes to the connection geometry or forces, it is necessary to exit the connection module and make the necessary changes in Masterframe and re-analyse the model.
The ability to change forces and sections in the connection module is intended as a method to allow some experimentation in the connection to determine what connection geometry is required to be created in Masterframe. However, changes in section sizes will affect the stiffness of the structure which, for continuous construction, is liable to affect the force distribution of the model and so affect the resulting forces on the connection. Therefore, for changes in connection geometry or for changes in the frame loading, these changes are required in the analytic model.
For Simple Connections, the design of the member is based on the applied shear force in the connection. In this case, the design of the connection is based on the maximum end reaction of the member. Thus the selection of the critical design case is simply a matter of identifying the maximum shear value. In the integrated design for simple connections, the software will, therefore, identify the critical design load case.
Selecting the critical design case for moment connections is less straightforward. The connection module will attempt to utilise the minimum number of bolt rows for each design case. Therefore, for loadcases with lower moment, the connection forces can be based on a lower number of bolt rows in tension, increasing the number of bolts in the shear zone. This can result in cases where the utilisation ratio for the bolts is higher for load cases with lower moments. Therefore, simply using the utilisation ratios for the connection to identify the most onerous design loadcase may not correctly identify the critical loadcase. Therefore, it is strongly advised to use the scan for failures function when designing moment connections, to ensure that the input connection is satisfactory for all loadcases.
When printing load cases, it is necessary for the user to identify the critical load case(s) for printing, since a scan of the utilisation ratios is not a reliable method to identify the most onerous load case.