Beam Eccentricities (Pro)
Go to Design > Beam Eccentricities (Pro).
MasterFrame uses an analytical model based on the centreline of members, so normally the centreline of a beam would line up with the centreline of a column. Therefore, no eccentricities are considered at the analysis stage.
Nominal Column Moments
In simple construction building modelling and design, where pin ended beams connect to a column, it is common practice to consider the 'nominal moment' in the column created by the eccentricity of the shear force from the beam, relative to its connection point to the column. For example, where a beam connects to an 'I' section column flange, it has an eccentricity 'e1' with the column major axis of D/2. Hence the shear force from the beam 'F' creates a moment in the column of F x e1. For the parallel to beam eccentricity direction it is also standard practice to add 100mm to e1. The moment may be distributed to a column above and below depending on the structural arrangement. This same beam may also be horizontally offset from the column centre within the flange width, with eccentricity 'e2' producing further minor axis moment in the column of F x e2.
Design Time Consideration
At steel design stage there is an option to consider these nominal moments in column members in the📄 Axial Loaded Members with Moments design brief and in the 📄 Columns in Simple Construction design brief. In this process the for a column design, connected beam members on all four faces, top and bottom, are identified along with their eccentricities. The nominal moment are calculated and their distribution to the column being designed added to analysis time forces. In the 'Axial with Moment' design brief, if the analysis column end moment are greater that the nominal moment, the nominal moments are not considered, or rather the design ensures the design moments are at leas the nominal member. Also in the 'Axial with Moment' the nominal moments can be turned off on a brief or global basis. Member end releases in the columns also affect the nominal moment distribution, with a column end release attracting no nominal moment in the release axis direction.
Control over Eccentricities
The 'Beam Eccentricities' provide member group properties, where members can be selected within a group, in which beam eccentricity properties are adjusted
At design stage the default assumptions is made that beam have the accepted standard 100mm parallel to member eccentricity plus the half column section depth relative to axis being considered. The beam is also assumed to connect centrally on the column in plan (XZ plane). Hence it is not necessary to place member in a 'Beam Eccentricities' group if the the default assumption is correct. In other cases the 'Beam Eccentricities' can be controlled in MasterFrame, affecting the additional design nominal column moments from the eccentricities of the shear forces of the connected beams.
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Parallel to Member Eccentricities
For the eccentricity parallel to the beam, where the beam connects to the web of an I section column, specifying a toe plate thickness assumes the beam is connected to a plate across the flange toes, and not into the web, hence adjusting the eccentricity from tw/2 to B/2 +Toe plate thickness. The default accepted standard additional nominal 100mm can also be adjusted ( a zero value will assume the default of 100).
Perpendicular to Member (On Plan XZ) Eccentricities
If a beam is offset from the centreline of the column to which it is attached, then you can specify the offset both in the direction laterally perpendicular to the beam. The beam can be aligned laterally to the centreline of the column or to the edge of the flange. An additional offset dimension can be added.
The other end (End 2) of the beam can be offset to different dimensions if required, otherwise it is assumed both ends are offset by the same amount.
The horizontal to member offsets are drawn in 3D member profile view.
Further Guidance
For more guidance on how nominal column moments are considered at design time, please see link below.