CAPÍTULO II. FUNDAMENTACIÓN TEÓRICA DE LA INVESTIGACIÓN
2.2 Marco referencial
2.2.7 Principales problemas en distintas partes del árbol
Essentially the assumptions in Fastrak Connections are those inherent in the design models for simple connections given in SCI P212(Ref. 1). However, a number of specific assumptions are made as given below.
Internal forces in the supporting member from the applied loading that it carries are assumed not to influence the connection design. For example, the axial force in a column from floors above is assumed not to affect significantly the performance or behaviour of the connection. Similarly, where simple connections and moment connections are connected to the same column it is assumed that the forces imparted by the one do not influence the other. A typical example might be moment connections to the column flanges with simple connections to the column web. Not only are the designs independent but also the detailing. Hence. in this example any stiffening required by the moment connections are assumed not to interfere with the simple connections framing into the web.
Any shear force out-of-plane of the beam web (minor axis shear force) is assumed not to influence the connection design. Similarly any axial force in the supported beam (other than due to tying action for structural integrity) is assumed not to be significant. In the Design
Options off the Design menu in Building Designer, you can specify limits for these forces below
which you believe the design will be unaffected. If Building Designer detects forces greater than these limits, the design will still proceed but the connection will be given a Warning
status and the value of these forces will be given in the results. The default values for these limits are:
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minor axis shear – 0.5 kN,•
axial force – 1.0 kN,•
major axis moment – 1.0 kNm•
minor axis moment – 0.1 kNm.The two moment limits are included even though simple connections are modelled in the analysis as pins. This is because the analysis model is a mathematical model and as such during the back substitution process very small moments that are effectively zero from an engineering point of view may exist mathematically.
Where connected beams are not orthogonal to the supporting member, Building Designer takes the following approach (see also Global analysis),
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where all angles are less than a given lower limit, then the angles are ignored and the design proceeds as if the members do connect orthogonally,•
where any angle is greater than a given upper limit, then Building Designer creates the connection but the design is not performed and the connection is given a Beyond Scopestatus,
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where one angle lies between the upper and lower limit, then the angle is ignored, the design proceeds as if the members do connect orthogonally and the connection is given aWarning status,
•
where more than one angle lies between the upper and lower limit, then Building Designer creates the connection but the design is not performed and the connection is given aBeyond Scope status.
In all cases the angle can be viewed either in the design results (on the Notes page) or by editing the connection. The upper and lower limits are,
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lower limit for slope, skew and rotation – 0.5°,•
upper limit for slope and skew – 10°, and•
for rotation – 5°.The definitions of slope, skew and rotation are depicted in the figure below.
For beam to beam connections in Building Designer, the top flanges of the supported and supporting beam are assumed to align. However, within the building model beams can be given an alignment relative to its cardinal points – the default is at the centre of the top flange. Any changes that you make to the cardinal points are not reflected in the simple connection design. For example if you set the cardinal point of a supported beam to the centroid of the section and connect this to a supporting beam with the cardinal point set as the centre of the bottom flange, the simple connection design will assume, still, that the top flanges align. Similarly, in beam to column connections the centre lines of beams and the column are assumed to node. However, within the building model both the column and the beams can be set off-grid. Any changes you make that set beams or columns off-grid will not be reflected in the simple connection design. For example, you might offset the beams on the edge of the building towards the outer face of the column to suit cladding details. Nevertheless, the simple connection design will assume, still, that the beams and the column node on the centre lines.
There are likely to be a number of positions in the building where braces connect to columns at the same position as a simple connection joining beams to the same column. The brace force is assumed to be transferred directly to the column and hence in the model has no influence on the force distribution in the connections. Similarly, the detailing of the brace end connection is assumed not to affect the configuration of the simple connections at the end of the co-joined beams.