Steel Designers Manual - TheBestFriend.org

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Steel Designers' Manual - 6th Edition (2003)
Fig. 26.25 Distribution of bolt forces
i.
Full Plastic distribution
Moment connections 757
Traditional
distribution
method adopted here is to use a plastic distribution of bolt forces. These two
approaches are illustrated in Fig. 26.25. In the traditional UK approach the centre
of compression is assumed to be in line with the compression flange of the beam,
with the bolt row furthest from the centre of compression attracting the most
tension.
In the Eurocode approach no assumption is made about the distribution of bolt
forces. Instead, each bolt row is allowed to attain its full design strength (on the basis
of the strength of the column flange or end-plate,whichever is the lowest).This model
relies on adequate ductility of the connecting part in the uppermost bolt rows to
develop the design strength of the lower bolt rows. To ensure adequate ductility a
limit is set on the thickness of the column flange or end-plate relative to the strength
of the bolts.Where S275 steel is used with grade 8.8 bolts the thickness of either endplate
or column flange should be less than 18.3 mm,21.9 mm or 27.5 mm for M20,M24
and M30 bolts respectively. If this criterion is not satisfied then the force in the lower
bolt rows is limited to a value resulting from the linear distribution.
The design method in Eurocode 3 uses what is called the component approach.
In this approach the potential resistance of each component is calculated. These
potential forces are then converted to the actual forces by considering equilibrium.
If there is a surplus capacity in the bolt forces then these forces should be reduced,
starting with the lowest row of bolts and working upwards progressively until equilibrium
is achieved. The moment capacity of the connection is then calculated by
summing the product of all the bolt row forces and their distance from the centre
of compression.
In all, there are 15 principal checks to be made on the beam, the column, the bolts
and the welds. These checks are shown in Fig. 26.26 and can conveniently be split