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Steel Designers' Manual - 6th Edition (2003)
Mode 3 Bolt failure
The potential resistance of the bolts in the tension zone is give by the following
expression:
Pr =SPt
In each of the above modes no specific mention is made of prying action nor are
any equations given to calculate its value. This is because prying action is implicit
in the expressions for the calculation of the effective length Leff.The principal author
of this method, Zoetemijer, addresses the problem of prying action in a background
publication. 12 In this publication Zoetemijer develops the following three expressions
for the equivalent effective length of an unstiffened column flange taking into
account different levels of prying action:
For prying force = 0.0 Leff = (p + 5.5m + 4n)
For maximum prying force Leff = (p + 4m)
For an intermediate value Leff = (p + 4m + 1.25n)
where p is the bolt pitch
Zoetemijer explains that the first expression has an inadequate margin of safety
against bolt failure while the margin of safety in the second is too high. He therefore
suggests using the third equation, which allows for approximately 33% prying
action. This approach simplifies the calculations by omitting complicated expressions
for determining prying action.
BS 5950: Part 1 allows two approaches for calculating the tension capacity of a
bolt in the presence of prying forces. The simple method given in clause 6.3.4.2
places certain restrictions on the centre-to-centre bolt spacing and on the capacity
of the connected part to reduce prying action. A reduced bolt capacity is also used
by including a 0.8 factor in the expression for calculating the nominal tension capacity
(pnom) of the bolt. One advantage of this approach is that it obviates the need to
calculate prying forces directly. In the more exact approach given in clause 6.3.4.3
bolt tension capacities (pt) are allowed provided the connection is not subject to
prying action or the prying forces are included in the design method. As explained
above the design method described here allows for prying action without the need
to calculate prying forces directly and therefore the enhanced bolt capacities of the
more exact method can be used.
Beam web/column web in tension
The resistance of either an unstiffened beam or column web in tension, Pt, at each
row or group of bolt rows is given by the following expression:
Pt = Lt ¥ tw ¥ py
Moment connections 761
where Lt is the effective length of web assuming a maximum spread at 60° from the
bolts to the centre of the web