Advisory Desk
Advisory Desk 2019
AD 425
Full depth stiffeners and lateral
torsional buckling
The SCI Advisory Desk sometimes receives questions about the
potential to use full depth stiffeners to restrain lateral torsional buckling,
suggesting that the stiffeners prevent relative movement of the
compression and tension flanges. Whilst this is true, lateral torsional
buckling is a displacement and
twist of the complete section, which
stiffeners alone do nothing to prevent.
The American Institute of Steel
Construction notes that “transverse
stiffeners are simply along for the ride”
as the sketch indicates.
Contact: Richard Henderson
Tel: 01344 636525
Email: advisory@steel-sci.com
AD 426
Bolt head protrusion through
nuts and threads in grip lengths
To ensure that bolt threads are fully engaged in the nut, BS EN 1090-2 clause
8.2.2 specifies that the protrusion must be at least one thread pitch. This
is because the very end of the bolt may be slightly convex, leading to a
reduced resistance if threads are not fully effective.
The same clause specifies the necessary numbers of threads within the
grip length (between bolt head and the nut). For non-preloaded bolts, one
full thread is required – to ensure the nut can be properly tightened. For
preloaded bolts according to BS EN 14399-3 (HR system, generally used in
the UK in preference to the HV system) or according to BS EN 14399-10 (HRC
system, commonly known as a ‘tension control bolt’), a minimum of four
threads within the tensioned length is specified. The reason for the threads
in the tensioned length is to encourage ductile behaviour – AD 268 (which
related to the BS 5950 requirements) reproduces a figure from Owens and
Cheal (Butterworths), showing significantly more elongation when there
are more threads in the tensioned length. Incidentally, BS 5950-2 required
three and five threads in the tensioned length, for class 8.8 and 10.9 bolts
respectively.
Contact: Richard Henderson
Tel: 01344 636525
Email: advisory@steel-sci.com
AD 427
Typographical error in P419
A few eagle-eyed readers have noticed a typographical error in SCI
publication P419 Brittle fracture: selection of steel sub-grade to
BS EN 1993-1-10, within the expression to determine the design crack
growth ad presented in section 3.1.1.
28 NSC
Technical Digest 2019
The sign of the fourth term in the expression should be negative and read
-6.3837 × 10-⁴t2
The typo is repeated in the numerical example in Appendix A section A.2
where the expression is stated again. However, the result of the expression
(a design crack depth of 2.26 mm) is correctly stated, having been calculated
respecting the correct sign. The tabulated values in the publication have
also been determined using the correct expression.
Contact: David Brown
Tel: 01344 636525
Email: advisory@steel-sci.com
AD 428:
Draft guidance: lateral and
torsional vibration of halfthrough
truss footbridges
Purpose of this guidance
This note alerts designers to the potential susceptibility of narrow halfthrough
footbridges to excitation by pedestrians in a lateral-torsional mode.
Eurocodes and UK National Annexes do not currently fully address this
mode of vibration, so there is a danger it may be discounted without proper
consideration. This gap in the standards has led to the need to retrofit
dampers and/or provide additional stiffening to some recently constructed
footbridges where excitation occurred due to pedestrians walking eccentric
to the deck centreline and, more significantly, from deliberate shaking of
the deck.
Affected mode of vibration
Half-through footbridges, without plan bracing to the top chord, often have
as their lowest natural mode of vibration a lateral-torsional mode. A typical
example is shown in Figure 1. The mode occurs because the open bridge
cross-section has a low torsional stiffness with a shear centre below the
deck level about which axis the rotation occurs.
Figure 1 – Lateral and torsional mode of vibration
Current UK design criteria and their interpretation
The criteria for assessing the dynamic behaviour of footbridges are outlined
in the following Eurocodes (BS EN) and BSI Published Documents (PD):
• BS EN 1990:2002+A1:2005 as modified by UK National Annex
• BS EN 1991-2:2003 as modified by UK National Annex
• PD 6688-2: 2011
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