Advisory Desk
NSC 29
Technical Digest 2018
AD 420:
Minimum values of shear and
bending moment in beams with
web openings
Table 3.1 of SCI publication P355 gives minimum values of co-existent shear
and bending moment to be used at beam openings. This AD provides clarity
on how these minimum values are to be used.
The concern behind the minimum values was to allow for non-uniform
loading, to guard against the situation when the shear force at an opening
could theoretically be zero. Table 3.1 therefore includes minimum values of
the shear force to be allowed for in design. The minimum values of shear force
in Table 3.1 have an associated bending moment.
The intention was that the minimum shear force and associated bending
moment from Table 3.1 should only be applied if the theoretical shear at an
opening was less than the minimum quoted. There is no requirement to apply
the minimum bending moment at all openings – the minimum bending
moment should only be applied if the minimum shear force is used in design.
Contact: Prof Mark Lawson
Tel: 01344 636555
Email: advisory@steel-sci.com
AD 421:
Design responsibility for welds in
fabricated plate girders
In recent months the SCI has received a number of questions about
responsibility for the design of the welds between the web and flanges of
a plate girder. These longitudinal welds are an integral part of the member
design – and should therefore be sized by the engineer responsible for the
design of the beam.
Contact: Richard Henderson
Tel: 01344 636555
Email: advisory@steel-sci.com
AD 422:
Punching shear check for fin
plates in P358
This AD note relates to Check 10 for fin plates in P358 Simple Joints to Eurocode
3 (the Eurocode “Green Book” on simple connections). Check 10 includes two
checks for punching shear (conservative and rigorous), but the value of γM2
is not specified in the text. Confusion is possible because γM2 appears in both
BS EN 1993-1-1 and BS EN 1993-1-8, but with different values (1.1 and 1.25
respectively, as given in the relevant UK National Annex).
Since the check does not concern the bolts or welds, but does concern the
ultimate material strengths of the fin plate and supporting member, the value
of γM2 should be taken as 1.1 from the UK NA to BS EN 1993-1-1.
Contact: Richard Henderson
Tel: 01344 636555
Email: advisory@steel-sci.com
AD 423:
Reduction in bending resistance
due to high shear
When considering the resistance of cross sections under combined bending
and high shear, (where the shear is equal to or exceeds half the plastic shear
resistance), the resistance moment of the section should be reduced: see BS
EN 1993-1-1 cl. 6.2.8(3). A reduced yield strength (1-ρ)fy where
=
2VEd
Vpl,Rd
( 1)2
should be used to calculate the contribution of the shear area to the
resistance moment of the section. For an I section, the shear resistance
is mainly provided by the web. In cl. 6.2.8(5) the alternative approach
calculates the bending resistance by deducting ρ times the plastic modulus
of the web from the full plastic modulus of the section, equivalent to using
a reduced web thickness.
Similarly, a reduced yield strength (1-ρ)fy applied to the shear area
should be used when considering combined bending, shear and axial
force, when the design shear exceeds half the plastic shear resistance of the
section.
A reduced plate thickness for the relevant part of the cross section may be
used as an alternative. Clause 6.2.10(3) of BS EN 1993-1-1 refers.
Contact: Richard Henderson
Tel: 01344 636555
Email: advisory@steelconstruction.org
AD 424:
Shear stud length
SCI has been advised that shear studs which are shorter than usual have
been placed on the market in the UK, and this AD warns against using them
unless the length has been reflected in the design, and unless the studs
meet the necessary material specification.
AD 380 indicates that a stud that starts with a manufactured length
of 105 mm would typically have a length after welding (LAW) of 100 mm
when welded directly to a beam flange and 95 mm when welded through
decking. The studs are identified as nominally 100 mm studs. AD 380 also
indicates that studs of diameter d = 19 mm and a nominal length of 100
mm may be deemed to satisfy the requirement that a stud extends at
least 2d above the height of the decking, when that height is 60 mm. UK
practice in composite construction for buildings generally involves the use
of through deck welded shear studs. Tests have shown that through deck
welded studs of 100 mm nominal length, with 60 mm decking, perform
satisfactorily.
A complication is that studs identified as nominally 100 mm long
have actual lengths “out of the box” which differ from manufacturer to
manufacturer. It is understood that the shorter studs referred to in the
opening paragraph are 90 mm before welding, so are likely to be less than
85 mm LAW when welded through decking. Clearly they should not simply
be substituted for nominal 100 mm studs unless the design is verified with
the shorter length.
All shear studs should conform to EN ISO 13918, as noted in the National
Structural Steelwork Specification (NSSS). Composite beam design generally
assumes a certain level of slip between the steel and concrete so the studs
must be ductile, regardless of the fact that failure is normally in the concrete
(at least for the grades of materials typically found in buildings). Annex B of
BS EN 1994-1 describes the stud test arrangement to demonstrate ductility.
Contact: Eleftherios Aggelopoulos
Tel: 01344 636555
Email: advisory@steel-sci.com
so is the edge to edge spacing of the openings
tw is the web thickness
fy is the yield strength of the steel
Contact: Prof Mark Lawson
Tel: 01344 636555
Email: advisory@steel-sci.com
link
link
link
/Steel_material_properties#Yield_strength
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