Technical Digest 2018
Artificially reducing the effective
width of slab to satisfy shear
In composite construction, the effective width of slab to be used in composite
beam design, as calculated from BS EN 1994-1-1 (clause 220.127.116.11) and the
former BS 5950-3.1+A1, is based on results from experimental and analytical
studies. In the past, designers would sometimes use a smaller effective
width in their design in an attempt to satisfy the minimum degree of shear
connection requirements (BS EN 1994-1-1, 18.104.22.168). This method has been a
matter of controversy as it could lead to situations where the actual number
of studs provided is not adequate.
The minimum degree of shear connection requirement is a complex
problem which is associated with the overall behaviour of the composite
beam, and the stiffness and ductility (slip capacity) of the shear connectors.
Therefore, due to the various unknowns and nonlinearities present, it is
difficult to justify a relaxation to the codified requirements for a minimum
degree of shear connection without proper analysis. For example, a number of
parameters have been known to have an effect on shear connection demands
such as the span, any asymmetry in the steel flange areas, the steel grade,
the construction method (propped vs unpropped) and the utilisation of the
beam in bending. As one can imagine, simplified methods such as the one in
question cannot possibly account for all these in a quantifiable manner.
The most recent guidance in SCI P405 was developed based on the results
from tests and extensive numerical analyses that accounted for the effects of
the above mentioned parameters. A set of alternative shear connection rules
that cover different practical cases is provided to complement the rules in BS
Contact: Dr Eleftherios Aggelopoulos
Tel: 01344 636555
Resistance of sections to
combined shear and bending
This Advisory Desk note reminds designers that the form of the section has a
significant impact on the reduction of bending resistance under high shear.
Clause 6.2.8 of BS EN 1993-1-1:2005 deals with the resistance of cross
sections to combined bending and shear and first of all states:
(1) Where the shear force is present allowance should be made for its effect
on the moment resistance.
It then goes on to say:
(2) Where the shear force is less than half the plastic shear resistance its
effect on the moment resistance may be neglected except where shear
buckling reduces the section resistance, see EN 1993-1-5.
(3) Otherwise the reduced moment resistance should be taken as the
design resistance of the cross-section, calculated using a reduced yield
strength … for the shear area.
The reduced yield strength depends on the ratio of design shear force to
the shear resistance of the section.
For an I section, the shear area approximates to the area of the web and the
flanges still provide their full resistance moment so the reduction in bending
resistance may not be more than about 20% when the design shear force
equals the shear resistance. For a rectangular section, the full section forms
the shear area so the bending resistance reduces to zero under the same
circumstances. A Tee section would also behave in a similar way.
Contact: Dr Richard Henderson
Tel: 01344 636555
Web-post buckling in composite
beams with rectangular and
elongated web openings
The design of composite beams with large web openings is presented in
SCI P355, which has been adopted in the development of software for the
design of both hot rolled and fabricated steel sections with openings of
various shapes and sizes. In P355, the method for addressing web buckling
next to or between rectangular or elongated openings identifies two cases;
closely spaced and widely spaced openings. For rectangular openings,
the transition between the two cases is taken at an edge-to-edge spacing
s, equal to the length of the opening ℓ. For elongated openings, this
o o transition occurs at an equivalent opening length, which may be taken as
ℓo - 0.55h.
o For widely spaced openings, web buckling next to an opening is checked
by considering the local transfer of the vertical shear force in the Tees acting
on a strut of width equal to half the opening depth.
For closely spaced openings, the relevant compression force acting on
the equivalent strut is taken as equal to the horizontal shear force in the
web-post and the check for web-post buckling is based on an inclined strut
whose slenderness depends on the spacing of the openings.
The issue in the design of beams with large web openings is the
potentially high ‘step’ in the shear resistance at the transition between
closely and widely spaced openings, which occurs due to the high
slenderness of the inclined strut. To partly reduce this issue, some changes
in the application of P355 are now appropriate, which relax the current rules
for long openings. These relaxations align with the current work to provide
normative clauses on the design of beams with large web openings in
Eurocodes 3 and 4.
Web-post buckling in P355
In P355, the buckling length of the web-post for closely spaced openings is
ℓw = 0.7(h2 + s2)0.5 for rectangular openings (1)
ℓw = 0.5(ho
2 + so
2)0.5 for circular or elongated openings (2)
ho is the opening height
so is the edge-to-edge distance between the openings.
For rectangular and elongated openings, the maximum opening length
is ℓo ≤ 2.5 ho for unstiffened openings and the minimum edge-to-edge
spacing, so should exceed 0.5 ℓo . In comparison, for circular openings, so ≥
0.1ho for steel beams and ≥ 0.3ho for composite beams.
Relaxation for adjacent rectangular openings
For adjacent rectangular openings, it is now accepted that to align with the
work on large web openings in the new part of Eurocode 3, EN 1993-1-13,
the maximum buckling length for web-post buckling between rectangular
openings of the same height may be taken as:
ℓw ≤ ho (3)
This leads to an upper bound nondimensional slenderness of the webpost
1 = (E/fy )0.5
λwp is used to obtain χwp , which is the reduction factor due to buckling
of the web-post acting as a strut. For rolled sections, buckling curve ‘a’ in
EN 1993-1-1 may be used and for fabricated sections, buckling curve ‘c’
should be used. The buckling resistance of the web-post is given by:
Nwp,Rd = χwp tw,min so fy/γM1 (5)