Advisory Desk
AD 418: 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
so , equal to the length of the opening ℓo . For
elongated openings, this transition occurs at an
equivalent opening length, which may be taken
as ℓo - 0.55ho .
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.
30 NSC
May 18
Web-post buckling in P355
In P355, the buckling length of the web-post for
closely spaced openings is given by:
ℓw = 0.7(h2 + s2)0.5 for rectangular openings (1)
o
o
ℓw = 0.5(ho
2 + so
2)0.5 for circular or elongated
openings (2)
where:
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 edgeto
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 web-post given by:
wp
3.5ho
tw1 (4)
where:
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:
NRd = χwp tmin so f/γM1 (5)
wp,w,ywhere:
tmin is the smaller web thickness above/
w, below the opening
fy is the yield strength of the steel
This buckling resistance is compared to the
horizontal shear force, VEd , acting in the
wp,web-post. The upper bound shear resistance is
given by χwp = 1/√3 = 0.577, which corresponds
to pure shear resistance of the web-post rather
than buckling.
For rectangular openings, a further check should
be made on the in-plane moment acting at the
top or bottom of the web-post due to the effects
of horizontal shear, which may control for narrow
web-posts. For a symmetric section, this moment
is given by 0.5VEd h, which should not exceed
wp,othe in-plane bending resistance of the web-post,
which is taken as tmin s2 fy /(6γ).
w,o
M1
Relaxation for adjacent elongated openings
or circular and elongated openings
The maximum buckling length for web-post
buckling between circular or circular and
elongated openings of the same height may be
taken as:
ℓw ≤ 0.7ho
This leads to an upper bound nondimensional
slenderness of the web-post given by:
wp
2.4ho
tw1 (6)
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