AD 419: Composite beams with
different positions of web openings
SCI publication P355 is widely used to design beams with large web
openings. It is adopted in the development of software to design hot rolled
and fabricated steel sections with openings of various shapes and sizes.
The purpose of this Advisory Desk note is to address some common
practical problems related to adjacent openings of different heights and
positions.
1. Unequal adjacent opening heights
In P355 and in the AD 418, the buckling length of the web post for buckling
between closely spaced openings on the same horizontal axis is given by:
ℓw = 0.7(ho
Circular openings Rectangular openings
Low Shear Side (LSS) High Shear Side (HSS) Low Shear Side (LSS) High Shear Side (HSS)
e02
e01
Low Shear Side (LSS) High Shear Side (HSS)
Low Shear Side (LSS) High Shear Side (HSS)
26 NSC
June 18
2 + so
2)0.5 ≤ ho for rectangular openings (1)
ℓw = 0.5(ho
2 + so
2)0.5 ≤ 0.7ho for circular or elongated openings (2)
where:
ho is the opening height (or average height, as defined below)
so is the edge-to-edge distance between the openings
For unequal adjacent opening heights, it is proposed that the average
height of the openings, ho,eff , may be used to determine the slenderness for
web post buckling with a lower limit of 0.75 of the larger opening height.
This corresponds to the smaller opening height being taken as not less than
half the larger opening height. Therefore, the effective opening height, ho,eff
replaces ho in the above equations and is taken as:
ho,eff = 0.5 (ho,1 + ho,2) ≥ 0.75 ho,1 (3)
where:
ho,1 is the height of the larger opening
ho,2 is the height of the smaller opening
2. Different eccentricities of adjacent openings
The eccentricity of the opening, eo , is defined as positive when the centre
line of the opening is above the centre line of the beam and negative when
it is below. For the checks on web-post buckling, the effective opening
height in the above equations for web-post buckling should include the
worst case of the difference in eccentricities, which is as follows:
ho,eff = 0.5 (ho,1 + ho,2) + | eo,1 – eo,2 | ≥ 0.75 ho,1 + | eo,1 – eo,2 | (4)
where:
| eo,1 – eo,2 | is taken as its absolute value, in which eo,1 and eo2 can have
different signs depending on the position of adjacent openings relative to
the centre line of the beam and the heights of the adjacent openings are
defined as above.
The use of the absolute value of | eo,1 – eo,2 | is the worst case for checking
web-post buckling. A more precise treatment that takes account of the
buckling length is given below.
3. More precise treatment of eccentricities or unequal adjacent
opening heights
For unequal adjacent opening heights and positions, the buckling length
should be calculated from the dimension, ℓ which is the diagonal distance
from the low edge of the opening in the High Shear Side (HSS) to the high
edge of the opening at the Low Shear Side (LSS). Various cases are shown in
Figure 1. The buckling length for web-post buckling is taken as:
For circular or elongated openings: ℓw = 0.5ℓ
For rectangular openings: ℓ= 0.7ℓ
wFor adjacent circular and rectangular openings: ℓw = 0.6ℓ
The dimension ℓ should be calculated by taking h2 ≥ 0.5h1 to be consistent
o,o,with the limit in equation (4).
For adjacent rectangular openings, it is also necessary to check the in plane
bending resistance of the web-post due to the horizontal force acting at the
mid height of the beam. The position of the critical section will depend on
the relative position of the openings in the beam depth. For simplicity, the
in plane moment in the case of symmetric steel sections is determined from:
MEd = 0.5 (0.5(h1 + h) + e1 + e2 ) VEd
wp,o,o,2o,o,wp,where:
VEd is the horizontal shear force acting at the mid height of the beam
wp,This moment should not exceed the elastic bending resistance of the web
post which is given by:
MEd = tw s2 fy /(6γ)
wp,o
M0where:
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
Advisory Desk
Low Shear Side (LSS) High Shear Side (HSS)
Low Shear Side (LSS) High Shear Side (HSS)
Figure 1:
Treatment of the diagonal distance for web-post buckling between adjacent openings
A forward looking
concept in bridge
The problem is not an unfamiliar one:
an interharbour bridge is to be built
as part of an interchange between
interstate highways in the United
States – in Baltimore, Maryland, to be
precise. Considerable investigation
has been carried out and unusual
thoughts have been forthcoming.
The bridge envisaged consists of
three decks at approximately the
same level: two decks each with five
lanes, the third having four lanes.
Pedestrians would have rights of way
through this third deck.
All decks would be of orthrotropic
design, constructed of steel and
be equipped with resilient asphalt
driving surfaces: the decks to be
supported by steel cables similar
to suspension bridges, but in a very
different manner. The design allows
design
for criss-crossing cables in various
planes, supported by Y-shaped
abutments at each end of the bridge.
Considerably less steel per sq ft is
required than for a conventional
bridge, bringing desirable economies.
The decks are approximately 800 ft
long terminating at the first concrete
supports of the approaches.
The Y-shaped abutments are narrow
and straddle only the middle deck:
they are more economical to build
than the more usual vertical towers
but are less bulky than four towers
situated at the faces of the three
decks. Since they do not obstruct the
view of the bridge at its entrance, not
only does the approach to the bridge
become more convenient but the
abutments also add lightness and
grace.
/Design
/Steel_construction_products#Plate_girders
/Advisory_Desk_Notes
/Steel_material_properties#Yield_strength
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/Design
/Steel_construction_products#Plate_girders
/Advisory_Desk_Notes
/Steel_material_properties#Yield_strength
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