Section
3 Design loading
3.1 General
3.1.1 This
Section describes the various types of loadings to be applied to Linkspans
when assessing their structural adequacy at the intended service location.
3.2 Hydrostatic loads
3.2.1 The
scantlings of pontoons, buoyancy tanks and buoyancy spaces in linkspan
bridge sections are to be assessed on the basis of a static pressure
load equivalent to the geometric depth, D of the pontoon
or buoyancy tank.
3.2.2 Submerged
buoyancy tank scantlings are to be based on the maximum depth of water
above the base of the tank.
3.3 Dead loads
3.3.1 The
self weight, including weight of steel, welding, any surfacing or
cladding material, or machinery items is to be taken into account
in the calculations of bridges, ramps, walkways, support structures,
bearings, connections or joints, as appropriate.
3.4 Vehicle loads
3.4.1 Individual
tyre prints (load and patch area) are to be considered when establishing
the deck plate thickness of pontoons, ramps and bridges.
3.4.2 Axle
loads and spacings are to be considered for the adequacy of deck strength
members.
3.4.3 To
allow for the possibility of emergency braking or skidding incidents,
a horizontal load of 0,2 × vehicle weight is to be considered
in conjunction with the vertical vehicle loadings.
3.4.4 A
design uniformly distributed load (UDL) may be considered for establishing
the overall, global strength of vehicle carrying decks, ramps or bridges.
Such a UDL is to be used for defining the rated load, or SWL, where
appropriate, of any deck, ramp or bridge structure and for determining
the appropriate proof test load.
3.4.6 When
requested the loadings from trains and railway wagons will be specially
considered.
3.5 Walkway loading
3.5.1 Walkways
are generally assessed on the basis of a pedestrian UDL of 5 kN/m2 acting
over the internal floor area, unless otherwise stated by the designer.
3.6 Wind loading
3.6.1 Each
linkspan, and its mooring or tethering arrangements, is to be capable
of withstanding:
-
the specified
maximum wind speed in which the linkspan will continue in normal service,
and
-
an extreme,
out of normal service, wind speed - based on a 1 in 50 years return
period.
3.6.2 The
design wind speeds are to be based on local climatological data.
Where the wind speeds are not defined by reliable local meteorological
records, the following values may be used:
-
20 m/s for
the normal in-service condition
-
63 m/s for
the out-of-service condition.
3.6.3 The
wind force, F
w acting on the linkspan structure
is to be calculated from the expression:
where
|
A
|
= |
the
effective area of the structure i.e. the solid area projected on a
plane perpendicular to the wind direction, in m2
|
|
P
|
= |
wind
pressure in N/m2
|
where
|
v
|
= |
wind
speed, in m/s |
|
C
f
|
= |
force coefficient in the direction of the wind. |
3.7 Current loading
3.7.1 Each
linkspan is to be capable of withstanding the maximum prevailing current
speed without loss of effective station-keeping capability.
3.7.2 The
maximum current speed is to be based on the most unfavourable combinations
of tide, surge and wind induced currents for a return period of 1
in 50 years, or other equivalent available data.
3.7.3 The
current force, F
c acting on the linkspan is
to be calculated from the expression.
where
|
C
D
|
= |
drag coefficient in direction of current |
|
ρ |
= |
density of
water, in t/m3
|
|
v
|
= |
incident
current velocity impinging on the pontoon, in m/s |
|
A
|
= |
submerged
area of pontoon normal to the current flow direction, in m2.
|
3.8 Wave loading
3.8.1 In
general, linkspans are to be located in sheltered positions where
the effects of wave impingement will not be severe. The forces due
to any wave action are to be considered in the design of the mooring
or tethering system.
3.8.2 Where
it can be assumed that waves impinging on pontoons will be reflected,
forces should be calculated as for a standing wave with the pontoon
assumed to be located at a node point.
3.8.3 The
maximum pressure, P
y on the pontoon elevation, y, relative to the still water level may be taken as:
|
P
y
|
= |
ρgy + ρgH
inc cosh
[2π(y + d)/L]/cosh (2πd/L) kN/m2
|
where
|
ρ |
= |
mass density
of water, in t/m3
|
|
g
|
= |
acceleration
due to gravity, in m/s2
|
|
y
|
= |
elevation
on pontoon relative to still water level measured positively upwards,
in metres |
|
H
inc
|
= |
incident wave height, in metres |
|
d
|
= |
still
water depth, in metres |
|
L
|
= |
wavelength
(crest-to-crest), in metres. |
3.8.4 For
locations where wave drift occurs the mean wave drift force, F
wd for an irregular sea may be calculated using the following
equation:
where
|
H
s
|
= |
significant wave height, in metres. |
3.8.5 For
linkspans being specially considered under Pt 1, Ch 2, 1.2 Application 1.2.3 a more rigorous
treatment of the effects of wave action is required. Reference should
be made to BS 6349: Parts 1 and 6, or any other relevant National
Standard.
3.9 Ship induced loadings
3.9.1 Where
appropriate, linkspans are to be designed to accommodate any horizontal
and vertical forces from the end berthing manoeuvres of ships for:
-
normal, operational
berthing contact;
-
abnormal,
or heavy berthing contact.
3.9.2 The
berthing energy, E of the ship is to be calculated from
the equation:
where
|
m
|
= |
ship's
maximum displacement, in tonnes |
|
V
|
= |
ship's
contact speed, in m/s |
|
α |
= |
approach
angle of ship. |
3.9.3 The
reaction force, R imparted into the linkspan's structure
by the fender's absorption of the berthing energy is to be obtained
from the manufacturer's published performance curves for the particular
fender installed. It is recommended that R is increased
by 10 per cent, or otherwise as suggested by the manufacturer, to
allow for possible variations in the nominal values obtained from
the performance curves.
3.9.4 To
allow for heavy berthing incidents caused by possible accidental occurrences,
the ultimate energy absorption capacity of fenders is to be twice
that for normal conditions. Any heavy berthing incident resulting
in damage, defect or breakdown which could adversely affect the ability
of the linkspan to accommodate the conditions for which a Class has
been assigned is to be reported to LR without delay, see
Pt 1, Ch 2, 1.1 General 1.1.7.
3.9.5 Where
appropriate (see
Pt 1, Ch 2, 1.2 Application 1.2.2), linkspans are also to be considered for the
effects of the following other ship induced loadings:
-
Mooring forces
transmitted to the linkspan, see
Pt 1, Ch 2, 1.2 Application 1.2.2.
-
For linkspans
that derive support from the berthed ship, any forces applied through
the connecting strops or links caused by motion of the ship.
-
The forces
of water jets or other propulsion units that may impinge on submerged
parts of the linkspan.
-
The effects
of wash from passing marine craft.
-
The effects
of vehicles braking or trains striking the buffers when on board the
berthed ship.
3.10 Towage loading
3.10.1 Where
it is intended to tow the linkspan from its construction site to port
of operation, it will be necessary to assess the pontoon structure
for the appropriate loadings associated with the tow voyage, in addition
to the hydrostatic load and other loads defined previously.
3.11 Snow and ice loading
3.11.1 Where
appropriate, and unless otherwise stated by the designer, snow and
ice loading of 2 kN/m2 is to be applied to surfaces within
12 per cent of the horizontal.
|