geomechanics
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SurchargeLoadDistribution.pdf
AS 4678—2002
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111
J10.2 Design surcharge loads
The distribution of stresses on a retaining structure due to surcharge loads should be
carefully assessed in design. The design load factors are 1.5 for strength and stability limit
states, and 0.7 for the serviceability limit state. See Table J1 for examples of load
components for each limit state.
J10.3 Assessment of the effect of surcharge for conventional retaining structures
Each pressure induced by surcharge loads will depend on the load spreading properties of
the retained earth and the stiffness of the wall. Two approaches may be used to assess the
magnitude and distribution of lateral pressures induced by surcharge loading, as follows:
(a) Rankine active earth pressure theory—the trial wedge method and associated force
polygon methods may be employed (see Figure J6).
(b) Elasticity theory supported by experimental measurements—the earth pressure design
charts given by Terzaghi (Ref. 4) given in Figure J7 may be used to estimate lateral
pressure due to vertical line loads, point loads and horizontal line loads. These have
been modified from the Boussinesq (Ref. 5) solution for distribution of stresses in an
isotropic semi-infinite elastic medium based on experimental evidence.
Numerical modelling approaches (finite element analysis) may be used to assess the
magnitude and distribution of earth pressures for complex loaded retaining structures.
J10.4 Assessment of the effect of applied loadings on reinforced soil structures
For reinforced soil-retaining structures, the internal stresses are derived from two separate
load conditions, resulting from—
(a) the supporting function, and
(b) the retaining function.
These stresses should be superimposed for the determination of the total stresses due to
applied loadings and for the calculation of reinforcement stresses.
The supporting function requires the determination of the additional vertical and horizontal
stresses resulting from the diffusion of the applied loadings.
For vertical loadings, the distribution of vertical strip loadings through the fill may be
determined by a lateral dispersion defined by a slope of 2 vertically to 1 horizontally (see
Figure J8), or by calculating an appropriate stress distribution such as defined by
Boussinesq (Ref. 5). Vertical load dispersion has to take into account the effect of the face
on the available dispersion area.
For horizontal loadings, the distribution of horizontal strip loadings through the fill may be
determined by a vertical dispersion over the outside surface or facing of the structure
defined by a slope from the rear of the contact surface. The horizontal pressure distribution
on the outside surface or facing has to vary linearly from a maximum at the level of the
contact surface to zero at its lowest point (see Figure J8).
The retaining function takes into account the overall retaining structure loadings such as
self weight, superstructure and general surcharge loadings. These should then be taken in
combination with the overturning moments generated by the applied loads on the structure.
The diffusion of stresses imposed by applied loadings depends on the stiffness
characteristics of the structure. Further guidance on the selection and use of appropriate
design methods is provided in BS 8006.
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AS 4678—2002
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112
FIGURE J4 SIZING OF WALLS WITH VARIOUS GEOMETRIES A1
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AS 4678—2002
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113
LEGEND:
K = earth pressure coefficient (see Note 2)
Q1 = intensity of effective line load imposed by compaction plant (see Note 3)
zc,hc = critical depths as shown
γ = soil unit weight
Phm′ = maximum horizontal earth pressure induced by compaction
Ph′ = horizontal earth pressure induced by overburden stress
NOTES:
1 Figure based on Ingold (1979a and b).
2 For retaining walls that can move forward sufficiently to mobilize active condition in the fill, K = K a . For unyielding
rigid structures, K = K o . For walls supporting a fill slope, it may be assumed that the compaction-induced earth
pressure is the same as that given by the diagram above for a horizontal final surface, except z c should be taken as
zero.
3 For dead weight rollers, the effective line load is the weight of the roller divided by its roll width, and for vibratory
rollers it should be calculated using an equivalent weight equal to the dead weight of the roller plus the centrifugal
force generated by the roller’s vibrating mechanism. The centrifugal force may be taken to be equal to the dead
weight of the roller in the absence of trade data.
4 The compaction-induced earth pressure assumed in the design should be clearly stated on the drawings.
FIGURE J5 SIMPLIFIED METHOD FOR THE EVALUATION OF
COMPACTION-INDUCED EARTH PRESSURES
A1
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AS 4678—2002
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114
FIGURE J6 TRIAL WEDGE METHOD OF ANALYSIS FOR SURCHARGE LOADING
BEHIND RETAINING STRUCTURES
A1
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AS 4678—2002
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FIGURE J7 CALCULATION OF LATERAL PRESSURE ON A VERTICAL RETAINING
WALL DUE TO VERTICAL AND HORIZONTAL LOADS A cc
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AS 4678—2002
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FIGURE J8 APPLIED LOAD DISTRIBUTION IN REINFORCED SOIL STRUCTURES
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AS 5100.2 Supp 1-2007 24
C6.I0 Load factors
Generally, the loads correspond to the serviceability limit state, i.e., load factor is 1.0. The exceptions are the braking forces which have been reduced due to their low probability of occurrence (see Clause C6.8.2).
The load factor for the ultimate limit state live load has been set at 1.8. This is a I 0% reduction from the 2.0 used in ABDC-1992 (HB 77 .2-1996) (Ref. I). The reduction is consistent with international trends, the fact that as legal axle loads increase the proportion of vehicles that can overload decreases, i.e., their weight is limited by the volume of the vehicle rather than limits on axle mass and the amount that they can overload is limited.
For centrifugal and braking forces to exist, vehicles should be on the structure. Consequently, there are co-existing horizontal and vertical loads. This is recognized in the Standard and presented as a combination that needs to be considered. For example-
1.8 [F, and tALF; xM1600; x (1 + a)]
requires the centrifugal forces (F,) to be combined with the vertical forces associated with the Ml600 moving traffic load. Note that the centrifugal force relates to the acceleration of the mass and is thus independent of dynamic effects. This is combined with the vertical effects of the Ml 600 moving traffic load including the allowance for dynamics.
C6.11 Deflection
The specification of limits on live load deflections and span to depth ratios have traditionally been used in road and railway bridge design codes to attempt to control vibration, prevent fatigue, limit stresses in secondary members and allow for dynamic loading. Although this Standard provides specific clauses aimed at addressing these serviceability limit states, it has also retained limitations on live load deflections.
Relaxed deflection limits have been introduced in this version of the Standard as part of the transition to designing bridges for SM 1600 loading and the uncertainty about appropriate serviceability limits for controlling vibration.
For bridges with walkways, the criteria for limiting vibration in Clause 12 of the Standard will generally control.
For other bridges, the magnitude of allowable deflection limits have been increased from 1/800 of the span or 1/400 of the cantilever projection in ABDC-1992 (HB 77.2-1996) (Ref. I) to 1/600 and 1/300 respectively and the method of calculation has also been made less conservative, as per AASHTO LRFD (2004) (Ref. 15).
These limits are similar to the values that have been used for railway bridges for many years and reflect the magnitude of the SM 1600 loading. They are also representative of the magnitude of live load deflections in typical prestressed concrete bridges designed for SM! 600. These deflection limits, together with the fatigue criteria specified in Clause 6.9 of the Standard, are likely to prove to be controlling serviceability limit states for steel beam bridges, particularly when high strength steel is used.
C6.12 Distribution of load traffic loads through fill
Figure C6.12 illustrates the distribution of wheel loads from road traffic through fill to the top of buried structures as specified in the Clause and covers individual and overlapping conditions.
The load distribution is based on that contained within AS 1597.2, making its application more general to all types of buried structures beneath roadways, as generally recommended by Standards Australia Committee CE-025.
© Standards Australia www .standards. org. au
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Rectangular wheel contact area at the road surface
25 AS 5100.2 Supp 1-2007
Rectangular distribution area on the surface of the structure
la) For depths of fill from O to 200 mm
Rectangular wheel contact area at the road surface
b +100 mm+ 11.2 < xlh-2~v
> mm+ 11.2
Rectangular distribution area on the surface of the structure
X lh -200))
lb) For depths of fill greater than 200 mm
Rectangular wheel contact area at the road surface,~---;,:')<
(c) Overlapping load distribution areas
Overlap of distribution area
FIGURE C6.12 DISTRIBUTION OF WHEEL LOADS THROUGH FILL
More refined methods of analysis may be used to derive more accurate distributions, if approved by the relevant authority.
Whilst the Clause specifies the distribution through fill of the SM! 600 design loads, the requirements of the Clause also apply to heavy load platforms.
The depth of fill includes the pavement and should be measured to the top of the finished wearing surface.
www. standards. o rg. au © Standards Australia
AS 5100.2 Supp 1-2007 26
The uniform load components of 6 kN/m (1.875 kPa) of the Ml600 moving traffic load and 24 kN/m (7 .5 kPa) of the S 1600 stationary traffic load are not dispersed longitudinally, but are dispersed transversely as for wheel loads, as the structure width may be wider than the roadway width.
The width of the distribution cannot be wider than the structure supporting the roadway.
For buried structures, the dynamic load allowance should be applied to the road traffic loads as specified in Clause 6. 7 .3 of the Standard.
C7 PEDESTRIAN AND BICYCLE-PATH LOAD
C7.1 General
It is unlikely that full maximum load will occur over a large walkway area, except in extreme crowds, e.g., major sporting events.
For walkways attached to and supported by the main bridge superstructure members, the design pedestrian load to be carried by these members decreases as the area of walkway increases. Individual components supporting smaller portions of the walkway area are to be designed for the high crowd loads possible over these small areas.
For pedestrian bridges and independent walkways, the structural members are to be designed to carry the higher pedestrian loads, especially if the location of the bridge is such that crowd loads are likely.
C7.2 Service live load on walkways
The additional load for walkways and service platforms is to provide for the stacking of materials during construction and servicing.
C7.3 Load factors
The Clause specifies a load factor of 1.8 for pedestrian load, which is greater than the value of 1.5 specified in AS/NZS 1170.1, Structural design actions-Permanent, imposed and other actions. The value of 1.8 is consistent with providing an acceptable probability that the design load will not be exceeded during the I 00 year design life taking into consideration unexpected crowd loading and the like. It also ensures an increased level of robustness which is important for road and bridge related structures that may be exposed to accidental collision loads.
CS RAILWAY TRAFFIC
C8.1 General
The railway traffic loads specified in the Standard, including the various factors, have been determined to cover the effects of-
(a) the maximum expected loading to be applied; and
(b) a large number of load repetitions of the operating loads which defines the fatigue loading spectra.
Live loads other than the specified 300LA may be used at the discretion of the railway authority, but this should be a factored version of the 300LA loading. This should not be done without good reason, and only after noting that lines which are currently only lightly loaded may have their loading profiles changed at a future time.
C8.2 300LA railway traffic load
The 300LA loading combines the two alternatives of the 300-A-12 loading from ABDC-1992 (HB 77.2-1996) (Ref. I), by adding the 360 kN single axle load 2 min front of the vehicle loading. This is to simulate six axle coupled locomotives and better represent their loading in the 15 to 22 m span range. This combination produces moment and shear envelopes more nearly proportional to that of existing trains over the whole range of spans [see Marcer (2002) (Ref. 16)].
© Standards Australia www.standards.org. au
AS 5100.2—2004 40
Standards Australia www.standards.com.au
FIGURE 12.4 DYNAMIC AMPLITUDE LIMITS FOR PEDESTRIAN BRIDGES
12.5 Special structures
This Standard does not provide acceptance criteria for the dynamic behaviour of bridges
with spans in excess of 100 m, or suspension and cable-stayed bridges. The dynamic
behaviour of such structures under the action of traffic, wind or other loadings shall be the
subject of special investigations.
13 EARTH PRESSURE
13.1 General
The load effects on a retaining structure due to earth pressure loads shall be determined in
accordance with AS 5100.3.
13.2 Surcharge loads from road traffic loads
Where highway live loads can approach within a distance equal to the effective height of
the wall from the backface of the structure, an equivalent load caused by an additional
height of fill, which diminishes over the height of the wall, as shown in Figure 13.2, shall
be assumed for the purpose of calculating design earth pressure. This load shall be assumed
to act above the finished grade and over the entire length of the retaining structure. The
effect of foundations or other loads placed in or on the backfill, within a distance equal to
the effective height of the wall, shall also be included.
The live load surcharge shall be applied irrespective of whether or not there is provision for
an approach slab in the bridge design.
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41 AS 5100.2—2004
www.standards.com.au Standards Australia
FIGURE 13.2 EQUIVALENT LOAD DUE TO LIVE LOAD SURCHARGE
13.3 Surcharge loads from railway loads
Where sleepers supporting railway traffic load are located within a distance from the back
face of a retaining wall or abutment equal to the effective height of the retaining structure,
an additional surcharge load equal to the railway traffic load shall be applied as a uniform
load at the level of the underside of the sleepers as shown in Figure 13.3. An equivalent
load caused by an additional height of fill shall be applied, or an alternative method of
allowing for surcharge shall be used.
In determining the distribution of rail loads at the underside of sleepers, it is assumed that
the total train load over any given length of track shall be uniformly distributed over the
area defined by the length of sleepers and the length of track considered. The length of
track shall be selected to produce the worst design effects. The resulting distributed loads
shall be considered in the design as discrete areas of surcharge.
These areas of surcharge shall be distributed with increasing depth below the underside of
sleepers. The width of the distribution perpendicular to the track centre-line shall be
increased in each direction at a slope of 1 horizontally to 2 vertically, to a maximum width
of 4.5 m, to determine the maximum vertical earth pressures at depth as a result of
surcharge.
When adjacent rail traffic load distributions overlap, the total load shall be considered to be
uniformly distributed over the area defined by the outside limits of the individual rail load
distributions at that depth. The total width of the distribution so determined shall not exceed
the total width of the structure supporting the fill and, if the centroid of the load is not
coincident with the loaded area, the load distribution shall be taken to vary linearly to
satisfy statics.
When determining lateral earth pressures on retaining walls and abutments, the areas of
surcharge at the underside of sleepers shall be taken to apply pressures to the structure if
they are located within the zone of a 45° projection from the heel or base of the retaining
structure.
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