When a bridge crosses a river, stream or any other body of water, it shall be designed to resist the effects of water flow and wave action, as applicable. The design shall include an assessment of how the water forces may vary in an adverse manner under the influence of debris, log impact, scour and buoyancy of the structure.
Tidal and wave actions shall be considered on bridges across large bodies of water, estuaries and open sea.
15.2 Limit states
15.2.1 Ultimate limit states
The ultimate limit states define the capability of a bridge to withstand, without collapse, any flood of a magnitude up to and including that with a 2000 year average return interval, whichever produces the most severe effect. It can be accepted that scour of the stream bed and considerable damage to approaches and embankments may take place, provided that the structural integrity of the bridge is maintained.
As the critical design condition may occur at the flood level which just causes overtopping of the superstructure, an estimate of the return interval of such a flood shall be made and, if appropriate, this condition shall be considered in the design.
Where the critical design condition occurs at an average return interval of less than 2000 years, the ultimate load factor (γWF) shall be obtained from Figure 15.2.1, but shall be not greater than 2.0.
FIGURE 15.2.1 ULTIMATE LOAD FACTOR (γWF)
15.2.2 Serviceability limit states
The serviceability limit states define the capability of the road and bridge systems to remain open during a serviceability design flood or to sustain an overtopping flood without damage to bridges, culverts, floodways or embankments within the system. The serviceability design flood shall be that with a 20 year average return interval.
15.3 Forces on piers due to water flow 15.3.1 Drag forces on piers
In bridge structures subjected to water flow effects, the fluid forces on the piers are dependent on the pier shape, the water velocity and the direction of the water flow. The design drag forces parallel to the plane containing the pier (as shown in Figure 15.3.1) shall be calculated as follows:
(a) Ultimate design drag force
( )
Fdu* : d 2 u d * du 0.5C V A F = . . . 15.3.1(1) A1(b) Serviceability design drag force
( )
F : ds* d 2 s d * ds 0.5C V A F = . . . 15.3.1(2) whereCd = drag coefficient, depending upon pier shape
Vu = mean velocity of water flow for ultimate limit states at the level of the superstructure or debris as appropriate
Vs = mean velocity of water flow for serviceability limit states at the level of the superstructure or debris as appropriate
Ad = area, equal to the thickness of the pier normal to the direction of the water flow, multiplied by the height of the water flow
In the absence of more exact estimates, the value of Cd shall be assumed as follows: Cd = 0.7 (semi-circular pier nosing)
= 1.4 (square end pier nosing)
= 0.8 (wedge, sharper than 90°, nosing) 15.3.2 Lift forces on piers
The design lift forces, perpendicular to the plane containing the pier (as shown in Figure 15.3.1) shall be calculated as follows:
(a) Ultimate design lift force
( )
F : Lu* L 2 u L * Lu 0.5C V A F = . . . 15.3.2(1)(b) Serviceability design lift force
( )
F : L*s L 2 s L * s L 0.5C V A F = . . . 15.3.2(2) whereCL = lift coefficient, which depends on the angle between the water flow direction and the plane containing the pier
AL = area, equal to the width of the pier parallel to the direction of the water flow, multiplied by the height of the flow
In the absence of more exact estimates, the value of CL shall be assumed as follows: CL = 0.9 for θw≤ 30°
= 1.0 for θw > 30°
where θw is the angle between the direction of the water flow and the transverse centre-line of the pier.
NOTE: In plate or wall-type piers angled to the direction of flow, transverse lift-type forces can be significant.
FIGURE 15.3.1 DRAG AND LIFT FORCES ON PIERS 15.4 Forces on superstructures due to water flow
15.4.1 General
A superstructure that is partially or fully submerged in a flood is subjected to— (a) a drag force normal to its longitudinal axis;
(b) a vertical lift force (positive upwards); and
(c) a moment about the girder soffit level (clockwise positive with the water flow from left to right).
The loads specified in Items (a), (b) and (c) shall be determined in accordance with Clauses 15.4.2, 15.4.3 and 15.4.4, as appropriate.
15.4.2 Drag force on superstructures
The drag force on superstructures shall be calculated as follows: (a) Ultimate design drag force
( )
Fd*u :s 2 u d * du 0.5C V A F = . . . 15.4.2(1)
(b) Serviceability design drag force (F ): ds* s 2 s d * ds 0.5C V A F = . . . 15.4.2(2) where Cd = drag coefficient
As = wetted area of the superstructure, including any railings or parapets, projected on a plane normal to the water flow
The value of Cd for superstructures shall be obtained from Figure 15.4.2(A). The relative submergence (Sr) and the proximity ratio (Pr) shall be calculated as follows:
sp wgs r d d S = . . . 15.4.2(3) ss gs r d y P = . . . 15.4.2(4) where
dwgs = vertical distance from the girder soffit to the flood water surface upstream of the bridge (see Figure 15.4.2(B))
dsp = wetted depth of the superstructure (including any railings or parapets) projected on a plane normal to the water flow (see Figure 15.4.2(B))
ygs = vertical average distance from the girder soffit to the bed assuming no scour at the span under consideration (see Figure 15.4.2(B))
dss = wetted depth of the solid superstructure (excluding any railings but including solid parapets) projected on a plane normal to the water flow (see Figure 15.4.2(B))
FIGURE 15.4.2(A) SUPERSTRUCTURE Cd
FIGURE 15.4.2(B) DIMENSIONS 15.4.3 Lift force on superstructures
The lift force on a superstructure shall be calculated as follows: (a) Ultimate design lift force
( )
F : Lu*L 2 u L * Lu 0.5C V A F = . . . 15.4.3(1)
(b) Serviceability design lift force
( )
F : Ls* L 2 s L * Ls 0.5C V A F = . . . 15.4.3(2) where CL = lift coefficientAL = plan deck area of the superstructure
The value of CL shall be obtained from Figure 15.4.3. Two lift forces shall be calculated at each Sr. The upper value of CL shall be used when determining the resistance of the structure to overturning and the tie down requirements for the superstructure. Where upward lift forces on the superstructure are possible, a positive tie-down system shall be provided.
The lower value of CL, a downward force, shall be considered in the design of the deck, girders, substructure and foundations. In determining the design flood load for each of these components, the downward force shall be combined with the moment as described in Clause 15.4.4.
FIGURE 15.4.3 SUPERSTRUCTURE CL
15.4.4 Moment on a superstructure
The drag and lift forces generate a moment about the longitudinal axis of the superstructure. The moment at the soffit level at the centre-line of the superstructure shall be calculated as follows:
(a) Ultimate design moment
( )
Mg*u : sp s 2 u m * gu 0.5C V A d M = . . . 15.4.4(1)(b) Serviceability design moment
( )
Mg*s : sp s 2 s m * s g 0.5C V A d M = . . . 15.4.4(2) where Cm = moment coefficientThe value of Cm shall be obtained from Figure 15.4.4.
FIGURE 15.4.4 SUPERSTRUCTURE Cm
15.4.5 Loads on superstructures with superelevation
The loads on a superstructure with a positive superelevation (upstream face raised) of up to 4% shall be calculated as described in Clauses 15.4.2 to 15.4.4. The loads on a superstructure with a negative superelevation of up to −4% shall be calculated as described in Clauses 15.4.2 to 15.4.4, but with the following adjustments to the coefficients:
(a) The value of Cd shall be increased by 5%. (b) The magnitude of CL shall be increased by 20%.
(c) The value of Cm shall be the same as for a level superstructure.
If the superelevation is greater than 4%, the upward lift force shall be calculated in the same manner as for wall type piers (see Clause 15.3.2) except that AL shall be taken as the plan deck area. Values of CL shall be calculated by interpolation of the values given in Clause 15.3.2.
For superelevations outside this range, study of specialist literature or physical model testing shall be undertaken.
15.5 Forces due to debris 15.5.1 Depth of debris mat
The depth of a debris mat varies depending on factors such as catchment vegetation, available water flow depth and superstructure span. In the absence of more accurate estimates, the minimum depth of debris mat for design shall be 1.2 m and the maximum depth shall be 3 m.
15.5.2 Debris acting on piers
A debris load acting on piers shall be considered for bridges where the flood level is below the superstructure. The length of a debris mat shall be taken as one half the sum of the adjacent spans or 20 m, whichever is the smaller. The debris load shall be applied at mid- height of the debris mat, assuming the top of the debris mat is at the flood level.
15.5.3 Debris acting on superstructures
A debris load acting on superstructures shall be considered for bridges where the flood level is above a level of 600 mm below the soffit level. The length of the debris mat shall be the projected length of the superstructure. The debris load shall be applied at mid-height of the superstructure, including any railing or parapets.
15.5.4 Calculation of debris load
The ultimate and serviceability design forces due to debris shall be calculated using Equations 15.5.4(1) and 15.5.4(2) respectively:
(a) Ultimate design drag force
( )
F : du* deb 2 u d * du 0.5C V A F = . . . 15.5.4(1)(b) Serviceability design force
( )
F : ds* deb 2 s d * ds 0.5C V A F = . . . 15.5.4(2) whereCd = obtained from Figure 15.5.4(A), for debris acting on piers
obtained from Figure 15.5.4(B), for debris acting on superstructures Adeb = projected area of debris
Debris forces shall not be used concurrently with water flow forces except that, in determining the resistance of the structure to overturning, an upward lift force shall be assumed when the debris is acting on the superstructure. The upward lift force shall be the sum of the lift force, calculated using Equations 15.4.3(1) and 15.4.3(2) given in Clause 15.4.3 and the buoyancy force. A value of 0.5 for CL shall be used.
FIGURE 15.5.4(A) PIER DEBRIS Cd
FIGURE 15.5.4(B) SUPERSTRUCTURE DEBRIS Cd
15.6 Forces due to log impact
Where floating logs are possible, the ultimate and serviceability design forces exerted by such logs directly hitting piers or superstructure shall be calculated on the assumptions that a log with a minimum mass of 2 t will be stopped in a distance of 300 mm for timber piers, 150 mm for hollow concrete piers, and 75 mm for solid concrete piers. If fender piles or sheathing, to absorb the energy of the blow, are placed upstream from the pier, the stopping
distance shall be increased. The design forces shall be calculated using the mean velocity of water flow at flood level Vs for serviceability limit states, or Vu for ultimate limit states, as appropriate.
The forces due to log impact and debris shall not be applied concurrently. Log impact shall be applied with such other water flow forces as appropriate.
15.7 Effects due to buoyancy and lift
In assessing the effects of buoyancy and lift on bridge structures, consideration shall be given to the following:
(a) The effects of buoyancy and lift on substructure, including piling, and superstructure dead loads. Buoyancy shall be applied concurrently with other water flow forces. (b) For beam and slab or box girder bridges, the provision of horizontal bleed holes in
webs or diaphragms, or both, or vertical bleed holes in the deck to dissipate air, which may be trapped between high water level and the underside of the deck slab. Several escape paths and a minimum diameter of 50 mm for vertical bleed holes and 75 mm for horizontal bleed holes shall be used.
(c) Provision of drainage from internal cells.
The provision of a positive tie-down system for the superstructure shall be provided for an ultimate force equal to 1.5FLu* +Buoyancy− γgDL, where γg shall be the lower value given in Table 5.2.
16 WIND LOADS 16.1 General
This Clause specifies design wind loads for conventional bridge structures. For wind- sensitive structures, such as suspension or long-span cable-stayed bridges, which may be subject to wind excited oscillations, special investigations into the dynamic behaviour of the structure shall be carried out. Wind loads on lighting, traffic signal and traffic sign structures shall be in accordance with Clause 23. Wind loads on noise barriers shall be in accordance with Clause 24.
16.2 Design wind speed 16.2.1 General
The design wind speed shall be derived from the appropriate regional basic design wind speeds, after adjustment for—
(a) average return interval; (b) geographical location; (c) terrain category; (d) shielding; and (e) height above ground.
The average return interval shall be as specified in this Clause. The values and factors for Items (b) to (e) shall be obtained from AS/NZS 1170.2.
16.2.2 Average return interval
The average return interval to be adopted shall be as follows:
(a) For ultimate limit states ... 2000 years. The regional basic design wind speed for a 2000 year average return interval shall be as specified in AS/NZS 1170.2 for that interval.
(b) For serviceability limit states ... 20 years, (for wind in conjunction with permanent effects only).
For serviceability limit state wind loads in conjunction with road traffic loads on a structure, the selection of a wind speed for a specified return interval is not appropriate and the design wind speed shall be taken as 35 m/s in all locations. The effect of wind on road traffic load need not be considered.
16.3 Transverse wind load
16.3.1 Calculation of transverse wind load
The transverse wind load shall be taken as acting horizontally at the centroids of the appropriate areas, and shall be calculated as follows:
(a) Ultimate design transverse wind load (W ): tu* d t 2 u * tu 0.0006V AC W = . . . 16.3(1)
(b) Serviceability design transverse wind load (W ): ts* d t 2 s * ts 0.0006V AC W = . . . 16.3(2) where
Vu = design wind speed for ultimate limit states Vs = design wind speed for serviceability limit states At = area of the structure for calculation of wind load Cd = drag coefficient
16.3.2 Area of structure for calculation of transverse wind load (At)
The area of the structure or element under consideration shall be the solid area in normal projected elevation subject to the following:
(a) Superstructures with solid parapets The area of the superstructure shall include the area of the solid windward parapet, but the effect of the leeward parapet need not be considered.
(b) Superstructures with open parapets The total load shall be the sum of the loads for the superstructure, the windward barrier and the leeward barrier considered separately. Where there are more than two parapets or safety fences, irrespective of the width of the superstructure, only those two elements having the greatest unshielded effect shall be considered.
(c) Piers Shielding shall not be considered. 16.3.3 Drag coefficient (Cd)
The drag coefficient (Cd) shall be determined as follows:
(a) Drag coefficient for all superstructures with solid elevation For superstructures with or without traffic load, Cd shall be as shown in Figure 16.3.3—
where
b = overall width of the bridge between outer faces of parapets d = depth of superstructure, including solid parapet, if applicable
(b) Aerodynamic shape factor for truss girder superstructures The wind force on truss girder superstructures shall be calculated by considering each component individually, using the aerodynamic shape factor specified in AS/NZS 1170.2.
(c) Drag coefficients for beams during erection The drag coefficient for beams and girders during erection shall be calculated for individual beams as shown in Figure 16.3.3. Shielding shall not be considered for individual beams, but may be allowed for when two or more beams are connected, provided the ratio of the clear distance between beams to the depth is not be greater than 7. Where the ratio of the clear distance between connected beams to the depth is greater than 7, the drag coefficient for the combination shall be taken as 1.5 times the value for an individual beam.
(d) Aerodynamic shape factor for parapet railings, parapet barriers and substructures Aerodynamic shape factors shall be obtained from AS/NZS 1170.2.
NOTES:
1 The values given assume a vertical elevation and a horizontal wind.
2 Where the windward face is inclined to the vertical, the drag coefficient (Cd) may be reduced by 0.5% per degree of inclination from the vertical, subject to a maximum reduction of 30%.
3 Where the windward face consists of a vertical and a sloping part or two sloping parts inclined at different angles, the wind load shall be derived as follows:
(a) The basic drag coefficient (Cd) shall be calculated using the total depth of the structure.
(b) For each non-vertical face, the basic drag coefficient calculated above shall be reduced in accordance with Note 2.
(c) The total wind load shall be calculated by applying the appropriate drag coefficients to the relevant areas.
4 Where a superstructure is superelevated, Cd shall be increased by 3% per degree of inclination to the horizontal, but not by more than 25%.
5 Where a superstructure is subject to wind inclined at not more than 5° to the horizontal, Cd shall be
increased by 15%. Where the angle of inclination exceeds 5°, the drag coefficient shall be derived from tests.
6 Where a superstructure is superelevated and also subject to inclined wind, the drag coefficient shall be the subject of special investigation.
FIGURE 16.3.3 DRAG COEFFICIENT (Cd) FOR SUPERSTRUCTURES
WITH SOLID ELEVATION 16.4 Longitudinal wind load
For piers, truss bridges and other superstructure forms, which present a significant surface area to wind loads parallel to the longitudinal centre-line of the structure, a longitudinal wind load shall be considered. The ultimate and serviceability design longitudinal wind loads shall be calculated in a manner similar to those for transverse wind loads.
NOTE: Longitudinal wind loads on the superstructure may also be significant during the construction stage of some bridge types, which are not affected by these loads during service. A1
16.5 Vertical wind load
An upward or downward vertical wind load, acting at the centroid of the appropriate area, shall be calculated as follows:
(a) Ultimate design vertical wind load (W ): vu* 3 L p 2 u * vu =0.6V A C 10− W . . . 16.5(1)
(b) Serviceability design vertical wind load (W ): vs* 3 L p 2 s * vs =0.6V A C 10− W . . . 16.5(2) where
Vu = design wind speed for ultimate limit states Vs = design wind speed for serviceability limit states Ap = bridge area in plan
CL = lift coefficient = 0.75
Equations 16.5(1) and 16.5(2) may be used provided the angle of inclination of the wind to the structure is less than 5°. For inclinations greater than 5°, the lift coefficient shall be investigated by testing.
16.6 Wind on railway live load
The effect of wind on railway live load shall be included in both ultimate and serviceability limit state load combinations and shall be considered to act with the design railway traffic load.
The area to be considered in the calculation of the wind load on railway live load shall be the solid area in normal projected elevation of the train area where it protrudes beyond the projected elevation of the bridge structure. For the calculation of the projected area, a train on the bridge shall be assumed to be 3.7 m in height, taken from the top of rails. The point of application shall be taken as 1.85 m above the top of the rails.
The drag factor to be used in calculating the force for wind on the bridge plus live load shall be obtained from Clause 16.3.3(a), with the height d taken as the projected area of the train and the bridge, and the width b as specified in Clause 16.3.3(a).
17 THERMAL EFFECTS 17.1 General
Daily and seasonal fluctuations in air temperature and solar radiation cause both variations in average bridge temperature and differential temperature gradients across structural members.
Variation in average bridge temperature shall be used as a basis for— (a) assessment of bearing and deck joint movement requirements; and
(b) evaluation of design loads or load effects resulting from the restraint of associated expansion or contraction by either the form of the structure, e.g., as in portal frames and arches, or by the support and bearing stiffnesses.
Differential temperatures within bridge superstructures result in load effects within the section. In the case of statically indeterminate or restrained structural forms, these differential temperatures also cause both longitudinal and transverse parasitic load effects, which shall be taken into account in the design.
17.2 Variation in average bridge temperature
Extremes of shade air temperature appropriate to the structure location shall be as given in Table 17.2(1).
Consideration shall be given to particular site characteristics, e.g., frost pockets and sheltered low-lying areas where the minimum shade air temperature may be substantially lower; and in urban and coastal areas where the minimum values may be higher than the values given in Table 17.2(1).
For major or special structures, extreme shade air temperatures for the actual site shall be