LOADS IN BRIDGE DESIGN
BY : AYAZ M ALIK
ABLE OF CONTENTS T
ACTORS & LOAD COMBINATIONS (CSA S6-06, CLAUSE 3.5) .......................... 1. LOADS F ......................................... ..................... ......4
2. DEAD LOADS (CSA S6-06, CLAUSE 3.6) ........................... ......................................... ............................ ............................. ............................. ........................... .............6 3. E ARTH LOADS & LOCKED-IN ERECTION LOADS (CSA S6-06, CLAUSE 3.7) ........................... .................................... .........7 ATIGUE LOADS (CSA S6-06, CLAUSE 10.17).......................... 4. F ......................................... ............................. ............................ ............................ .................. ....8
5. LIVE LOADS (CSA S6-06, CLAUSE 3.8)............................ .......................................... ............................ ............................ ............................ ............................ .................. 9 5.1
RUCK (CSA S6-06, CLAUSE 3.8.3.2) ............................ CL-W T RUCK .......................................... ............................ ............................ ....................... .........9
5.2
CL-W L ANE LOAD (CSA S6-06, CLAUSE 3.8.3.3) ............................ .......................................... ............................ ......................... ...........10
5.3
CL-625-ONT L CL-625-ONT LIVE LOADING (CSA S6-06, A S6-06, ANNEX A3.4).............. A3.4)............................ ............................. ......................... ..........10
5.4
DESIGN LANES & MULTI-L ANE LOADING (CSA S6-06, CLAUSE 3.8.2 & CLAUSE 3.8.4.2) ............................ ........................................... ............................. ............................ ............................ ............................ ............................ ............................ ............................ ............................ .................... ...... 11
5.5
DYNAMIC LOAD ALLOWANCE , DLA (CSA S6-06, CLAUSE 3.8.4.5)........................... ........................................ .............11
5.6
WHEELS ON THE SIDEWALK (CSA S6-06, CLAUSE 3.8.4.4)............................ ........................................... ......................... ..........13
5.7
ORCE (CSA S6-06, CLAUSE 3.8.5) ........................... CENTRIFUGAL F ORCE ......................................... ............................ ........................... .............13
5.8
BRAKING FORCE (CSA S6-06, CLAUSE 3.8.6)........................... ......................................... ............................ ............................ ...................... ........ 14
5.9
CURB LOAD (CSA S6-06, CLAUSE 3.8.7)........................... .......................................... ............................. ............................ ............................ ................ 14
5.10
PEDESTRIAN AND BICYCLE BARRIERS (CSA S6-06, CLAUSE 3.8.8.2) ............................ ................................ ....14
5.11
PEDESTRIAN LOAD (CSA S6-06, CLAUSE 3.8.9) ........................... ......................................... ............................ ........................... .............14
5.12
M AINTENANCE ......................................... .................. ...14 AINTENANCE V EHICLE EHICLE LOAD (CSA S6-06, CLAUSE 3.8.11) ..........................
6. W ATER LOADS (CSA S6-06, CLAUSE 3.11)........................... ......................................... ............................ ............................. ............................ ................... ...... 16 6.1
STATIC PRESSURE (CSA S6-06, CLAUSE 3.11.2) ........................... ......................................... ............................ ............................ ................ 16
6.2
BUOYANCY (CSA S6-06, CLAUSE 3.11.3) ............................ .......................................... ............................ ............................ ........................... .............16
6.3
STREAM PRESSURE (CSA S6-06, CLAUSE 3.11.4) ........................... ......................................... ............................ ........................... .............16
6.4
W AVE ACTION (CSA S6-06, CLAUSE 3.11.5) ........................... ......................................... ............................ ............................ ...................... ........ 17
6.5
SCOUR ACTION (CSA S6-06, CLAUSE 3.11.6) ............................ .......................................... ............................ ............................ .................... ...... 17
1/32
LOADS IN BRIDGE DESIGN 6.6
BY : AYAZ M ALIK
DEBRIS TORRENTS (CSA S6-06, CLAUSE 3.11.7)........................... ......................................... ............................ ............................ ................ 18
7. WIND LOADS (CSA S6-06, CLAUSE 3.10)........................... .......................................... ............................. ............................ ............................ ...................... ........ 19 7.1
GUST EFFECT COEFFICIENT (CSA S6-06, CLAUSE 3.10.1.3) .......................... ......................................... ......................... ..........19
7.2
WIND E XPOSURE COEFFICIENT (CSA S6-06, CLAUSE 3.10.1.4)........................... .......................................... .................. ... 19
7.3
NON-UNIFORM LOADING (CSA S6-06, CLAUSE 3.10.1.5)........................... ......................................... ........................... .............20
7.4
DESIGN OF THE SUPERSTRUCTURE (CSA S6-06, CLAUSE 3.10.2) ........................... ......................................... ................ 20
7.5
DESIGN OF THE SUBSTRUCTURE (CSA S6-06, CLAUSE 3.10.3)........................... ......................................... .................... ...... 21
7.6
AEROELASTIC INSTABILITY (CSA S6-06, CLAUSE 3.10.4) ........................... ......................................... ............................ ................ 22
8. ICE LOADS (CSA S6-06, CLAUSE 3.12) ........................... ......................................... ............................ ............................ ............................ ........................... .............24 8.1
BUOYANCY (CSA S6-06, CLAUSE 3.11.3) ............................ .......................................... ............................ ............................ ........................... .............24
8.2
DYNAMIC ICE FORCES (CSA S6-06, CLAUSE 3.11.3).......................... ........................................ ............................ ......................... ...........24
8.3
ICE IMPACT FORCES (CSA S6-06, CLAUSE 3.12.2.3) ............................ .......................................... ............................ ...................... ........ 26
8.4
STATIC ICE FORCES (CSA S6-06, CLAUSE 3.12.3)............................ .......................................... ............................ ........................... .............26
8.5
ICE JAMS (CSA S6-06, CLAUSE 3.12.4) ............................ ........................................... ............................. ............................ ............................ ................ 27
8.6
ICE ADHESION FORCES (CSA S6-06, CLAUSE 3.12.5) ........................... ......................................... ............................ ...................... ........ 27
8.7
ICE ACCRETION (CSA S6-06, CLAUSE 3.12.6)............................ .......................................... ............................ ............................ .................... ...... 27
ESSEL COLLISIONS (CSA S6-06, CLAUSE 3.14) ........................... 9. V ESSEL ......................................... ............................ ............................. ......................... ..........29
9.1
DESIGN VESSEL (CSA S6-06, CLAUSE 3.14.5) ........................... ......................................... ............................ ............................ .................... ...... 30
10.
V EHICLE ........................................ ............................ ......................... ...........30 EHICLE COLLISION LOAD (CSA S6-06, CLAUSE 3.15)..........................
11.
CONSTRUCTION LOADS (CSA S6-06, CLAUSE 3.16) ........................... ......................................... ............................ ............................ ................ 31
11.1
SEGMENTAL CONSTRUCTION (CSA S6-06, CLAUSE 3.16.4) ............................ .......................................... .................... ......31
2/32
LOADS IN BRIDGE DESIGN 6.6
BY : AYAZ M ALIK
DEBRIS TORRENTS (CSA S6-06, CLAUSE 3.11.7)........................... ......................................... ............................ ............................ ................ 18
7. WIND LOADS (CSA S6-06, CLAUSE 3.10)........................... .......................................... ............................. ............................ ............................ ...................... ........ 19 7.1
GUST EFFECT COEFFICIENT (CSA S6-06, CLAUSE 3.10.1.3) .......................... ......................................... ......................... ..........19
7.2
WIND E XPOSURE COEFFICIENT (CSA S6-06, CLAUSE 3.10.1.4)........................... .......................................... .................. ... 19
7.3
NON-UNIFORM LOADING (CSA S6-06, CLAUSE 3.10.1.5)........................... ......................................... ........................... .............20
7.4
DESIGN OF THE SUPERSTRUCTURE (CSA S6-06, CLAUSE 3.10.2) ........................... ......................................... ................ 20
7.5
DESIGN OF THE SUBSTRUCTURE (CSA S6-06, CLAUSE 3.10.3)........................... ......................................... .................... ...... 21
7.6
AEROELASTIC INSTABILITY (CSA S6-06, CLAUSE 3.10.4) ........................... ......................................... ............................ ................ 22
8. ICE LOADS (CSA S6-06, CLAUSE 3.12) ........................... ......................................... ............................ ............................ ............................ ........................... .............24 8.1
BUOYANCY (CSA S6-06, CLAUSE 3.11.3) ............................ .......................................... ............................ ............................ ........................... .............24
8.2
DYNAMIC ICE FORCES (CSA S6-06, CLAUSE 3.11.3).......................... ........................................ ............................ ......................... ...........24
8.3
ICE IMPACT FORCES (CSA S6-06, CLAUSE 3.12.2.3) ............................ .......................................... ............................ ...................... ........ 26
8.4
STATIC ICE FORCES (CSA S6-06, CLAUSE 3.12.3)............................ .......................................... ............................ ........................... .............26
8.5
ICE JAMS (CSA S6-06, CLAUSE 3.12.4) ............................ ........................................... ............................. ............................ ............................ ................ 27
8.6
ICE ADHESION FORCES (CSA S6-06, CLAUSE 3.12.5) ........................... ......................................... ............................ ...................... ........ 27
8.7
ICE ACCRETION (CSA S6-06, CLAUSE 3.12.6)............................ .......................................... ............................ ............................ .................... ...... 27
ESSEL COLLISIONS (CSA S6-06, CLAUSE 3.14) ........................... 9. V ESSEL ......................................... ............................ ............................. ......................... ..........29
9.1
DESIGN VESSEL (CSA S6-06, CLAUSE 3.14.5) ........................... ......................................... ............................ ............................ .................... ...... 30
10.
V EHICLE ........................................ ............................ ......................... ...........30 EHICLE COLLISION LOAD (CSA S6-06, CLAUSE 3.15)..........................
11.
CONSTRUCTION LOADS (CSA S6-06, CLAUSE 3.16) ........................... ......................................... ............................ ............................ ................ 31
11.1
SEGMENTAL CONSTRUCTION (CSA S6-06, CLAUSE 3.16.4) ............................ .......................................... .................... ......31
2/32
LOADS IN BRIDGE DESIGN
BY : AYAZ M ALIK
LIST OF F IGURES IGURES F IGURE IGURE 1 CL-W T RUCK RUCK .............................................................. ............................................................................................. ............................................................. ............................................................. ................................... 9 F IGURE IGURE 2 CL-W L ANE LOAD .......................................................... ......................................................................................... ............................................................. ....................................................... ......................... 10 F IGURE IGURE 3 CL-625-ONT T CL-625-ONT T RUCK RUCK & L ANE LOAD ............................................................ .......................................................................................... .................................................. .................... 10 F IGURE IGURE 4 F REE REE-BODY DIAGRAM FOR CENTRIFUGAL F ORCE ORCE (REF . 1).......................................................................... .............................................................................. .... 13 F IGURE IGURE 5 M AINTENANCE V EHICLE EHICLE LOAD ............................................................ ........................................................................................... .............................................................. ................................... .... 15 F IGURE IGURE 6 PIER NOSE ANGLE & SUBTENDED NOSE ANGLE FOR C ALCULATING F ORCES ORCES DUE TO MOVING ICE (CSA S6-06 F IGURE IGURE 3.7) ........................................................... .......................................................................................... ............................................................. ........................................ .......... 25 F IGURE IGURE 7 ICE ACCRETION (CSA S6-06 F IGURE IGURE A3.1.4) ............................................................ ........................................................................................... ................................... .... 28
LIST OF T ABLES T ABLE 1 LOAD F ACTORS & LOAD COMBINATIONS (CSA S6-06 T ABLE 3.1)................................................................ ................................................................ 5 T ABLE 2 PERMANENT LOADS - M AXIMUM & MINIMUM V ALUES OF LOAD F ACTORS FOR ULS (CSA S6-06 T ABLE 3.2) ............................................................ ........................................................................................... ............................................................. ........................................... ............. 5 T ABLE 3 UNIT WEIGHT OF M ATERIALS (CSA S6-06 T ABLE 3.3) ............................................................ ................................................................................... ....................... 6 ABLE 4 AVERAGE D AILY T RUCK RUCK T RAFFIC RAFFIC ............................................................. T ............................................................................................ .............................................................. ................................... 8
T ABLE 5 NUMBER OF DESIGN L ANES (CSA S6-06 T ABLE 3.4).................................................................................. .................................................................................. 11 T ABLE 6
MODIFICATION F ACTOR FOR MULTI-L ANE LOADING (CSA S6-06 T ABLE 3.5) ......................................... ......................................... 11
T ABLE 7 LONGITUDINAL DRAG COEFFICIENT , CD (CSA S6-06 T ABLE 3.10)............................................................. ............................................................. 16 T ABLE 8 L ATERAL LOAD COEFFICIENT , CL (CSA S6-06 T ABLE 3.11) ........................................................ ....................................................................... ............... 17 ABLE 9 WIND E XPOSURE COEFFICIENT , CE (CSA S6-06 T ABLE 3.8).............................................................. T ........................................................................ .......... 20
3/32
LOADS IN BRIDGE DESIGN ACTORS & LOAD COMBINATIONS 1. LOADS F
BY : AYAZ M ALIK (CSA S6-06, CLAUSE 3.5)
As per CSA S6-60, Clause 3.5, the loads to be considered in bridge design are as follows, A
=
Ice accretion load
D
=
Dead load
E
=
Loads due to earth pressure and hydrostatic pressure, including surcharges but excluding dead load
EQ
=
Earthquake loads
F
=
loads due to stream pressure and ice forces or to debris torrents
H
=
Collision load arising from highway vehicles or vessels, excluding barrier loads
K
=
All strains, deformations, and displacements and their effects, including the effects of their restraint and the effects of friction or stiffness in bearings. Strains and deformations include strains and deformations due to temperature change and temperature differential, concrete shrinkage, differential shrinkage, and creep, but not elastic strains
L
=
Live load (including the dynamic load allowance, when applicable), including barrier loads
P
=
Secondary prestress effects
S
=
load due to differential settlement and/or movement of the foundation
V
=
wind load on traffic
W
=
wind load on structure
As per CSA S6-06, clause 3.16.3, live loads shall include the weights of workers, vehicles, hoists, cranes, other equipment, and structural components that are subject to movement during the construction stage considered. The live load factor to be used for construction live loads shall be 85% of the value specified for L in the following table.
4/32
LOADS IN BRIDGE DESIGN
BY : AYAZ M ALIK
ABLE 1 LOAD F ACTORS & LOAD COMBINATIONS (CSA S6-06 T ABLE 3.1) T LOADS
PERMANENT LOADS
D
E
T RANSITORY LOADS
P
L
K
E XCEPTIONAL LOADS
W
V
S
EQ
F
A
H
0
0
0
0
0
0
0
F ATIGUE LIMIT STATE
FLS combination
11.00
1.00
1.00
1.00
0
SERVICEABILITY LIMIT STATES
SLS combination 1
1.00
1.00
1.00
0.90
0.80
0
0
1.00
0
0
0
0
SLS combination 2
0
0
0
0.90
0
0
0
0
0
0
0
0
ULTIMATE LIMIT STATES
ULS combination 1
αD
αE
αP
1.70
0
0
0
0
0
0
0
0
ULS combination 2
αD
αE
αP
1.60
1.15
0
0
0
0
0
0
0
ULS combination 3
αD
αE
αP
1.40
1.00
0.45
0.45
0
0
0
0
0
ULS combination 4
αD
αE
αP
0
1.25
1.5
0
0
0
0
0
0
ULS combination 5
αD
αE
αP
0
0
0
0
0
1.00
0
0
0
ULS combination 6
αD
αE
αP
0
0
0
0
0
0
1.30
0
0
ULS combination 7
αD
αE
αP
0
0
0.80
0
0
0
0
1.30
0
ULS combination 8
αD
αE
αP
0
0
0
0
0
0
0
0
1.00
ULS combination 9
1.35
αE
αP
0
0
0
0
0
0
0
0
0
T ABLE 2 PERMANENT LOADS - M AXIMUM & MINIMUM V ALUES OF LOAD F ACTORS FOR ULS ABLE 3.2) (CSA S6-06 T DEAD LOAD
Factory-produced components, excluding wood Cast-in-place concrete, wood, and all non-structural components Wearing surfaces, based on nominal or specified thickness Earth fill, negative skin friction on piles Water DEAD LOAD IN COMBINATION WITH E ARTHQUAKES
M AXIMUM αD
1.10 1.20 1.50 1.25 1.10 M AXIMUM αD
All dead loads for ULS Combination 5
1.25
E ARTH PRESSURE & HYDROSTATIC PRESSURE
M AXIMUM αE
Passive earth pressure, considered as a load At-rest earth pressure Active earth pressure Backfill pressure Hydrostatic pressure PRESTRESS
Secondary prestress effects
MINIMUM
αD
0.95 0.90 0.65 0.80 0.90 MINIMUM
αD
0.80 MINIMUM
αE
1.25 1.25 1.25 1.25 1.10
0.50 0.80 0.80 0.80 0.90
M AXIMUM αP
MINIMUM αP
1.05
0.95
5/32
LOADS IN BRIDGE DESIGN
BY : AYAZ M ALIK
2. DEAD LOADS (CSA S6-06, CLAUSE 3.6) The dead load of the structural components and nonstructural attachments are definitely permanent loads and must be included. Here structural components refer to those elements that are part of the load resistance system. Nonstructural attachments refer to such items as curbs, parapets, barrier rails, signs, illuminators, and guard rails. The weight of such items can be estimated by using the unit weight of the material combined with the geometry. For third-party attachments, for example, the guard rail, the manufacture’ s literature often contains weight information. In the absence of more precise information, the unit weights given in the following table may be used. ABLE 3 UNIT WEIGHT OF M ATERIALS (CSA S6-06 T ABLE 3.3) T
M ATERIAL
Aluminum alloy Bituminous wearing surface Low-density concrete Semi-low-density concrete Plain concrete Prestressed concrete Reinforced concrete Coarse-grained (granular) soil Crushed rock Fine-grained sandy soil Glacial till Rock-fill Air-cooled slag Water-cooled slag Steel Fresh water Salt or polluted water Hardwood Softwood
UNIT WEIGHT , kN/m3
27.0 23.5 18.1 21.0 23.5 24.5 24.0 22.0 22.0 20.0 22.0 21.0 11.0 15.0 77.0 9.8 10.5 9.5 6.0
6/32
LOADS IN BRIDGE DESIGN
BY : AYAZ M ALIK
3. E ARTH LOADS & LOCKED-IN ERECTION LOADS (CSA S6-06, CLAUSE 3.7) The dead load of earth fills must be considered for buried structures such as culverts. This load is determined by multiplying the unit weight times the depth of materials. Soil – structure interaction effects may apply. Soil retained by a structure such as a retaining wall, wing wall, or abutment creates a lateral pressure on the structure. The lateral pressure is a function of the geotechnical characteristics of the material, the system geometry, and the anticipated structural movements. Most engineers use models that yield a fluid-like pressure against the wall. Earth loads, other than those applied as dead loads, shall be as specified in CSA S6-06, section 6. The requirements of CSA S6-06, section 7 shall apply to buried structures. Locked-in erection stresses are accumulated force effects resulting from the construction process. They include secondary forces from posttensioning and downdrag. Downdrag is a force exerted on a pile or drilled shaft due to soil movement around the element. Such a force is permanent and typically increases with time.
7/32
LOADS IN BRIDGE DESIGN
BY : AYAZ M ALIK
ATIGUE LOADS (CSA S6-06, CLAUSE 10.17) 4. F
The strengths of various components of the bridge are sensitive to repeated stressing or fatigue. When the load is cyclic, the stress level that ultimately fractures the material can be significantly below the nominal yield strength. The fatigue strength is typically related to the range of live-load stress and the number of stress cycles under service load conditions. The live-load stress range is estimated by using a single design truck. The dynamic load allowance (DLA) must be included and the bridge is assumed to be loaded in a single lane. The number of stress–range cycles is based on traffic surveys. The average daily truck traffic (ADTT) in a single lane may be estimated as:
f p
where p is the fraction of traffic assumed to be in one lane as defined in table 4. Because the traffic patterns on the bridge are uncertain, the frequency of the fatigue load for a single lane is assumed to apply to all lanes. The ADTT is usually available from the bridge owner, but in some cases only the average daily traffic (ADT) is available. In such cases, the percentage of trucks in the total traffic must be estimated. This percentage can vary widely with local conditions, and the engineer should try to estimate this with a survey. The fatigue truck is applied in the same manner as the other vehicles and the range of extreme stress (actions) are used. Note that the number of stress–range cycles is not used in the structural analysis directly. The number of stress–range cycles is used to establish the available resistance (CSA S6-06, Clause 10.17.2.3). T ABLE 4 AVERAGE D AILY T RUCK T RAFFIC CLASS OF HIGHWAY
ADTT
A B C D
4000 1000 250 50
8/32
LOADS IN BRIDGE DESIGN
BY : AYAZ M ALIK
5. LIVE LOADS (CSA S6-06, CLAUSE 3.8) 5.1
CL-W T RUCK (CSA S6-06, CLAUSE 3.8.3.2)
The CL-W truck is the idealized five-axle truck shown in Figure 3.2. The W number indicates the gross load of the CL-W Truck in kilonewtons. Wheel and axle loads are shown in terms of W and are also shown for the CL-625 Truck. The wheels spacing, weight distribution, and clearance envelope of the CL-W Truck shall be as shown in figure 1. The CL-W and the CL-625-ONT Truck shall be placed centrally in a space 3.0 m wide that represents the clearance envelope for each Truck. A XLE NO.
CL–W CL–625
1
2
4
3
0.04W 0.08W
0.1W 0.1W 0.2W 0.2W
0.14W 0.28W
2 5
62.5 62.5 12 125
87.5 175
3.6m
1.2m
5
0.12W Wheel loads 0.24W Axle loads 7 15
6.6m 18m
Wheel loads, kN Axle loads, kN
6.6m
0.25m (Typ.)
0.25m (Typ.)
0.25m (Typ.)
1.80m
2.40m
(Typ.)
Clearance envelope 3.0m
Curb
0.6m
0.60m
1.8m
0.6m
F IGURE 1 CL-W T RUCK
9/32
LOADS IN BRIDGE DESIGN 5.2
BY : AYAZ M ALIK
CL-W L ANE LOAD (CSA S6-06, CLAUSE 3.8.3.3)
The CL-W Lane Load consists of a CL-W Truck with each axle reduced to 80% of the value specified in CL-W Truck loading, superimposed within a uniformly distributed load of 9 kN/m, and 3.0 m wide. The CL-W Lane Load is shown in figure 2. Uniformly distributed load 9 kN/m 0.032 0.064
0.08W 0.08W 0.16W 0.16W
3.6m 1.2m
0.112 0.224
0.096 0.192
Wheel loads Axle loads
6.6m
6.6m 18m
F IGURE 2 CL-W L ANE LOAD
5.3
CL-625-ONT LIVE LOADING (CSA S6-06, ANNEX A3.4)
In Ontario, the CL-625-ONT truck and the CL-625-ONT lane load shown in figure 3 shall be used instead of the CL-625 truck and CL-W lane load, respectively. A XLE NO.
1
Wheel loads, kN Axle loads, kN
2 5
2
3
4
70 70 14 140
3.6m 1.2m
5
87.5 175
6.6m
6 120
6.6m
18m Uniformly distributed load 9 kN/m
Wheel loads, kN Axle loads, kN
20 40
56 56 112 112
3.6m 1.2m
70 140
6.6m 18m
48 96
6.6m
F IGURE 3 CL-625-ONT T RUCK & L ANE LOAD
10/32
LOADS IN BRIDGE DESIGN 5.4
BY : AYAZ M ALIK
DESIGN LANES & MULTI-L ANE LOADING (CSA S6-06, CLAUSE 3.8.2 & CLAUSE 3.8.4.2)
The number of design lanes, n, shall be determined as per CSA S6-06, Clause 3.8.2. The number of lanes for a given deck width shall be decided based on the values below. ABLE 5 NUMBER OF DESIGN L ANES (CSA S6-06 T ABLE 3.4) T
DECK WIDTH, WC , m
n
6.0 or less
1
Over 6.0 to 10.0
2
Over 10.0 to 13.5
2 or 3
Over 13.5 to 17.0
4
Over 17.0 to 20.5
5
Over 20.5 to 24.0
6
Over 24.0 to 27.5
7
Over 27.5
8
Trucks will be present in adjacent lanes on roadways with multiple design lanes, but it is unlikely that three adjacent lanes will be loaded simultaneously with the heavy loads. Therefore, some adjustments in the design loads are necessary. To account for this effect, CSA S6-06 provides an adjustment factor for the multiple presences. When more than one design lane is loaded, the traffic load shall be multiplied by the applicable modification factor. Design lanes that are loaded shall be selected to maximize the load effect. ABLE 6 T
5.5
ACTOR FOR MULTI-L ANE LOADING (CSA S6-06 T ABLE 3.5) MODIFICATION F
NUMBER OF LOADED DESIGN L ANES
MODIFICATION F ACTOR
1
1.00
2
0.90
3
0.80
4
0.70
5
0.60
6 or more
0.55
DYNAMIC LOAD ALLOWANCE , DLA (CSA S6-06, CLAUSE 3.8.4.5)
The roadway surface is not perfectly smooth, thus the vehicle suspension must react to roadway roughness by compression and extension of the suspension system. This oscillation creates axle forces that exceed the static weight during the time the acceleration is upward and is less than the static weight when the acceleration is downward. Although commonly
11/32
LOADS IN BRIDGE DESIGN
BY : AYAZ M ALIK
called impact, this phenomenon is more precisely referred to as dynamic loading. Based on the study by Hwang and Nowak, 1991, the dynamic load allowance can be expressed as; dditional deflection due to dynamic effects ynamic oad lloance, aximum static deflection It is important to observe that this ratio varies significantly with different vehicle positions. Thus, it is quite possible to observe impact factors that greatly exceed those at the maximum deflections. However, it is reasonable to define DLA based on extreme values. The principal parameters that affect the impact factor are the dynamic characteristics of the truck, the dynamic characteristics of the bridge, and the roadway roughness. These characteristics are expected as all transient structural dynamic problems involve stiffness, mass, damping, and excitation. As per CSA S6-06, Clause 3.8.4.5, dynamic load allowance shall be applied to the CL-W Truck. A dynamic load allowance shall not be applied to the CL-W Lane Load, including that part of the CL-W Lane Load represented by axle loads. A dynamic load allowance shall be included in loads on the superstructure and loads transferred from the superstructure to the substructure, but shall not be included in loads transferred to footings that are surrounded with earth or to those parts of piles that are below ground. The dynamic load allowance for loads on arch-type buried structures with a depth of earth cover, DE, between the riding surface and the highest point of the structure shall be 0.40(1 – 0.5DE), but not less than 0.10. The dynamic load allowance for box-type buried structures shall be the value obtained from the values listed below multiplied by the factor (1 – 0.5DE), but not less than 0.10. For components other than buried structures, the dynamic load allowance shall be a. 0.50 for deck joints; b. 0.40 where only one axle of the CL-W Truck is used (except for deck joints); c. 0.30 where any two axles of the CL-W Truck, or axles nos. 1 to 3, are used; or d. 0.25 where three axles of the CL-W Truck, except for axles nos. 1 to 3, or more than three axles, are used. For wood components, the dynamic load allowance mentioned above shall be multiplied by 0.70.
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LOADS IN BRIDGE DESIGN 5.6
BY : AYAZ M ALIK
WHEELS ON THE SIDEWALK (CSA S6-06, CLAUSE 3.8.4.4)
When sidewalks and other areas adjacent to a roadway are separated from it only by curbs and not by a traffic barrier, local responses shall be computed by considering a CL-W Truck with each axle load reduced to 70% and with its wheel centers not less than 0.30 m from the face of the railing or barrier on the outer edge. This requirement shall apply only at the ultimate limit states and shall not apply to longitudinal effects in slab bridges or to main girders. 5.7
CENTRIFUGAL F ORCE (CSA S6-06, CLAUSE 3.8.5)
A truck can increase speed, decrease speed, and/or change directions as it moves along a curvilinear path. All of these effects require an acceleration of the vehicle that causes a force between the deck and the truck. As a truck moves along a curvilinear path, the change in direction of the velocity causes a centrifugal acceleration in the radial direction. The forces and accelerations involved are illustrated in figure 4. F r is the force on the truck directed towards the center of the curve (outward on the bridge) and is given as
r r
where W is weight of the vehicle. The position of this force shall be at the center of each design lane in horizontal direction, at right angles to the direction of travel. This force is assumed to be acting 2000 mm above the roadway/ deck surface. No dynamic load allowance shall be included with this force. V
W = mg
r F r
m r
m r
F r 2m
N
F IGURE 4 F REE-BODY DIAGRAM FOR CENTRIFUGAL F ORCE (REF . 1)
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LOADS IN BRIDGE DESIGN 5.8
BY : AYAZ M ALIK
BRAKING FORCE (CSA S6-06, CLAUSE 3.8.6)
Braking force shall be considered only at the ultimate limit states. Braking force shall be an equivalent static force of 180 kN plus 10% of the uniformly distributed load portion of the lane load from one design lane, irrespective of the number of design lanes, but not greater than 700 kN in total. The braking force shall be applied at the deck surface. 5.9
CURB LOAD (CSA S6-06, CLAUSE 3.8.7)
Curb load shall be considered only at the ultimate limit states. For continuously supported curbs, the design load shall be a uniformly distributed lateral load of 20 kN/m. For curbs supported at discrete points, the design load shall be a concentrated lateral load of 32 kN. Curb loads shall be applied at the top of the curb or 250 mm above the deck surface, whichever is lower. 5.10 PEDESTRIAN AND BICYCLE BARRIERS (CSA S6-06, CLAUSE 3.8.8.2)
The load on pedestrian and bicycle barrier railings shall be a uniform load of 1.20 kN/m applied laterally and vertically simultaneously. 5.11 PEDESTRIAN LOAD (CSA S6-06, CLAUSE 3.8.9)
For pedestrian bridges and sidewalks on highway bridges, the pedestrian load applied to the walkway area, p, shall be a p
s
a
For highway bridges with sidewalks, traffic loads in design lanes shall be considered together with the pedestrian load only at the ultimate limit states, with the pedestrian load reduced by 20%. The traffic load due to wheels on the sidewalk and the pedestrian load shall not be considered to act simultaneously on a sidewalk. 5.12 M AINTENANCE V EHICLE LOAD (CSA S6-06, CLAUSE 3.8.11)
If the width of a sidewalk on a highway bridge, or of a pedestrian bridge, is greater than 3.0 m and access is provided for maintenance vehicles, the maintenance vehicle load shown in figure 5 shall be considered on the walkway area. For sidewalks on a highway bridge, the maintenance vehicle load shall be considered only at the ultimate limit states. The maintenance vehicle load shall not be considered to act simultaneously with the pedestrian live load or with the loading from wheels on the sidewalk
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LOADS IN BRIDGE DESIGN Wheel loads, kN Axle loads, kN
BY : AYAZ M ALIK
12 24
28 56 Gross Load, 80 kN 2.0m
Axle
Axle ELEVATION
0.15m
0.25m
Travel 0.15m
0.25m
0.15m
l e e h W
l e e h W
0.3m h t d i w k c u r t
0.15m
PLAN
F IGURE 5 M AINTENANCE V EHICLE LOAD
m 2 . 2
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LOADS IN BRIDGE DESIGN
BY : AYAZ M ALIK
6. W ATER LOADS (CSA S6-06, CLAUSE 3.11) 6.1
STATIC PRESSURE (CSA S6-06, CLAUSE 3.11.2)
Static water pressure shall be assumed to act perpendicular to the surface that is retaining the water. The pressure of water at a specific point shall be calculated as the product of the height of water above that point and the density of water. 6.2
BUOYANCY (CSA S6-06, CLAUSE 3.11.3)
The effects of immersion in water or exposure to water pressure shall be considered. The beneficial effects of buoyancy shall be included, provided that they are always in existence. The non-beneficial effects of buoyancy shall be included unless the possibility of their occurrence can be excluded with certainty. Buoyancy shall be taken as the vertical components of the static forces as calculated in accordance with CSA S6-06, Clause 3.11.2. Buoyancy shall be considered as an uplift force equivalent to the volume of water displaced. 6.3
STREAM PRESSURE (CSA S6-06, CLAUSE 3.11.4)
Water flowing against and around the substructure creates longitudinal as well as lateral force directly on the structure as well as debris that might accumulate under the bridge. Flood conditions are the most critical. The forces created are proportional to the square of velocity and to a drag coefficient. The load due to flowing water acting longitudinally on a substructure element, P, shall be taken as T ABLE 7 LONGITUDINAL DRAG COEFFICIENT , CD (CSA S6-06 T ABLE 3.10) UPSTREAM SHAPE OF PIER
LONGITUDINAL DRAG COEFFICIENT , CD
Semi-circular nosed
0.7
Square ended
1.4
Wedge nosed at 90o
0.8
Pier with debris lodged
1.4
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LOADS IN BRIDGE DESIGN
BY : AYAZ M ALIK
If the substructure is oriented at an angle to the stream flow, then adjustments must be made. The lateral load due to water flowing at angle, θ, against a substructure element, PP, shall be taken as
T ABLE 8 L ATERAL LOAD COEFFICIENT , CL (CSA S6-06 T ABLE 3.11) ANGLE, BETWEEN DIRECTION OF F LOW AND
LONGITUDINAL PIER A XIS, DEGREES
COEFFICIENT , CL
0
0.0
5
0.5
10
0.7
20
0.9
6.4
L ATERAL LOAD
30
1.0
W AVE ACTION (CSA S6-06, CLAUSE 3.11.5)
Force effects due to wave action on bridge substructure elements exposed to environments where significant wave action can occur shall be evaluated in accordance with site-specific conditions. In the absence of such evaluations, the force against a flat surface substructure element, F w, due to wave action, as a function of the wave height, H w, shall be taken as 10Hw2. F w shall be considered to act at mid-height of the wave, H w /2, above the still water elevation. For aerodynamically curved frontal surfaces, a value of F w /2 shall be used. 6.5
SCOUR ACTION (CSA S6-06, CLAUSE 3.11.6)
Although not a force, the scour of the stream bed around the foundation can result in structural failure. Scour is the movement of the stream bed from around the foundation, and this can significantly change the structural system, creating a situation that must be considered in the design. Local conditions and past records of floods shall be consulted in designing foundation elements when scour is expected to occur. Changes in foundation conditions resulting from the design flood shall be considered at serviceability and ultimate limit states.
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LOADS IN BRIDGE DESIGN 6.6
BY : AYAZ M ALIK
DEBRIS TORRENTS (CSA S6-06, CLAUSE 3.11.7)
Debris torrent loads shall be considered on exposed superstructures and substructures in accordance with site-specific conditions. Sites subject to heavy rainfall of short duration, earthquakes, landslides, and rock-falls shall be investigated for debris torrents when the following conditions exist: a. The creek channel gradient is greater than 25° for an extended length along the channel profile; b. Boulders and debris exist in the channel; c. There is a history of such events.
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LOADS IN BRIDGE DESIGN
BY : AYAZ M ALIK
7. WIND LOADS (CSA S6-06, CLAUSE 3.10) The velocity of the wind varies with the elevation above the ground and the upstream terrain roughness, and therefore pressure on a structure is also a function of these parameters. Velocity increases with elevation, but at a decreasing rate. If the terrain is smooth, then the velocity increases more rapidly with elevation. All wind loads based on the reference wind pressure, q, shall be treated as equivalent static loads. The hourly mean reference wind pressure, q, shall be as specified in CSA S6-06, Table A3.1.1 for a return period of a. 100 years for bridge structures with any span 125 m long or longer; b. 50 years for bridge structures with a maximum span shorter than 125 m, luminaire support structures higher than 16 m, and overhead sign structures; c. 25 years for luminaire and traffic signal support structures 16 m high or shorter, and for barriers; and d. 10 years for roadside sign structures where a long life expectancy is not required, or for any of the structures specified in Items (a) to (c) during construction. If the topography at the structure site can cause funneling of the wind, the reference wind pressure shall be increased by 20%. 7.1
GUST EFFECT COEFFICIENT (CSA S6-06, CLAUSE 3.10.1.3)
For highway bridges that are not sensitive to wind action (which includes most bridges of spans less than 125 m except those that are cable supported), the gust effect coefficient, C g, shall be taken as 2.0. For slender, lighter structures, e.g., pedestrian bridges, luminaire, sign, and traffic signal supports, barriers, and slender structural elements, C g shall be taken as 2.5. For structures that are sensitive to wind action, the gust factor approach shall not be used and the wind loads shall be determined on the basis of a detailed analysis of dynamic wind action, using an approved method that includes the effects of buffeting. 7.2
WIND E XPOSURE COEFFICIENT (CSA S6-06, CLAUSE 3.10.1.4)
The wind exposure coefficient, C e, shall not be less than 1.0 and shall be taken from table 9 or calculated as (0.10xH) 0.2, where H is the height above ground of the top of the
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LOADS IN BRIDGE DESIGN
BY : AYAZ M ALIK
superstructure. For luminaire, sign, and traffic signal supports, and for barriers, H shall be taken to the top of the standard, support, or structure considered. The height above ground shall be measured from the foot of cliffs, hills, or escarpments when the structure is located in uneven terrain or from the low water level for structures over water. T ABLE 9 WIND E XPOSURE COEFFICIENT , CE (CSA S6-06 T ABLE 3.8) HEIGHT ABOVE GROUND OF THE T OP OF THE
7.3
STRUCTURE, H, m
WIND E XPOSURE COEFFICIENT , CE
0 to 10
1.0
Over 10 to 16
1.1
Over 16 to 25
1.2
Over 25 to 37
1.3
Over 37 to 54
1.4
Over 54 to 76
1.5
Over 76 to 105
1.6
NON-UNIFORM LOADING (CSA S6-06, CLAUSE 3.10.1.5)
Wind loads shall be applied uniformly or non-uniformly over the entire structure, whichever produces the more critical effects. Unless an analysis of non-uniform wind loads specific to the structure is undertaken, the non-uniform loading shall be 0.75 times the effective uniformly distributed load over any portion of the structure and the full effective uniformly distributed load applied over the remaining portion. When the prescribed loads in the design of members are being applied, overturning, uplift, and lateral displacement shall be considered. 7.4
DESIGN OF THE SUPERSTRUCTURE (CSA S6-06, CLAUSE 3.10.2)
The superstructure shall be designed for wind-induced vertical and horizontal drag loads acting simultaneously. The assumed wind direction shall be perpendicular to the longitudinal axis for a straight structure or to an axis chosen to maximize wind-induced effects for a structure curved in plan. The following wind load per unit exposed frontal area of the superstructure shall be applied horizontally: F h = q CeC gCh
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LOADS IN BRIDGE DESIGN
BY : AYAZ M ALIK
where q, Ce and C g are as specified in CSA S6-06, Clauses 3.10.1.2, 3.10.1.4, and 3.10.1.3, respectively, and C h = 2.0. In the case of truss spans, this load shall be taken to act on the windward truss simultaneously with a load on the leeward truss equal to the load on the windward truss in the through-trusses and 75% of the load on the windward truss in other trusses unless a recognized method is used to calculate the shielding effect of the windward truss. The following wind load per unit exposed plan area of the superstructure shall be applied vertically: F v = q CeC gCv where q, Ce and C g are as specified in CSA S6-06, Clauses 3.10.1.2, 3.10.1.4, and 3.10.1.3, respectively, and C v = 1.0. The vertical load shall be taken to act either upwards or downwards. In addition to the application of F v as a uniformly distributed load over the whole plan area, the effect of possible eccentricity in the application of the load shall be considered. For this purpose, the same total load shall be applied as an equivalent vertical line load at the windward quarter point of the transverse superstructure width. The horizontal wind load per unit exposed frontal area of the live load shall be calculated in accordance with CSA S6-06, Clause 3.10.2.2, except that C h shall be taken as 1.2. The exposed frontal area of the live load shall be the entire length of the superstructure, as seen in elevation in the direction of the wind as specified in CSA S6-06, Clause 3.10.2.1, or any part or parts of that length producing critical response, multiplied by a height of 3.0 m above the roadway surface for vehicular bridges and 1.5 m for pedestrian bridges. Areas below the top of a solid barrier wall shall be neglected. 7.5
DESIGN OF THE SUBSTRUCTURE (CSA S6-06, CLAUSE 3.10.3)
The substructure shall be designed for wind-induced loads transmitted to it from the superstructure and for wind loads acting directly on the substructure. Loads for wind directions both normal to and skewed to the longitudinal centerline of the superstructure shall be considered. The horizontal drag load specified in CSA S6-06, Clause 3.10.2.2 shall be resolved into transverse and longitudinal components using the skew angle modification coefficients
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LOADS IN BRIDGE DESIGN
BY : AYAZ M ALIK
specified in table 10. These loads shall be applied as equivalent horizontal line loads at the elevation of the centroid of the exposed frontal area of the superstructure. The vertical load specified in CSA S6-06, Clause 3.10.2.3, modified for skew angle using appropriate coefficients from Table 10, shall be applied as an upward or downward line load along the centerline of the superstructure or along the windward quarter point, whichever produces the more critical effect. The vertical load and the longitudinal and transverse horizontal loads shall be applied simultaneously and the combination leading to maximum load effects in the substructure shall be used. The requirements of CSA S6-06, Clause 3.10.2.4 shall apply in determining the wind load on the live load that is to be transferred to the substructure. The modifications specified for “Other spans” in table 10 shall apply to skewed wind loads on the live load on any type of span. Longitudinal loads shall be determined for winds parallel to the longitudinal axis of the bridge (i.e., at a skew angle of 90°) using the projected area to the wind of the bridge superstructure in the longitudinal direction. The substructure shall be designed for directly applied horizontal drag loads. The wind load on a unit frontal exposed area of the substructure shall be calculated in accordance with CSA S6-06, Clause 3.10.2.2. The horizontal drag coefficient, C h, shall be taken as 0.7 for circular piers, 1.4 for octagonal piers, and 2.0 for rectangular and square piers. For wind directions skewed to the substructure, the loads shall be resolved into components taken to act perpendicularly to the end and side elevations of the substructure. These load components shall be assumed to act horizontally at the centroids of the exposed areas of the end and side elevations and shall be applied simultaneously with the loads transmitted from the superstructure. 7.6
AEROELASTIC INSTABILITY (CSA S6-06, CLAUSE 3.10.4)
Aerodynamic force is exerted on a body by the air (or some other gas) in which the body is immersed, and is due to the relative motion between the body and the gas. Aerodynamic force arises from two causes, a. Normal force due to the pressure on the surface of the body, and b. Shear force due to the viscosity of the gas, also known as skin friction.
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LOADS IN BRIDGE DESIGN
BY : AYAZ M ALIK
Pressure acts locally, normal to the surface, and shear force acts locally, parallel to the surface. The net aerodynamic force over the body is due to the pressure and shear forces integrated over the total exposed area of the body. When an airfoil (or a wing) is moving relative to the air it generates an aerodynamic force, in a rearward direction at an angle with the direction of relative motion. This aerodynamic force is commonly resolved into two components. a. Drag is the force component parallel to the direction of relative motion, b. Lift is the force component perpendicular to the direction of relative motion. Aeroelastic instability, in which the motion of the structure in wind produces aerodynamic forces augmenting such motion, shall be taken into account in the design of bridges and structural components apt to be wind sensitive. The aeroelastic phenomena of vortex shedding, galloping, flutter, and divergence shall be considered where applicable. For a wind-sensitive structure affected by the wind actions specified in CSA S6-06, Clause 3.10.4.1, it shall be shown that the performance of the structure without further application of load factors is acceptable up to a wind speed higher than the reference wind speed, V ref . Unless alternative rational procedures are available, the reference wind speed shall be taken as
ref α qe where, αw = the load factor for wind specified in CSA S6-06, Clause 3.5.1 The reference wind velocity shall be taken at deck height. Bridges and their structural components, including cables, shall be designed to be free of fatigue damage due to vortexinduced or galloping oscillations.
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LOADS IN BRIDGE DESIGN
BY : AYAZ M ALIK
8. ICE LOADS (CSA S6-06, CLAUSE 3.12) 8.1
BUOYANCY (CSA S6-06, CLAUSE 3.11.3)
The effects of immersion in water or exposure to water pressure shall be considered. The beneficial effects of buoyancy shall be included, provided that they are always in existence. The non-beneficial effects of buoyancy Ice forces on bridge substructure elements shall be determined by considering the prevailing site conditions and expected form of ice action. The following interaction modes between ice and structure shall be considered: a. Dynamic forces due to collision of moving ice sheets or floes carried by the stream current or driven by wind action (both horizontal and vertical components shall be considered); b. Static forces due to thermal movements of continuous stationary ice sheets; c. Lateral thrust due to arching action resulting from ice dams and ice jams; and d. Static or dynamic vertical forces along the substructure element due to the effects of fluctuating water levels or the dynamic effects of colliding ice floes. Data related to the anticipated thickness of ice, its direction of movement, its speed of impact, and the height of its action on the substructure element shall be obtained or derived from field surveys and records of measurements made at or near the site. 8.2
DYNAMIC ICE FORCES (CSA S6-06, CLAUSE 3.11.3)
Unless more precise data is available, the following values for the effective crushing strength of ice, p, shall be used: a. The ice breaks up at melting temperature and is substantially disintegrated: 400 kPa; b. The ice breaks up at melting temperature and is somewhat disintegrated: 700 kPa; c. The ice breaks up or ice movement occurs at melting temperature and is internally sound and moving in large pieces: 1100 kPa; and d. The ice breaks up or ice movement occurs at temperatures considerably below the melting point or the ice: 1500 kPa.
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LOADS IN BRIDGE DESIGN
BY : AYAZ M ALIK
The horizontal dynamic ice force on a pier shall be determined in accordance with CSA S6-06, Clause 3.12.2.2.3 using the ice failure forces in accordance with CSA S6-06, Clause 3.12.2.2.2. Ice failure forces shall be determined as follows: a. Bending force, F b: F b = Cn pt 2 where, Cn = 0.5 tan (α + 15°), with α as shown in figure 6 b. Crushing force, F c: F c = Ca pt w where,
[( √ ) ]
t a
c. Bending/crushing transition force F bc: c n p
α
Flow
β
F IGURE 6 PIER NOSE ANGLE & SUBTENDED NOSE ANGLE FOR C ALCULATING F ORCES DUE TO MOVING ICE (CSA S6-06 F IGURE 3.7)
The horizontal force, F, due to the pressure of moving ice shall be taken as, a. When F c F b F = F c
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LOADS IN BRIDGE DESIGN
BY : AYAZ M ALIK
b. When F c > F b i.
F = F c if F bc F c
ii.
F = F b if F bc F b
iii.
F = F bc if F c > F bc > F b
In small streams where it is unlikely that large-size ice floes will form, the force, F, may be reduced by up to 50% of the above values. 8.3
ICE IMPACT FORCES (CSA S6-06, CLAUSE 3.12.2.3)
Where the longitudinal axis of the pier is reasonably parallel to the direction of the movement of ice, the force, F, as derived from CSA S6-06, Clause 3.12.2.2, shall be considered to act along the longitudinal axis of the pier. The following design cases shall be investigated: a. Case 1: a longitudinal force, F, plus a transverse force, 0.15F ; and b. Case 2: a longitudinal force, 0.5F, plus a transverse force, F t , where t
tan β θf
In the absence of more precise data, θf shall be taken as 6°. For a round-nosed edge, β shall be taken as 100°, where β is as shown in figure 6. For piers with their longitudinal axis at an angle to the direction of flow, the total collision forces shall be considered to act on the projected pier width and resolved into components parallel and perpendicular to the pier shaft. The component that acts transversely on the pier shaft shall not be taken as less than 20% of the total force. Where ice forces are significant, slender and flexible piers and their components, e.g., piles exposed to ice action, shall be used only when a specialist on the mechanics of ice and structure interaction is consulted. 8.4
STATIC ICE FORCES (CSA S6-06, CLAUSE 3.12.3)
Where ice sheets are exposed to non-uniform thermal stresses and strains relative to the pier due to unbalanced freezing, the resulting forces on the piers shall be calculated using a compressive crushing strength of ice of not less than 1500 kPa when the ice temperature is significantly below the freezing point.
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LOADS IN BRIDGE DESIGN 8.5
BY : AYAZ M ALIK
ICE JAMS (CSA S6-06, CLAUSE 3.12.4)
For clear openings of less than 30 m between piers or between a shoreline and a pier located in bodies of water where floating ice can occur, a pressure of 10 kPa shall be considered to act against the exposed substructure element. This force shall be applied above the level of still water for the expected thickness of the ice jam, both laterally and in the direction of the ice flow. For clear openings of more than 30 m, this force may be reduced to 5 kPa against the exposed faces. 8.6
ICE ADHESION FORCES (CSA S6-06, CLAUSE 3.12.5)
The vertical force due to water level fluctuations, F v , on a pier frozen to an ice formation shall be calculated as follows: a. For circular piers: F v = 1250t 2 (1.05 + 0.13R/t 0.75 ); and b. For oblong piers: F v = 15Lpt 1.25 + 1250t 2 (1.05 + 0.13R/t 0.75 ). 8.7
ICE ACCRETION (CSA S6-06, CLAUSE 3.12.6)
Ice accretion loads shall be taken to occur on all exposed surfaces of superstructure members, structural supports, traffic signals, luminaires, and railings. In the case of sign panels, bridge girders, and solid barriers, ice accretion shall be considered to occur on one side only. The design ice thickness for ice accretion shall be the value specified in figure 7. A unit weight of 9.8kN/m 3 shall be used in calculating ice accretion loads.
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LOADS IN BRIDGE DESIGN
F IGURE 7 ICE ACCRETION (CSA S6-06 F IGURE A3.1.4)
BY : AYAZ M ALIK
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LOADS IN BRIDGE DESIGN 9. V ESSEL COLLISIONS
BY : AYAZ M ALIK
(CSA S6-06, CLAUSE 3.14)
In a navigable waterway crossing where there is a risk of vessel collision, all bridge elements that could be hit shall be designed for vessel impact or adequately protected from vessel collision. The design procedure for vessel collision shall be as specified in CSA S6-06, Annex A3.3. The following general requirements shall apply: a. In navigable waterways where vessel collision is possible, structures shall be I.
Designed to resist the design vessel collision forces;
II.
Evaluated to meet a minimum level of safety; or
III.
Adequately protected by fenders, dolphins, berms, islands, or other devices, as appropriate.
b. Consideration shall be given to the relationship of the bridge (including its structural dynamic response) to the following: I.
Waterway geometry;
II.
Size, type, loading condition, and frequency of vessels using the waterway;
III.
Navigable water depth; and
IV.
Vessel speed and direction.
Bridges shall be classified as follows: a. Class I: bridges that are of critical importance, including those that have to remain open to all traffic after a vessel collision. b. Class II: bridges that are of regular importance, including those that have to remain open to emergency and security vehicles after a vessel collision. Two methods, specified in CSA S6-06, Annex A3.3, may be used for assessing the classification criteria, the selection of the design vessel, and the calculation of the vessel collision forces. Method I is a simplified approach. Method II is a probabilistic approach based on AF max, the maximum annual frequency of collapse for the whole bridge. The annual frequency of collapse, AF, for each pier and span component susceptible to ship collision shall be determined by distributing the total bridge acceptance criterion, AF max, over the number of piers and span components located in the navigable waterway.
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LOADS IN BRIDGE DESIGN 9.1
BY : AYAZ M ALIK
DESIGN VESSEL (CSA S6-06, CLAUSE 3.14.5)
The number of vessels, N, passing under the bridge shall be developed for each pier and span component being evaluated in accordance with the size, type, and loading condition of the vessels and the depth of navigable water. For Method I, the selection of the design vessel shall be based solely on the frequency distribution of vessel traffic. For Method II, a design vessel for each pier or span component shall be selected, such that the estimated annual frequency of collapse due to vessels equal to and larger than the design vessel is less than the maximum permitted annual frequency, AF max. Forces shall be applied as equivalent static forces for superstructure and pier design (see CSA S6-06, Annex A3.3). Protection may be provided to reduce or eliminate the exposure of bridge piers to vessel collision. Physical protection systems may include fenders, pile clusters, pile-supported structures, dolphins, islands, and combinations thereof. Such protection systems shall be considered sacrificial and be capable of stopping the vessel before contact with the pier or redirecting the vessel away from the pier.
10. V EHICLE COLLISION LOAD (CSA S6-06, CLAUSE 3.15) Highway bridge piers located less than 10 m from the edge of the road pavement shall be designed for a collision load equivalent to a horizontal static force of 1400 kN. The collision load shall be applied horizontally 1.20 m above ground level at the pier, and at 10° to the direction of travel.
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LOADS IN BRIDGE DESIGN 11. CONSTRUCTION
BY : AYAZ M ALIK
LOADS (CSA S6-06, CLAUSE 3.16)
The weights of materials, workers, and equipment supported during construction shall be considered dead loads or live loads in accordance with CSA S6-06, Clauses 3.16.2 and 3.16.3. The possibility of occurrence of loads due to wind, ice, and stream flow shall be determined in accordance with the expected life of the structure or the duration of the construction stage considered. A ten-year return period shall be used for these loads when they are applied. Dead loads shall include the weights of formwork, falsework, fixed appendages, stored material, and lifting and launching devices, or parts thereof, which are not subject to movement during the construction stage considered. Live loads shall include the weights of workers, vehicles, hoists, cranes, other equipment, and structural components that are subject to movement during the construction stage considered. The live load factor to be used for construction live loads shall be 85% of the value specified for L in Table 3.1. 11.1 SEGMENTAL CONSTRUCTION (CSA S6-06, CLAUSE 3.16.4)
Erection loads assumed in design shall be shown on the Plans. Erection loads shall include all induced forces due to the anticipated system of temporary works, erection equipment, construction sequence, and closure forces due to misalignment corrections. Consideration shall be given to the effects of any changes to the statics of the structural system occurring during construction and the imposition, change, and removal of any temporary supports, erection equipment, or assumed loads, including residual built -in forces, deformations, post-tensioning effects, creep, shrinkage, and thermal and any other strain-induced effects. Except for bridges constructed by incremental launching, a uniformly distributed load of not less than 500 Pa over the constructed deck area of the bridge shall be considered, to allow for the weight of miscellaneous equipment and machinery. For balanced cantilever construction, the load shall be not less than 500 Pa on one cantilever and shall be 250 Pa on the other.
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