LOADS ON BRIDGES: DESIGN LOADS Flashcards
Effect of acceleration, including that of due to gravity, imposed deformation or volumetric change
LOAD
An arbitrary selected design load level
NOMINAL LOAD
Coefficient expressing the probability of variations in the nominal load for
the expected service life of the bridge
LOAD FACTOR
Deformation or stress resultant (i.e. shear, torque/moment) caused by applied loads, imposed deformation or volumetric change
FORCE EFFECTS
IMPORTANCE OF LOAD PREDICTION
A structural engineer has to make a structure safe against failure
Reasons for structure being susceptible to failures are:
▪ Loads that structure will be called upon to sustain CANNOT be predicted with certainty
▪ The strength of various components CANNOT be assessed with full assertion
▪ The condition of structure may deteriorate with time causing it to lose strength
design of the bridge superstructure is based on a set of loading conditions which the component or element must withstand
DESIGN LOADS
Variety of loads are taken into consideration based on
▪Duration (permanent or temporary)
▪Direction (vertical, longitudinal, etc)
▪Deformation (creep, expansion)
▪Effect (shear, bending, torsion)
The following ___________________ shall be considered for design of bridges where applicable. The load provisions may also be applied to the structural evaluation of existing bridges
permanent and transient loads and forces
Categories of Loads:
▪PERMANENT LOAD
▪TRANSIENT LOADS
▪Loads that always remain and act on a bridge throughout its life
PERMANENT LOAD
▪TEMPORARY LOAD
▪Placed on bridge for only a short time
▪TRANSIENT LOADS
DD
DownDrag force
DC
Dead load of structural Components and nonstructural attachments
DW
Dead load of Wearing surfaces and utilities
EH
Horizontal Earth pressure load
EL
Miscellaneous Locked-in force Effects resulting
from construction process, including jacking
apart of cantilevers in segmental construction
ES
Earth Surcharge load
EV
Vertical pressure from dead load of Earth fill
PS
Secondary forces from Post-tensioning for
Strength limit states; total Prestress forces for
Service limit states
SH
Force effects due to SHrinkage
CR
Force effects due to CReep
BL
BLAST LOADING
BR
VEHICULAR BRAKING FORCE
CE
VEHICULAR CENTRIFUGAL FORCE
CT
VEHICULAR COLLISION FORCE
CV
VESSEL COLLISION FORCE
EQ
EARTHQUAKE
FR
FRICTION LOAD
IC
ICE LOAD
IM
VEHICULAR DYNAMIC LOAD ALLOWANCE
LL
VEHICULAR LIVE LOAD
LS
LIVE LOAD SURCHARGE
PL
PEDESTRIAN LIVE LOAD
SE
FORCE EFFECT DUE TO SETTLEMENT
TG
FORCE EFFECT DUE TO TEMPERATURE GRADIENT
TU
FORCE EFFECT DUE TO UNIFORM TEMPERATURE
WA
WATER LOAD AND STREAM PRESSURE
WL
WIND ON LIVE LOAD
WS
WIND ON STRUCTURE
CATEGORIES OF PERMANENT LOADS:
- DEAD LOAD
- SUPERIMPOSED LOAD
- PRESSURE
▪Aggregate weight of all superstructure elements
DEAD LOAD
▪Loads placed on superstructure after the deck has cured and begun to work with primary members in resisting loads
SUPERIMPOSED LOAD
▪Pressure due to earth or water
▪Note: Though it primarily affect substructure elements, it has potential of
affecting superstructure elements as wells
PRESSURE
▪Include the weight of all components of the structure, appurtenances and
utilities attached thereto, earth cover, wearing surface, future overlays, and
planned widening
DEAD LOAD
deadload of structural components and nonstructural attachments
DC
▪Refer to the elements that are part of load resistance system
▪STRUCTURAL COMPONENT
▪Refers to items such as curbs, parapets, barriers, rails, signs, etc
▪Weight of such items can be estimated by using unit weight of material and its geometry
▪NONSTRUCTURAL ATTACHMENTS
▪Include the weight of all components of the structure, appurtenances and
utilities attached thereto, earth cover, wearing surface, future overlays, and
planned widening
DEAD LOAD
deadload of wearing surfaces and utilities
▪DW
▪Estimated by taking the unit weight times the thickness of the surface
▪This value is combined with DC loads
▪Weight of utilities carried by the bridge is also included
▪Example: include pipes for sewage, oil, gas, communication wires, etc
DW
DL for wearing surfaces and utilities are treated as separate DL group which have higher load factors due to its _______
larger uncertainty
dead load of earth fill
EV
▪Represents vertical earth pressure applied to substructure components by refill after the components are completed and buried as designed
▪Typical bridge component subjected to EV is the footing of an abutment or pier
▪MUST be considered for buried structures such as culverts
▪Determined by multiplying the unit weight time the depth of the materials
EV
▪Includes earth loads
PRESSURE
ES
earth surcharge load
▪Refers to those loads and their effects on a wall buried in soil due to forces applied on the surface of backfill soil behind the wall
▪Calculated like EV loads with the only difference being in load factors
▪Difference is attributed to variability
▪Part of all load could be removed in future or surcharge material loads could be change
ES – earth surcharge load
DD
downdrag
▪Force exerted on a pile or drilled shaft due to soil movement around the element
▪Typically increases with time
DD – downdrag
▪Example situations where possible downdrag
will be evaluated
▪Sites are underlain by compressible
material such as clay, silts or organic soils ▪Fill will be or has recently been placed
adjacent to the piles or shafts, such as
frequently the case of bridge approach fills ▪Groundwater can be substantially lowered ▪Liquefaction of loose sandy soil can occur
EH
Horizontal Earth Pressure Load
Refers to the horizontal earth
pressure normally relevant to the
substructure components such as
an abutment
EH – Horizontal Earth Pressure Load
▪Refers to loads due to moving vehicles that are dynamic
▪Loads that change their positions with respect to time
▪For modern bridges, service lives are generally decades or even more than a hundred years
▪For highway bridges, the live load includes vehicle load and sidewalk load
LIVE LOADS
▪hypothetical design vehicles based on truck loadings developed by AASHTO
VEHICLE LIVE LOAD (LL)
VEHICLE LIVE LOAD (LL)
▪Three Categories:
▪Design Truck Load
▪Design Tandem Load
▪Design Lane Load
1935 AASHO LOADING SCHEME
❏ H20-35
❏ H15-35
▪1944 AASHTO LOADING SCHEME
❏ H10-44
❏ H15-44 ❏ HS15-44
❏ H20-44 ❏ HS20-44
▪To account for higher loading conditions
▪25% increase in loading over the HS 20 -44 truck (90,000lb or 400kN)
HS -25
“design vehicular live load”
HL – 93 loading
▪composed of a design truck or tandem (identical to HS – 20 or tandem) which ever gives a larger force, combined with a 0.64 k/ft (9.34kN/m) design lane load
▪Design tandem : 2 - 25k axles spaced at 4.0 ft (1.2m)
▪HL – 93 loading
▪Former Highway semitrailer 20-ton design truck (HS 20-44)
DESIGN TRUCK LOAD
▪Consist of two 110-kN axles spaced at 1.20 m on center
▪Transverse spacing of wheels shall be taken as 1.8 m
▪Need to multiply this by dynamic allowance
factor (IM)
DESIGN TANDEM LOAD
Lead to larger moment than the HS 20 truck for simple support beam with span length less 13. m
▪*tandem
– can be defined as two closely spaced and mechanically interconnected axles of equal weight
DESIGN TANDEM LOAD
▪Number of lanes a bridge may accommodate must be established
DESIGN LANE LOAD
▪The number of lanes of traffic that the traffic engineer plans to route across the bridge
TRAFFIC LANE
▪A lane width is associated with a traffic lane and is typically __ m
3.6
▪Lane designation used by bridge engineer for live load placement
▪Design lane width may or may not be the same as traffic lane
▪DESIGN LANE
Number of Design lanes
= 𝐈𝐧𝐭𝐞𝐠𝐞𝐫 𝐨𝐟 𝐰/𝟑𝟔𝟎𝟎𝐦𝐦
≥ 𝐧𝐮𝐦𝐛𝐞𝐫 𝐨𝐟 𝐚𝐜𝐭𝐮𝐚𝐥 𝐭𝐫𝐚𝐟𝐟𝐢𝐜 𝐥𝐚𝐧e
▪For roadway width from __ m to ___ m, there should be 2 design lanes
6 to 7.2
______ is used in conjunction with design truck or tandem
lane load
Lane load is spread over a 3m wide are in a standard __ m lane
3.6
▪no dynamic allowance (IM) for this load
DESIGN LANE LOAD
▪Live loads created by pedestrians and/or bicycles
PEDESTRIAN LOAD (PL)
▪ A _________ is applied to sidewalks simultaneous
with the vehicular live load
0.075 KSF (3.6 kPa)
a design live load of ______________ is used if bridge is designed only for pedestrian (including bicycle traffic)
0.085 ksf (4.07 kPa)
▪If sidewalk is designed for vehicular load, ________ need not to be considered concurrently
pedestrian load
No IM factor (neglect dynamic effect of pedestrians)
PEDESTRIAN LOAD
▪Accounts for the dynamic effects of vehicle riding over a structure
▪An impact factor is used as multiplier for certain structural elements
IMPACT (DYNAMIC LOAD ALLOWANCE, IM)
2 Sources of IMPACT (DYNAMIC LOAD ALLOWANCE, IM)
- Hammering Effect
- Dynamic response of bridge as a whole to passing vehicles
Dynamic response of the wheel assembly to riding surface discontinuities, such as deck points, cracks, potholes and delamination
Hammering Effect
Due to long undulations in the roadway pavements such as caused by settlement of fill, resonant excitation as result of similar frequencies of vibration between bridge and vehicle. Frequency of vibration of bridge should not exceed 3 Hz
Dynamic response of bridge as a whole to passing vehicles
