LOADS ON BRIDGES: DESIGN PHILOSOPHIES Flashcards
-
-
With due regard to issues of:
-
constructability
safety
serviceability
inspectability
economy
aesthetics
➢ Refers to the ability to successfully complete construction of bridge being designed
➢ A prerequisite for the bridge to start its design life by entering the stage of operation
➢ Thus, discussed before other general design issues
constructability
➢ New provisions included in LRFD specifications:
➢ Need to design bridges so that they can be fabricated and built without undue difficulty and with control over locked-in construction force effects
➢ Need to document one feasible method of construction in the contract documents unless type of construction is self-evident
➢ Clear indication of the need to provide strengthening and/or temporary bracing or support during erection, but not requiring complete design thereof
constructability
Public safety is the primary responsibility of the design engineer
safety
- structure should not collapse under design event
STRUCTURAL SURVIVAL
- structure should remain stable under designated emergency vehicular live loading
LIMITED SERVICEABILITY
- structure may be reopened to all traffic after inspection following an extreme event
IMMEDIATE USE
3 elements/considerations under safety
STRUCTURAL SURVIVAL
LIMITED SERVICEABILITY
IMMEDIATE USE
Ensure bridge safety such that minimum resistances exceed the potential maximum demands or force effects due to various loads during its design life
BRIDGE DESIGN SPECIFICATIONS
BRIDGE DESIGN SPECIFICATIONS
IN TERMS OF:
strength
stiffness
stability of structural system (component and the entire bridge
meaning of ASD
Allowable Stress Design
Working stress Design or Service Load Design (AASHTO)
Allowable Stress Design (ASD by AISC)
Ultimate/ Strength Design
LOAD FACTOR DESIGN
meaning of LRFD
LOAD AND RESISTANCE FACTOR DESIGN
“Philosophies Of Safety”
DESIGN METHODS
3 types of DESIGN METHODS
- Allowable Stress Design (ASD by AISC)
- LOAD FACTOR DESIGN
- LOAD AND RESISTANCE FACTOR DESIGN
❖also known as
❖Allowable Stress Design (ASD by AISC)
❖ Service Load Design (AASHTO)
❖does not recognize that some loads are more variable than others
❖treats each load in a given load combination as equal
WORKING STRESS DESIGN (WSD)
CONCEPT:
❖the maximum applied stress in a structural component does not exceed a certain allowable stress under normal service or working conditions
WORKING STRESS DESIGN (WSD)
CONCEPT:
❖Based on the premise that one or more factors of safety can be established based primarily on experience and judgement that will assure safety of a bridge component over its design life
WORKING STRESS DESIGN (WSD)
WORKING STRESS DESIGN (WSD)
EQUATION OF SUFFICIENCY:
summation of Qi <= Re/FS
WHERE:
Qi = Load
RE = elastic resistance
FS = factor of safety
DISADVANTAGES of WORKING STRESS DESIGN (WSD)
- Resistance concepts are based on elastic behavior of materials
- It does not embody reasonable measure of strength, which is more fundamental measure of resistance is allowable stress
- Safety factor is applied only to the resistance. Loads are considered to be deterministic (without variation)
- Selection of safety factor is subjective, and it does not provide a measure of reliability in terms of probability of failure
meaning of LFD
LOAD FACTOR DESIGN
❖also known as
❖Ultimate Design ( older ACI code)
❖ Strength Design (ACI and AASHTO)
LOAD FACTOR DESIGN (LFD)
CONCEPT of LOAD FACTOR DESIGN (LFD)
❖mainly recognizes that the live load (vehicular loads and wind forces), in particular, is more variable than the dead load
❖Was developed to overcome drawbacks of ASD method
❖Loads are multiplied by factors greater than unity and added to other factored loads to produce load combinations for design purposes
LOAD FACTOR DESIGN (LFD)
EQUATION OF SUFFICIENCY:
summation of YiQi <= phi R
Where:
γi = load factor; Qi = load; R = resistance; φ = strength reduction factor
DISADVANTAGE of LOAD FACTOR DESIGN (LFD)
From a probabilistic design point of view,
➢ load factors and resistance factors were not calibrated on a basis that takes into account statistical variability of design
➢ Design methodology where applicable failure and serviceability conditions can be evaluated considering uncertainties associated with loads by using load factors and material resistances by considering resistance factors
LOAD AND RESISTANCE FACTOR DESIGN (LRFD)
ADVANTAGE of LOAD AND RESISTANCE FACTOR DESIGN (LRFD)
- Accounts for variability and uncertainty in both resistance and loads
- Achieves relatively uniform levels of safety different limit states and material type to possible extent
- Provides more rational and consistent design method
- Consistent with other design specifications
LIMITATION of LOAD AND RESISTANCE FACTOR DESIGN (LRFD)
- The most rigorous method for developing and adjusting resistance factors to meet individual situations
- Requires a change in design philosophy (from previous AASHTO methods)
- Requires an understanding of the basic concepts of probability and statistics
- Requires availability of statistical data and probabilistic design algorithms
LOAD AND RESISTANCE FACTOR DESIGN (LRFD)
SAFETY CRITERION
summation of ni Yi Qi <= phi Rn = Rr
Where:
ni = load modifier (ductility, redundancy, operational importance)
gamma (or Y)i = load factor (statistically based multiplier applied to force effects)
Qi = force effect
phi = resistance factor (statistically based multiplier applied to nominal resistance)
Rn = nominal resistance
Rr = factored resistance
n
LOAD MODIFIER
LOAD MODIFIER, n
for maximum Yi values:
for minimum Yi values:
ni = nD nR nI >= 0.95
ni = 1/nD nR nI <= 1.0
nD
DUCTILITY
Response of structural components or connections beyond elastic limit can be characterized by either brittle or ductile behavior
DUCTILITY, nD
DUCTILITY, nD
For Strength Limit State:
For non ductile :
For conventional designs :
beyond AASHTO requirements:
> = 1.05
= 1.0
= 0.95
DUCTILITY, nD
For other Limit State:
= 1.0
nR
REDUNDANCY
Significantly affects safety margin of bridge structure;
*a statistically indeterminate structure is redundant, that is, more restraints than necessary to satisfy equilibrium
REDUNDANCY
REDUNDANCY, nR
For Strength Limit State
for non redundant members:
for conventional redundancy levels:
for exceptional redundancy levels:
> = 1.05
= 1.0
= 0.95
REDUNDANCY, nR
For other limit state:
= 1.0
nI
OPERATIONAL IMPORTANCE
Bridge of operational importance are those with shortest path between residential areas and hospital or schools or provide access for police, fire and rescue vehicles to home, business and industrial plants
OPERATIONAL IMPORTANCE
OPERATIONAL IMPORTANCE, nI
For Strength Limit State
for bridge of operational importance:
for typical bridges:
for relatively less important bridge:
> = 1.05
= 1.0
= 0.95
OPERATIONAL IMPORTANCE, nI
For other limit state:
= 1.0
Ability of a bridge to serve specified functions at an acceptable level over the design life
serviceability
serviceability
From the specification:
➢ Durability
➢ Inspectability
➢ Maintainability
➢ Rideability
➢ Controlled deformation
➢ Facilitating utilities
➢ Allowance for future widening
Contract documents should call for high quality materials and require that those material subject deterioration from moisture content and/or salt attack be protected
Durability
➢ Highway bridges need adequate maintenance over their design lives
➢ Maintenance of traffic during rehabilitation or replacement of bridge components or entire bridge is often required since completely closing the road for such maintenance operations is unacceptable to the traveling public
Maintainability
➢ Relevant to the bridge deck since it provides driving surface of the bridge.
➢ Deck is required to be designed to permit smooth movement of vehicle traffic
Rideability
Bridge shall be made to support and maintain conveyance for utilities
Facilitating utilities
➢ Load carrying capacity of exterior beams not be less than that of an interior beam unless future widening is virtually inconceivable
➢ When future widening can be anticipated, consideration should also be given to designing the substructure for widened condition
Allowance for future widening
➢ Bridges should be designed to avoid undesirable structural effects due to deformations
➢ While deflection and depth limitations are optional, except for orthotropic plate decks, any large deviation from past successful practice regarding slenderness and deflections should be cause for review of design to determine adequate performance
Controlled deformation
Recommended deflection limits for steel, aluminum and concrete structure
Vehicular load, general = span/800
Vehicular and pedestrian loads = span/1000
Vehicular load on cantilever arms = span/300
Vehicular and pedestrian loads on cantilever arms = span/375
To be assured through adequate means for permitting inspectors to view all parts of structure that have structural or maintenance significance
inspectability
AASHTO specifications also explicitly require inspection ladders, walkways,
catwalks, covered access holes and lightings, if necessary, where other means
of inspection are not practical
inspectability
Where practical, the specifications also requires access to permit manual or visual inspection, including adequate headroom in box sections, to the inside of cellular components and interface areas, where relative movement may occur
inspectability
➢ Economic consideration is required at every stage and step of bridge design
➢ Starting from preliminary design to taking into account the location and dimensions of members and amount of reinforcement in concrete components
economy
under economy,
_______ IS ALWAYS A SIGNIFICANT FACTOR
COST SAVING
➢ Bridges due to their significant geometric dimensions, become part of environment or landscape after construction
➢ Design engineer should be conscious about possible impact of bridge to the surrounding
aesthetics
Aesthetic qualities of design are intangible, perceived qualities arising from relationships of design elements
➢ Properties of aesthetic qualities are:
➢ Proportion
➢ Rhythm
➢ Order
➢ Harmony
➢ Balance
➢ Contrast
➢ Scale
➢ Unity
______ complements the natural surroundings
Color
