Means and Methods for
Superstructure Evaluation
CG Gilbertson
March 22, 2011
Overview of load rating procedure
• Bridge Load ratings:
• Establish the safe load capacity
• Should be reviewed following inspections
• Are controlled by the weakest link
• Weakest link might not be within the superstructure – consider substructure
• Have a lower target reliability index to reflect reduced exposure period, consideration of site realities, and the economic considerations of rating vs. design
Superstructure
• In this workshop we will look only at
the bridge superstructure
• If other portions of the bridge system
are suspect, they should be fully
analyzed and considered in the load
rating calculations.
When are bridges load rated?
When a bridge is new and has not been rated
the bridge has had a significant alteration that may affect the capacity of the bridge
the bridge has incurred damage that affects the capacity
a key component of the structure has deteriorated such that the previous load rating is no longer valid
a request has been made to permit an overload vehicle to use the bridge
Qualifications
• The person charged with overall responsibility for load-rating bridges should have:
• PE
• 5-years bridge design/inspection experience
• Knowledge and skills for proper evaluation
Ref: The Manual for Bridge Evaluation
Governing References
• The Manual for Bridge Evaluation, Second Edition, 2011
• Bridge Analysis Guide, 2005
Edition w/ 2009 Interim Updates
MDOT Construction and
Technology Support Area
Additional References
• AASHTO LRFD Bridge Design Specifications, Customary U.S. Units: 5th Edition with 2010 Interim Revisions
• AASHTO Standard Specifications for Highway Bridges: 17th Edition
Overview of load rating procedure
• Three rating methods are available:
• Allowable Stress Rating (ASR)
• Load Factor Rating (LFR)
• Load and Resistance Factor Rating (LRFR)
• LFR was the previous standard for MDOT & the National Bridge Inventory System, HOWEVER, LRFR is now required for bridges designed with LRFD (designed after October 1, 2010)
General Procedure
• Determine applied loads
• Determine capacity of member*
• Conduct load rating calculations
• Submit results to MDOT
* Member capacity MUST include the affects of
deteriorated or damaged sections
Documentation
Hand calculations need to be well documented with appropriate references, inspection reports, and test data used to perform calculations
When software is used, input and output files should be printed and checked for “reasonableness” with at least rough hand calculations
Assumptions need to be clearly identified as well as the underlying reason for such assumptions.
Required Background Info
Design/construction info from original
design plans and as-built plans
Detailed information gathered during
bridge inspections
Information on modifications to original
structure (deck overlays for example)
Distress or deterioration to the structural
components
Four Components to Load Ratings
Federal Inventory
Federal Operating
Michigan Operating
Overload (Permit) Vehicles
Note: In LRFR these are referred to as:
Design (Federal)
Legal (Michigan)
Permit (Overload)
Load Rating Calculations
• Inventory
• Corresponds to customary design level of
stresses
• Incorporates existing conditions
• Results in a live load which can safely utilize
structure for an indefinite period of time
(HS-20 Truck) (NOTE: HL-93 for LRFR)
Load Rating Calculations
• Operating
• Describes maximum permissible live load for
bridge
• Limited load applications
• Loading at this level may shorten life of bridge
(HS-20 or HL-93 +Michigan’s 28 legal loads)
General Equation
Reserve Capacity for Live Load
Maximum Legal Live Load RF =
• The rating factor (RF) is useful in
understanding the factor of safety
on the bridge.
General Equation (LFR)
)(2
1
ILA
DACRF
RF = Rating factor for live load capacity
C = Capacity of the member
D = Dead load on member
L = Live load on member
I = Impact factor
A1 = Factor for dead load
A2 = Factor for live load
LFR Load Factors
• For both inventory
and operating:
• A1 = 1.3
• For inventory:
• A2 = 2.17
• For operating:
• A2 = 1.3
General Equation (LRFR)
RF = rating factor
C = capacity
γDC = LRFD load factor for structural components and attachments
DC = dead load effect due to structural components and attachments
γDW = LRFD load factor for wearing surfaces and utilities
DW = dead load effect due to wearing surfaces and utilities
γP = LRFD load factor for permanent loads other than dead loads = 1.0
P = permanent loads other than dead loads
γLL = evaluation live load factor
LL = live load effect
IM = dynamic load allowance
𝑅𝐹 =𝐶 − 𝛾𝐷𝐶 𝐷𝐶 − 𝛾𝐷𝑊 𝐷𝑊 ± 𝛾𝑃 (𝑃)
𝛾𝐿𝐿 (𝐿𝐿 + 𝐼𝑀)
LRFR Resistance Factors
Condition Factor
Good or Satisfactory – 1.00
Fair – 0.95
Poor – 0.85
System Factor
Reflects level of redundancy in structural
components
Table 6A.4.2.4-1
LRFR Load Factors
Load factors vary with:
Bridge type
Limit state
Type of vehicle
Rating level of evaluation (Design, Legal, Permit)
Table 6A.4.2.2-1 is provided in AASHTO
Manual for Bridge Evaluation to identify the
proper load factors
Live Load Factors in LRFR:
Table 6A.4.2.2-1
LRFR Load Factors (MI trucks)
The live load factors vary with the
average daily truck traffic and the weight
of the truck being analyzed
Tables 4a-1 through 4a-6 of the BAG
provide the live load factors for each of
the MI legal load vehicles for normal,
designated, and special designated
loadings, and ADTT of 5000, 1000, and
100
Table 4a-1 (BAG)
Finding Loads
• Dead Load
• Compute according to existing conditions
• Material unit weight must be at least the
value specified in the AASHTO Design
Specs.
• Composite beams:
• Separate loads acting on non-composite and
composite sections
Finding Loads (Cont.)
• Dead Load (Cont.)
• Be careful with decks built prior to 1965 due
to significant deck thickness variations
• Nominal dead load values should be based
on plan dimensions with allowance for
construction tolerances
Finding Loads (Cont.)
• Live Load
• HS-20/HL-93 Truck or AASHTO Design
Specifications lane load
• 28 Michigan Legal Trucks
• Wheel Load – If the deck is the weakest link,
this is normally not the case, wheel loads
are used to find the bending moment stress
of the deck
Finding Loads (Cont.)
• Truck Loads
• Number of loaded lanes and transverse placement of wheels in accordance with AASHTO Design Specs. and:
• Roadway widths 18-20 ft, 2 design lanes, each half the width, centered live load
• Widths less than 18 ft, 1 design lane
• Fewer traffic lanes may be warranted depending on traffic moments and volume
Finding Loads (Cont.)
• Normal
• Lowest set of max loadings that apply to all
roads in Michigan
• Designated (typical for counties and cities)
• Local or state owned roads that allow loads
heavier than normal
• Special Designated
• Typically applies to interstate roadways
Finding Loads (Cont.)
• Lane Loads
• AASHTO lane loads may be used for all span
lengths where it will result in greater effects than
the standard truck.
Load effects
• BAG Tables • Simple spans
• HS-20 truck
• 28 Michigan legal trucks
• Moment and shear effects
• AASHTO Manual for Bridge Evaluation • Simple spans, trusses, through girders
• HS-20 truck
• HL-93 truck
Overload Trucks
BAG Tables
Simple spans, 5 to 300 feet
Class A, B, and C
20 vehicles for each class
Moment and shear effects
How to use the Tables (BAG: Chapter 10)
• Determine the controlling vehicle
• Search all vehicles listed in tables for the largest
moment or shear effect of each truck type
(be sure you are in the proper table)
• This vehicle will produce the lowest rating factor
(but not necessarily the lightest vehicle weight)
• We will talk about determination of lightest vehicle
weight (posted weight) later
How to use the Tables (Cont.)
Controlling 2-Unit Truck
Load Distribution
• The amount of live load transferred to a
single member of the bridge structure
should be in accordance with current
AASHTO Design Specs.
Impact Affect
• The affects of impact should be accounted
for in the load rating assessment
• For LFR
• Determine in accordance with current AASHTO
Design Specs
• For LRFR analysis:
• IM = 33% of live load effect
• May be eliminated for permit vehicles if they are
restricted to less than 10 mph.
Other Loads
• Deflection
• Do not consider except in special
circumstances
• Longitudinal
• Operating level
• Reduce speed if inadequate
• May be needed to evaluate substructure
Other Loads (Cont.)
• Wind
• Only for moveable bridges, suspension
bridges, high-level structures
• Thermal
• Only for long-span bridges, concrete arches
Other Loads (Cont.)
• Environmental
• Operating level
• Stream Flow
• Only if critical for structure stability
• Ice Pressure
• Only for substructure
Capacity of Member
• Nominal capacity based AASHTO Specifications
• Structural steel
• Reinforced concrete
• Prestressed concrete
• Load factor methods for timber and masonry are
not available – use ASR or LRFR
Condition of Bridge Members
• Bridge ratings should be based on
recent and thorough bridge inspections
• Adequate data should be available to
evaluate the affect of damage.
• Description
• Location
• Extents
It is EXTREMELY important
to consider distress!
Map Cracks Hairline Cracks
Spalling or
Delamination
Narrow Cracks w/
Water or Corrosion Water Stains at
Joints
Longitudinal Cracks
on Deck
Common Forms of Deterioration
Common Forms of Deterioration (Cont.)
Medium Cracks w/o
Water
Evidence of
Displacement
between Beams
Medium Cracks with
Water or Corrosion
Wide cracks w/ Water
or Corrosion
Spalling w/ Exposed or
Corroded Reinforcement Shear or Flexure
Cracking
Incorporating Distress into Load
Rating
Material and Structural Distress
• Material Related Distress
• Chemical attack
• Weathering
• Structural Distress
• Spalls
• Cracks
• Corrosion
• System malfunction (shear keys, diaphragms)
Incorporating Distress
• Material Distress
• Change f’c to represent actual concrete
strength
• May use core tests to establish
Incorporating Distress (Cont.)
• Structural distress
• Change cross-sectional properties to reflect loss of section
• Moment of inertia
• Area of beam
• Area of reinforcement
• Don’t forget the location on the beam (moment is critical near mid-span, shear at supports)
Structural Distress Example
rating design distressA A A
2( )rating design distress distressI I I A d
Impact of Distress on Load Rating
Description of Distress
Inventory
Load
Rating
Percent of
Original
Rating
Controlling
Equation
No Distress 1.26 100% Prestress
steel tension
Spalling of concrete (5.9%) 0.97 77.0% Prestress
steel tension
Spaling of concrete (5.9%)
Corrosion of strands (30%)
0.72
57.1%
Flexural
Strength
Impact of Distress on Load Rating
Legal-Load Truck
Max Weight (U.S. Tons)
1-unit 42
2-unit 77
3-unit 82
Note: Above table represents
Designated Loading
Posting Load Restrictions
• Determine which vehicles have a load effect greater than allowable, they cannot safely use the bridge at max load • Set RF=1 and solve for the moment/shear
• Check tables for trucks exceeding this moment/shear
• The “controlling” truck in this group is the one that has the largest M/W or V/W ratio
• Divide the allowable moment/shear by this ratio to determine the max weight of that truck type under the operating/legal load level
Posting Load Restrictions (Cont.)
• The posted limit is the total weight on the
controlling truck which will produce the
allowable load effect
Posting Load Restrictions (Cont.)
)(2
1
ILA
DACRF
RF = Rating factor for live load capacity
C = Capacity of the member
D = Dead load on member
L = Live load on member
I = Impact Factor
A1 = Factor for dead load
A2 = Factor for live load
Use the same equation and values used to conduct the load
rating. Substitute the controlling truck for the live load. Multiply
the result by the weight of the truck to get the maximum
allowable weight for that particular truck classification.
Basic Submittal Process
• Owner or appointee enters data into the
Online Inventory Inspection Program
• MDOT manages database
• MDOT submits reports to the Federal
Highway Administration for the National
Bridge Inventory Records
Records
• All assumptions used in the calculations
must be clearly identified at the
beginning of the load rating
• A summary table should be provided
with the load rating calculations to
clearly summarize the results
References:
Bridge Analysis Guide, 2005 Edition with 2009 Interim Update. MDOT Construction and Technology Support Area
The Manual for Bridge Evaluation, Second Edition, AASHTO, Washington DC. (2011)