What is bridge load rating Purpose of load rating Who is qualified Load rating methods Live Loads What should be load rated Reasons for reload rating Federal Code of Regulations ID: 758357
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Slide1
Bridge Load Rating
1Slide2
Outline
What is
bridge
load rating? Purpose of load rating Who is qualified?Load rating methodsLive LoadsWhat should be load rated?Reasons for re-load ratingFederal Code of RegulationsMinnesota StatutesBridge load posting What do you submit to MnDOT?
2Slide3
What is bridge load rating?
Bridge Load Rating – The determination of the live load carrying capacity of a bridge
3
RF = Rating factor for live load
capacity
C
= Capacity of the
member
D
= Dead load effect on
member
L
= Live load effect on
member
I
= Impact
factor
A1
= Factor for dead
load
A2
= Factor for live loadSlide4
What is bridge load rating? (continued)
Generally a RF ≥ 1.0 indicates that the bridge has adequate live load carrying capacity
Generally a RF < 1.0 indicates that the bridge has inadequate live load carrying capacity
4Slide5
What is bridge load rating? (continued)
Two primary types of loads used in load ratings.
Permanent or Dead Load
Structure self weightSuperimposed dead loads (barriers, overlays, utilities, etc.)Transient or Live LoadsVehicular loadsImpact loads5Slide6
What is bridge load rating? (continued)
Inventory
Rating – the load level that a structure can safely sustain for an indefinite period.
Operating Rating – the absolute maximum permissible load level to which a structure may be subjected.6Slide7
What is bridge load rating? (continued)
7Slide8
Who is qualified?
8
Individual in charge of load rating bridges shall have:
Professional Engineer License5-year bridge design and inspection experienceKnowledge and skills for proper evaluationSlide9
Purpose of load rating
Ensure public safety and extend life of structures
Comply with federal regulations – National Bridge Inspection
Standards - FHWA Metric 13Emphasis on Maintaining and Preserving Bridges: Accurate Load Ratings essentialPosting needsEmergency responseProcessing overweight permits9Slide10
Load rating methods
Allowable Stress
Rating
Timber bridgesLoad Factor Rating Existing bridges (structures designed prior to Oct. 2010)Load & Resistance Factor Rating (LRFR)New bridge including timber bridgesLRFR has been required for all new bridges since Oct. 201010Slide11
Live Loads
Design Loads
H20
, HS20-44, HS25, HL-93Minnesota Legal Loads (posting loads)Type M3: single unit vehicles including SHV trucksType M3S2-40: Semi trucksType M3S3-40: Semi trucks with dual trailersPermit Loads & Superloads11Slide12
Live Loads (continued)
Design Loads
H20
HS20-4412Slide13
Live Loads (continued)
Design Loads
H20-44
13Slide14
Live Loads (continued)
Design Loads
HS25
14Slide15
Live Loads (continued)
Design Loads
HS25
15Slide16
Live Loads (continued)
Design Loads
HL93
16+Slide17
Live Loads (continued)
Design Loads
HL93
17Slide18
Live Loads (continued)
Minnesota Legal Loads
18
Type M3
16’
48 kips
Type M3S2-40
51’
80 kips
Type
M3S3-40
47’
80 kipsSlide19
Live Loads (continued)
Minnesota Legal Loads
19
Type M3
16’
48 kipsSlide20
Live Loads (continued)
Minnesota Legal Loads
20
Type M3S2-40
51’
80 kipsSlide21
Live Loads (continued)
Minnesota Legal Loads
21
Type M3S3
47’
80 kipsSlide22
Live Loads (continued)
Permit Loads
22
Single Trip Permit
Annual Permit
6 Axle = 90,000
lbs.
GVW
7 Axle = 97,000
lbs.
GVW
104,000 - 500,000+ lbs. GVWSlide23
Live Loads (continued)
Emergency Vehicles (EV)
23
EV2
EV3Slide24
What should be load rated?
Superstructures - primary load-carrying members are required to be load rated
Steel Girder Structures
Interior and exterior girdersCross frames in a curved girder structurePrestressed Concrete Girder StructuresInterior and exterior girders24Slide25
What should be load rated? (continued)
Superstructures: primary
load-carrying members are required to be load rated
Timber Girder or Slab StructuresTimber girdersTimber slabsConcrete Box or Channel StructuresAdjacent box beamsConcrete channelsConcrete Slab StructuresStructural concrete slab25Slide26
What should be load rated? (continued)
Substructures
Generally do not control the load rating
Scenarios where element conditions may prompt a load rating:Significant deterioration, particularly those with a lack of redundancyScour, undermining, or settlementLarge concrete cracks, tipping, displacement, or other movements26Slide27
What should be load rated? (continued)
Substructures
27Slide28
What should be load rated? (continued)
Substructures
28Slide29
Reasons for re-load rating
29
Physical change to structure
Deterioration (severe corrosion)Bridge accident (high load hit)Slide30
Reasons for re-load rating (continued)
Physical change to structure
30
Bowing along truss gusset plate free edge
Section loss along truss gusset plate shear zone
Corrosion within compression zone of gusset plateSlide31
Reasons for re-load rating (continued)
Change in dead load of the structure
Widening or additional loads on the superstructure
Bituminous overlayGravelIncrease weight of railingNew deckNew beam or girderChange in State truck weight lawsTimber Haulers BillChange in Federal Regulations31Slide32
Reasons for re-load rating (continued)
Change in legal live loads
Specialized Hauling Vehicles (SHV
)Change in load rating method ASR, LFR, LRFR32Slide33
Federal Code of Regulations
Non-divisible
Single axle weight limit
Tandem axle weight limitEmergency vehicle 33Overweight Permit VehicleSlide34
Federal Code of Regulations (continued)
Non-divisible
Single axle weight limit
Tandem axle weight limitEmergency vehicle 34
80 kipsSlide35
Federal Code of Regulations (continued)
Federal Bridge Formula
Congress enacted the Bridge Formula in 1975
Limit the weight-to-length ratio of a vehicle crossing a bridgeIncrease the distance between axles or Spreading weight over additional axles35Slide36
Federal Code of Regulations (continued)
Federal Bridge Formula
W = 500 [NL/N- l + 12N + 36]W =the overall gross weight on any group of two or more consecutive axles to the nearest 500 pounds.L =the distance in feet between the outer axles of any group of two or more consecutive axles.N =the number of axles in the group under consideration. 36Slide37
Federal Code of Regulations (continued)
Example
The truck shown satisfies the single-axle weight limit (12,000 pounds are less than 20,000 pounds)
The tandem-axle limit (30,000 pounds are less than 34,000 pounds) and the gross-weight limit (57,000 pounds are less than 80,000 pounds)With these restrictions satisfied, a check is done for Bridge Formula requirements, axles 1 through 4. Actual Weight= 12,000 + 15,000 + 15,000 + 15,000 = 57,000 pounds.37Slide38
Federal Code of Regulations (continued)
Example
continued
Minnesota’s Gross Weight Schedule:Maximum weight (W) = 57,500 pounds L =23 feet and 4 axles. Checks out!38Slide39
Federal Code of Regulations (continued)
Example continued
Since axles 1 through 4 are satisfactory, check axles 2 through 4:
Actual weight = 15,000 + 15,000 + 15,000 = 45,000 pounds.39Slide40
Federal Code of Regulations (continued)
Example
continued
Minnesota’s Gross Weight Schedule:From Table: Maximum weight (W) = 43,500 pounds for L of 10 feet and 3 axles.This is a violation because the actual weight (45,000 lbs.) exceeds the weight allowed by the Bridge Formula. The load must either be reduced, axles added, or spacing increased to comply with the Bridge Formula.40
40
NGSlide41
Minnesota Statutes
Definitions
TIRE WEIGHT LIMITS: Pneumatic-tired vehicle
9,000 pound wheel/18,000 pound axle on an unpaved street or highway10,000 pound wheel/20,000 pound axle on a paved street or highwayGROSS WEIGHT SCHEDULEAxle weight limits table for the distance between the centers of any group of two or more consecutive axles Gross weights in excess of 80,000 pounds require an overweight permit41Slide42
Minnesota Statutes (continued)
Definitions
Implements of Husbandry
A self-propelled or towed vehicle designed for timber-harvesting, agricultural, horticultural, or livestock42Slide43
Minnesota Statutes (continued)
Weight
Laws
Weight Increases10% winter increase10% harvest increaseWeight RestrictionsLoad limit on bridgeWeight limits set by other law43Slide44
Minnesota Statutes (continued)
Weight Exemptions
Weight limits not applicable to certain vehicles:
Tow trucks and utility vehicles for municipal electric, gas, or water systemFire Engines: Emergency vehicles used by fire departments44Slide45
Minnesota Statutes (continued)
Annual Overweight
Permits
Excess weightWhen 10% increase is in effectWhen gross vehicle weight exceeds 80,000 poundsWhen any axle group weight exceeds gross weight scheduleWhen single axle weight in exceeds 20,000 pounds and travels on interstate routes45Slide46
Minnesota Statutes (continued)
Annual Overweight
Permits
Forest ProductsIncludes hauling raw or unfinished forest productsGVW not exceed 90,000 pounds GVW not exceed 99,000 pounds during seasonal increasesVehicle must have six or more axles, and not exceed 20,000 pounds on any single axle.Must comply with all bridge and roadway posting limitsTypically not allowed on interstate highways46Slide47
Minnesota Statutes (continued)
Annual Overweight
Permits
Special Paper ProductsMaximum gross vehicle weight of 90,000 pounds with six or more axles Maximum gross vehicle weight of 97,000 pounds with seven or more axles. Maximum gross vehicle weight of 99,000 pounds during seasonal weight increases period Must have local approval to operate on streets or highways under the control of local authorities Vehicle is subject to bridge load limits posted Vehicle is subject to axle weight limitations Vehicle may not be operated on the interstate highway system47Slide48
Minnesota Statutes (continued)
Annual Overweight
Permits
Special Farm ProductsHaul raw or unprocessed agricultural productsMaximum gross vehicle weight of 90,000 pounds with six or more axles Maximum gross vehicle weight of 97,000 pounds with seven or more axles. Maximum gross vehicle weight of 99,000 pounds during seasonal weight increases period Must have local approval to operate on streets or highways under the control of local authorities 48Slide49
Minnesota Statutes (continued)
Annual Overweight
Permits
Special Farm ProductsVehicle is subject to bridge load limits posted Vehicle is subject to axle weight limitations May only be operated on local paved streets and highways Vehicle may not be operated on the interstate highway system49Slide50
Minnesota Statutes (continued)
Annual Overweight
Permits
Road Construction Materials Special PermitMaximum gross vehicle weight of 90,000 pounds with six or more axles Maximum gross vehicle weight of 97,000 pounds with seven or more axles. Maximum gross vehicle weight of 99,000 pounds during seasonal weight increases period Vehicle is subject to bridge load limits posted Vehicle is subject to axle weight limitations May only be operated on paved or unpaved streets and highwaysVehicle may not be operated on the interstate highway system50Slide51
Bridge load posting
Why do you load post?
Minnesota Statutes
165.84Preserve and maintain bridges safely open to trafficProtect from overstressing that could lead to possible damage or failure51Slide52
Bridge load posting (continued)
Reasons for posting
Original design weight
Size of the vehicles use in load rating analysis have increaseChange in Federal or State weight lawsSpecialized Hauling Vehicles, SHVs165.12.c Maintenance of Bridge on Town RoadMajor deterioration, e.g. beam members or timber piles(s) Bridge HitMissing timber pile(s)52Slide53
Bridge load posting (contined
)
Example
53Slide54
What do you submit to MnDOT?
Bridge load rating
Form RC-CL & PIR-CL: Forms must be completed, signed and dated by the Program Administrator and a registered professional engineer.
Form 90 (Culvert rating form): Form must be signed by Program Administrator or inspection team leaderSoftware program data file used in the load rating analysis. AASHTOWare BridgeRating, BrR file (previously know as VIRTIS)MDX fileCANDE file for arch structuresMnDOT rating documents54Slide55
What do you submit to MnDOT? (continued)
Overweight permit
Truck axle spacing & weight schematic
Form OW-CL - for single trip overweight permit over TH bridges on local roadway MnDOT rating documents55Slide56
What do you submit to MnDOT? (continued)
Overweight permit
56Slide57
Questions?
Dave Conkel, P.E.
dave.conkel@state.mn.us
651-366-449357
Moises Dimaculangan, P.E.
moises.dimaculangan@state.mn.us
651-366-4522