Bridge Load Rating 1 Outline What is bridge
Bridge Load Rating 1
Outline • What is bridge load rating? • Purpose of load rating • Who is qualified? • Load rating methods • Live Loads • What should be load rated? • Reasons for re-load rating • Federal Code of Regulations • Minnesota Statutes • Bridge load posting • What do you submit to Mn. DOT? 2
What is bridge load rating? Bridge Load Rating – The determination of the live load carrying capacity of a bridge 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 A 1 = Factor for dead load A 2 = Factor for live load 3
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 4
What is bridge load rating? (continued) Two primary types of loads used in load ratings. • Permanent or Dead Load • Structure self weight • Superimposed dead loads (barriers, overlays, utilities, etc. ) • Transient or Live Loads • Vehicular loads • Impact loads 5
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. 6
What is bridge load rating? (continued) 7
Who is qualified? • Individual in charge of load rating bridges shall have: • Professional Engineer License • 5 -year bridge design and inspection experience • Knowledge and skills for proper evaluation 8
Purpose of load rating • Ensure public safety and extend life of structures • Comply with federal regulations – National Bridge Inspection Standards FHWA Metric 13 • Emphasis on Maintaining and Preserving Bridges: Accurate Load Ratings essential • Posting needs • Emergency response • Processing overweight permits 9
Load rating methods • Allowable Stress Rating • Timber bridges • Load Factor Rating • Existing bridges (structures designed prior to Oct. 2010) • Load & Resistance Factor Rating (LRFR) • New bridge including timber bridges • LRFR has been required for all new bridges since Oct. 2010 10
Live Loads • Design Loads • H 20, HS 20 -44, HS 25, HL-93 • Minnesota Legal Loads (posting loads) • Type M 3: single unit vehicles including SHV trucks • Type M 3 S 2 -40: Semi trucks • Type M 3 S 3 -40: Semi trucks with dual trailers • Permit Loads & Superloads 11
Live Loads (continued) Design Loads • H 20 • HS 20 -44 12
Live Loads (continued) Design Loads • H 20 -44 13
Live Loads (continued) • Design Loads • HS 25 14
Live Loads (continued) • Design Loads • HS 25 15
Live Loads (continued) • Design Loads • HL 93 + 16
Live Loads (continued) • Design Loads • HL 93 17
Live Loads (continued) Minnesota Legal Loads Type M 3 48 kips 16’ 80 kips Type M 3 S 2 -40 51’ Type M 3 S 3 -40 80 kips 47’ 18
Live Loads (continued) Minnesota Legal Loads Type M 3 48 kips 16’ 19
Live Loads (continued) Minnesota Legal Loads Type M 3 S 2 -40 80 kips 51’ 20
Live Loads (continued) Minnesota Legal Loads Type M 3 S 3 80 kips 47’ 21
Live Loads (continued) Permit Loads Single Trip Permit Annual Permit 104, 000 - 500, 000+ lbs. GVW 6 Axle = 90, 000 lbs. GVW 7 Axle = 97, 000 lbs. GVW 22
Live Loads (continued) Emergency Vehicles (EV) EV 2 EV 3 23
What should be load rated? Superstructures - primary load-carrying members are required to be load rated • Steel Girder Structures • Interior and exterior girders • Cross frames in a curved girder structure • Prestressed Concrete Girder Structures • Interior and exterior girders 24
What should be load rated? (continued) Superstructures: primary load-carrying members are required to be load rated • Timber Girder or Slab Structures • Timber girders • Timber slabs • Concrete Box or Channel Structures • Adjacent box beams • Concrete channels • Concrete Slab Structures • Structural concrete slab 25
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 redundancy • Scour, undermining, or settlement • Large concrete cracks, tipping, displacement, or other movements 26
What should be load rated? (continued) Substructures 27
What should be load rated? (continued) Substructures 28
Reasons for re-load rating Physical change to structure • Deterioration (severe corrosion) • Bridge accident (high load hit) 29
Reasons for re-load rating (continued) Physical change to structure • Bowing along truss gusset plate free edge • Section loss along truss gusset plate shear zone • Corrosion within compression zone of gusset plate 30
Reasons for re-load rating (continued) • Change in dead load of the structure • Widening or additional loads on the superstructure • Bituminous overlay • Gravel • Increase weight of railing • New deck • New beam or girder • Change in State truck weight laws • Timber Haulers Bill • Change in Federal Regulations 31
Reasons for re-load rating (continued) • Change in legal live loads • Specialized Hauling Vehicles (SHV) • Change in load rating method • ASR, LFR, LRFR 32
Federal Code of Regulations • Non-divisible • Single axle weight limit • Tandem axle weight limit • Emergency vehicle Overweight Permit Vehicle 33
Federal Code of Regulations (continued) • Non-divisible • Single axle weight limit 80 kips • Tandem axle weight limit • Emergency vehicle 34
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 bridge • Increase the distance between axles or Spreading weight over additional axles 35
Federal Code of Regulations (continued) Federal Bridge Formula W = 500 [NL/N- l + 12 N + 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. 36
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 = 57, 000 pounds. 37
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! 38
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 = 45, 000 pounds. 39
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. NG 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
Minnesota Statutes Definitions • TIRE WEIGHT LIMITS: Pneumatic-tired vehicle • 9, 000 pound wheel/18, 000 pound axle on an unpaved street or highway • 10, 000 pound wheel/20, 000 pound axle on a paved street or highway • GROSS WEIGHT SCHEDULE • Axle 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 permit 41
Minnesota Statutes (continued) Definitions • Implements of Husbandry • A self-propelled or towed vehicle designed for timber-harvesting, agricultural, horticultural, or livestock 42
Minnesota Statutes (continued) Weight Laws • Weight Increases • 10% winter increase • 10% harvest increase • Weight Restrictions • Load limit on bridge • Weight limits set by other law 43
Minnesota Statutes (continued) Weight Exemptions • Weight limits not applicable to certain vehicles: • Tow trucks and utility vehicles for municipal electric, gas, or water system • Fire Engines: Emergency vehicles used by fire departments 44
Minnesota Statutes (continued) Annual Overweight Permits • Excess weight • When 10% increase is in effect • When gross vehicle weight exceeds 80, 000 pounds • When any axle group weight exceeds gross weight schedule • When single axle weight in exceeds 20, 000 pounds and travels on interstate routes 45
Minnesota Statutes (continued) Annual Overweight Permits • Forest Products • Includes hauling raw or unfinished forest products • GVW not exceed 90, 000 pounds • GVW not exceed 99, 000 pounds during seasonal increases • Vehicle must have six or more axles, and not exceed 20, 000 pounds on any single axle. • Must comply with all bridge and roadway posting limits • Typically not allowed on interstate highways 46
Minnesota Statutes (continued) Annual Overweight Permits • Special Paper Products • Maximum 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 system 47
Minnesota Statutes (continued) Annual Overweight Permits • Special Farm Products • Haul raw or unprocessed agricultural products • Maximum 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 48
Minnesota Statutes (continued) Annual Overweight Permits • Special Farm Products • Vehicle 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 system 49
Minnesota Statutes (continued) Annual Overweight Permits • Road Construction Materials Special Permit • Maximum 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 highways • Vehicle may not be operated on the interstate highway system 50
Bridge load posting Why do you load post? • Minnesota Statutes 165. 84 • Preserve and maintain bridges safely open to traffic • Protect from overstressing that could lead to possible damage or failure 51
Bridge load posting (continued) Reasons for posting • Original design weight • Size of the vehicles use in load rating analysis have increase • Change in Federal or State weight laws • Specialized Hauling Vehicles, SHVs • 165. 12. c Maintenance of Bridge on Town Road • Major deterioration, e. g. beam members or timber piles(s) • Bridge Hit • Missing timber pile(s) 52
Bridge load posting (contined) Example 53
What do you submit to Mn. DOT? • 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 leader • Software program data file used in the load rating analysis. • AASHTOWare Bridge. Rating, Br. R file (previously know as VIRTIS) • MDX file • CANDE file for arch structures • Mn. DOT rating documents 54
What do you submit to Mn. DOT? (continued) • Overweight permit • Truck axle spacing & weight schematic • Form OW-CL - for single trip overweight permit over TH bridges on local roadway • Mn. DOT rating documents 55
What do you submit to Mn. DOT? (continued) Overweight permit 56
Questions? Dave Conkel, P. E. Moises Dimaculangan, P. E. dave. conkel@state. mn. us moises. dimaculangan@state. mn. us 651 -366 -4493 651 -366 -4522 57
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