Bridge Load Rating.

Bridge Load Rating

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 MnDOT?

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 A1 = Factor for dead load A2 = Factor for live load 𝑹𝑭= (𝑪− 𝑨 𝟏 𝑫) 𝑨 𝟐 𝑳+𝑰 What is Bridge load Rating? Bridge load rating is the determination of the live load carrying capacity of a bridge The general equation is shown. It is expressed as a rating factor (RF). The RF is simply the structure's capacity at material yield (Capacity) less the dead load (DL) divided by the live load (LL) plus Impact In other words, the structure must have enough strength remaining after the dead load is subtracted to carry the live load.

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 By definition, the adequacy or inadequacy of a structural element to carry a specified truck load will be indicated by the value of its rating factor (RF); that is, whether it is greater or smaller than 1.0. For a bridge with a RF of greater than or equal to 1.0, the bridge has adequate capacity to carry the live load For a bridge with a RF of less than 1.0, the bridge does not have enough capacity to carry the live load, therefore resulting in load posting of the bridge. So for a example, if you have a rating factor of 1.0 , it means that the bridge’s capacity is exactly equal to the live load that’s put on it. If you have a rating factor of 1.5, that would mean that you have 50% extra capacity more than the live load that being place on it. And if you have a rating factor of 0.9, that would not be good because the bridge’s capacity is less than the live load you’re putting on it, which would require the bridge to be posted.

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 Next let’s look at the “Load” aspect of Bridge Load Rating Two primary types of loads use in load ratings, dead load and live load. Permanent loads are loads and forces that are constant for the life of the structure. They consist of the weight of the materials used to build the bridge. Permanent load includes both the self-weight of structural members and other permanent external loads. They do not move and do not change unless the bridge is modified. For example, beams, deck, railing, overlay, sidewalk etc…. On the other hand, a transient load is a temporary load and force that is applied to a structure which changes over time. In bridge applications, transient live loads are moving vehicular or pedestrian loads. For example, trucks, cars, pedestrians, etc. To account for the affects of speed, vibration, and momentum, truck live loads are typically increased for vehicular dynamic load allowance (impact loads). This is the bouncing effect of vehicles when they drive across a bridge. Vehicular dynamic load allowance is expressed as a percentage of the static truck live load effects.

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. Bridges are rated at two different stress levels, Inventory level and Operating level. Inventory rating as defined by AASHTO is equivalent to the design level of stress (It corresponds to the rating at the design level at which the bridge is safe for an infinite period of time) Operating Rating is the maximum permissible live load that can be placed on the bridge. So it is the absolute maximum load that should be allowed on the bridge under any circumstances. Operating rating is always higher than the inventory rating because it has a lower factor applied to the loads. In other words, it has a lower factor of safety because the loads are specifically known. Also, the Operating level is used for load posting and for evaluation of overweight permits.

In the structure inventory report all bridge load rating information is located on the lower right hard corner of the sheet under capacity rating. Information about the bridge’s design load, inventory and operating rating, posting information, rating date and overweight permit codes are all given in this section. Bridges load rated using Load Factor Rating (LFR) and Allowable Stress Rating (ASR), Inventory and Operating Ratings are expressed in terms of an equivalent HS-truck and reported in tons. Bridges load rated using Load Resistance Factor Rating (LRFR), the inventory and Operating ratings are reported in rating factor. HL-93 rating factor values cannot be converted to tonnage because it’s a notional load that includes a lane load. The design load, inventory and operating rating shown in further discussed in under live loads of this presentation. For this example shown, the bridge was load rated using LFR. HS-20 as the live load, Inventory Rating equals to 39.2 Tons, Operating Rating equals to 23.4 Tons. To convert the HS inventory and operating rating values to rating factors (RF), simply take the Inventory Rating or Operating Rating from the structure inventory report and divide them by the live load (HS-20). Inventory rating factor = 23.4/20 = (This means that the particular bridge is estimated to be able to support 1.17 HS 20 Loadings before being stressed to the Inventory Stress Level. ) Operating rating factor = 39.2/20 = 1.96 (This means that the particular bridge is estimated to be able to support 1.96 HS 20 Loadings before being stressed to the Inventory Stress Level. )

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 Anyone performing a bridge load rating has to be an engineer. However if that individual is not a registered engineer, he/she can do the rating, but the person that signs the rating form must be a licensed professional engineer in the state of MN. So an engineer in training can perform the rating but a licensed registered engineer has to be the one that signs the rating form. Ultimately, the bridge owner is responsible to get his/her bridges load rated. He/she can either perform the load rating if the above requirement is met or hire an engineering consultant to perform the load rating analysis. Also, the engineer in charge must have a 5 year experience in bridge design, bridge inspection and have the knowledge and skills to perform a proper load rating analysis.

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 What is the purpose of load rating? Why do we need to load rate bridges? First and most obvious reason is to ensure that our bridges are safe Next, comply with the National Bridge Inspection Standards (NBIS) requirements To determine rehab or replacement needs, so if a bridge has a low rating, the engineer may decide to rehab or replace the bridge to keep it in service. To check if the bridge needs to be posted Bridge models and load rating results are very valuable during emergencies: Emergency response to Fire, Flood, Accidents,…etc. And finally we load rate our bridges for processing overweight permits. We need to know whether a given customize special permit truck can drive over a bridge. We use a rating analysis to determine that.

Load rating methods Allowable Stress Rating Load Factor 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 There are 3 load rating methods to consider when performing a bridge load rating. The Allowable Stress Rating (ASR) is only applicable to timber bridges. Allowable stress is basically a method where you would apply the factor of safety to the material strength portion of the structure capacity only. Next is the Load factor rating or LFR – This method is applicable to structures designed prior to October Also, with this method the Safety factors are applied to the dead load and live load only. LRFR method– Should be used on New bridges (including timber bridges) that have designs completed after October 1, The ratings shall be based on the HL-93 truck and with this method the Safety factors are applied to both the bridge capacity and to the loads. It’s the combination of the previous two.

Live Loads Design Loads Minnesota Legal Loads (posting loads)
H20, HS20-44, HS25, HL-93 Minnesota Legal Loads (posting loads) Type M3: single unit vehicles including SHV trucks Type M3S2-40: Semi trucks Type M3S3-40: Semi trucks with dual trailers Permit Loads & Superloads Design or inventory loads = Truck configurations developed by design codes such as AASHTO. Standard AASHTO vehicle live loads do not represent actual vehicles, but it does provide a good approximation for bridge design and rating. Minnesota Legal Loads = Truck configurations developed by AASHTO that are used exclusively for load rating. The feds impose live load limitations based on a formula they call the “Federal Bridge Gross Weight Formula”. These Legal Loads are intended to cover the limits of this formula. It envelopes all legal trucks that are out on the road today. And finally… Permit and Superloads are the extra heavy loads that require special permission to travel Mn highways. 6 & 7 axle annual trucks (Timber Trucks, Agricultural trucks), single and annual overweight permits with weight exceeding 155,000 lbs.

Live Loads (continued)
Design Loads H20 HS20-44 Throughout the history of highway bridge design, live loads have evolved from a single unit vehicle into a tractor–semi-trailer combination. The 1st edition of the AASHTO Standard Specification for highway design came out in 1931 and it contained a representation of a truck and/or a group of trucks for use in bridge design. The basic design truck was a single unit weighing up to 40 kips, which was known as the H20 truck. Then in the early 40’s, the truck was extended into a tractor-semi-trailer combination, known as HS20. It is identical to the H truck, but with an extra 32,000-pound axle representing the rear axle of the trailer.

Design Loads H20-44 For this example, the bridge was load rated using LFR. HS-20 as the live load, Inventory Rating equals to 39.2 Tons, Operating Rating equals to 23.4 Tons. To convert the HS inventory and operating rating values to rating factors (RF), simply take the Inventory Rating or Operating Rating from the structure inventory report and divide them by the live load (HS-20). Inventory rating factor = 23.4/20 = (This means that the particular bridge is estimated to be able to support 1.17 HS 20 Loadings before being stressed to the Inventory Stress Level. ) Operating rating factor = 39.2/20 = 1.96 (This means that the particular bridge is estimated to be able to support 1.96 HS 20 Loadings before being stressed to the Operating Stress Level. )

Design Loads HS25 In the 50’s (start of interstate era) and in the 60’s, trucks have gotten bigger and heavier trucks and in the late 70’s and early 80’s some states responded to increase the design vehicle and adopted a scaled-up version of the HS20 vehicle (increasing the HS20 wheels load by 25%, thus HS25. Since then, this has become a common design vehicle.

Design Loads HS25 For this example, the bridge was load rated using LFR. HS-25 as the live load, Inventory Rating and operating rating values are 21.8 Tons and 32 Tons respectively. To convert the HS inventory and operating rating values to rating factors (RF), simply take the Inventory Rating or Operating Rating from the structure inventory report and divide them by the live load (HS-20). Even though the bridge was designed for HS-25 truck, we would still load rate the bridge using the HS-20 live load vehicle. Inventory rating factor = 21.8/20 = (This means that the particular bridge is estimated to be able to support 1.09 HS 20 Loadings before being stressed to the Inventory Stress Level. ) Operating rating factor = 32/20 = 1.6 (This means that the particular bridge is estimated to be able to support 1.6 HS 20 Loadings before being stressed to the Operating Stress Level. )

Design Loads HL93 + Three decades after the start of the Interstate era, AASHTO Highway subcommittee on Bridges and Structures developed and updated bridge design specification which became known as the AASHTO LRFD. Knowing that design vehicle configurations and weights have evolved, it became clear that a new design live load model was needed, hence in 1993, HL93 was born. It is similar to HS20 but with the addition of 640 lbs/ft of uniform load was added to the design. This is now used as the design load for highway structures in the US and other countries where AASHTO code is followed.

Design Loads HL93 Here is an example of a bridge that was load rated using the LRFR method. HL-93 as the live load, Inventory Rating and operating rating values are 1.15 and 1.72 respectively. Both are reported in rating factors. As stated earlier, HL-93 rating factor values cannot be converted to tonnage because it’s a notional load that includes a lane load. Inventory rating factor = (This means that the bridge has 15% more capacity to carry the live load before being stressed to the Inventory Stress Level. ) Operating rating factor = (This means that the bridge has 72% more capacity to carry the live load before being stressed to the operating Stress Level. )

Minnesota Legal Loads Type M3 16’ 48 kips Type M3S2-40 51’ 80 kips Type M3S3-40 47’ In determining if your bridge is going to need a posting, here are the 3 models of legal loads that we use in the load rating analysis, M3, M3S2-40, M3S These represent all the combinations of legal trucks that are out on the road today. In order for a vehicle to be considered legal under federal law, the vehicle has to have a total weight of less than or equal to 80,000 pounds or 80 kip.

Minnesota Legal Loads Type M3 16’ 48 kips For the Type M3 single unit vehicles – FHWA defines “Single Unit” trucks as all vehicles on a single frame including trucks, camping and recreational vehicles, motor homes, etc. Additionally, truck-tractor units travelling without a trailer should be considered single-unit trucks. Examples are gravel trucks, concrete pump trucks, buses. These are vehicles that are on a single frame. Single unit vehicles include Specialized Hauling Vehicles (SHVs). These are single unit trucks that have liftable axles than can weight up to 80,000 lbs and produce significantly higher stresses.

Minnesota Legal Loads Type M3S2-40 51’ 80 kips Next figure is called combinations - FHWA defines combinations as semi-truck units travelling with a trailer or a combination of multiple trailers. For example, Minnesota’s M3S2-40 truck shown above.

Minnesota Legal Loads Type M3S3 47’ 80 kips FHWA defines combinations as semi-truck units travelling with a trailer or a combination of multiple trailers. For example, Minnesota’s M3S3-40 truck shown above.

Permit Loads Single Trip Permit Annual Permit Now, single trip permits are your oversize/overweight permit vehicles. These are trucks that have a total weight of 104K up to excess of 500k depending on the axle configuration. Example of loads are mobile cranes, farm equipment or generator for wind turbines. And for annual permit trucks, these are timber trucks and ag trucks that have six or seven axles with a total weight of 90kips and 97kips, respectively. Annual permits can weigh as much as 155,000 lbs. depending on the axle configuration. 6 Axle = 90,000 lbs. GVW 7 Axle = 97,000 lbs. GVW 104, ,000+ lbs. GVW

Emergency Vehicles (EV) EV2 EV3 On December , President Obama signed into law the Fixing America’s Surface Transportation ACT (FAST Act) On November 3, 2016, FHWA sent out a memo to provide guidance on maintaining compliance with the load rating/posting requirements for Emergency vehicles (EV). In accordance with the National Bridge Inspection Standards (NBIS), all bridges on public roads must be load rated and posted for unrestricted legal and routine permit loads. Since EV’s are considered unrestricted legal loads, then Fast ACT law says that EV’s must now be considered when load rating and posting bridges on the interstate and within the range of reasonable access to the interstate (1 mile radius) and for non-interstate bridges EV’s must also be considered in load rating/posting if the state statute allows EV’s to cross bridges without restrictions, meaning they are exempt from any load posting law. Mn statute exempt EV’s from load posting.

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 - Concrete deck as it relates to any composite action with the girder (and potentially reinforcing steel in the deck for negative moment applications - Secondary elements may be load rated if there is question regarding the original design capacity. - If the load rating engineer, utilizing engineering judgment, determines that certain members or components will not control the rating, then a full analysis of the non-controlling element is not required

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 - Concrete deck as it relates to any composite action with the girder (and potentially reinforcing steel in the deck for negative moment applications - Secondary elements may be load rated if there is question regarding the original design capacity. - If the load rating engineer, utilizing engineering judgment, determines that certain members or components will not control the rating, then a full analysis of the non-controlling element is not required

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 - Prompting a load rating from significant deterioration of exposed steel pile bents with section loss from corrosion, or exposed timber pile bents with rot. Timber elements are particularly susceptible to deterioration, particularly if wet conditions are present. If timber deterioration is significant, these substructure elements may control the rating. In the case of timber piles, calculated ratings may be low, even with little or no deterioration. - The load rating engineer should thoroughly review inspection reports when making the decision on what substructure elements may require a load rating.

Substructures Picture on the left is an example of a split timber pile Picture on the right is an example of corroded H-pile at the water line

Substructures Normally, we would rate the superstructure but this is a case where the substructure will control the ratings. Left picture, Rotted timber pile Right picture, rotted timber pile that exploded from pressure

Reasons for re-load rating
Physical change to structure Deterioration (severe corrosion) Bridge accident (high load hit) Changes in the conditions of the primary structural components may cause a need for performing a load rating. There are a number of scenarios for which a load rating is required. As bridge gets older and begins to change, a couple of reasons you would want to rerate your bridge. Examples would be deterioration or to steel structural elements resulting in section loss or damage to primary structural component of a bridge caused by an accident. Accident like this, would require the bridge to be restricted and have the beam replaced ASAP or if left in service, a new rating must be perform to reflect the loss of capacity. Other examples are: Cracks in steel beam or girder tension zones Loss of gusset plates, holes in gusset plates Severed or corroded strands in prestressed box beams Severe cracking and spalling in concrete superstructure Beam sag and differential settlement Drop in condition rating of a primary superstructure to 4 or below Critical Finding showing severe section loss, cracks, strand loss

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 Changes in the conditions of the primary structural components may cause a need for performing a load rating. There are a number of scenarios for which a load rating is required. As bridge gets older and begins to change, a couple of reasons you would want to rerate your bridge. Examples would be deterioration or to steel structural elements resulting in section loss or damage to primary structural component of a bridge caused by an accident. Accident like this, would require the bridge to be restricted and have the beam replaced ASAP or if left in service, a new rating must be perform to reflect the loss of capacity. Other examples are: Cracks in steel beam or girder tension zones Loss of gusset plates, holes in gusset plates Severed or corroded strands in prestressed box beams Severe cracking and spalling in concrete superstructure Beam sag and differential settlement Drop in condition rating of a primary superstructure to 4 or below Critical Finding showing severe section loss, cracks, strand loss Corrosion within compression zone of gusset plate

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 A load rating is also performed if there is a change in the dead load on the structure such as widening of additional loads on the superstructure such as a deck overlay or addition of sidewalk, etc. Also, if there is a change in truck weight laws. An example of this is the Timber Haulers Bill and we rerate a lot of our bridges as a result of it. SPECIAL FARM PRODUCTS PERMITS

Change in legal live loads Specialized Hauling Vehicles (SHV) Change in load rating method ASR, LFR, LRFR Another reason, if there is a change in live loads. Example would be the special hauling vehicles. And finally, if there is a change in the load rating method. (There are different methodology and specifications for load ratings that AASHTO has and when those change, we have to perform a new load rating.)

Federal Code of Regulations
Non-divisible Single axle weight limit Tandem axle weight limit Emergency vehicle Overweight Permit Vehicle Non-divisible- Means any load or vehicle exceeding applicable length or weight limits which, if separated into smaller loads or vehicles, would; (1) Compromise the intended use of the vehicle, i.e., make it unable to perform the function for which it was intended; (2) Destroy the value of the load or vehicle, i.e., make it unusable for its intended purpose; or (3) Require more than 8 workhours to dismantle using appropriate equipment. The applicant for a non-divisible load permit has the burden of proof as to the number of workhours required to dismantle the load. A State may (but is not required to) treat emergency response vehicles, casks designed for the transport of spent nuclear materials, and military vehicles transporting marked military equipment or material as non-divisible vehicles or loads. A “non-divisible load” is one piece or item which cannot be separated into units of less weight without affecting the physical integrity of the load. Examples of non-divisible loads include, but are not limited to single poles, girders, columns, wood or metal trusses, buildings, houses. By statute, a non-divisible load includes bulk milk, coiled steel, transformers, wind farms and sealed shipping containers. A “divisible load” any cargo being carried which can be separated into units of legal weight without affecting the physical integrity of the load. Examples of divisible loads include: aggregate (sand, top soil, gravel, stone), logs, scrap metal, fuel, milk, trash/refuse/garbage, etc.

Federal Code of Regulations (continued)
Non-divisible Single axle weight limit Tandem axle weight limit Emergency vehicle 80 kips Single axle weight limit - The total weight transmitted to the road by all wheels whose centers may be included between two parallel transverse vertical planes 40 inches apart, extending across the full width of the vehicle. The Federal single axle weight limit on the Interstate System is 20,000 pounds. Tandem axle weight limit - The total weight transmitted to the road by two or more consecutive axles whose centers may be included between parallel transverse vertical planes spaced more than 40 inches and not more than 96 inches apart, extending across the full width of the vehicle. The Federal tandem axle weight limit on the Interstate System is 34,000 pounds. Emergency vehicle - A vehicle designed to be used under emergency conditions- to transport personnel and equipment; and to support the suppression of fires and mitigation of other hazardous situations.

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 Bridge Formula - Congress enacted the Bridge Formula in 1975 to limit the weight-to-length ratio of a vehicle crossing a bridge. This is accomplished either by spreading weight over additional axles or by increasing the distance between axles. As trucks grew heavier in the 1950s and 1960s, something had to be done to protect bridges. The solution was to link allowable weights to the number and spacing of axles. Axle spacing is as important as axle weight in designing bridges. Figures: Axle spacing is as important as axle weight in designing bridges. In Figure 1 A, the stress on bridge members as a longer truck rolls across is much less than that caused by a short vehicle as shown in Figure 1 B, even though both trucks have the same total weight and individual axle weights. The weight of the longer vehicle is spread out, while the weight of the shorter vehicle is concentrated on a smaller area. In addition to Bridge Formula weight limits, Federal law states that single axles are limited to 20,000 pounds, and axles closer than 96 inches apart (tandem axles) are limited to 34,000 pounds. Gross vehicle weight is limited to 80,000 pounds.

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. Notes: Compliance with Bridge Formula weight limits is determined by using the following 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.

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 = 57,000 pounds.

Example continued Minnesota’s Gross Weight Schedule: Maximum weight (W) = 57,500 pounds L =23 feet and 4 axles. Checks out! From Table: Maximum weight (W) = 57,500 pounds (Bridge Table for "L" of 23 feet and "N" of 4 axles). This axle spacing is satisfactory. GROSS WEIGHT SCHEDULE,

Example continued Since axles 1 through 4 are satisfactory, check axles 2 through 4: Actual weight = 15, , ,000 = 45,000 pounds. Next we check axles 2 through 4 to make sure that is satisfies the gross weight table for the 3 axle group column.

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. NG From Table: Maximum weight (W) = 43,000 pounds (Bridge Table for "L" of 9 feet and "N" of 3 axles). The actual weight of axles 2-4 is 45,000 lbs, clearly this is a violation because the actual weight 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. GROSS WEIGHT SCHEDULE, 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 TIRE WEIGHT LIMITS: Pneumatic-tired vehicle, No vehicle or combination of vehicles equipped with pneumatic tires shall be operated upon the highways of this state: 1. where the gross weight on any wheel exceeds 9,000 pounds on an unpaved street or highway or 10,000 pounds on a paved street or highway, unless posted to a lesser weight 2 where the gross weight on any single axle exceeds 18,000 pounds on an unpaved street or highway or 20,000 pounds on a paved street or highway, unless posted to a lesser weight GROSS WEIGHT SCHEDULE, § Subdivision 1.Table of axle weight limits. (a) No vehicle or combination of vehicles equipped with pneumatic tires shall be operated upon the highways of this state where the total gross weight on any group of two or more consecutive axles of any vehicle or combination of vehicles exceeds that given in the following axle weight limits table for the distance between the centers of the first and last axles of any group of two or more consecutive axles under consideration. Unless otherwise noted, the distance between axles must be measured longitudinally to the nearest even foot, and when the measurement is a fraction of exactly one-half foot the next largest whole number in feet shall be used, except that when the distance between axles is more than three feet four inches and less than three feet six inches the distance of four feet shall be used. The gross weights shown without parentheses in the axle weight limits table are allowed on unpaved streets and highways, unless posted to a lesser weight The gross weights shown in this table, whether within or without parentheses, are allowed on paved streets and highways, unless posted to a lesser weight Gross weights in excess of 80,000 pounds require an overweight permit under this chapter, unless otherwise allowed under section Gross Weight Seasonal Increases, an increase of 10% in load limits subject to implementation of springtime load restrictions.

Minnesota Statutes (continued)
Definitions Implements of Husbandry A self-propelled or towed vehicle designed for timber-harvesting, agricultural, horticultural, or livestock Implement of husbandry: An implement of husbandry that is not self-propelled and is equipped with pneumatic tires may not be operated on a public highway with a maximum wheel load that exceeds 500 pounds per inch of tire width. After December 31, 2009, a person operating or towing an implement of husbandry on a bridge must comply with the gross weight limitations provided in section Also a person operating or towing an implement of husbandry must comply with a sign that limits the maximum weight allowed on a bridge

Weight Laws Weight Increases 10% winter increase 10% harvest increase Weight Restrictions Load limit on bridge Weight limits set by other law Weight Increases: Winter increase amounts. The limitations provided in sections to (tire weight limits, GW schedule, GW limits Forest Products) are increased by ten percent between the dates set by the commissioner for each zone established by the commissioner based on a freezing index model each winter. Harvest season increase amount permit: Beginning of harvest to November 30 each year for the movement of sugar beets, carrots, and potatoes from the field of harvest to the point of the first unloading, load limit may be increased by ten percent. Weight Restrictions: Load Limit on Bridge: The gross weight of any vehicle or combination of vehicles driven onto or over a bridge on any highway shall not exceed the safe capacity of the bridge, as may be indicated by warning posted on the bridge. Weight limits set by other law: In cases where gross weights in an amount less than that set forth in sections to are restricted on a bridge, the lesser gross weight as restricted may not be exceeded.

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 Weight Exemptions: Weight limits not applicable to certain vehicles: Sections to do not apply to tow truck and utility vehicles for municipal electric, gas, or water system. Fire Engines: The provisions of sections to governing size, weight, and load shall not apply to a fire apparatus. Note an emergency vehicle is a vehicle of a fire department MN STATURTE Subdivision 1e, The provisions of sections to governing size, weight, and load shall not apply to a fire apparatus (FIRE TRUCKS), or to a vehicle operated under the terms of a special permit issued as provided by law. Therefore LOAD LIMIT ON BRIDGE does not apply to fire trucks.

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 Excess weight: permit needed, when the ten percent increase is in effect, a permit is required for a motor vehicle, trailer, or semitrailer combination that has a gross weight in excess of 80,000 pounds, an axle group weight in excess of that prescribed in section , or a single axle weight in excess of 20,000 pounds and which travels on interstate routes.

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 Forest Products: To haul raw or unfinished forest products" include wood chips, paper, pulp, oriented strand board, laminated strand lumber, hardboard, treated lumber, untreated lumber. GVW not exceed 90,000 pounds or 99,000 pounds during the time when seasonal increases are authorized. Must be equipped and operated with 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. A vehicle operated under this section may exceed the legal axle weight limits listed in section by not more than 12.5 percent; except that, the weight limits may be exceeded by not more than percent during the time when seasonal increases.

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 Special Paper Products: A truck-tractor and a single semitrailer that may exceed 48 feet, but not 53 feet, and has a maximum gross vehicle weight of 90,000 pounds if the vehicle combination has a total of six or more axles or 97,000 pounds if the vehicle combination has a total of seven or more axles. The maximum gross vehicle weight is 99,000 pounds during seasonal weight increases period. The vehicle may be operated on streets or highways under the control of local authorities only upon the approval of the local authority. This vehicle is subject to bridge weight limit, axle weight limitations and may not be operated on the interstate highway system.

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 Special Farm Products: A road authority may issue an annual permit authorizing a vehicle or combination of vehicles with a total of six or more axles to haul raw or unprocessed agricultural products and be operated with a gross vehicle weight of up to 90,000 pounds and 99,000 pounds during seasonal weight increases period. Also a road authority may issue an annual permit authorizing a vehicle or combination of vehicles with a total of seven or more axles to haul raw or unprocessed agricultural products and be operated with a gross vehicle weight of up to 97,000 pounds and 99,000 pounds during seasonal weight increases period.

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 This vehicle is subject to bridge weight limit, axle weight limitations and may only be operated on paved streets and highways other than interstate highways.

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 Road Construction Materials: A road authority may issue an annual permit authorizing a vehicle or combination of vehicles with a total of six or more axles to haul road construction materials and be operated with a gross vehicle weight of up to 90,000 pounds and 99,000 pounds during the seasonal weight increases period. A road authority may issue an annual permit authorizing a vehicle or combination of vehicles with a total of seven or more axles to haul road construction materials and be operated with a gross vehicle weight of up to 97,000 pounds and 99,000 pounds during the seasonal weight increases period. This vehicle is subject to bridge weight limit, axle weight limitations and may only be operated on paved or unpaved streets and highways other than interstate highways.

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 Why do we load post our bridges? It’s the law but, according to the Minnesota Statutes , when a load rating shows that a bridge does not have enough capacity to carry legal loads (including SHVs), the bridge must be posted for load. You post your bridges when the maximum legal load exceeds the capacity of the structure to support that load with safety factors. Also, we have to preserve and maintain the condition of our bridges to keep them open safely to traffic. And finally, to protect our bridges from overstressing that could lead to possible damage or failure of the structure Picture: This picture of a bridge in Iowa that collapsed due to heavy load, the bridge was properly posted but the tractor and load exceeded the capacity of the bridge (the bridge had an 18Ton limit and the load was 35tons). The other is a picture of a bridge in Nobles Co. The bridge had a bad pile on the outside of the pier and did not have the capacity to carry the weight of the paving machine. Damaged Bridge due to Failure to Comply with Bridge Posting

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 c Maintenance of Bridge on Town Road Major deterioration, e.g. beam members or timber piles(s) Bridge Hit Missing timber pile(s) There are many factors as to why bridges become load posted. Original design weight that was used was less than now Size of the vehicles are much greater now (This applies to legal loads) Change in Federal or State weight laws Examples are SHV trucks. Special Hauling Vehicles. SHV’s are legal single unit trucks that have liftable axles. They can weigh up to 80,000 lbs. These are trucks that have 4axles up to 7 axles. (In 2007, FHWA implemented the SHV’s as a new posting vehicles therefore we have to load rate our bridges for SHVs) Another example is Mn Statute , that applies to township bridges, if the township fails to provide the required posting within 30 days of notice, the county is authorized to provide the required posting and bill the town for all related expenses. Again, it is mandatory the posting signs be erected within 30 days after notification of their requirements; otherwise temporary sign should be erected in the interim or the bridge owner elects to provide expedited repairs to strengthen the bridge to carry legal loads. Major deterioration in beam members or timber pile rotting from the inside Bridge Hit - high load hit, this often happens to TH bridges And finally missing timber piles caused by scouring or being hit by debris during flooding So keep in mind that the last three bullets, these could potentially result in putting weight restrictions on your bridge. There load rating analysis should be done right away to verify if the bridge is still capable of carrying legal loads.

Bridge load posting (contined)
Example Good example of having your bridge load posted. Picture of a 15ton John Deere Combine crossing a local bridge. If you notice the bridge is posted. I don’t even think the farmer even think twice when crossing this bridge.

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 leader Software program data file used in the load rating analysis. AASHTOWare BridgeRating, BrR file (previously know as VIRTIS) MDX file CANDE file for arch structures MnDOT rating documents For every bridges/culverts in Minnesota that needed a new a load rating, completed forms must be sent to MnDOT along with software program file used in the analysis such as AASHTO BrR files, CANDE, MDX etc. Copies of the load rating forms are located on our MnDOT bridge website. MnDOT rating documents -

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 documents For any overweight permit application -Drawing of overweight truck with weight and dimension information. -For single trip overweight permit over TH bridges on local roadway, we ask that the local agency notify the trucker to fill out Form OW-CL A legal truck can weigh up to 80,000 lbs, anything above 80,000 is considered a permit vehicle requiring. An annual permit vehicle are trucks that have a GVW of 90,000 lbs up to 155,000 lbs Single trip permits (super loads) are trucks that have a GVW greater than 150,000 lbs.

What do you submit to MnDOT? (continued)
Overweight permit Permit vehicles are overweight vehicles which, in order to travel a state’s highways, must apply for a permit from that state. They are usually heavy trucks (e.g., combination trucks, construction vehicles, or cranes) that have varying axle weights and spacings depending upon the design of the individual truck. To ensure that these vehicles can safely operate on existing highways and bridges, most states require that bridges be designed for a permit vehicle or that the bridge be checked to determine if it can carry a specific type of vehicle. For safe and legal operation, agencies issue permits upon request that identify the required gross weight, number of axles, axle spacing, and maximum axle weights for a designated route

Moises Dimaculangan, P.E.
Questions? Dave Conkel, P.E. Moises Dimaculangan, P.E.

Bridge Load Rating.

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