Section 3: Load RatingsAnchor: #i1005569
Definition of Load Ratings
The Load Rating is a measure of bridge live load capacity and has two commonly used categories:
- Inventory Rating, as defined by the current AASHTO Manual for Bridge Evaluation,3 is that load, including loads in multiple lanes that can safely utilize the bridge for an indefinite period of time.
- Operating Rating, defined by the same manual, is the maximum permissible live load that can be placed on the bridge. This load rating also includes the same load in multiple lanes. Allowing unlimited usage at the Operating Rating level will reduce the life of the bridge.
Determination of Load Ratings
Currently, all Inventory and Operating Ratings are expressed in terms of an equivalent HS-truck as shown in the Manual for Bridge Evaluation.4 Prior to about 1995, many ratings were for an equivalent H-truck, shown in Manual for Bridge Evaluation.5 The H-truck directly corresponds to single-unit trucks, which used to be common on rural highways. Today, even rural Farm- or Ranch-to-Market highways and many off-system highways are exposed to much larger semi-trucks; therefore, the HS-truck is more realistic.
Traditionally Inventory or Operating Ratings were determined using either Load Factor (LF) or Allowable Stress (AS) methods. Since 2000, LF is to be used for all on-system bridges, except for timber bridges. It is difficult to assign an ultimate strength to timber. Therefore, both on- and off-system timber bridges are rated using only AS methods. AASHTO has included Load and Resistance Factor Rating (LRFR) as an acceptable method for load rating bridges. Calculate load ratings using LRFR methods if the bridge was designed using the Load and Resistance Factor Design (LRFD) methodology.
Either AS or LF may be used for all off-system bridges.Anchor: #i1005614
Inventory Rating and Design Load Considerations
The Inventory Rating (Item 66) can be initially estimated to be at least equal to the design loading if no damage or deterioration exists and the original design was made using an HS or HL-93 (LRFR) load pattern. Assumed load ratings based on original design loads require plan sheets with design the design load identified to be added to the Bridge Record. Many old plans have a design loading shown as H-20 S-16, which some raters have misinterpreted as meaning H-20. AASHTO replaced the H-20 S-16 designation in 1965 with the HS-20 designation. Re-rating these bridges using LF procedures will usually increase the Inventory Rating above HS-20. Rating bridges designed between 1946 and about 1958 by current LF procedures may result in significantly different values than the original design loading. Although the plans may say designed to H-20 S-16 and THD Supplement No. 1, the bridge may rate significantly less than HS-20 loading. This difference is due to the more liberal effects of THD Design Supplement No. 1 described below.Anchor: #i1005629
THD Design Supplement No. 1
In 1946, the Bridge Division of TxDOT (then called THD) issued what is commonly called THD Supplement No. 1.6 Texas was influential in the development of the AASHTO Bridge Design Specifications. However, not all the Texas opinions were immediately accepted by the AASHTO Bridge Committee, which includes all states. As a result, TxDOT used the supplement for a number of years to amend portions of the 1944 and 1949 AASHTO Standard Specifications for Highway Bridges78 for use in Texas.
The first version of Supplement No. 1 was dated June 1946.9 The second version of Supplement No. 1 was dated September 195310 and included only those items of the 1946 version that had not been incorporated into the 1949 AASHTO Standard Specifications for Highway Bridges.11 The primary subjects of the supplement that affected bridge design can be summarized as follows:
- Crown Width Bridges. The 1944 AASHTO Bridge Specifications12 required curbs on all bridges. Texas initiated the concept of crown-width bridges with the following: “On non-restrictive bridges the curbs may be omitted provided the guard fence or an equivalent member is carried continuously through the structure.” The 1949 AASHTO Bridge Specifications13 allowed the condition of no curbs with certain additional width limitations. Texas continued the crown-width, no-curb concept with the retention of the provision in the second version of Supplement No. 1 dated September 1953.14
- Design Overload. The 1944 AASHTO Bridge Specifications15 required an overload to be considered for all bridges designed for less than an H-20 (40,000 lbs) or H-20 S-16 (72,000 lbs) loading, now called HS-20 loading. The overload was to be the design truck (usually H-15) increased by 100 percent, but without concurrent loading of adjacent lanes, thus allowing single-lane load distribution. The allowable stress was to also be increased to 150 percent of the basic allowable. Texas modified this provision specifically to apply the same overload to truss counter members for all design loadings. Truss counters are those members that, for some positions of live load, will change from tension to compression. If a truss was designed H-15, H-20, or H-20 S-16, the overload was applied in determining the size of counter member.
- Lane Load Negative Moments. The 1944 AASHTO Bridge Specifications16 required for H-10, H-15, or H-20 lane loads an additional concentrated load in one other span in a continuous unit positioned to produce maximum positive and negative moments. Texas limited the distance between the concentrated loads for the lane load to a maximum of 30 ft. This is probably based on the fact that the AASHTO 1944 bridge specifications17 did not require an additional concentrated load for H-20 S-16 lane loadings. The H-20 S-16 truck loadings have a second axle spaced from 14 to 30 ft from the first heavy axle. This is probably the rationale for the limit of 30 feet in THD Supplement No. 1.18 The 1949 AASHTO bridge specifications19 made the lane loading negative moment requirement the same for HS-trucks. However, the 1953 THD Supplement No. 120 continued modifying the provision for continuous spans subjected to lane load by limiting the spacing between the additional concentrated load to 30 ft. This limit had the effect of reducing the lane load negative moment maximums for some continuous spans. The 30-ft limit may also have been in recognition that the second large axle for an HS-load pattern is spaced at a maximum of 30 feet from the first large axle, or it might have been because the lane load approximately represents a train of trucks with a headway distance of 30 feet between trucks. Placing the second concentrated load at least 30 ft from the first instead of a maximum of 30 ft would have been more logical. Current specifications do not limit the distance between the two loads for negative moment lane loadings.
- Impact Load Provision. The 1944 AASHTO Bridge Specifications21 required that the shortest length of adjacent spans in a continuous unit be used for the negative moment impact value. In 1949, AASHTO changed this to the current provision of using the average length of the adjacent spans. Both versions of THD Supplement No. 12223 changed the impact provision for continuous units or other structures where discontinuous lane loadings are applied to be the loaded length as indicated by the influence line for the section of member considered. This change had the effect of slightly increasing the impact value.
- Special Axle Loads. The 1946 THD Supplement No. 124 added a provision that no axle load in excess of 24,000 lbs should be considered in the design of floor slabs. It further required that either a single 24,000-lb axle or two 16,000-lb axles spaced four ft apart must be used for the design of H-20 and H-20 S-16 bridge floors (slabs, grids, timber) instead of the 32,000 lb axle. The provision was dropped in the 1953 THD Supplement No. 125 because the 1949 AASHTO Bridge Specifications26 included the provision specifically for concrete bridge slabs. The AASHTO Bridge Specifications further limited the 24,000-lb axle to slab spans under 18 ft and the two 16,000 lb axles for slab spans over 18 ft. This provision had the effect of reducing the design load for many slab spans designed during that time. It has been found that some beams have been designed in Texas using the single 24,000-lb axle. It is believed to bean error for beams to have been designed this way. For this reason, carefully evaluate any plans prepared during the period between approximately 1949 and 1961 with a design load of H20 or H20 S-16 that also had the THD Supplement No. 127 notation.
Customary Rating Procedures
When a bridge was originally designed, the designer often had to select the next size of reinforcing bar, size of steel beam, or thickness of cover plate to meet the design stress criteria. Sizes that were larger than the theoretically perfect size of member result in Inventory Ratings significantly higher than the design loading. However, the design loading and date of original construction are important parts of the bridge data since they often provide a basis for determining initial routing of overload permits.
If the original design was made using an H-load, such as H-15 or H-20, then the equivalent HS Inventory Rating will usually be significantly less numerically. For example, an H-15 design might rate at HS-12. However, this difference means that the total inventory HS-load capacity is 43,200 lb (two 19,200 pounds axles and one 4,800 lb axle totaling 21.6 tons) as compared to the H-15 design of 30,000 pounds (15 tons).
Determine the original design load from a review of the bridge plans if available. If the structure essentially matches an old TxDOT standard bridge, then the design load for that standard can be used for the Design Load (Item 31). Enter appropriate notation about this in the Bridge Record, and update the electronic Bridge Inventory File. However, use caution accepting the design load in plans that used the THD Design Supplement No. 12829 due to circumstances described above.
AS rating procedures are usually set at 55 percent of the material yield stress for steel structures and 50 percent of the material yield stress for Grade 40 reinforcing steel in concrete structures. When AASHTO first introduced the use of Grade 60 reinforcing steel in the 1970 Interim Bridge Design Specifications,30 the allowable of 24 ksi for Grade 60 was assigned based approximately on the ratio of the Grade 60 ultimate strength to that of Grade 40. Thus, the AS procedures were still compatible in factor of safety for concrete members.
LF rating procedures usually assign a dead load factor of 1.3 and live load factors of 2.17 (when computing Inventory Ratings) and 1.3 (when computing Operating Ratings). The resulting stresses or bending moments are compared to the yield of steel members or the ultimate capacity of concrete members also considering appropriate phi strength reduction factors.
Note that the value of 2.17 is the dead load value of 1.3 times 1.67. The load factor of 1.3 accounts for a 30 percent increase in all loadings, either dead or live, so as to provide a uniform safety factor. The factor of 1.67 accounts for the variability of live load configurations other than a standard HS-load pattern and further provides for potential overloads or loads in excess of the State Legal Loads.
Specific analysis of structures for over-weight loads, particularly superheavy permits over 254,300 pounds, is usually done with a load multiplier consistent with the restricted speed of the vehicle. Commonly this factor is about 1.1, with total stresses compared to an allowable of 75 percent of the yield for steel bridges or 75 percent of the ultimate capacity for concrete bridges including prestressed beam bridges. This procedure is explained more fully in Chapter 6, Routing and Permits.
Do not consider temporary repairs for Inventory or Operating Ratings. However, take temporary repairs into account when assigning the operational status code of Item 41 to the structure. Temporary repairs are to be considered for the operational status code only until a more permanent repair is made. Do not use temporary repairs for more than four years. The Inventory Rating directly affects the Sufficiency Rating, so therefore do not assign any weight to temporary repairs in the Load Rating calculations.
Use all field information and conventional analysis techniques when the design loading is unknown or deterioration exists. Even when the design loading is known, the only acceptable method for accurate load rating is to do calculations based on the plans and known field measurements.Anchor: #i1005766
Rating Concrete Bridges with No Plans
A concrete bridge with unknown reinforcing details (no plans) can be rated for the State Legal Load (HS-20) at the Operating Level, which is currently defined for load rating purposes as an HS-20 design load, provided that the following two considerations are met:
- It has been carrying unrestricted traffic for many years.
- There are no signs of significant distress.
Three additional considerations for rating concrete bridges with unknown reinforcing are:
- Ensure bridge exhibits proper span-to-depth ratios of the main members, which indicates that the original design was by competent engineers. In general, this consideration means that for simple span structures the span-to-depth ratio of main members should not exceed approximately 20. Span-to-depth ratios exceeding this ratio may indicate that the designer did not properly consider reasonable design truck loadings.
- Construction details such as slab thickness and reinforcement cover over any exposed reinforcing to specifications current at the time of the estimated construction date.
- Appearance of the bridge shows that construction was done by a competent builder.
A comparative original design rating can be used to estimate the amount of reinforcing in the main members. Normally, if the design was done prior to about 1950 and the above five considerations are met exist, then the amount of reinforcing can be estimated based on a percentage of the gross concrete area of the main beams (if tee-beam construction), or depth of slab (if slab construction). Two of the examples below describe this method, and a third example describes a method that can be used for prestressed beam bridges with no plans or other documentation.
Figure 5-2. Load Ratings for Concrete Bridges without Plans
NOTE: *Permit Trucks with gross or axle weights that exceed the state legal load limits will not be allowed to use these bridges.
I.F. - Inspection Frequency.
Refer to AASHTO Manual for Bridge Evaluation, Chapter 6, Section B.Anchor: #i1005830
Examples of Rating Concrete Bridges with No Plans
Example 1. A flat-slab bridge designed between about 1930 and 1960 can be assumed to have approximately 0.7 percent tension steel based on the total slab depth. Calculations with this amount of steel using AS procedures with stresses, materials, covers, and live load distribution appropriate to the AASHTO Bridge Specifications for the estimated date of construction should give at or very near an H-10, H-15, or perhaps an H-20 theoretical rating. Any other value would make the assumptions suspect. After this analysis is made, an analysis using LF procedures, HS loading, and current load distributions should give an acceptable rating. Flat-slab bridges constructed off-system can also often be rated by this procedure providing the above five considerations are also met. This method is not suitable for evaluation of FS slabs, which may be recognized as those with narrow roadways and tall integral curbs.
Example 2. A multi-beam concrete bridge built between about 1940 and 1965 can be estimated to have approximately 0.3 percent tension steel based on beam spacing and an estimated depth to the center of the steel group of 0.9 D where D is the total depth of the tee-beam. As in Example 1, an old AS rating can first be calculated for comparison. If reasonable, then a modern LF rating can be made with HS loading and the estimated amount of reinforcing steel. The amount of steel can be adjusted slightly so the AS design exactly matches an H-rating of H-10, H-15, or H-20.
Example 3. Some bridges built since about 1955 are composed of prestressed beams and no plans exist. This condition is often found for off-system bridges. The ratings should be done using conservative assumptions and good engineering judgment. One procedure would be to assume that the beams were designed to an H-15 loading in conformance with the estimated date of specifications. Using this assumption, an AS calculation can be made to estimate the even number of 7/16-in. 250 K strands. An LF rating using the HS-loading can then be performed based on this number and size of strand. In Texas, prestressed beams were probably never designed to less than H-15. Most beams have been designed to H-20 or HS-20. Texas prestressed beam fabricators keep good records of their products, and identification of the design loading may sometimes be tracked down.
All three of these examples should give H-ratings using AS procedures that are close to a realistic design load. For instance, a calculated value of H-14.4 could reasonably be assumed to verify that the original design was H-15. A calculated AS value of H-13 would be suspect and further investigation will be required.Anchor: #i1005857
Ratings for Unusual Bridges
Unusual bridges, such as those composed of old railroad flat cars, can be rated, but ensure that the critical rating component is considered. For instance, flat cars were originally designed for a maximum point load combined with a uniform load over the whole car. When used for traffic loadings, even though the main two-girder members may give a good equivalent HS load rating, the transverse stiffening members and floor beams often control the live load capacity.
Another unusual type of bridge in Texas is the continuous cast-in-place (CIP) flat slab. Most of these bridges were designed in the 1940s and 1950s with an H-15 or H-20 load pattern. Unfortunately, the design negative moments were from the single truck load in one span. As a result, these bridges may be under-designed for HS-loadings and, as a consequence, may require a load restriction. Design procedures using an HS-20 design load; use a lane load with two concentrated loads in adjacent spans for the controlling negative moment case for longer continuous bridges. For shorter, continuous bridges, an HS-20 design uses two heavy axles of the HS-20 load pattern at variable spacing in adjacent spans. However, the current AASHTO Bridge Specifications do not differentiate between single-and multiple-lane distribution factors for slab bridges. As a result, this type of bridge has greater strength for multiple trucks positioned in the middle of the bridge span. Some structural evaluators make live load distribution adjustments based on the number of lanes loaded for flat slab bridges. Exercise care and properly correlate it to two- or three-dimensional methods of analysis to use this procedure.Anchor: #i1005875
H- and HS-Load Ratings
Previously, all ratings were done with the equivalent H-truck, shown in Figure 5-1, or the HS-truck shown in Figure 5-1. Currently, all ratings are only with the HS-truck. A moment equivalency conversion from H- to HS-ratings is not recommended since this process would assume that the structure was exactly designed for the given H-loading. In addition, continuous spans cannot be converted by this process. Most structures have a degree of capacity past the design H-load, particularly since load distribution assumptions of the AASHTO Bridge Specifications31 have been made more liberal since the time many structures were commonly designed using H-loads. However, as previously explained, some bridges were intentionally designed with AS methods to a 5 percent overstress for some components.
It is not acceptable to ratio the design live load moments for an H-truck to the same moment for an equivalent HS-truck. For instance, if a 48-ft simple-span bridge has a design load of H-15, the design load for moment equivalency would be HS-10.8. However, due to the above reasons, the actual rating based on LF methods might easily be HS-9 or HS-13. Generate an LF rating in this case.