Inventory Rating and Design Load Considerations

The Inventory Rating Item 66 (SNBI: B.LR.05) 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 load pattern for Load Factor Design (LFD) or HL-93 load pattern for Load and Resistance Factor Design (LRFD). Many old plans have a design loading shown as H-20 S16, which some raters have misinterpreted as meaning H-20. AASHTO replaced the H-20 S16 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 Texas Highway Department (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.
In 1946, the THD issued what is commonly called THD Supplement No. 1.1 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 Bridges
23 for use in Texas. The first version of Supplement No. 1 was dated June 1946.4 The second version of Supplement No. 1 was dated September 19535 and included only those items of the 1946 version that had not been incorporated into the 1949
AASHTO Standard Specifications for Highway Bridges
.6 The primary subjects of the supplement that affected bridge design loading can be summarized as follows:
  • Design Overload
    . The 1944 AASHTO Bridge Specifications7 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 Specifications8 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. The H-20 S-16 truck loadings have a second axle spaced from 14 to 30 ft from the first heavy axle. The 1949 AASHTO bridge specifications9 made the lane loading negative moment requirement the same for HS-trucks. However, the 1953 THD Supplement No. 110 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. Current specifications do not limit the distance between the two loads for negative moment lane loadings.
  • Impact Load Provision
    . The 1944 AASHTO Bridge Specifications11 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. 11213 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. 114 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. 115 because the 1949 AASHTO Bridge Specifications16 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 be an 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. 117 notation.
There are three acceptable approaches to determining inventory and operating ratings: assigned, assumed, and calculated.
1. THD Supplement No. 1, TxDOT, September 1953.
5. THD Supplement No. 1, TxDOT, September 1953.
6. Standard Specifications for Highway Bridges, AASHTO, 1949.
7. Standard Specifications for Highway Bridges, AASHTO, 1944.
10. THD Supplement No. 1, TxDOT, September 1953.
13. THD Supplement No. 1, TxDOT, September 1953.
14. THD Supplement No. 1, TxDOT, June 1946.
16. Standard Specifications for Highway Bridges, AASHTO, 1949.
17. THD Supplement No. 1, TxDOT, September 1953.