TABLE I COMPUTED GROSS WEIGHT TABLE Page 11 BASED ON AASHO HI5 (44) BRIDGES WITH OVERSTRESSES NOT TO EXCEED 30 PERCENT Weight Formula W = 500 (LN+12N + 32 32) Distance in feet between the extremes of any group of two or more consecutive axles Maximum computed weight in pounds for any group of two or more consecutive axles (wheelbases) 2 axles 3 axles 4 axles 5 axles 6 axles 32000 Not Permitted Not Permitted Not Permitted The modification consists in limiting the maximum load on any single aule to 20.000 lbs and the load on any two axles spaced 8 ft. or less to 12,000 lbs. Loaded vehicles of 7 or more axles regardless of type and of wheelbase are not permitted. The weight formula given above should be applied to determine the permissible loads of vehicles whenever the load on the front axle in considerably low frat the permissible loads. TABLE II PERMISSIBLE GROSS LOAD TABLE Page 12 BASED ON AASHO HIS(44) BRIDGES WITH OVERSTRESSES NOT TO EXCEED 30 PERCENT FOR CERTAIN TYPICAL VEHICLES IN REGULAR OPERATION ILLUSTRATING WHEN TOTAL COMPUTED GROSS WEIGHT OR THE AXLE LOADINGS CONTROL THE PERMISSIBLE GROSS LOAD OF THE VEHICLE. Note: The loads are computed to the nearest 500 lbs. The modifications consist in limiting the maximum load on any single axle to 20,000 lbs. and the lead on any two axles spaced 8 ft. or less on confors to 32,000 lbs. The permissible load on any group of 2 or more asles spaced R. or less between the centers of the extreme axles of the group is 32,000 lbs. For the updier of this table the steering arie is assumed to be 10,000 lbs. and & feet to subtracted from the maximum permissible vehicle length to arrive of the darguse befoose the centers of extreme eles. 90-908 O-68-15 structure affected except floor slab." The question arises as to when the conditions of traffic change from an infrequent heavy load to a load which should be the design loading. This question can only be answered by an analysis of the traffic at each bridge. It would be worthwhile to explain here that although the design load has remained relatively constant at H15 for the major percentage of the bridges, the load carrying capacity on newer bridges has generally increased. This is due to the increase in roadway width. For the typical I-Beam or R.C.D. Girder bridge with four or more members the loads for an extremely heavy loading in one lane are generally distributed more uniformly than our specifications provide. The AASHO road test has provided basic information on the equivalent effects of single-and tandem-axle weights on both rigid and flexible pavements for the conditions of the road test. This information is a basic requirement in establishing axle weight limits. The equivalence relationship for a range of single and tandem axles on rigid and flexible pavements, based on the road test data, has been established. There are many formulas that would approximate the maximum desirable loads. One is the present simplified bridge formula which may be used to determine permissible gross weights and axle weights. This formula in its present form as stated in "Policy on Maximum Dimensions and Weights of Motor Vehicles to be Operated Over the Highways of the United States" conforms with the results of the AASHO road test mentioned earlier. I wish to emphasize the present formula was not devised by the AASHO Bridge Committee, was developed in its general form prior to the AASHO road tests, and was never approved by a 2/3 vote of the AASHO Bridge Committee. It was the opinion of those voting against, that the formula allowed gross loads that were too high. The basic axle loads used for development of this formula are 18,000# for a single axle and 32,000# for a tandem axle. The present AASHO Policy is modified to allow 20,000# on a single axle when spaced 8 feet or more apart. The change in the proposed formula from the present formula is a change from 32,000# to 36,000# for a tandem axle. This changes the permissible gross load of the vehicles, even though it may not be a vehicle with tandem axles. Chart No. 1 shows graphically the comparison of the proposed loading S.B. 2658 and the three (3) common types of AASHO Standard Loadings (H15, H20 and HS20). Chart No. 2 shows the percentage of the moments produced by the proposed loading as a percent of the standard loadings. span. The maximum percentage for the H15 loading is 176% which is for the 83' You will note that the effect of the new load on bridges with HS 20 design loading is minor. Bridges designed for H20 loading have an overload of approximately 32% which will result in a maximum overstress of probably 15%. The effects on H15 bridges is much more critical. With 76% overload for the live load moment an overstress of approximately 40% can be anticipated. While no definite statement can be made as to this overstress we feel that this is in excess of what can be tolerated without exceeding the proper limits of safety and structural life. The Until recently, the AASHO Specifications considered that only loadings which produce a change in sign of the stress were critical in fatigue. specifications also assumed that the loads would be repeated at least 2 million times, and therefore, allowable stresses were kept below the fatigue limit. Recent research and study indicated that these limits were not adequate. |