section 1.2 Truck Characteristics Affecting Pavements; (d) Axle Spacing

Navigation: Page 1, Cover Page | Page 2.. Phase 1.1, Background. | 1.2 Truck Characteristics Affecting Pavements. (a) Axle Weights | Page 4, Section 1.2 [b] Tire Characteristics | Page 5, Section 1.2 (c) Suspension Systems | Page 6, Section 1.2; (d) Axle Spacing | Page 7, Section 1.2; (e) Liftable Axles | Page 8, Section 1.2; (f) Tridem Axles | Page 9, Section 2.1 Axle Weight Limits | Page 10, Section 2.2 Bridge Formula | Page 11, Section 2.3 - 80,000 Pound GVW Cap | Page 11, Section 2.4 Policies to Encourage Tridems | Page 11, section 2.5 Weight Limits Per Unit of Tire Width | Page 12, section 2.6 Turner Trucks | Page 13, section 2.7-New Approach; TRB Truck Weight Study | Page 14, section Section 3.0; Knowledge Gaps and Research Needs | Page 15, section 4.0 References for Pavements Working Paper

Comprehensive Truck Size and Weight (TS&W) Study

Phase 1—Synthesis

Working Paper 3—Pavements and TS&W Regulations

1.2 Truck Characteristics Affecting Pavements

(d) Axle Spacing

Two primary load effects on flexible pavement performance are rutting and fatigue. For rutting, bringing axles closer together is unlikely to significantly affect the critical stresses and pavement performance. Thus, the effect of a tandem axle on rutting is expected to be identical to the cumulative effects of the two single axles of which it is composed. For fatigue, when widely separated loads are brought closer together, the stresses they impart to the pavement structure begin to overlap and they cease to act as separate entities. While the maximum deflection of the pavement surface continues to increase as axle spacing is reduced, maximum tensile stress at the underside of the surface layer (considered to be a primary cause of fatigue cracking) can actually decrease as axle spacing is reduced. However, effects of the overlapping stress contours also include increasing the duration of the loading period. Thus, the beneficial effects of stress reduction are offset to some largely unknown degree by an increase in the time or duration of loading. In short, the net effect of changes in axle spacing on pavement wear is complex and highly dependent on the nature of the pavement structure.

Hajek and Agarwal studied the influence of spacing on pavement damage associated with dual and triple axles on thick flexible pavements (SN=5.7). They examined six different measures related to pavement damage and two different axle spacings each for tandems and tridems. For the pavements studied, AASHTO load equivalence factors indicate that two 10,000 kilogram single axles would have the same effect as a tandem axle weighing 21,600 kilograms. For tandems with a 1.0 meter spacing, Hajek and Agarwal found that lower tandem weights would have the same effect: ranging from 14,900 kilograms to 20,600 kilograms depending on the damage measure used. For the pavements studied, AASHTO load equivalence factors indicate that three single axles weighing 10,000 kg. would have the same effect as a tridem axle weighing 34,300 kg. For tridems with a 2.0 meter spacing (from the first to third axle), Hajek and Agarwal found that lower tridem weights would have the same effect, ranging from 20,300 kg. to 31,000 kg. Based on these results, they concluded that the AASHTO ESAL values appear to understate the damaging effect of dual and triple axles in comparison to single axles.

[continued on next page, Page 7, Section 1.2; (e) Liftable Axles ]

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