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41 promoting and implementing regional truck size and weight · New "truck only" lanes allowing increased productivity policies. Participants recognized, however, that to avoid states and improved safety through separation of commercial ve- having total control over truck size and weight limits, a federal hicles from personal vehicles; umbrella would be needed to impose some overall limits on · "Last mile" military base, port, airport, inland waterway flexibility. and rail connections; · Tunneled and elevated roads and railways on existing 3.3.10 The Corridor Perspective right-of-way; · International gateways; The American Road and Transportation Builders Associa- · Bottleneck relief; tion (ARTBA) recently articulated a vision for the future to · Multimodal freight transfer centers; and be included in the Safe, Accountable, Flexible, Efficient · Integrated telecommunications corridors. Transportation Equity Act: A Legacy for Users (SAFETEA- LU) legislation. The heart of that vision was a new, more ac- The issue of trade corridors was also discussed at length at countable, structure for the federal highway program consist- the stakeholder workshop noted above (77). These partici- ing of two components, one to preserve and improve the pants believe that, under carefully controlled conditions, such current highway and transit systems through a significant in- corridors could be candidate sites for operations of larger ve- crease in federal user fees and the second, and more critical to hicles at higher weights. Some expressed concerns, however, this topic, the creation of an integrated, national strategy that about cost and the potential for off-corridor operations, such ARTBA refers to as "Critical Commerce Corridors." The pro- as trucks not staying in specified lanes or corridors. On high gram, sometimes referred to as the "3C Proposal," would volume freight corridors there may be opportunities for sep- facilitate the safe and efficient movement of freight and re- arate truck lanes. Some workshop participants felt that con- duce the impact of truck traffic (78). sideration should be given to liberalizing federal truck size This ARTBA vision was borne, at least partly, out of a sense and weight limits for some trade corridors. Due to uncertain- of global competition. China has a massive strategic trans- ties concerning potential impacts of liberalizing size and portation plan underway to build 68,000 km (42,000 mi) of weight limits, there was a consensus that any initiative would new interstate highways in 20 years, India is building 40,000 probably have to be in the form of a demonstration project km (25,000 mi) of expressways, and the European Union is with a well defined termination date and strong controls so adding nearly 16,000 km (10,000 mi) of new highway and rail that the project could be ended if necessary (77). capacity. The ARTBA initiative stresses that the United States is in a global economy and must also establish a competitive plan to meet future transportation needs. 3.3.11 The Bridge Perspective The Critical Commerce Corridors Proposal is intended to 18.104.22.168 Design Loadings address America's freight challenges and handle the expected doubling of truck traffic over the next 25 years. The 3C pro- The H and HS truck live-load model was adopted in 1944, gram would provide new surface transportation system capac- and since then, variations of this basic model (H15-44, ity and operational improvements focused on safe and effi- H20-44, HS15-44, HS20-44) have been the basis of the live cient movement of freight. A secondary use of these corridors load model used by designers of almost all bridges in the might be for evacuation purposes in times of national emer- United States. The "-44" indicates the series by year of adop- gencies or disasters. tion, 1944. The HS model consists of a two-axle truck plus a Financing of the program could come from dedicated and semitrailer with a variable trailer wheelbase of 4.27 to 9.14 m protected user fees levied on freight shipments and could (14 to 30 ft). The total weight on the first two axles of the H involve public-private partnerships and debt financing. The and HS trucks is designated (in tons) by the numeral follow- U.S.DOT would lead this effort in collaboration with public ing the H or HS designator, with 20% on steering axle and and private sector stakeholders. This cooperative public- 80% on the drive axle, with the weight on the third (trailer) private sector process would develop the costs and specific axle identical to that on the tractor drive axle. The gross vehi- components of the program. Components of this program as cle weight of an HS20-44 then is 32,658 kg (72,000 lb), or envisioned by ARTBA are: 36 tons, with 20 tons on the tractor and a 16 ton trailer axle. The HS20 live-load model also includes a uniformly distrib- · Most, if not all, of the existing Interstate highway system uted lane loading of 952 kg/m (640 lb/ft) plus a concentrated and a portion of the non-Interstate national highway load of 8.164 kg (18,000 lb) when checking moment, or system; 11,793 kg (26,000 lb) when checking shear. The live-load · New multimodal trade corridors; model uses the truck or lane loading that creates the maximum
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42 Figure 25. Comparison of simple span moments of Canadian design vehicle versus HS20. value of the load effect being checked. For example, when and 27 report the ratio of the simple span moments of the checking moments in a simple span bridge, the truck governs OHBD design vehicle and the HS20 design vehicle. The dif- for spans of 140 ft and shorter, while the lane load governs for ference, due to the most recent changes in the OHBD vehicle, longer spans. Most on-system bridges designed today in the is most noticeable in short span bridges (10 to 20 m), whereas United States are designed for HS-20 loading. the ratio to HS20 moments has increased significantly with Canadian bridges are designed for one of two live load the recalibration. models, the Ontario Bridge Design Code (OBDC) within On- Bridge design in other Canadian provinces follows the tario or the CSA in other Canadian provinces. The OBDC CAN/CSA-S6-06 Code. The design vehicle in this Code is cur- uses the Ontario Highway Bridge Design (OHBD) live-load rently the CL-625 5-axle vehicle with a gross weight of 625 Kn model, which is a 5-axle vehicle currently of gross vehicle (140,456 lb) and an 18-m (59-ft) wheelbase. There are minor weight 740 Kn (166,400 lb). This vehicle has evolved and has differences in the application of the design vehicle between the been calibrated to surveys of actual truck traffic, particularly CSA and OHBD codes, specifically in the way superimposed maximum observed overloads. Figure 25 compares the sim- lane loadings are handled, in the way impact or dynamic load ple span moments caused by the then current OHBD design allowances are handled, and in the load factors used in load vehicle with the HS20-44 design vehicle (79). factor design (LFD). The end result is that the two Canadian The current OHBD design vehicle is heavier after a recent design codes result in very similar design moments on sim- recalibration against current truck weight surveys. Figures 26 ple span bridges of a given span length, and the moments re- sulting from these two design codes are significantly greater 2.25 than those resulting from the AASHTO LFD design process 2.00 that uses the HS20 design vehicle. With the exception of short span bridges, with a span up to 15 m (49 ft), designed Moment Ratio 1.75 for CL-625 loadings, Canadian bridges are designed for sig- 1.50 nificantly greater loadings than U.S. bridges designed for 1.25 No Impact HS20 loadings. 1.00 With Impact The OHBD design vehicle is based on maximum observed 0.75 overloads, and is multiplied by a live-load factor of 1.40, 0.50 whereas the CAN/CSA-S6 design vehicle is based on regula- 0 10 20 30 40 50 60 tory loadings and is multiplied by a live-load factor of 1.60. Span (m) The HS20 loads are multiplied by a live-load factor of 1.67 in Figure 26. Ratio of unfactored design moments by the AASHTO LFD design procedure. current OHBD and HS20 design vehicles on simple It is observed in Figure 28 that the design loadings for spans, with and without impact. HS20, OHBD, and CL-625 are also different in the way in
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43 Ratio of Factored Moments 2.0 Moment Ratio 1.5 1.0 OHBD/HS20 0.5 CL-625/HS20 0.0 0 10 20 30 40 50 60 70 Simple Span (m) Figure 27. Comparison of factored design moments by Canadian design Loads on simple spans to HS20 factored design moments, with impact. which truck loads and lane loads are combined. In the case of that few bridges were constructed during World Wars I and AASHTO design with HS20 vehicles, truck loads are used II, and much of the current inventory dates from the 1960s, alone until lane loads with a single concentrated 8,164 kg when most of the Interstate highway system was under con- (18,000 lb) axle load take precedent. For simple spans, this struction. Figure 29 also shows the bridges classified as struc- means that truck loads govern for spans shorter than 42.67 m turally deficient as of May 2006. (140 ft), and lane loads govern for spans longer than this. Table 4 shows a summary of the design loadings used for Both the OHBD and CL-625 design vehicles are combined design of the nation's 682,482 bridges in the National Bridge In- with lane loadings (the loading may be governed by an unfac- ventory as of 2006 (80). Nationwide, about 14% of the nation's tored truck or a reduced truck combined with a lane loading). existing bridges were designed for HS15 or lighter loadings, For the OHBD, the combined truck/lane loading begins to with another 18% having "other or unknown" design load- govern at about 35 m (115 ft), and the CL-625 combined ings. Of all currently existing bridges in the nation, 45% are loading governs at even shorter spans. (See Figure 28.) designed for HS20 or a modified HS20 loading. However, only 3.30% are known to be designed for HS25 or heavier loadings. Most bridges built in the United States today are designed 22.214.171.124 Bridge Population in the United States for HS20 loadings, but this is not true for all bridges in the ex- The National Bridge Inventory documents the bridge pop- isting bridge inventory. As an indication, consider the follow- ulation of the United States (80). Data from that inventory ing statistics for the inventory of 49,593 bridges in Texas from are summarized in Figure 29, which breaks down the approx- the National Bridge Inventory data files (80). Seventy-four imately 597,000 bridges by year of construction. This shows percent of the 10,237 bridges built in Texas in 19902006 were Factored (LL+I) Moment (kN-m) 25000 20000 OHBD*1.4 15000 CL-625*1.6 10000 HS25*1.67 HS20*1.67 5000 0 0 10 20 30 40 50 60 70 Simple Span (m) Figure 28. Factored design moments by current Canadian and HS20 design vehicles on simple spans, with impact.