Comprehensive Truck Size and Weight Limits Study: Comparison of Results Report

Chapter 6: Bridge Comparative Analysis

6.1 Purpose

The purpose of this section is to compare principal results of the Bridge Comparative Analysis with other similar studies available in the literature. This involves two main objectives. First, those documents summarized in the revised desk scan that contain quantitative results pertaining directly to bridge analysis (i.e., the main objectives of the current 2014 CTSW Study) are identified. Second, the results from each of the selected documents are reviewed and objectively compared with the results of the 2014 CTSW Study.

6.2 Comparison of Bridge Study Findings

6.2.1 Structural Impacts Due to Overweight Trucks

6.2.1.1 Strength Limit State

The results of studies of impacts to bridges in terms of the strength limit state due to overweight trucks have been presented as the dollar value of resulting bridge replacements. Due to the variety of loadings (truck configurations studied), analysis methods used, roadway and bridge networks considered, etc., the direct comparison of reported bridge replacement costs do not yield meaningful results. The limited comparison presented in Table 6-1 is focused on the scale of study, the analysis approach, and the truck types investigated in two previous studies, compared to the 2014 CTSW Study.

Table 6-1: Major Bridge Study Analyses
Name of Study	USDOT Comprehensive Truck Size and Weight Study, 2000	Wisconsin Truck Size and Weight Study, 2009	USDOT Comprehensive Truck Size and Weight Limits Study, 2014
Scale of study	Used NBI data to screen bridges from 11 states.	85 bridges including 25 slab bridges, 25 pre-stressed girder bridges, 25 steel bridges, and 10 specialty bridges.	500 representative bridges taken from eleven states, representative of the bridges on the national networks and comprised of the twelve most common bridge types spanning from less than 50' to over 500'.
Analysis Approach	Used the WINBASIC program to analyze idealized (not real) bridges and compared results.	Used SEP analysis to record the maximum vehicle weight allowable on the 85 bridges.	Used AASHTO ABrR (VIRTIS) analysis program to conduct LRFR ratings
Types of Trucks	Base Case Uniformity Scenario North American Trade Scenario Longer Combination Vehicles Nationwide Scenario H.R. 551 Scenario Triples Nationwide Scenario Trucks varied from 3 axle 54 kips GVW up to 9 axle 148 kips GVW	6-axle 90 kips GVW 6-axle 98 kips GVW 7-axle 97 kips GVW 8-axle 108 kips GVW 7-axle 80 kips GVW (6 axle and Pup with 98 kips GVW)	3S2-80 kips Control Vehicle 3S2-88 kips Scenario 1 3S3-91 kips Scenario 2 3S3-97 kips Scenario 3 2S1-2-80 kips Control Vehicle (28.5' trailers) 2S1-2-80 kips Scenario 4 (33' trailers) 2S1-2-2 105.5 kips Scenario 5 3S2-2-2-129 kips Scenario 6

Results of 2014 CTSW Study:A threshold Rating Factor (RF) value of 1.0 establishes a potential need for bridge strengthening or replacement. The results are presented in Table 6-2.

Table 6-2: 2014 CTSW Study Bridge Results
Number of Bridges in the NBI				LOAD RATING RESULTS in terms of percent of bridges with posting issues (in need of strengthening or replacement)
# of IS Bridges in the NBI	# of Other NHS Bridges in the NBI	# of IS Bridges Rated	# of Other NHS Bridges Rated	Vehicle Configuration	Vehicle Configuration	% of IS Bridges Rated with RF < 1.0	% of Other NHS Bridges Rated with RF < 1.0
45417	43528	153	337	Scenario 1	5 axle, 88 kips	3.3%	5.0%
				Scenario 2	6 axle, 91 kips	3.3%	7.7%
				Scenario 3	6 axle, 97 kips	4.6%	9.5%
				Scenario 4	5 axle, 80 kips LCV, 33' trailers	2.6%	3.0%
				Scenario 5	7 axle, 105.5 kips LCV	2.0%	0.9%
				Scenario 6	7 axle, 129 kips LCV	6.5%	5.6%

The desk scan reveals differences in analysis approach or methodology, etc. that render direct comparisons of structural impacts between the current study and previous ones untenable. The bridge team can note that various other studies included some of the same scenario vehicles. For instance, in the 2000 CTSW Study, the North American Trade Scenario featured a six-axle tractor-semitrailer combination weighing 97,000 lbs. This vehicle is essentially the same as the Scenario 3 vehicle in the 2014 CTSW Study. Similarly the Triples Nationwide Scenario (7 axle triple trailer, 132,000 lb. GVW) is similar to the current Scenario 6 vehicle (tractor with three 28' trailers, 129,000 lb. GVW).Similarly, the 6 axle 90,000 truck and the 6-axle 98,000 lb. truck featured in the 2009 Wisconsin Truck Size and Weight Study correspond fairly closely with Scenarios 2 and 3 of the 2014 CTSW Study. However, differences in analysis method, determination of control or base vehicles, governing threshold criteria, study limits (networks), presentation of results, etc. prevent direct comparison.

6.2.1.2 Bridge Fatigue Limit State

According to the results of the Desk Scan, it can be concluded that actual truck traffic closely correlates the effects of the fatigue design truck and that heavy traffic will not cause severe fatigue problems on steel girders with fatigue details of categories A, B and C. therefore, analysis focused on the categories D, E and E' (E-prime) will be more meaningful. Previous studies on overweight truck effects, have primarily been a product of state sponsored research using limited WIM data in accordance with the state's needs. Due to the variety of study purpose and needs, analysis methods, fatigue trucks, etc., there are not widely available results for direct comparison to the results obtained for the specific scenario vehicles considered in the 2014 CTSW Study. Only the 2003 Minnesota DOT "Effects of Increasing Truck Weight on Steel and Prestressed Bridges", (Altay et al., 2003) study evaluated the effects of increasing the legal truck weight on fatigue details categories E and E'. The results of this study can yield meaningful comparison with the 2014 CTSW Study and detailed compassions are listed in Table 6-3.

Table 6-3: Major Bridge Study Results
Study	2003 Minnesota DOT Study	2014 CTSW Study
Fatigue Trucks	54 kip Truck (HS15) 58 kip Truck 66 kip Truck	3S2-80 kip Truck 3S2-88 kip Truck 3S3-91 kip Truck 3S3-97 kip Truck 2S1-2-80 kip Truck (28.5' trailer) 2S1-2-80 kip Truck (33' trailer) 2S1-2-2 105.5 kip Truck 3S2-2-2-129 kip Truck
Bridge Data	4 span continuous bridge (Category E') 3 span continuous bridge (Category E) Multiple span continuous plate girder (Category C) 2 span continuous bridge (Category E')	Short span (42') simply supported bridge (Category E') Long span (133') simply supported bridge (Category E) 3 span continuous bridge (Category E) 5 span continuous bridge (Category E)
Results	Bridges that did not have E or E' details had infinite fatigue lives under all situations including a 10% increase in truck weight; bridges with category D or better details and with connection plates attached to both flanges are not as susceptible to fatigue. An increase in truck weight of 20% would lead to a reduction in the remaining life in these older steel bridges of up to 42% and a 10% increase would lead to a 25% reduction in fatigue life.	12% higher main axle weights result in an incremental 25 to 27% negative effect on fatigue life. The addition of the third axle to the rear axle grouping results in a negative effect on fatigue life on the order of 29 to 54%. A negative incremental effect on fatigue life will be up to 66% due to the closely spaced axles.

6.2.1.3 Service Limit State

Numerous transportation entities at state and national levels have conducted highway cost allocation studies (HCAS). The scale and breadth of these studies varied from urban settings to highway corridors, to state to region or national levels. Bridge costs were either studied separately or determined as a portion of the overall highway pavement costs as indicated below. Methods and means of conducting the cost studies depended on the purpose of the study and the availability of the data. Tables 6-4 and 6-5 are presented on the following pages. Table 6-4 includes HCAS conducted in the United States at national or state levels. Table 6-5 includes HCAS studies conducted in other countries at national levels. The final costs themselves are not depicted due to the disparity in the cost ranges. This disparity arises not only from the scale of the study, but also methods, purpose of the study, and the composition of the costs. For example would a corridor study devised to determine user fees (tolls) be comparable to a regional study looking for costs attributable to overweight trucks to establish permit fees and fines for violating weight limits. However, the bridge team has provided a listing of the methods, allocators and other parameters used for each study as applicable.

**Table 6-4: Highway Cost Allocation Studies (as applied to bridges) (US)**
Owner Agency/Year	Scale/Type of Study	Method	% of Cost Attributed To All Trucks	Key Allocators	Axle Load Power	Cost Category
2000 CTSW Study	National Study	Federal	-	VMTs used to distribute cost between truck types	NA	Pavement & Bridges
Arizona/2005	State HCAS	Federal- Hybrid	-	VMT	NA	Pavement & Bridges
Ohio/2009	State HCAS	Federal & AASHTO	35%	ESAL/LEFs	4th	Pavement & Bridges
Oregon/2013	State HCAS	Modified Federal	30%	VMT	NA	Bridges
New York/2013	Corridor HCAS	Federal	NA	WIM	NA	Bridges
District of Columbia/2010	Trucking Routes in City Limits HCAS	Modified AASHTO	41%	ESAL/LEFs	2.9th	Pavement & Bridges
South Carolina/2011	State HCAS	Fatigue Limit State	-	Stress levels in Deck Reinforcement or Pre-stressed Tendons	NA	Bridges
Vermont/Maine Pilot Study/2012	Interstate Corridor HCAS	Fatigue Limit State	-	Stress Levels in Weld Detail C	NA	Bridges

Notes:

AASHTO Method: the determination of ESAL factors (LEFs) to allocate accrued damage costs to different truck types

Federal/Incremental Method (as defined for bridges): the analysis of determining the cost of constructing bridges at design loadings (AASHTO H & HS Trucks) in 5 T load increments of 15 T, 20 T & 25 T Based on 1997 Federal State HCAS Method as formalized in NCHRP Report 495 (2003).

Federal Method: Variation and Refinement of Incremental Approach

Fatigue Limit State: The allocator is an AASHTO Fatigue Detail Category - (C), or deck reinforcement for which a remaining fatigue life is determined based on stress range in the element detail and the number of repetitions using Miner's Principles

ESAL - Equivalent Single Axle Load & LEF - Load Equivalent Factor - Also referred to as the "AASHTO" method

VMT - Vehicle Miles Traveled

**Table 6-5: National (Highway) Cost Allocation Studies (Foreign)**
Country	Method/Allocator (for Weight Dependent Costs)	Axle Load Power	% Attributed to Bridges	Cost Category
Australia	ESALs were used (by way of axle load factors - LEFs) to distribute Highway Costs. Bridge Costs were determined as a portion of total Highway Costs & PCUs used to proportion costs between truck types.	4th	15 %	Pavement & Bridge Improvements
Switzerland	ESALS (LEFs)	2.5th		Pavement & Bridge Maintenance Costs
Finland	ESALs were used (by way of axle load factors - LEFs) to distribute Highway Costs. Bridge Costs were determined as a portion of total Highway Costs & VKMs used to proportion costs between truck types.	4th	25%	Pavement & Bridge Maintenance Costs
Germany - 2 Studies	Game Theory (Known as the Maut Study) ESAL (LEFs)/VKMs (Ministry of Transport)	NA 4th	15%	Pavement & Bridge Maintenance Costs
Sweden	ESAL (LEFs)/VKM	4th	20%	Pavement & Bridge Maintenance Costs
Netherlands (Dutch Study)	ESAL (LEFs) (Does not separate out bridges)	2nd	NA	Highway & Bridges
UK	Truck Average Gross Mass (AGM)	NA	NA	Bridges

Notes:

PCU or PCE - Passenger Car Units or Passenger Car Equivalents

VKM - Vehicle Kilometers Traveled

6.2.2 Bridge Deck Deterioration, Service File and Preventative Maintenance:

The bridge deck subtask analysis in the 2014 CTSW Study was charged with investigating the potential effects of the proposed alternative configuration vehicles on bridge decks. Secondly, it was to investigate the measures owner agencies can take to maintain and preserve bridge decks and for what costs. The bridge team did not find correlative studies dealing with the effects of specific truck configurations (and loadings) or axle loads in quantitative terms on bridge decks. Therefore, a direct comparison of results with respect to the scenario vehicles cannot be made. However, the findings of the report indicate that more long term empirical research on the combined effects of truck axle loads and adverse climatic effects (such as chloride contamination and chemical attacks) is needed on bridge decks. The research then should be augmented with data-driven predictive deterioration models and life-cycle cost analysis methods.

In general, due to design considerations of reinforced concrete bridge decks, wheel loads were applied to localized areas of the slab to find the controlling loading condition. Studies simulated static or dynamic wheel loads. The variables in these studies were typically with respect to deck thickness, reinforcement size and spacing, girder support spacing, and simulating climatic conditions such as moisture and the long term effects of chloride use in cold climates.

Research studies on bridge decks have not investigated the effect of specific truck configurations or the dynamic effects of multiple wheel or axle configurations on the bridge decks in quantitative terms.