Cost-Effective Practices for Off-System and Local Interest Bridge (2004) / Chapter Skim
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From page 48... ...
46 CHAPTER FOUR BRIDGE REPLACEMENT ALTERNATIVES This chapter explores the various aspects of off-system bridge replacements. Included in the discussion are the design rules as they apply to the geometric and structural design of the bridges; experiences and opinions of bridge owners regarding decisions made in the bridge replacement process; an extensive discussion of literature relevant to off-system bridge replacements; a discussion related to innovative concepts for bridge replacements; and finally a section on the use of software, standard plans, and design aids to expedite the design and construction of off-system bridges.
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From page 49... ...
47 Following this information gathering process, several new bridge types were developed that met the objectives of county engineers. Additionally, standard solutions already in use were presented along with a brief discussion of the design and construction characteristics of each type.
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48 was economic and was based on the anticipated savings in bridge materials. The selection of proper structure types for low-volume road bridges was approached from the perspective of economics and durable choices for bridge decks, superstructures, and substructures.
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49 and have a minimum service life of 15 years. Bridge replacements should be in accordance with the latest AASHTO standards.
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50 TABLE 3 RECOMMENDED MINIMUM GEOMETRIC AND STRUCTURAL CAPACITIES FOR LOCAL RURAL ROADS (A Policy of Geometric Design of Highways and Streets 2001) Criteria Design Volume (vpd)
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51 widths and design loading for low-volume rural or urban bridges should be provided. Agency Survey Responses Agencies were queried to provide insights into their geometric design policies.
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52 In 1981, NCHRP published NCHRP Report 230: Recommended Procedures for the Safety Performance Evaluation of Highway Appurtenances, which outlined the crash test requirements for roadside hardware. At that time, NCHRP Report 230 procedures did not mandate the use of crash testing in the design of roadside hardware.
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53 0% 5% 10% 15% 20% 25% S tr uc tu ra l S te el R ei nf or ce d C on cr et e C on cr et e B ox C ul ve rt s S te el P ip e / A rc h C ul ve rt s P re st re ss ed C on cr et e T im be r O th er P er ce nt R an ki ng T yp e as M os t P re fe rr ed States Locals FIGURE 11 Structure type preferences from project survey. 0% 10% 20% 30% 40% 50% 60% S tr uc tu ra l S te el R ei nf or ce d C on cr et e C on cr et e B ox C ul ve rt s S te el P ip e / A rc h C ul ve rt s P re st re ss ed C on cr et e T im be r P er ce nt I nd ic at in g L oc al F or ce s T ha t W ou ld B e U se d States Locals FIGURE 12 Construction capabilities of local forces.
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54 higher-volume roads, namely structural steel and prestressed concrete structures. Local agencies on the other hand have a greater preference for reinforced concrete and timber bridges and steel pipe arches and culverts.
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55 States 35% Locals 30% 25% 20% 15% 10% 5% 0% T im be r D ec ki ng S te el D ec ki ng FIGURE 13 Bridge deck type preferences from project survey.
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56 State has a significant number of approved railings and a variety of choices for off-system bridges; non-NHS structures is their criteria for alternate railings. Examples of railings approved for use on non-NHS structures include thrie beam railings, double box beam curbless railings, timber railings for longitudinally laminated timber decks, timber rails on concrete decks, thrie beam transitions to timber rails, and standards for upgrading numerous existing bridge railings.
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57 0% 10% 20% 30% 40% 50% 60% C .I .P . C on cr et e S te el P il es & T im be r L ag gi ng S te el P il es & C on cr et eL ag gi ng T im be r P il es & T im be r L ag gi ng S te el P il e B en t T im be r P il e B en t States Locals FIGURE 15 Relative preference of bridge pier types.
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58 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% S he et in g S to ne F il l S tr ea m be d L in er s C on cr et e P av er s In cr ea se d C ov er States Locals FIGURE 16 Scour countermeasures employed by survey respondents. FIGURE 16 Scour countermeasures employed by survey respondents.
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59 others describe the use of locally produced commercial products. A short synopsis by material type is presented in the following paragraphs.
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60 structures constructed in Tallahassee indicated the following. Transverse post-tensioning to create an average stress of 1034 kPa (150 psi)
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61 (granular soil groups A-1 through A-4) with low plasticity are acceptable.
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62 stream. Additionally, state requirements were such that because of the new alignment, the bridge would have to be designed for a 100-year storm, whereas if the bridge alignment was maintained, the new structure would only have to convey flows similar to the existing bridge.
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63 The replacement structure is a precast concrete arch having a clear span over the stream of 18.3 m (60 ft) , a total length of 27.4 m (90 ft)
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64 The U.S. Bridge system is similar to the Acrow system in that prefabricated trusses are used; however, the method of fabrication and construction is significantly different.
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65 grid of the longitudinal stringers and intermediate diaphragms with the entire unit fabricated upside down. The slab is cast using formwork, which is hung from the beams.
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66 in the lower half of the system. This system can be made to act compositely with the concrete by drilling or torching regularly spaced holes in the beam web so that a concrete "dowel" is formed when the CIP concrete flows through the beam web hole.
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67 are a tangible benefit, whereas in high traffic areas such as the Middle Atlantic states, they are not a prevalent bridge type. Another economic advantage is that owing to their light weight they can be used on old substructures and be more readily fabricated and installed by local workers.
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68 lack of AASHTO standards (which have now been developed) for the design of such bridges.
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69 weighed approximately 1360 kg (3,000 lb)
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70 Faller et al.
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71 neers (USACE) , California Department of Fish and Game, U.S.
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72 al.
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73 Because construction of each unit is simple and inexpensive, the county can cast units during winter seasons or other down times in anticipation of scheduled or emergency bridge replacements. Additionally, because of the quick turnaround time for casting, these units are an attractive option compared with commercially precast or prestressed concrete products.
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74 bridge configurations, are composed of glass fibers and polyester or vinyl ester resins. These decks are available in two common configurations, pultruded tubes and sandwich systems.
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75 example, results in a more costly stringer because of the loss of composite action and reduced load distribution. However, such a deck requires no special pockets for shear connectors.
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76 Both sections [each weighing approximately 71 kN (16 kips)
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77 the system has been used include the rehabilitation of the Schroon River Truss Bridge in Warrensburg New York; a truss bridge with steel stringers in Harford County, Maryland; and in the Lewis & Clark Bascule lift bridge in Astoria, Oregon. In a significantly more involved project, an old steelstringer bridge with a timber deck was replaced with FRP-pultruded wide-flange shapes and an FRP deck ("Light but Strong" 1997)
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78 Bridge Elimination for Low Water Stream Crossings As previously noted, the focus of this chapter is to find new and innovative ways of constructing low-cost and lowmaintenance short-span bridges to replace aging and deteriorating structures. However, the new bridge will still have an initial expense and associated maintenance costs.
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79 roads. Roadway slopes should be gradual, a rational design storm should be used (i.e., a 5-year storm)
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80 consult with various material suppliers and fabricators regarding regional factors that might affect the selected bridge type. Consultation with the state agency may also reveal the existence of bridge standards very similar to the FHWA plans, but updated to current codes and the agency's preferences.
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81 PennDOT Standard Plans for Low-Cost Bridges (BLC Series) Through the years PennDOT has maintained a series of design aids intended to expedite the design process and result in standardized, economical designs for ordinary bridge structures.
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82 Finally, Standard Plans for Low Cost Bridges Series BLC-500 Span 90'–130' (1983c) are intended to replace what could be considered the longest simple spans for which standard designs are a reasonable solution.
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83 tion of beam spacing, span lengths, concrete deck types (normal or lightweight concrete) , and beam types results in more than 1,100 standard designs in the plan set.
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84 accordance with the AASHTO Standard Specifications for design loads of MS 18 or MS 22.5 (HS 20 or HS 25)
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85 precasting the beams remotely or on site. If desired, several sets of forms can be built to minimize total construction time.
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86 of off-system bridges. Although not specifically described herein, nor included in the reference list, a vast number of electronic and print resources are readily available that provide guidance in everything from foundation design, bridge inspection and rehabilitation, bridge hydraulics, and other areas.
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87 available in both the AASHTO Standard Specifications and LRFD versions and depending on the software version can work in either US or US and SI units (International System of Units)
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88 Standard Specifications, the software is an extension of the standard plans and allows for the design of bridges of almost any width and length with user-specified dimensions. Additionally, the program can be used to compute the inventory and operating ratings of existing steel structures.
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89 data that can include speeds in the workzone, accident rates, driver costs, vehicle operating costs, accident costs, and others, rehabilitation and replacement options that require various workzone lane closures can be explored. The construction costs can be considered along with user costs in selecting an optimum solution.
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