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5 CHAPTER TWO LITERATURE REVIEW Review of OS/OW permits may require evaluation of the level (i.e., the inventory or the operating rating) are used. bridges that the permit vehicle would cross before approval This formula should also be applied to all of the critical cross is granted. For those who are not familiar with bridge evalu- sections of the bridge component, and the lowest resulting ation, its concept and process are briefly reviewed here to LRF is taken as the LRF for that bridge component. In addi- provide the background needed for the following literature tion, for a bridge structure with a number of structural com- review. ponents, the same formula needs to be applied repeatedly to all the components of concern. The lowest of the resulting Bridge evaluation for permit review is very similar to LRFs for these components and failure modes is typically bridge load rating. Both are used to understand the bridge's taken as this bridge's LRF. This is an important index in the safe load carrying capacity, with the former targeting the per- jurisdiction's inventory for the particular bridge. It is also re- mit vehicle as the reference and the latter referring to some quired by FHWA, as included in the National Bridge Inven- standard vehicle loads meant to represent the general truck tory. traffic. Because of their common objective to understand bridge capacity, these two phrases or processes are some- When the LRF of the bridge is found to be 1.0 or higher times interchanged. for a standard vehicular load LL, the bridge is said to be able to carry that standard load. Alternatively, the bridge is also Bridge load rating in the United States is guided by the said to have a (safe) load carrying capacity equal to the AASHTO specifications Manual for Condition Evaluation of standard load's tonnage times the LRF. For example, if a Bridges (MCEB) (2000). The safe load carrying capacity bridge is found to have a LRF of 1.20 for the AASHTO here as the result of load rating refers to a load level that the standard HS20 live load (gross vehicle weight or GVW = bridge can safely carry, but not the ultimate capacity. 36 tons or 72,000 lb) as shown in Figure 1, it is said to have a capacity of 1.20 × 36 tons = 43.2 tons (86,400 lb). As seen The bridge load rating process is usually analytical (i.e., it in Figure 1, the HS20 load consists of the standard truck does not involve the physical testing of the material or the en- and the lane load. The lane load may induce a higher load tire bridge system). When estimating the quantities needed, effect than the truck depending on the span length. When such as the material strength and load effect distributed to the this happens, the larger load effect is taken and used in structural component, the AASHTO MCEB refers to the de- Eq. 1 as LL for load rating. sign specifications (Standard Specifications . . . 2002). This also reflects the conceptual consistency between the design The H20 load is another AASHTO standard load as and rating processes for highway bridges. The following load shown in Figure 2, which is sometimes also used as a refer- rating factor (LRF) is the result of bridge load rating for a ence when stating the load carrying capacity. Note that the bridge component with respect to a specific failure mode load carrying capacity in tonnage depends on the reference (bending moment, shear, etc.), according to the MCEB: standard vehicle load used, because the (vehicular) live load effect used in Eq. 1 is not proportional between different Load rating factor = (R - A1 DL)/(A2LL) (1) standard loads. For example, Figure 3 shows the maximum load effects (bending moments and shear forces) of simply where supported bridge spans for the AASHTO HS20 load, and R is the bridge component's resistance for that particular Figure 4 for the H20 load for comparison. Both are taken failure mode, and from the AASHTO design specifications (Standard Specifi- DL and LL, respectively, are the total dead and live (vehic- cations. . . 2002), which specify the HS20 truck GVW as ular) load effects in that component. 72,000 lb and the H20 truck GVW as 40,000 lb. However, the ratio of the two maximum load effects is not always 40/72 A1 and A2 are the dead and live load factors that cover possi- for every bridge span. This also illustrates that different per- ble uncertainty involved in estimating DL and LL. The spec- mit vehicles may induce different load effects in a bridge's ifications (MCEB 2000) give specific values for each de- components. Therefore, for heavier truck loads, bridge eval- pending on which limit state (i.e., the load factor method or uation is required to understand their individual effects to the the service load/allowable stress method) and which rating bridge and associated risk of bridge failure.
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6 FIGURE 1 AASHTO standard HS20 vehicle live load: (top) truck load; (bottom) lane load. As mentioned earlier, bridge evaluation for permit re- and critical cross sections are taken into account, the permit view also has the same purpose of understanding the vehicle is then considered to be permissible for that evalu- bridge's load carrying capacity, but for a specific target of ated bridge. the permit vehicle. Namely, it is to answer the question whether the bridge is able to carry the particular permit ve- In practice and literature, there have been many differ- hicle. Thus, bridge evaluation for overweight permit review ent phrases used to refer to bridge evaluation for permit re- typically replaces the standard vehicle load's load effect LL view. For example, "bridge load rating" is one term for the with the permit vehicle's load effect in Eq. 1. Similarly, if obvious reason of using the same equations and identical the bridge's LRF is 1.0 or higher, after all the components quantities. Other terms that were found to refer to bridge