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Live Load Analysis for Route Previewer
The RoutePreviewer determines the approximate load carrying capacity of a bridge for a permit vehicle by comparing the live load moments and shears for the permit vehicle against the live load moments and shear for the design vehicle specified in the FHWA National Bridge Inventory (NBI) data.
This method is approximate and is not a substitute for a complete load rating analysis using a structural model of the bridge. Although approximate, the method is intended to provide a more dynamic assessment of bridge capacity using data from the NBI along with the aforementioned live load analysis rather than using an “envelope” vehicle approach where gross vehicle weight is compared to bridge capacity.
Assumptions and Limitations:
- This is only an approximate weight analysis based solely on the comparison of live load moments and shears for a design vehicle and a permit vehicle. It is only meant to provide a general assessment of the ability of bridges along a route to safely carry a permit load. Ultimately a full detailed route analysis must be done according to the standards of the individual permitting agencies.
- The analysis performed herein is generally based on the operating level of analysis.
- Typically a factor of 1.67 (5/3) determines the difference between inventory and operating ratings for LFD analysis. Similarly a factor of 1.363 (.75/.55) determines the difference between inventory and operating for ASD analysis. Since this procedure is an approximate analysis and does not take into account the actual structural makeup or actual condition of the bridge, along with many other variables, the value of FACTOR used in the analysis, which is based purely on simple span live load analysis, is reduced from 1.67 to 1.50.
- The computed moments and shears are based on the maximum span length as stored in NBI item 48 since the NBI does not provide sufficient information on span arrangements or continuity.
- Live load moments used for the design and permit vehicles include impact and will be based on an AASHTO HS20 vehicle regardless of the actual design vehicle specified in the NBI for each bridge.
- The live load analysis only takes place for bridge types in NBI item 43 fitting the criteria in number 3 of Given Information (below).
Given Information:
- Defined permit vehicle
- Design vehicle/load from NBI item 31. As stated above (i.e. FACTOR=1.5), always use HS20.
- Maximum span length from NBI item 48 (MaxSpanLength).
- Bridge Type from NBI item 43. Currently the analysis is only performed for bridges where NBI item 43 meets the following criteria:
- digit 1 is a "1" thru "7"
- digits 2 and 3 are "01" through "08"
Procedure:
- Using a standard HS20 vehicle axle configuration, compute the live load moments (LLMDes) and live load shears (LLVDes) for a simple span of length MaxSpanLength.
- Using the permit vehicle axle information, compute the live load moments (LLMPer) and live load shears (LLVPer) for a simple span of length MaxSpanLength.
- Compute the moment ratio using the value of FACTOR as stated above:
- RATIO_M = (FACTOR * LLMDes) / LLMPer
- RATIO_V = (FACTOR * LLVDes) / LLVPer
- If RATIO_M > 1.0, PASS for Moment
- If RATIO_V > 1.0, PASS for Shear
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