manufacturers create products based on ASTM and organizations such as ASTM and ACI. . loads, refer back to AASHTO or ASTM C standards. Find the most up-to-date version of ASTM C at Engineering HS • HS – AASHTO Standard Specification. • A – ASTM C, C HL 93 AASHTO LRFD Specification. • HL – AASHTO LRFD Specification.
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First, what is LRFD?
It is intended as a means to provide a more uniform and consistent level for bridge safety under a wide range of load and resistance models. It is based on the statistical reliability of a structural element exceeding a given limit state.
A limit state is a condition beyond which the bridge or component ceases to satisfy the provisions for which it was designed. To put the LRFD methodology into perspective, we need to take a trip back into the history of reinforced awtm design. Loads and forces, flexure, shear and axial were computed axtm service level and the resulting steel and concrete stress was determined and compared to code-mandated allowables.
In the ASD method, there is no distinction between types of loads. This method places different factors based on the type of load such as dead, live, wind, c8900, etc. The combination of factored loads results in an ultimate load being placed on the element.
The ultimate load is then compared to the nominal strength of the element. Once the strength determination is made, the engineer has to go back and compute service load stresses, just like the ASD method, in order to check serviceability criteria for crack control.
While there may have been some differences in load factors and serviceability criteria between the AASHTO Standard Specification and the American Concrete Institute, the method of design and computations for many years were similar. However, sometime afterACI applied its research and started modifying some if its load factors and certain calculations such as crack control and rebar development.
Coincidentally, this is about the time that personal computers were becoming more commonplace, which is a good thing because the number of computations needed to complete an LRFD design increased exponentially from those required using ASD and traditional load factor design.
While the LRFD code has been in existence sinceit was not widely astmm until it was mandated for all federally funded bridges going into the design phase beginning in the fall of It is important to understand that the LRFD code is nearly 2, pages long and has been meticulously calibrated for every structural component specifically defined in the code to ensure that it meets a reliability index of between 3.
It is well beyond the scope of this adtm to describe what a reliability index is, but the above numbers translate to a failure probability range of roughly 2 in 10, H and HS revolve around a 16,pound dual truck wheel. This definition works well for precast vault and manhole designs because you generally apply one wheel over the structure.
By contrast, HL is geared toward determining the load effects from an entire axle and driving lane. Getting back to our original question, is there any rational basis for converting precast inlet, manhole c980 vault designs from HS to HL?
Arguably, the answer is no and for several good reasons:. Therefore, it is logical to assume that they have not been calibrated to the reliability index and yet there is little doubt that these products have a long history of being extremely reliable in service. Also, a simple analysis shows that the tandem axle does not control over the standard truck for spans less than about 14 feet.
Very few precast manholes, inlets and vaults fall into this category. These standards define specific loading requirements for traffic conditions that are consistent with AASHTO standard specification and they reference ACI for design methodology.
Does the LRFD Standard Work for all Precast?
ASTM standards are universally specified for ready-mixed concrete and constituent materials, structural awtm reinforcing steel, bolts and hardware, testing methods and many other products that are used in both highway and commercial construction. Likewise, specifying the appropriate ASTM standards for precast products rather than converting to LRFD is a much better way to ensure consistency, reliability, structural integrity and quality for any project.
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