Precast circular manhole sections for drainage per AASHTO M199 (ASTM C478) positioned for DOT deep burial.
By Ron Thornton, P.E.
If you are a precaster who makes any products related to highway or bridge construction, you have probably at least heard of American Association of State Highway and Transportation’s Load Resistance Factor Design and HL-93 loading. If you produce box culverts, bridge beams, three-sided or arch bridges, and/or wingwalls, you have likely accepted LRFD as the new norm and are designing your products accordingly. However, if you are making manholes, inlets and utility vaults, you’re probably wondering what is going on with this new standard and why you have to change your designs that have served the industry well for many years.
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. AASHTO LRFD defines four limit states including strength, extreme event, service and fatigue.
To put the LRFD methodology into perspective, we need to take a trip back into the history of reinforced concrete design. Prior to the early ‘70s, concrete was designed using a method called allowable stress or working stress. Loads and forces, flexure, shear and axial were computed at service level and the resulting steel and concrete stress was determined and compared to code-mandated allowables. By today’s standards, the computation method was reasonably simple, which was good because the most advanced tool for performing these calculations at the time was the slide rule. In the ASD method, there is no distinction between types of loads.
During the ‘70s, as electronic calculators became more prevalent, more engineers began using the load factor design method. This method places different factors based on the type of load such as dead, live, wind, ice, 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 after 1999, ACI applied its research and started modifying some if its load factors and certain calculations such as crack control and rebar development. AASHTO, however, was moving in a completely different direction when it officially rolled out LRFD in 1993. 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 since 1993, it was not widely used until it was mandated for all federally funded bridges going into the design phase beginning in the fall of 2007. It is important to understand that the LRFD code is nearly 2,000 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 3.3 and 3.8. It is well beyond the scope of this article to describe what a reliability index is, but the above numbers translate to a failure probability range of roughly 2 in 10,000.Not only has LRFD changed our design methodology, but we now have a whole new load designation to deal with. Gone is HS20-44 that, like an old shoe, we have become comfortable with for so many years. We now have something called HL-93, which is not so much an actual load but rather a set of loads that includes a standard truck that actually is the same as an HS-20 vehicle, a tandem axle and a uniform lane load.
H-20 and HS-20 revolve around a 16,000-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-93 is geared toward determining the load effects from an entire axle and driving lane. This is an appropriate and logical approach for bridges but immaterial to the precast box sitting on the side of the road.
Getting back to our original question, is there any rational basis for converting precast inlet, manhole and vault designs from HS-20 to
HL-93? Arguably, the answer is no and for several good reasons:
• Nowhere in the nearly 2,000 pages of the LRFD specification are any of these products specifically referenced. 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.
• The HL-93 standard truck is identical to an HS-20 vehicle and the uniform lane load does not apply to spans less than 15 feet. 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.
• Typically, only two of the four limit states – service and strength – as required by LRFD apply to these units, which is essentially no different than traditional load factor design.
• The design and manufacturing of precast products is well covered by various ASTM standards. These standards define specific loading requirements for traffic conditions that are consistent with AASHTO standard specification and they reference ACI 318 for design methodology.
ASTM standards are universally specified for ready-mixed concrete and constituent materials, structural and 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.
Ron Thornton is Precast Concrete Association of New York’s executive director.