Utility structures and traffic loading simplified
By Alex Morales, M.Ed.
When underground precast concrete structures are used on a project, they are often one part of many components of the total project. Choosing a material for the access hatches for those structures, then, could become another detail. However, it’s a decision that carries a lot of weight, pun intended.
Access hatches come in a variety of performance strengths based on the type of material (e.g. aluminum, steel, cast iron) and the amount of it used to cast the hatch. Specifying the exact load requirement of a hatch will help both standardize the cost for owners and ensure proper performance throughout the life of the structure.
Planning for the unexpected
Although every effort is made to keep surface traffic away from underground structures, sometimes it is necessary. In a previous Precast Solutions article1, the issue of traffic-rated precast tank design was addressed, and concluded that the entire precast structure carries live loads (e.g. wheel loads) and must be properly designed to handle stated traffic ratings. That discussion was specific to wastewater treatment structures, however, and access points for them are usually designed for only pumping equipment to enter the tank. A proper traffic-rated circular lid does the job. When looking at underground precast concrete utility structures, access to the utility equipment by humans is expected, and a hatch is used rather than a lid.
Utility hatches are square or rectangular and come in various sizes. The strength of the hatch must coincide with the strength of the precast concrete in order for the entire structure to function properly and handle the anticipated live loads. The surface area of a hatch exposed to live loads can be greater than a standard lid. Therefore, the type of traffic (e.g. live load) is important to address for preventing a needless overdesign or, worse, an under designed hatch.
AASHTO traffic ratings
The American Association of State Highway and Transportation Officials (AASHTO) publishes specifications and guidelines in highway design and construction throughout the United States. The Standard Specification for Highway Bridges addresses load bearing requirements for manhole lids that had traditionally been applied to utility hatches. AASHTO has established the following categories for weight limits for regular vehicular traffic:
H20/HS20 = 16,000-lb. wheel load, 32,000-lb. axle
H25/HS25 = 20,000-lb. wheel load, 40,000-lb. axle
It is important to note that lids and hatches designed to meet these AASHTO loading requirements need to be tested to meet these minimums. If a specification requires H20/HS20 loading, it simply requires a cover meet the design load of 16,000 pounds per wheel without a safety or impact factor or dynamic load allowance.
AASHTO M306, “Standard Specification for Drainage, Sewer, Utility, and Related Castings,” was first published in 1989 and began requiring a 2.5 times safety factor in a proof load test, as stated in section 6.2 of the specification. A closer look at AASHTO M306, however, reveals the document specifically applies to cast iron and ductile iron castings. What about the other castings in the industry?
CSA load classifications
A single-wheel HS20 or HS25 footprint load analysis is appropriate for structures using hatches in highways that may experience truck or semi-trailer wheel loads constantly. But what about lanes of traffic where semi-trailer traffic is prohibited? Or in a drive-thru where large trucks simply can’t fit?
The Canadian Standards Association’s specification B481 Series-12 (reaffirmed in 2017) addressed these scenarios in load testing for lids of grease interceptors, which must be installed as close as possible to the sources of fats, oils and greases and many times end up in various traffic conditions. In an effort to standardize the lid and cover requirements for various traffic scenarios, Section 6.1.1 of B481 Series-12 requires that tops and rims be rated according to the Load Classification Table below:
In Table 1 of CSA’s specification, a reference to various live load classifications exhibits a keen understanding that not all grease interceptors are exposed to large, heavy trucks. An H20 or HS20 requirement would be an overdesign in a load classification H (light trucks) scenario, and even more so in a load classification M (light vehicular traffic) scenario.
ASTM International load levels
The same overdesign concerns have existed with utility structures. If a utility structure was placed near a stoplight in an area that was periodically mowed with commercial equipment, it would only be exposed to actual traffic wheel loads (in the event a vehicle left the actual roadway). If that same structure was placed in a field away from traffic, it may never be subjected to traffic-rated live loads aside from the periodic lawn mower. Would the hatches need to be specified precisely the same?
ASTM C1802-18, “Standard Specification for Design, Testing, Manufacture, Selection, and Installation of Fabricated Metal Horizontal Access Hatches for Utility, Water, and Wastewater Structures,” does not think so. This standard provides the designer added flexibility by ranking load levels from Load Level 1 for light pedestrian loads to Load Level 10 for special equipment loads.
With each increase in ASTM C1802 load level, the cost of a fabricated metal hatch will increase the total cost of a structure. If no specific hatch load is specified, an H20 load might be assumed, but chances are the owner will be looking at quotes of various load levels, a situation ripe for comparing apples to oranges. Specifiers of fabricated metal hatches for underground structures who define their requirements to the specific loading described within ASTM C1802 can not only ensure they are getting a hatch properly designed for their particular situation but can better control their costs as they avoid unnecessary overdesigns.
Alex Morales, M.Ed., is NPCA’s Director of Workforce Development.