Pinpointing exact hatch needs to control costs.
By Alex Morales, M.Ed.
Precasters can receive a wide range of responses when shopping for utility hatches. You, or the project owner, may have preferences about its design features or operation, but meeting in-service strength criteria is independent from mechanisms for opening and closing the door. It’s important to ensure you are comparing the exact same strength of hatches from different manufacturers with respect to the type and thickness of metal used that complies with project specifications.
Specifiers and owners need to be on the same page regarding the exact requirement of the hatch to help precasters bid more competitively. ASTM C1802-18, “Standard Specification for Design, Testing, Manufacture, Selection, and Installation of Fabricated Metal Horizontal Access Hatches for Utility, Water, and Wastewater Structures,” seeks to accomplish that goal.
Access hatches can be made of varying performance strengths based on:
✓ Type of material (aluminum, steel, cast iron, etc.)
✓ Amount of material used to manufacture the hatch (thickness)
✓ Added structural components within the frame and lids (frame configuration, added angle and channel shapes)
Specifying the exact load requirement of a hatch will help standardize the cost for owners and ensure proper performance throughout the life of the structure.
The load capacity of the hatch must coincide with the strength of the precast concrete unit for the entire structure to function properly and handle the anticipated live loads. The surface area of a hatch exposed to live loads, and the specific type of traffic, is important to address to prevent either over or under design.
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. AASHTO Standard Specifications for Highway Bridges addresses load bearing requirements for manhole covers that had traditionally been applied to utility hatches. AASHTO has established the following categories for design truck loads for regular vehicular traffic:
H20/HS20 = 16,000-pound wheel load, 32,000-pound per axle
H25/HS25 = 20,000-pound wheel load, 40,000-pound per axle
A single-wheel footprint load analysis is appropriate for structures using hatches in highways that 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 can’t fit? Although the industry standard for H20 loading is for highway traffic exposure, utility hatches are frequently found in areas where semi-traffic is prohibited. An H20- or HS20-rated utility hatch may be an overdesign for an application adding substantial unnecessary expense to the structure. Realizing this, some manufacturers may propose to install a hatch that meets the off-highway requirement of a particular application, while others may propose a hatch complying with H20 or HS20 requirements. Comparing these proposals will reveal a wide range in costs.
ASTM International load levels
If a utility structure was placed near a stoplight off the pavement in a grassy area that was periodically mowed with commercial equipment, it would rarely 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 occasional lawn mower or maintenance pickup truck. Would these need to be specified the same as hatches in or next to the roadway?
ASTM C1802-18, “Standard Specification for Design, Testing, Manufacture, Selection, and Installation of Fabricated Metal Horizontal Access Hatches for Utility, Water, and Wastewater Structures,” provides a variety of loading options to fit the actual condition. As seen in Table 1, the specification ranks load levels from Load Level 1 for light pedestrian loads to Load Level 10 for special equipment loads.
Load Levels 1-3 are all below the H20 loading of 16,000-pound duel wheel loads. They are designed for areas with only pedestrian traffic or lightweight vehicles, like golf carts or four-wheelers. An H20 loading requirement would be considered an overdesign in these applications.
Load Levels 5-7 include the 16,000-pound H20 wheel load as defined by AASTHO specifications. However, these load levels also include an impact factor that is not part of the AASHTO Bridge Design Specification requirement. The load levels are differentiated by whether or not the structure is placed in an actual two-lane highway. The impact factor and allowable deflection when loaded differentiates these load levels.
Above Load Level 7 are several levels for specialty situations, such as airport, or special construction and industry installations. In many of these instances, an H20 loading might be considered an under design.
With each increase in ASTM C1802 load level, the cost of a fabricated metal hatch will increase the total cost of a structure. You could be looking at a wide range of quotes if some fabricators are adhering to an H20 requirement, which doesn’t include an impact factor, and others are aware of various ASTM C1802 load levels. It’s a situation ripe for comparing apples to oranges.
Ultimately, the owner or specifier will determine what load level is required. While the H20 loading requirement is well understood, many owners don’t necessarily understand that a heavier, stronger hatch may also require a stronger, more expensive precast unit so the entire structure performs as intended. Precast structures requiring utility hatches could be overdesigned for lower ASTM C1802 load level applications, which increases costs and makes precast options less competitive.
Partner with suppliers
Precasters and hatch suppliers are wise to work together to educate specifiers on ASTM C1802 load levels. The best practice is to begin the hatch selection process in the project specification stage. This will ensure a structure properly meets the in-service requirements rather than needlessly overdesigning underground structures installed away from highway traffic loads. Specifiers who define underground structures that require fabricated metal hatches by load level can ensure they are getting a hatch designed for their particular situation and can better control costs while avoiding unnecessary overdesigns
Alex Morales, M.Ed., is NPCA’s director of workforce development.