By Eric Carleton, P.E.
Precast concrete manholes have been a critical component of our infrastructure for decades. They provide relatively maintenance-free access for workers, are strong enough to withstand soil loads at great depths, offer long lifespans and can be manufactured in an almost endless array of sizes and configurations.
Manhole cones provide a transition from larger diameter risers to smaller diameter risers. The majority of cones are used at the top of manholes to provide a transition from the 48 inch-diameter riser to a smaller access hole provided by the frame and cover at grade (Figure 1). Designers must understand the arch action concept to feel comfortable that compression forces – not reinforcing steel in tension – will resist loads from the soil. Below, concepts enabling designers to create structurally sound, cost-efficient reducing cones are described.
The most common reducing cone is used at grade, just below the frame and cover. Cones are produced in two shapes – concentric and eccentric. With a concentric design, the top access hole is placed at the middle of the manhole, requiring a portable ladder for access. In an eccentric design, the top access hole is placed to one side, where permanent rungs provide interior access. Both designs require the same engineering principles.
Here, we will concentrate on the eccentric cone seen in Figure 1. Figure 2 depicts a typical eccentric cone used deeper below grade for transition from a 60 inch diameter or 72 inch diameter riser to a 48-inch riser. Making the transition between two different size risers can also be accomplished with a flat reducing slab. However, reinforcing steel is required to resist tensile forces.
Archetypal design properties
Cones rely heavily on the concept of arch design. Arches have been used for centuries in masonry construction because vertical loads are transmitted to supports via compression (Figure 3). The arch provides a big advantage for concrete, which has very little tension capacity compared to its great compression capacity. Examples of arch design can be seen in lintels above windows and in bridges built by the railroad industry in the 19th century, many of which are still in operation.
Today, some smaller diameter circular concrete pipe is made and successfully installed without reinforcing steel because of the load transfer ability of the arch. A 48-inch diameter riser with a 5-inch thick wall made with 4,000-psi concrete is strong enough to withstand soil loads 500 feet below grade without reinforcing steel thanks to arch design. The load is transmitted via arch action as a compressive force to the walls at the centerline of the manhole and resisted by an equal soil load from the opposite direction (Figure 4). As a result, some manhole standards – including ASTM C478 – do not require tensile or sheer steel in the barrel sections. In fact, for 48-inch diameter manholes, only minimal hoop steel in the top and bottom is required for resisting stresses experienced during handling.
With an eccentric cone design, one wall is vertical and resists loads in the same manner as the risers that support the cone. The opposite wall is sloped to make the conversion from large diameter to small diameter. This geometry is where confusion on assumptions for design comes into play.
Taller cones with large openings will have a sloped wall that is almost vertical. In that instance, it’s obvious the compressive arch action will work. However, design of cones where the slope is more severe is not as obvious. One such case is a 24-inch-high eccentric cone reducing 24 inches in diameter, creating a 45 degree wall slope (Figure 5).
For this type of slope, consider Beam A in Figure 6. In an eccentric cone, this beam is not independent and not supported at each end. Instead, it is supported along the side of the beam for the entire length. And, most importantly, if we look at the cross sections in Figure 6, the beam is in fact curved as an arch where the load is transferred to the continuous supports on each side. Resistance (R) will be large, as it is with a 4-foot diameter riser.
In an eccentric cone, the concrete is arched, with strength dependent on the compression capacity where the arch meets the remainder of the cone. In a 1-foot length of a 12-inch wide beam as shown in Figure 6, the compressive strength of 4,000-psi concrete with wall thickness equal to 6 inches results in support for the beam of:
R=4,000 psi x 12 inch x 6 inch = 288,000 pounds.
Since R exists on each side of the beam, the total resisting capacity is 576,000 pounds. Thus, the load can be as much as 576,000 pounds per square foot. If a factor of safety of two is applied, the load can be as much as 288,000 pounds per square foot, more than enough to resist loads from trucks traveling 60 mph. This capacity is available without the use of reinforcing steel.
Both concentric and eccentric cones have been produced and successfully used for more than 40 years. Success is based on the fact that concrete in compression can handle heavy loads. Additionally, reducer cones are simply arches that transfer loads to supports via compression without the need for reinforcing steel in tension. To check with local precast concrete manufacturers regarding the specific availability of precast concrete manhole sections and geometry for your area, visit precast.org/find.
Eric Carleton, P.E., is NPCA’s vice president of Technical Services. A special thanks goes to Gary Munkelt, P.E., for his consultation on this article.