In the world of construction products, there are always discussions between engineers, contractors or other industry parties about the strength of different products. To many people, products that don’t deflect under load are the strongest. We think of reinforced concrete walls or heavy steel beams when we’re asked to list construction elements that are strong.
However, an engineer working in the world of buried utilities might not see things the same way. An engineer working with buried structures might say that concrete walls and steel beams are elements that can add a lot of stiffness to buried structures, but that may not necessarily make them the strongest.
From an engineer’s perspective, stiffness is defined as a products ability to resist deflection. There is no question that a thick walled, rectangular box has a lot of stiffness. You can park a truck directly on top of the structure and it will show no visual effects from the load. By contrast, if you took a relatively thin walled corrugated steel pipe of the same size and parked a truck on top of it, you would distort and damage the pipe. It doesn’t possess nearly as much stiffness as the concrete box structure.
However, if you bury both of those structures with compacted structural fill and compared the amount of soil cover you could mound over each of the structures, you would likely find that the buried corrugated metal pipe could carry more soil cover than the rectangular concrete vault. By that measure, the corrugated metal pipe has more strength than the concrete vault.
The source of the apparent strength from the flexible structure as compared to the stiffer box structure comes from two primary sources: its geometry and its ability to engage and utilize the strength of the backfill around it.
On the geometry side, it’s well understood that arch structures provide a very effective means of carrying large, uniform loads. That’s because the arch converts much of the load placed over it into compression loads that are carried through the wall of the arch as opposed to beams that have to carry loads in bending. Carrying loads in compression allows the full depth of the structure wall to be engaged. Beams carrying loads in bending primarily engage the outer edges of the beam, leaving the majority of the center of the beam underutilized.
The other source of strength for the corrugated pipe is that it engages the strength and stiffness of the backfill around it. It was mentioned earlier that an unsupported corrugated steel pipe may be damaged by a vehicular load if it were parked directly on top of it. However, by burying the pipe with a competent backfill, when the same vehicular load is placed over the pipe it deforms slightly – thereby engaging the stiffness available from the backfill material around it. The combined stiffness of the metal pipe and backfill material is enough to support the load while utilizing a smaller, more efficient wall profile. That’s the basis behind all buried flexible structures, whether they are 6” diameter round pipes or 60’ span buried arches.
At Contech, we design our products to perform for the specific applications in which they are intended to be used. Some of the applications in which our products are used require a lot of stiffness in order to carry loads and perform properly. Conversely, where efficiencies are available by utilizing the strength of the backfill support around a structure, our products take full advantage of that as well. By offering a wide variety of engineered solutions – from reinforced concrete and tubular steel sections to corrugated metal, PVC or reinforced polyethylene pipes – Contech can always offer our customers a variety of cost effective solutions to meet their project needs.