Mechanics

Monthly Mechanics: Introduction to Septic Design

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So you want to design septic systems for a living? Your answer is probably “omg no.” But consider: almost every household in rural America lacks a sewer connection and needs to provide its own wastewater treatment. In Vermont, that applies to over 50% of homes – maybe 150,000 units in all. And the number of people licensed to design septic systems in the state is relatively tiny – about one hundred professional engineers, and a few hundred other individuals who have passed a qualifying exam.

Moreover, wastewater systems – unlike the houses themselves – are strictly enforced in all communities. It’s an issue of public safety. If your house collapses, you’ll only hurt yourself. But if you dispose of your waste improperly, you’ll poison the water supply, potentially sickening lots of people and wildlife for years to come. So those several hundred designers have a captive market.

Usually the system takes the form of a septic tank, where solids (aka POOP) are allowed to settle out, followed by a field where effluent (the remaining wastewater without solids) safely drips into the soil. The first step in designing a wastewater system is to run a percolation test, or perc test, to find out if the existing soil on site is sufficient.

Here’s how a perc test works. First, dig a hole 10 inches deep and 6-8 inches wide. Place 1 inch of crushed stone on the bottom. Then start your timer and pour 6 inches of water into the hole as steadily as possible. (Experts recommend using a siphon.) You need to record how long it takes for the water level to drop a certain amount, with the distance depending on the type of soil you have. As soon as the water level drops the required amount, refill to 6 inches. Repeat a total of 7 times. The percolation rate is the average speed at which the water level drops.

If the percolation rate is faster than a prescribed rate (often 120 minutes per inch), then you can say that the soil percs, and it may be possible to construct a leach field on the existing ground. There are other factors to consider, including ground slope and isolation distance, to determine if a site is appropriate as is. If the soil doesn’t perc, then a mound system is required, which means trucking in appropriate soil to a depth determined by the needs of the system.

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Every person living in a house is assumed to generate wastewater in the amount of 70 gallons per day. Since the number of occupants can change over time, the design flow for a house depends on how many bedrooms it has. In Vermont, the first three bedrooms count for two persons each, and additional bedrooms count for one person each. Thus, a three-bedroom house has a design flow of 6*70=420 gallons per day, and a four-bedroom house has a design flow of 7*70=490 gallons per day. This is the volume a septic designer must prove the system can handle.

Interested yet? Read about septic designer licensing at the Vermont ANR website.

Monthly Mechanics: Prestressed Concrete

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This is Part 4 of a series about building materials. Read Part 1, Part 2, and Part 3.

Note: On March 15 a prestressed concrete footbridge under construction at Florida International University collapsed, killing six. An investigation is underway, and I will comment on this tragedy when more information is made public.

If you read Monthly Mechanics regularly (thank you!), you might feel some déjà vu. We already explored reinforced concrete (and a little bit of unreinforced concrete) two months ago in Part 2. But prestressed concrete, while similar in appearance, behaves completely differently.

Recall that plain concrete is strong in compression and weak in tension. Reinforced concrete solves the mismatch by placing steel bars inside the formwork to resist tension once the concrete sets. Prestressed concrete attacks the problem a different way: by trading compressive strength for tensile strength.

How is that possible? Imagine putting a bathroom scale on the floor under a pull-up bar. If you stand on the scale, it reads your weight. (Duh.) Now if you grip the bar, you can take some weight off the scale (by pulling up), or add more weight (by pushing down). Prestressed concrete does the same thing. Instead of your body, prestressed concrete uses steel strands to put the concrete into compression and reset the “zero” point.

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Typical prestressed concrete cross-section

To make things even more confusing, there are two types of prestressed concrete: prestressed pre-tensioned, and prestressed post-tensioned. The difference is whether the strands are tensioned when the concrete is wet, or after it dries.

A pre-tensioned beam is cast around a network of parallel strands that have already been pulled tight, like violin strings. After the concrete sets, the strands are cut on the ends. The strands will then try to shorten to their original length, and since now they’re bonded to the concrete they pull the whole beam into compression.

For a post-tensioned beam, the concrete is cast around a series of PVC sleeves, leaving holes that run the length of the beam. Then, the strands are threaded through these holes, attached to the beam ends, and tightened. In this case they pull the concrete beam in from the ends, not its entire length, but the result is the same: the beam is in compression before it ever receives a structure load.

Here’s an example. A concrete beam has length L=10 feet (120 inches) and a structure load of P=500 pounds in the middle. We know that the bending moment in the middle is M=PL/4=15,000 inch-pounds. If the beam is 12 inches square, then the section modulus is S=123/6=288 in3 and there’s a stress of f=M/S=52.08 psi at the top and bottom fiber. Tension occurs at the bottom; compression at the top.

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If it’s a reinforced concrete beam, then the steel at the bottom of the beam does all the work in tension. A small portion of the concrete at the top of the beam resists the compression stress.

If it’s a prestressed concrete beam, then the beam starts out with a uniform compression stress of, say, 100 psi. Then the structure load is applied. The total stress is now 100+52.08=152.08 psi at the top fiber, compression, and 100-52.08=47.92 psi at the bottom fiber, also compression. In effect you’ve lost 100 psi of compressive strength but you’ve gained 100 psi of tension strength.

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Reinforced concrete beam (top) | Prestressed concrete beam (bottom)

There are high setup costs to manufacture prestressed concrete, but it’s an economical choice for many bridge decks and building floors. Typically the concrete is prefabricated in planks with a rectangular, hollow, or I shape, which makes them easy to work with on site with a crane.

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Prestressed concrete bridge beam under construction. (flickr – creative commons)

Monthly Mechanics: Wood

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This is Part 3 of a series about building materials. Read Part 1 (Steel) and Part 2 (Reinforced Concrete).

Wood is a convenient material for construction. It literally grows on trees, it’s soft enough to cut quickly with hand tools, and it accepts fasteners easily. That means wood framing has been around a lot longer than steel and concrete – probably as long as humans have built shelters. Wood is still one of the fastest materials to build with, especially with the advent of power tools, and has one of the smallest carbon footprints.

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The convenience of wood construction.

Foresters divide the myriad species of trees into hardwoods (deciduous) and softwoods (coniferous). Hardwoods are denser and best for fine woodworking like cabinetry. It’s softwoods, with their fast growth and straight grain, that are used for high-volume and high-strength applications like framing buildings.

Engineers also care about which wood species is used, but they care even more about the grade. Lumber is either visual graded (i.e. by actual people looking and feeling) or machine graded (i.e. by a mechanical stress test). The straightness of the grain, and the presence of knots (which are really just locations along the original tree trunk where branches grew), has a tremendous effect on the lumber’s structural properties.

If you can find a piece of dimensional lumber made from clear structural redwood, you can enjoy a bending strength of 1750 psi. At the other end of the spectrum is utility-grade eastern hemlock, with a bending strength of 175 psi – an order of magnitude weaker than the redwood. Your typical off-the-shelf dimensional lumber in New England is #2 grade southern pine, with a middle-of-the-road bending strength of 975 psi.

Engineers then adjust these strength values based on the geometry and environment of the structure they’re designing. Things like how big the lumber is, whether it’s braced along its length, whether it’s used on edge or flat, whether it’s repetitive (as in a series of floor joists spaced 16” or 24”), and whether it’s exposed to extreme temperatures or moisture all affect the safe strength to use for design calculations.

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Lumber stamp.

This Old House has a nice explanation here of how to read the stamp on a piece of dimensional lumber. The stamp tells you the species and grade, as well as the moisture content and the mill where it was cut.

For mostly archaic reasons, wood is rarely used for commercial construction in the US, but the tide may be turning. The development of stronger wood products like cross-laminated timbers (CLTs) – layers of lumber bonded together with the grain at right angles, sort of a super-plywood – enables wood construction to go far higher than the four or five stories at which dimensional lumber stick-framing tops out. (CLTs can have a bending strength of 2250 psi or more.) The tallest completed wood building is 18-story Brock Commons in Vancouver, and architects in Tokyo have plans to go as high as 70 stories, a true “plyscraper.”

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Cross-laminated timber fabrication.

In the US, where codes limit wood construction to 85 feet, the tallest wood building is a seven-story office tower in Minneapolis. But with advances in fireproofing, and the strength and stability of the newest wood products, the American Wood Council is lobbying for a change.

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T3 Tower in Minneapolis.