 From the Editors of The Journal of Light Construction
Introduction
Every good carpenter has a strong sense of structure – what framing details will create a sturdy building and which will feel flimsy or sag over time. Building on that instinctual knowledge with a fuller understanding of the engineering principles involved will make any tradesperson a better, more confident builder. That’s the goal of this book, which is adapted from articles originally published in The Journal of Light Construction. To that end, we’ve made every effort to translate the work of the engineer into practical principles and guidelines that builders can understand and put to use on the job site.
In addition, we’ve included the best framing articles from The Journal focusing on the tools, materials, and techniques that can bring greater efficiency to high-quality custom work.
To get the best information available, we sought out leading production framers – the real framing experts – to learn about the lightning fast techniques they’ve developed and honed to perfection and how to apply them to custom work.
Like all material in JLC, the information in this book comes directly from the field through practicing building and remodeling professionals with many years of job-site experience. So whether you’re a seasoned professional or a tradesperson just starting out, we think you’ll find the structural insights and framing pointers in these pages of real practical value.
Steven Bliss
Editorial Director
Section One
Structural Design
Chapter One
How To Read Span Tables
Wood is naturally engineered to serve as a structural material. The tree is fastened to the earth with its roots, the foundation; the trunk supports the weight of the branches, as a column; and it bends when loaded by the wind, as a cantilever beam. Wood's mechanical properties are complex, but if you understand a few basics of lumber strength you can easily size uniformly loaded joists and rafters with span tables.
Stiffness and Strength
A good set of tables includes a number of variables, the most basic of which are stiffness and strength. A house frame has to resist dead loads (the weight of materials), live loads (the weights imposed by use and occupancy), snow loads, and wind loads. Beams, studs, joists, and rafters must be strong enough and stiff enough to resist these loads.
Stiffness. A set of second-story floor joists can be strong enough to support all dead and live loads yet still be too bouncy. The joists won't break, but the first-story ceiling plaster may crack as the occupants walk across the second floor.
Stiffness requirements for joists or rafters are limited by their maximum allowable deflection, which is set by code. Deflection limits vary for different parts of the house and are based on the live loads experienced in each room. They're expressed as a fraction: the clear span in inches (L) over a specified number. Typical code-prescribed deflection limits are L/360 for all floors and any rafters with plaster on their underside, L/240 for rafters with drywall attached, and L/180 for rafters with no plaster or drywall. A floor joist that's appropriately selected to span 10 feet with an 1/360 limit will deflect no more than 1/3 inch (120 inches + 360) under its maximum design load.
The measure of a material's stiffness is "modulus of elasticity," or E. It's expressed in pounds per square inch, or psi. A material with a higher E value is stiffer. For example, No.2 eastern white pine has an E value of 1,100,000 psi, while No.2 hem-fir, which is stiffer, has an E value of 1,300,000 psi.
Strength is obviously important, too: Joists and rafters must be strong enough not to break when loaded. Strength is expressed as "extreme fiber stress in bending," or Fb (Figure 1).
Loads cause structural members like beams, joists, and rafters to bend. As a structural member bends, the wood fibers on its top and bottom edges are stressed more than the fibers along its centerline. The fibers along the top edge are squeezed in compression, while those along the bottom edge are stretched in tension. Fb is the design strength of those "extreme," or outermost, fibers; the higher the Fb, the stronger the wood.
How strong a structural member must be depends on the load it will carry. You can calculate the minimum design values required of a structural member by adding the live loads and dead loads carried by that member. The individual weights of drywall, strapping, floor joists, plywood, and carpet are listed in Architectural Graphic Standards and other reference books. But adding the weights of materials is rarely necessary except in unusual designs. The tables list a variety of average live and dead load combinations for floors, ceilings, and rafters. These combinations are more than adequate for most residential designs.
Other Considerations
Of course, stiffness and strength aren't the only factors that determine how a structural member responds to loading. That's why the tables also include several other variables. The ability to balance these lets you fine-tune a structure's cost and performance.
Depth of structural members. The deeper the joist or rafter, the more weight it can support. For example, 2x10 joists spaced 24 inches on-center often provide a stronger and stiffer floor assembly than 2x8s of the same grade and species spaced 16 inches on-center.
Lumber grade. A higher grade of a given species usually has a higher strength rating (Fb) and often a higher stiffness value (E), too.
Wood species. All species are not created equal. Southern pine, for example, is generally stronger and stiffer (higher Fb and E values) than spruce.
Duration of load. How long will the members be loaded? Full-time live loading (as with floor joists) serves as the benchmark value, so-called normal duration. Normal duration values are multiplied by 1.15 to yield snow-load values and by 1.25 for seven-day loading (explained below).
Over time, the load on a joist or rafter can cause it to bend permanently. This happens whether or not the load is continuous; the effect is cumulative. The normal duration Fb value assumes that, during its lifetime, a joist will be subjected to its full design load for a cumulative total of ten years. Using the normal duration Fb value for a given wood species ensures that the joist will not fail. In reality, actual loads on the joist are much less than the design loads. The cumulative effect of lighter loads drops off sharply as the load decreases, meaning that rarely are joists in danger of failure.
Likewise, the snow loading Fb value assumes that a roof will have to support the design snow load for a total of only two months during its lifetime. Snow load Fb values are increased 15% over normal duration values because shorter loading periods have less effect than loads of longer duration. This means, for example, that a 2x10 of a given species may have a higher assigned Fb value when used as a rafter than when used as a joist.
Seven-day loading assumes an even shorter loading period, and is applied in some code districts where there are no wind or snow loads on roofs. The "seven days" assumes that over the lifetime of the roof, construction workers may place full design loads on the roof for a cumulative total of a week – roofers storing shingles on the roof, for instance.
Calculations for normal duration, snow loading, and seven-day loading are automatically factored into the tables. You can apply them according to your local code.
What You Need
To use this information, you'll need three publications. The first is a building code book, which includes information about required grades, spans, bearing, lateral support, notching, etc. The One and Two Family Dwelling Code from the Council of American Building Officials (5203 Leesburg Pike, Suite 708, Falls Church, VA 22041; 703/931-4533) is a good choice. It has one appendix with span tables for joists and rafters and another appendix with design values for joists and rafters. Many local codes reference the CABO code as an acceptable option.
The other two publications are available from the American Forest & Paper Association (AF&PA, 1111 19th St. NW, Suite 800, Washington, DC 20036; 202/463-2700). They are Design Values for Joists and Rafters, which lists Fb and E values for various species, sizes, and grades of dimensional lumber, and Span Tables for Joists and Rafters, which assigns allowable spans to various combinations of E and Fb. I find the AF&PA documents easy to follow. And if you get stuck, the association's technical staff can help you. Western Wood Products Association (WWPA, Yeon Bldg., 522 S. W. 5th Ave., Portland, OR 97204; 503/224-3930) also publishes span tables. WWPA's tables are more flexible than AF&PA's, so some designers and engineers prefer them for calculating loads on complex structures. However, they're also harder to use, because they require the correct use of numerical multipliers. The AF&PA publications, by contrast, use a simplified approach that's suitable for most wood frame homes. This makes them a better tool for most architects and builders.
Sizing Floor Joists
Let's work through an example that illustrates the steps involved in using the tables. Let's say you're building a 16-foot addition and have to select the correct size and species of lumber for the floor joists. The joists will be 16 inches on-center. Their design span -- the exact length from face to face of the supports -- is 15 feet 1 inch (Figure 2).
Step 1: Check the Code
First, check the local code for allowable live load, dead load, and deflection (see Figure 3). For this example I'll use the CABO One and Two Family Dwelling Code, which serves as the model for many state and local codes. This sets an allowable first-floor live load of 40 psf, a dead load of 10 psf, and a deflection of L/360.
Step 2: Span Table
Select the appropriate table in Span Tables for Joists and Rafters. The table of contents indicates that Table F-2 watches these loading conditions. Using Table F-2 (Figure 4), check each lumber size to see if a 16-inch spacing will permit a span of 15 feet 1 inch. Start with the "16.0" line in the "Spacing" column at the left of the table, then go to the right until you reach an appropriate span (at least 15 feet 1 inch in this case). Then drop down to find the appropriate Fb value for that span.
As the table shows, no 2x8s meet the span and spacing requirements, but a 2x10 with an E of 1,300,000 psi and an Fb of 1093 psi can span 15 feet 3 inches - more than enough. A 2x12 with an E of 800,000 psi and Fb of 790 psi also works, since it can span 15 feet 10 inches.
Step 3: Wood Design Values
Now you must select a wood species and grade that meets the required Fb and E values, and that's available in your area. For this, use the tables in Design Values for Joists and Rafters. For this example, I've excerpted the relevant sections from tables for hem-fir, Douglas fir-larch, and spruce-pine-fir (Figure 5). In hem-fir, either a No.1 2x10 or a No.2 2x12 would work. In Douglas fir-Iarch, either a No.2 2x10 or a No.2 2x12 works. In spruce-pine-fir, a No.1&2 2x10 or 2x12 would do the job.
Step 4: Compression Check
The final step is to make sure the lumber you’ve chosen meets the required design value for compression perpendicular to the grain. The loads carried by floor joists, ceiling joists, and rafters are transferred through their end points to supporting walls and beams. The ends of these members must be able to resist these loads without crushing.
Table 9.1 in Span Tables for Joists and Rafters (Figure 6, previous page) gives a required compression value of 237 psi for a span of 16 feet and a bearing length of 1.5 inches. (The tables permit a bearing length of up to 3.5 inches, but since 1.5 is probably the worst case that you’ll encounter for joist or rafter bearing, it’s a safe value.) You can get the compression design value for various species selected from the addendum that comes with Design Values for Joists and Rafters. For instance, hem-fir has an acceptable value of 405 psi, spruce-pine-fir of 425.
Ceiling Joists and Rafters
Ceiling joists are sized like floor joists except that deflection limits vary depending on whether the joists will be used for attic storage or will have a plaster or drywall finish. Check your code and follow the AF&PA tables accordingly.
When using the tables to size rafters, there are two points to keep in mind. First, remember that the rafter’s span is not its actual length but its total horizontal projection (see Figure 7). Second, use the snow load value for your region in determining which rafter table to use. If your code book says your snow load is 40 psf, then you must use the 40 psf live load rafter table. The fact that snow loads only act part of the year has been taken into account in the rafter tables, but don’t forget to use the “Snow Loading” column to get the Fb design value.
By Paul Fisette, a wood technologist and director of the Building Materials Technology and Management program at the University of Massachusetts in Amherst.
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