In the last Practical Engineering column, we discussed how to trace load paths and translate roof, wall, and floor loads into pounds per lineal foot of supporting beam. With this information, you can then choose the right beam size and material to carry the load.
In this article, we’ll look at some more uniform-load case studies, then compare the performance and cost of sawn lumber with several engineered-lumber products in meeting these different applications.
Sample House
The five applications and spans that I have selected as examples of beams and headers are arbitrary but common ones (see illustration).
First, I worked out the uniform loads for each of the beams, following the procedure outlined in last month’s article. If you don’t follow the calculations, review that column. I’ve included calculations for two loading conditions. One design assumes a 50-pound snow load and the other is in a 20-pound non-snow climate (both loads are treated as live loads). The applications, as noted in the illustration and the chart below, are:
1) a structural ridge beam with a 20-foot span, as you might find in a cathedral-ceilinged master bedroom2) a second-floor header with a 4-foot span, which picks up roof loads only, since there is no ceiling in that room
3) a first-floor header with an 8-foot span, which picks up roof, wall, and second-floor loads
4) a basement girder with a 15-foot span
5) a garage-door header with an 18-foot span, which picks up loads from half the truss roof
Once I knew the loads, I sized and priced the required beams and headers five different ways, to see how the options compared with one another.
Beam Choices
There are countless choices when it comes to specifying a beam material. To simplify this presentation, I chose for comparison’s sake one species of sawn lumber, Doug Fir-Larch, and four engineered beam options.
Sawn lumber has its limitations. It doesn’t clear-span long distances (such as for the garage-door header), and select structural grades are not always available except by special order. Depending on the species and grade, its bending strength (Fb) values are often a half to a third that of engineered products. But overall, for short spans, sawn lumber is tough to beat. Note, however, that in the cases of the ridge beam and central girder, a triple 2x12 beam requires an intermediate post, which the engineered beams don’t require.
Laminated veneer lumber (LVL) is strong, stiff, and versatile. It easily spans long distances, and is modular like sawn lumber: To increase load capacity, you can just bolt on a second member. Labor has to be factored in, but on the positive side, two or three workers can often assemble a beam in place that would otherwise require a crane to place. LVL typically comes 13/4 inches thick and ranges in depth from 71/4 inches up to 18 inches.
Power Beam (APB) is a glulam (made by Anthony Forst Products) positioned to compete with LVL and Parallam. It comes in 31/2- and 51/2- inch widths; depths range from 71/4 to 18 inches. There is also a 7-inch-wide version available in depths up to 287/8 inches. APB comes as a full-size beam — that is, it requires no site lamination — so it is fairly heavy. For example, the 18- foot garage header for our sample house weighs in at 380 pounds.
Parallam (patented by MacMillan Bloedel (now Weyerhaeuser) is an assembly of long, thin strands of wood veneer glued together to form continuous lengths of beam. The wood fiber used is strong and stiff. Several widths from 13/4 inches to 7 inches are available in depths of 91/4 inches to 18 inches. Like APB, Parallam comes fully assembled and is relatively heavy.
TimberStrand is a “laminated strand lumber” header material (also by Weyerhauser now). It is made by upgrading low-value aspen and poplar fiber into high-grade structural material. The Fb and E values are certainly no match for LVL, Power Beam, and Parallam, but the performance of TimberStrand is still impressive. It worked for most of the applications in our case house, although the 18-foot garage-door header pushed TimberStrand beyond its structural limit. TimberStrand comes 31/2 inches wide in depths ranging from 43/8 to 18 inches.
Using Beam-Sizing Tables
I used the Wood Structural Design Data (available from the American Forest & Paper Association) to size the Doug Fir Larch beams for this article. Consisting primarily of tables that allow you to accurately size uniformly loaded beams and headers, this is a worthwhile book for any builder to have.
Engineered-lumber manufacturers will gladly send you their sizing guides for free. Engineered-wood span tables for uniform loads are used in much the same way as span tables for sawn lumber. The code allows reductions in live loads based on duration of load. Depending on which manufacturer’s literature you refer to, sometimes these reductions are already applied and sometimes not. Typically, shear values are incorporated into the tables, and the required bearing length at the end of the beam is given. Tables for most products are limited to whole-foot spans.
Sizing begins with pounds per foot of beam. But with engineered wood, unlike sawn-lumber tables, you use both the live and dead load values: Live load determines stiffness; total load determines strength. To size engineered lumber:
• determine the total load and live load per foot of beam
• identify the type of load you are supporting (roof snow, roof nonsnow, or floor) and choose the appropriate table.
• pick the span you need
• match the total load and live load values to the values listed in the tables. The thickness and depth of the member required to carry the load (or a choice of thicknesses and depths) will be listed.
No matter what product you specify, structural performance is controlled by bending strength (Fb) and stiffness (E). Builders who make up headers and girders out of multiple pieces of sawn lumber rarely think about these engineering design values. But when you start looking at engineered lumber design guides, it’s a good idea to be aware of them. Just as sawn lumber of different species varies in strength and stiffness, the same can be true of engineered lumber. An LVL product that has an Fb of 3,100 can handle a greater load than an LVL product with an Fb of 2,400. So be careful when you compare products.
