Sizing Engineered Beams and Headers
Once the loads acting on structural beams are calculated, the next step is to size and select the appropriate beam.
In Part 1, “Calculating Loads On Headers and Beams“, we learned how to trace load paths and translate roof, wall and floor loads into pounds per lineal foot of supporting beam. We know how to measure the forces acting on a beam, now we’ll use this information to choose the appropriate structural material to resist the loads. We will compare the performance and cost of sawn-lumber, LVL, Timberstrand, Parallam and Anthony Power Beam in several different applications.
Simplified Sizing Using Tables
No matter what material we specify, beams must provide adequate strength, stiffness, and shear resistance. Structural ability of sawn- and engineered-wood beams are predicted through mathematical calculation. Formulas that determine the allowable span and size of a beam rely on a host of variables like species, grade, size, deflection limit and type of load. You can do these calculations yourself or you can use span tables. Technical experts have computed many combinations of these variables and present a variety of solutions in the form of span tables.
Sawn-Lumber span tables are convenient tools. You merely look for the distance you need to span; match the load per foot of beam to the appropriate Fb(strength) and E(stiffness) values listed; and bang: you have a winner! Span tables are easy to use, but they have limitations. They don’t provide fine-tuned results. Most beam tables only list values for whole-foot spans like 11’0″, 12’0″, etc. And even though span tables provide limited data, they are very long. American Forest & Paper Association’s Wood Structural Design Data, provides span recommendations for solid-sawn wood beams up to 32 feet, but the table runs a hefty 140 pages. The WSDD is an extremely useful book (WSDD costs $20. Call 800-890-7732). Get it for your reference library. The WSDD tables only list values for solid wood beams at deflection limits of L/360. But you can trick WSDD tables into giving you values for double or triple 2-by beams with other deflection limits. Just do the following:
determine the total load per foot of beam
pick the span you want (pick 4’0″ for example)
select the Fb column of the lumber you intend to use
(in AF&PA Design Values for Joists and Rafters #2 hem-fir = Fb @1104 psi & E @1,300,000 psi— so use span table column Fb 1100)
choose the row for the size of lumber used in the double header: use 2×6 in this example. Note: a single 2×6 will support 347 pounds per lineal foot of beam. Therefore, a double 2×6 carries 2 x 347 = 694 pounds per lineal foot.
The required E-value does not change when you double the 2×6 because as you double the allowable load, you are doubling the thickness of the beam.
The table lists spans with a deflection limit of L/360, normal for floor loads. If you size a roof beam like a structural ridge that has a L/240 limitation, you would multiply the minimum E-value by 0.666 (785,000 x 0.666 = 522,810 in this case). For L/180 multiply by 0.5.
Make sure the shear value (Fv) for the species and grade you use exceeds the Fv listed in the span table. Fv does not change when you double the thickness.
Engineered Wood manufacturers are quick to point out that their products provide superior strength and stiffness. The claims are basically true, but you do pay for the improved performance. Strength-reducing characteristics like knots, grade and slope of grain are controlled during manufacturing process so that the end product represents a more efficient use of the wood fiber. Engineered wood is consistent from one piece to the next because each piece is made more-or-less the same. No matter what product you specify, structural performance is controlled by strength (Fb) and Stiffness (E). An LVL product that has an Fb of 3100 will carry more load than and LVL product with an Fb of 2400. So be careful when you compare products. All of these high-performance products are cost effective in some applications. And at times, they make or break a design.
Span tables for engineered wood are used in a very similar way as those for sawn lumber. Building codes allow reductions in live loads based on duration of load. For example a roof is subjected to a full snow-load only a small percentage of time during the course of a year, so this is factored into the roof’s load calculation. Usually, each manufacturer automatically applies these reductions and clearly labels the appropriate application in the various tables for floors and roof conditions. Be careful: some manufacturers require that you slope-adjust your roof loads. In other words, some manufacturers do not base roof loads on horizontal projection, but rather base loads on the actual length of the rafter. Look carefully at the literature before you assign roof loads per-foot of ridge beam or header. Typically shear values are incorporated into the tables, and required bearing length at the ends of beams are given too. Tables are limited to whole-foot spans, but the values can be interpolated for fractional lengths. The tables used to size engineered lumber are provided by manufacturers free of charge.
To size engineered beams and headers you begin with load per foot of beam. With engineered wood, you use both live load and dead load values. Live load determines stiffness and total load is used to determine strength. The sizing steps are:
determine the total load and live load per foot of beam
identify the type of load you are supporting (roof snow, non snow or floor)
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 required member will be listed.
There is an incredibly long list of options to consider when specifying sawn and engineered beams or headers. I have tried to simplify the process by choosing several popular materials and sizing them for a case-house. The applications and spans selected are arbitrary, but common. There certainly are many different loading scenarios than the ones demonstrated. You must verify the loading conditions for each application before sizing beams and headers. However, this exercise will give you a feel for how sawn-lumber, LVL, Parallam, Timberstrand, and Anthony Power Beam compare in various applications.
Using span tables, I have sized several structural elements for 2 climatic conditions. One set of elements is in a 50 pound snow-load climate and the other is in a 20 pound non-snow climate. Both loads are treated as live loads. The applications are: (see diagrams and calculations for each condition)
1) structural ridge beam with a 20-foot span
2) 2nd floor header with a 4-foot span
3) 1st floor header with an 8-foot span
4) basement girder with a 16-foot span
5) garage door header with an 18-foot span
Once I determined the loads, I sized and priced the beams that are required to carry the loads. I considered five different conditions, to see how the options compared to one another.
Sawn Lumber has it limitations. Its bending strength is often only 1/2 that of engineered wood products. As a result, it doesn’t clear-span long distances, comes in sizes only up to 2×12, and select structural grades are not always available. Select structural grades are special-ordered in many locations. Also, not every species is readily available. For example, Douglas-fir is difficult to buy in some eastern markets. But overall, for short spans, sawn-lumber is tough to beat.
Laminated Veneer Lumber (LVL) is strong, stiff and versatile. It spans long distances. I was able to use LVL for every application in the case-house. Typically, LVL comes 1 ¾” thick and ranges in depth from 7 ¼” up to 18″. To fine-tune the load-carrying potential of a LVL beam, just add another ply to the side of a beam. Labor is a factor. It takes time to laminate multiple layers of LVL. But the upside is that 2 workers can usually handle the weight of each lamination as it is assembled. LVL is carried as a stock item in most lumber yards and it is familiar to most building code officials and designers.
Anthony Power Beam (APB) is a relative newcomer to the structural beam market positioned to compete with LVL and Parallam. APB is a laminated beam product that comes in 3 1/2ö and 5 1/2ö widths to match standard 2×4 and 2×6 wall thicknesses. Depths range from 7 ¼” to 18″, matching standard I-joist depths. There is also a wider 7ö version available in depths up to 28 7/8″. APB requires very little labor because is comes “fully assembled”, but it is fairly heavy. The 18-foot garage header for our house weighs in at 380 pounds. APB is a new product and its penetration is somewhat limited so you may have to look for a local supplier. Call Anthony Forest Products direct to find a distributor.
Parallam, manufactured by Trus Joist MacMillan (TJM), virtually defines the term: parallel strand lumber (PSL). PSL 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 1 ¾” – 7″ are available in depths of 9 ¼” – 18″. Parallam dimensions are compatible with the other engineered wood products like I-joists and LVL. Parallam has been around for a while, but still — not all sizes are available in all regions. It is best to plan your design well ahead of schedule. Like APB, Parallam comes fully assembled and is comparably heavy. It is a good choice for long clear spans where sawn lumber is impractical.
TimberStrand FrameWorks Header, a laminated strand lumber (LSL) made by TJM, is the latest entry into the structural header and beam competition. LSL 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 APB, LVL and PSL, but the performance of TimberStrand is impressive. It worked for most of the applications in our case house. It is worth noting that the 18-foot garage-door header application pushed TimberStrand beyond its structural limit. TimberStrand Header comes only in 3 ½” widths in depths that range from 4 3/8″ to 18″. This product is new and distributors don’t want to stockpile inventory. It is a cost-effective option for many applications, but it can be very hard to find.
Comparison of Products
Table 1 consolidates loading, sizes and cost data for all of the applications. Header spans are typical for a window and a patio door. The structural ridge span represents the size of a large family room. The span for the girder is based on the size of an average-sized game room. And the garage door header is based on a 2-car garage-door opening.
All illustrations are provided through the courtesy of the Journal of Light Construction.
Last updated: November 10, 2012