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Sizing Engineered Beam

loadbearing

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.

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.

Case House

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.

engbeam_pic1b

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.

engbeam_pic2b

Considerations

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.

 

Understanding Lumber

loadbearing

Understanding Loads and Using Span Tables

Using span tables to size joists and rafters is a straight-forward process when you understand the structural principles that govern their use.

by Paul Fisette – ©2003

Wood is naturally engineered to serve as a structural material: The stem of a tree is fastened to the earth at its base (foundation), supports the weight of its branches (column) and bends as it is loaded by the wind (cantilever beam). A complete analysis of wood’s mechanical properties is complex, but understanding a few basics of lumber strength will allow you to size joists and rafters with the use of span tables.

Let’s start by taking a broad view. The structural goal of a house is to safely transfer building loads (weights) through the foundation to the supporting soil. Remember when your science teacher said: every action has an opposite and equal reaction? Well every building load has an equal “reaction load”. If, when the loads of the house are combined, the house weighs more than the soil can support – the house will sink until it reaches a point at which the soil can support the load. This article will focus on how simple beams like joists and rafters react to loading.

Residential Loading

The house acts as a structural system resisting dead loads (weight of materials), live loads (weights imposed by use and occupancy), like snow loads and wind loads. Beams, studs, joists and rafters act as a structural skeleton and must be strong enough and stiff enough to resist these loads.

Strength and stiffness are equally important. For example, first-floor ceiling plaster would crack as occupants walked across a second-floor bedroom that was framed with bouncy floor joists. Perhaps the joists were strong enough if they didn’t break! But lack of stiffness leads to costly problems.

Stiffness of structural members is limited by maximum allowable deflection. In other words, how much a joist or rafter bends under the maximum expected load. Only live loads are used to calculate design values for stiffness.

Maximum deflection limits are set by building codes. They are expressed as a fraction; clear span in inches (L) over a given number. For example: a floor joist appropriately selected to span 10 feet with an L/360 limit will deflect no more than 120″/360 = 1/3 inches under maximum design loads. Drywall attached to the underside of this system is not expected to crack when the floor joist system deflects 1/3″.

Typical deflection limits referenced in code books are L/360, L/240 or L/180. These limits are based on live loads and activities experienced in specific rooms of a house. Examples of code-prescribed deflection limits and live load values are:

  • Living room floors L/360 & 40 psf

  • Bedrooms and habitable attic floors L/360 & 30 psf

  • Attic floors with limited storage L/240 & 10 psf.

Strength of a material is obviously important. Joists, and rafters must be strong enough not to break when loaded. Unlike stiffness, live loads and dead loads are added together to determine minimum design values for strength.

To determine the dead load value for a given floor or roof system, the weight of all permanently installed materials in a given component are added together. For a floor system you can find the individual weights of drywall, strapping, floor joists, subfloor, underlayment and carpet in an architectural handbook like Architectural Graphic Standards. But for most cases there is a cookbook solution. Simply reference the Tables published by the American Forest & Paper Association’s (AF&PA), American Wood Council (AWC). AF&PA’s Appendix A lists a variety of live and dead load combinations for floors, ceilings and rafters. For example, Appendix A indicates that one type of clay tile roof system has a live load value of 20 psf and a dead load value of 15 psf.

Factors That Influence

Many factors influence how a system responds to loading. It is important to realize that the way you select and use materials will control costs and performance.

  • Depth of structural members. Often, 2×10 joists spaced 24-inches o.c. will provide a stronger and stiffer floor assembly than 2×8 joists of the same grade and species that are spaced 16-inches o.c.

  • E value or modulus of elasticity of the individual elements. E is a ratio that relates the amount a given load causes a material to deform. A material with a higher E value is stiffer. For example: No.2 grade eastern white pine has an E value of 1,100,000 and No.2 hem-fir has an E value of 1,300,000. Hem-fir is a stiffer material.

  • Fb value or extreme fiber stress in bending. Loads cause beams, joists and rafters to bend. As a beam bends the outermost (extreme) fibers are compressed along the top edge. And at the same time, fibers stretch along the bottom edge. The outermost (extreme) wood fibers on the top and bottom surfaces are stressed more than those fibers in the middle. An Fb value indicates design strength for those extreme fibers. The higher the Fb the stronger the wood.

  • Lumber grade. A higher grade of a given species has a higher strength rating (Fb) and often has a higher stiffness value (E) too.

  • Species of wood. All species are not created equal. For example southern pine is much stronger and stiffer than spruce.

  • Duration of load. How long will the members be loaded? Full-time loading (floor joists) serves as the benchmark value. Benchmark values are multiplied by 1.15 to yield snow-load values and by 1.25 for 7-day loading. Don’t worry about the calculations! Tables automatically handle this adjustment. You just read the numbers under the appropriate column heading. For example: A select structural, southern pine 2×8 floor joist has a 2650 Fb. While the same grade and species 2×8 has a 3040 Fb when used as a roof rafter in snow country. E values are unaffected by duration of load.


What You Need

Alright, so now you want to use this information. First you need to get a few things: Code book; AF&PA’s Span Tables for Joists and Rafters (this assigns allowable spans to various combinations of E and Fb); and a copy of Design Values for Joists and Rafters (this has Fb and E values for various species, sizes and grades of dimension lumber).

The code book can be purchased through your local code official. Building codes provide you with information about required grades, spans, bearing, lateral support, notching, etc. Purchase CABO One and Two Family Dwelling Code,5203 Leesburg Pike, Suite 708, Falls Church, VA 22041. CABO is referenced in most local building codes as an acceptable option to the local code. This code book has one appendix with span tables for joists and rafters and another with design values for joists and rafters.

The other publications I mentioned are referenced by most codes and can be purchased from AF&PA’s American Wood Council, PO Box 5364, Madison, WI 53705-5364, 1-800-890-7732. Or they can be ordered online at: http://www.forestprod.org/awc

These documents provide an expanded view of span-table use through “explanation” and “commentary” sections at the beginning and end of the publications. I find the AWC documents easy to follow. The technical staff at AWC is eager and able to help you understand the documents if you get stuck. You can contact the AWC Helpdesk at 800-AWC-AFPA (292-2372) or via email at awcinfo. Or visit their website at http://www.awc.org for more information.

There are other span tables and publications available too. Western Wood Products Association (WWPA) publishes tables, for example. But WWPA uses “base values” that make the job more complicated. Some designers may find WWPA’s tables useful. However, I think builders and architects are better served by AF&PA’s version.

PULLING IT ALL TOGETHER

Calculating Loads

For the most part, live load and dead load values for floor and roof systems are considered distributed loads. In other words, the weight is distributed or shared uniformly by the members in the floor or roof system. In order to establish proper sizes, grades and on-center spacing of joists and rafters you first need to determine what loading is acceptable to the building code.

Use your code book here. Look up the allowable loads and deflection limits imposed by your local code. For example: Massachusetts code book includes the following information.

Floors (joists)

Dwellings

live load (psf)

dead load

first floor

40

*

second floor

30

*

uninhabitable attics

20

*

* weights listed in code book appendix


Deflection

The code section on working load deflection states: The deflection of floor and roof assemblies shall not be greater than L/360 for plastered construction; L/240 for unplastered floor construction; and L/180 for unplastered roof construction. So these are the limits set by the code.

You can also use AF&PA’s “Span Tables for Joists and Rafters”. This is the easiest way to determine allowable dead loads, live loads and deflection limits. This publication has a much more extensive offering of possible joist and rafter conditions.

Once you find the appropriate table in the book, you determine acceptable Fb and E values for your particular span condition. Span is the distance from face to face of the supports.(for joists: from basement-side of sill to sill-side of center girder.)

Rafters

Rafters are sized the same way as joists: Establish live load, dead load and deflection limits; use the appropriate rafter table to determine acceptable Fb and E values; and then select the appropriate species, size and grade from AF&PA’s Design Values for Joists and Rafters publication.

Sizing rafters differs from sizing joists in 2 ways:

1) The span of a rafter is not based on the measurement along its length. Rather, the span is based on the rafter’s “horizontal projection”. This is the horizontal distance from the inside surface of the supporting wall to the inside surface of the ridge board. So consider a simple gable roof on a 24-foot wide ranch framed with 2×6 exterior walls and a 1 1/2 ridge: the span would be 11’5 3/4″.

2) You must determine the snow load for your region. This information is found in the code book. The snow load is treated as a live load when you use AF&PA’s tables. If your code book says your snow load is 40 psf, then you use the 40 psf live load rafter table. The fact that snow loads only act part of the year has been used to create the rafter tables.

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 “react” or resist these loads without crushing. AF&PA lists the required compression perpendicular to grain values for joists and rafters for various spans, on-center spacing and loading conditions in its Span Tables for Joists and Rafters. AF&PA’s Design Values for Joists and Rafters lists compression perpendicular to grain design values for a variety of species. Just be sure the species design value exceeds the required compression perpendicular to grain value for your structural condition.

SUMMARY

Step by Step

Here is a checklist of steps to follow when using span tables

1) check plans to determine span and on-center spacing (design conditions)
2) check codes for allowable live load, snow load, dead load and deflection
3) select appropriate span table
4) match span in table to design condition and determine minimum Fb and E values listed in the span table

  • NOTE: you will have options for on-center spacing and size

5) select appropriate species and grade from values listed in design values table

  • NOTE: you will have options regarding species and grade providing you with an economic opportunity

6) determine required compression perpendicular to grain design value in table
7) verify that the compression perpendicular to grain design value for the species selected in step 5 meets the required design value determined in step 6


EXAMPLE: A Test Case

Test your skill. 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 (see illustration – Figure #1)

Figure 1

image: figure 1

When sizing joists, use the clear span – the
length from support to support – not the full
length of the joist

Steps

Floor Joists

Step 1 Check The Code: First check the local code for allowable live load, dead load, and deflection (see Figure #2). 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.

Figure 2
Live loads and deflection limits are set by code.
These tables are from the CABO One and Two Family Dwelling Code.

MINIMUM UNIFORMLY DISTRIBUTED LIVE LOADS
Use Live Load
Balconies (exterior) 60
Decks 40
Fire escapes 40
Garages (passenger cars only) 50
Attics (no storage with roof slope no steeper than 3 in 12) 10
Attics (limited attic storage) 20
Dwelling Units (except sleeping rooms) 40
Sleeping Rooms 30
Stairs 40
ALLOWABLE DEFLECTION OF STRUCTURAL MEMBERS
Structural Member Allowable Deflection
Rafters with slope > 3/12 and no ceiling load L/180
Interior walls and partitions L**/180
Floors and plastered ceilings L/360
All other structural members L/240
Notes: L = span length, L** = vertical span

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 #3), 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 the span.

As the table shows, no 2×8’s meet the span and spacing requirements, but a 2×10 with an E of 1,300,000 psi and Fb of 1093 psi can span 15 feet 3 inches – more than enough. A 2×12 with an E of 800,000 psi and Fb of 790 psi also works, since it can span 15 feet and 10 inches.

Figure 3
Given a design span of 15 feet 1 inch and a 16 inch joist spacing, first determine which size lumber will work. Then find the required Fb value at the bottom of the column.

FLOOR JOISTS WITH L/360 DEFLECTION LIMITS
DESIGN CRITERIA:
Deflection – For 40 PSF live load.
Limited to span in inches divided by 360.
Strength – Live load of 40 psf plus dead load of 10 psf determines the required bending design value.
Joist Size
(in.)
Spacing
(in.)
Modulus of Elasticity, E, in 1,000,000 psi
0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6
2×6 12.0 8-6 8-10 9-2 9-6 9-9 10-0 10-3 10-6 10-9
16.0 7-9 8-0 8-4 8-7 8-10 9-1 9-4 9-6 9-9
19.2 7-3 7-7 7-10 8-1 8-4 8-7 8-9 9-0 9-2
24.0 6-9 7-0 7-3 7-6 7-9 7-11 8-2 8-4 8-6
2×8 12.0 11-3 11-8 12-1 12-6 12-10 13-2 13-6 13-10 14-2
16.0 10-2 10-7 11-0 11-4 11-8 12-0 12-3 12-7 12-10
19.2 9-7 10-0 10-4 10-8 11-0 11-3 11-7 11-10 12-1
24.0 8-11 9-3 9-7 9-11 10-2 10-6 10-9 11-0 11-3
2×10 12.0 14-4 14-11 15-5 15-11 16-5 16-10 17-3 17-8 18-0
16.0 13-0 13-6 14-0 14-6 14-11 15-3 15-8 16-0 16-5
19.2 12-3 12-9 13-2 13-7 14-0 14-5 14-9 15-1 15-5
24.0 11-4 11-10 12-3 12-8 13-0 13-4 13-8 14-0 14-4
2×12 12.0 17-5 18-1 18-9 19-4 19-11 20-6 21-0 21-6 21-11
16.0 15-10 16-5 17-0 17-7 18-1 18-7 19-1 19-6 19-11
19.2 14-11 15-6 16-0 16-7 17-0 17-6 17-11 18-4 18-9
24.0 13-10 14-4 14-11 15-4 15-10 16-3 16-8 17-0 17-5
Fb
Fb
Fb
Fb
12.0 718 777 833 888 941 993 1043 1092 1140
16.0 790 855 917 977 1036 1093 1148 1202 1255
19.2 840 909 975 1039 1101 1161 1220 1277 1333
24.0 905 979 1050 1119 1186 1251 1314 1376 1436

Note: The required bending design value, Fb, in pounds per square inch is shown at the bottom of each table and is applicable to all lumber sizes shown. Spans are shown in feet – inches and are limited to 26′ and less. Check sourcesof supply for availability of lumber in lengths greater than 20′.

EXCERPTED FROM SPAN TABLES FOR JOISTS AND RAFTERS, Copyright © 1993 AMERICAN FOREST & PAPER ASSN., WASHINGTON, D.C.

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 4). In hem-fir, either a No.1 2×10 or a No. 2 2×12 would work. In Douglas fir-larch, either a No 2 2×10 or a No. 2 2×12 works. In spruce-pine-fir, No. 1 7 2 2×10 or 2×12 would do the job.

Figure 4
After determining what size lumber to use, turn to the tables in Design Values For Joists and Rafters to select a species and grade that meets the required Fb and E values. The tables shown here are excerpts from the hem-fir, Douglas fir-larch, and spruce-pine-fir tables.

DESIGN VALUES FOR JOISTS AND RAFTERS
VISUALLY GRADED LUMBER

These Fb values for use where repetative members are spaced not more than 24 inches. For wider spacing, the Fb values shall be reduced 13%. Values for surfaced dry or surfaced green lumber apply at 19% maximum moisture content in use.

Species and Grade Size Design Value in Bending (Fb) Modulus of Elasticity (E)
Normal Duration Snow Loading 7 Day Loading
HEM-FIR
Select Structural 2×10 1770 2035 2215 1,600,000
No. 1 & Btr. 1330 1525 1660 1,500,000
No. 1 1200 1380 1500 1,500,000
No. 2 1075 1235 1345 1,300,000
No. 3 635 725 790 1,200,000
Select Structural 2×12 1610 1850 2015 1,600,000
No. 1 & Btr. 1210 1390 1510 1,500,000
No. 1 1095 1255 1365 1,500,000
No. 2 980 1125 1385 1,300,000
No. 3 575 660 720 1,200,000
DOUGLAS FIR-LARCH
Select Structural 2×10 1835 2110 2295 1,900,000
No. 1 & Btr. 1455 1675 1820 1,800,000
No. 1 1265 1455 1580 1,700,000
No. 2 1105 1275 1385 1,600,000
No. 3 635 725 790 1,400,000
Select Structural 2×12 1670 1920 2085 1,900,000
No. 1 & Btr. 1325 1520 1655 1,800,000
No. 1 1150 1325 1440 1,700,000
No. 2 1005 1155 1260 1,600,000
No. 3 575 660 720 1,400,000
SPRUCE-PINE-FIR
Select Structural 2×10 1580 1820 1975 1,500,000
No. 1/No. 2 1105 1275 1385 1,400,000
No. 3 635 725 790 1,200,000
Select Structural 2×12 1440 1655 1795 1,500,000
No. 1/No. 2 1005 1155 1260 1,400,000
No. 3 575 660 720 1,200,000

EXCERPTED FROM DESIGN VALUES FOR JOISTS AND RAFTERS, Copyright © 1992 AMERICAN FOREST & PAPER ASSN., WASHINGTON, D.C.

Step 4 Bearing 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 #5) gives a required compression value of 237 psi for a span of 16 feet and 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 perpendicular to grain 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 psi.

Figure 5
Check to see that the lumber species selected has the necessary compression strength perpendicular to the grain. This table, from Span Tables for Joists and Rafters, gives the required values for various design conditions; an addendum that comes with Design Values for Joists and Rafters gives the valies for specific species.

SPAN TABLES FOR JOISTS AND RAFTERS

Required compression perpendicular to grain values (Fc) in pounds per square inch for simple span joists and rafters with uniform loads

Bearing Length, in.
Span, ft. 1.5 2.0 2.5 3.0 3.5
8 119 98 71 59 51
10 148 111 89 74 63
12 178 133 107 89 76
14 207 156 124 104 89
16 237 178 142 119 102
18 267 200 160 133 114
20 296 222 178 148 127
22 326 244 196 163 140
24 356 267 213 178 152

Notes:
1) Bearing width is assumed to be 1.5″
2) Total uniform load is assumed to be 66.67 plf.
3) Alternate Fc perpendicular to grain values were possible by adjusting the tabulated values in direct proportion to the desired load.

1993 ADDENDUM TO DESIGN VALUES FOR JOISTS AND RAFTERS
Species1 Compression design value, psi. “Fc“perpendicular to grain
Douglas Fir-Larch 625
Eastern White Pine 350
Hem-Fir 405
Southern Pine, Dense 660
Southern Pine, Select Structural No.1, No.2, No.3, Stud, Construction, Standard, Utility 565
Southern Pine, Non-Dense 480
Spruce-Pine-Fir 425
Spruce-Pine-Fir (south) 335
1. Design values apply to all grades for the species listed unless otherwise indicated in the table above.
EXCERPTED FROM SPAN TABLES FOR JOISTS AND RAFTERS, Copyright © 1993 AMERICAN FOREST & PAPER ASSN., WASHINGTON, D.C.

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 #6). 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.

Figure 6

image: figure 6

Use the horizontal projection of a rafter, not
its actual length, when figuring rafter span

– See more at: http://bct.eco.umass.edu/publications/by-title/understanding-loads-and-using-span-tables/#sthash.rMdB0pIv.dpuf

Steel Beam

loadbearing

A steel I-beam is a type of joist or girder made from steel. I-beams are used as major I-Beam 10"- 25.4# Per Footsupport trusses in building, to ensure that a structure will be physically sound. Steel is one of the most common materials used to make I-beams, since it can withstand very heavy loads, although other materials, such as aluminum, are sometimes used. Composite I-beams are also available, with layers of other materials encasing the outside of the steel to disguise it as something else, such as wood.

The shape of a steel I-beam strongly resembles a capital “I” in cross section, which explains the name. It has a strong central core capped with flanges on either side. Various lengths of beam are available to suit construction project needs, and each beam also carries a rating, indicating how large it is and how much weight it is able to bear. When engineers are designing a structure, they determine what the load limits of the I-beams used in the structure should be.

PSL

loadbearing

Parallam® PSL Columns deliver our highest load capacity.

Parallam PSL columns are strong and consistent. Combine their great load capacity with the strength of Parallam PSL beams, and longer spans and open floor plans are possible. Parallam PSL columns can be stained or finished for a handsome look in exposed applications. They are an efficient use of wood resources and minimize waste.

Parallam PSL delivers on a host of fronts. Our manufacturing process uses veneer strands, allowing a significant percentage of each log to become a high-grade structural member.

For more information about product availability in your region, contact your local Trus Joist representative.

LVL

loadbearing

Laminated Veneer Lumber (LVL) is a high-strength engineered wood product used primarily for lp-lvl-product-4structural applications. It is comparable in strength to solid timber, concrete and steel and is manufactured by bonding together rotary peeled or sliced thin wood veneers under heat and pressure. LVL was developed in the 1970s and is today used for permanent structural applications including beams, lintels, purlins, truss chords and formwork. LVL can be used wherever sawn timber is used however one of the main advantages is that it can be manufactured to almost any length, restricted only by transportation to site.

Prior to lamination, the veneers are dried and the grains of each veneer are oriented in the same direction. This makes LVL stronger, straighter and more uniform than solid timber and overcomes some of timber’s natural limitations such as strength-reducing knots. This gives orthotropic properties (different mechanical properties against different axes) in a similar way to the properties of sawn timber, rather than the isotropic properties (the same mechanical properties in each direction) in the plane of plywood. The added durability of being an engineered wood product means LVL is less prone to shrinking or warping. LVL can also support heavier loads and span longer distances than normal timber.

Section sizes are then cut from 1200 m wide sheets or “billets”. The ability to cut different shapes from the LVL sheets allows for structural innovation using angular and curved shapes.

LVL provides a cost-effective and sustainable building material, delivering high structural reliability and strength.

Note: Some LVL members can be made with a few laminations laid up at right angles to enhance the shear strength of the member. These are known as Cross-Banded LVLs and may need to be specially ordered, as it is not a commonly stocked item.