Calculating Loads on Headers and Beams

Understanding how loads are transferred through a structure and act on structural members is the first step to sizing headers and beams

by Paul Fisette – © 2005

Most builders automatically choose double -2 x 8 or -2 x 10 headers to frame windows and doors in every house they build. These headers work to support most residential loads and coincidentally keep the window tops to a uniform height. A neat solution, but is this an efficient and cost effective use of material? The same is true for beams like structural ridge beams and center girders. Too often builders gang together 2-inch dimension lumber to support roof and floor loads without considering other options. You can’t beat sawn lumber for most small window headers, but as spans and loads increase, stronger materials are a better choice. Sawn lumber limits design potential and in some cases just doesn’t work. Parallam, Timberstrand, Laminated Veneer Lumber and Anthony Power Beam are examples of alternative materials that provide builders with some exciting choices.

In this 2-part series we will review how sawn lumber and these engineered materials measure up as headers and beams. Part I will show you how to trace structural loads to headers and beams. Part II will review sizing procedures, performance and cost of these materials for several applications (see “Sizing Engineered Beams and Headers” for part 2).

Doing Work

The job of headers and beams is a simple one. They transfer loads from above to the foundation below through a network of structural elements. The idea behind sizing headers and beams is straight-forward: Add together all live loads and dead loads that act on the member and then choose a material that will resist the load. The beam must be strong enough so it doesn’t break (Fb value) and stiff enough so that it doesn’t deflect excessively under the load (E value). However, the process for sizing these structural elements can be complicated if you are not an engineer. Here is a simplified approach that will help you specify the appropriate material for many applications.

The first step is the same for sawn- and engineered wood materials: add up all the loads acting on a header or beam and then translate this load into terms of how much load each lineal foot of header or beam will feel. In beam-speak you say: this header must carry X-pounds per lineal foot. This translation is the key to any structural sizing problem. Armed with this information you can determine the minimum size, span or strength of the beam. Engineered wood components are sized using span tables that match various spans to pounds per foot of beam. For sawn-lumber you must perform mathematical calculations.

Loads are considered to be either distributed or point loads. A layer of sand spread evenly over a surface is an example of a pure distributed load. Each square foot of the surface feels the same load. Live and dead loads listed in the building code for roofs and floors are approximations of distributed loads. Point loads occur when a weight is imposed on one spot in a structure, like a column. The load is not shared equally by the supporting structure. Analysis of point loading is best left to engineers. We will consider only distributed loads. This will enable us to size beams for most common applications.

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Figure 1

Let’s trace distributed loads for several different houses. Assume that all are located in the same climate, but have different loading paths because of the way they are built. These examples illustrate how distributed loads are assigned to structural elements. Our sample homes are in an area where the snow load is 50 pounds per square foot of roof area (treat snow as live load). It goes without saying that in a warmer climate, the snow load probably would be less, so you need to check your code book for live loads and dead loads in your region. All loads are listed as pounds per square foot of horizontal projection (footprint area). (SEE FIGURE 1)

Headers

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Figure 2

Header Example #1

Here, each square foot of roof system delivers 50 pounds of live load and 15 pounds of dead load (65 psf total) to the structural support system. Remember, these loads are distributed uniformly over the entire surface of the roof. The exterior wall (and the headers within) will carry all loads from the mid-point of the house (between the supporting walls) to the outside of the house (including the roof overhang). The distance in this case is 12 ft+ 2 ft = 14 ft. So, each lineal foot of wall must carry the loads imposed by a 1-foot wide strip in that 14 ft region. In technical terms, the wall has a tributary width of 14 ft. From this we can readily see that each lineal foot of wall supports:

Conditions:

live load (snow):

50 psf x 14ft = 700 pounds per lineal foot

roof dead load:

15 psf x 14ft = 210 pounds per lineal foot

total load:

= 910 pounds per lineal foot

It is important to list live load, dead load and total load separately because live load is used to compute stiffness and total load is used to calculate strength.

pic3a
Figure 3

Header Example #2

This house is identical to our first example except it is stick-built. As a result, the live load, dead load and distribution of forces are different. Unlike the trussed roof, live load and dead load of the rafters and ceiling joists must be accounted for as separate systems. Since it is possible to use the attic for storage, the live load of the attic floor is set at 20 psf according to code.

Conditions:

live load (snow):

50 psf x 14ft = 700 pounds per lineal foot

roof dead load:

10 psf x 14ft = 140 pounds per lineal foot

ceiling live load:

20 psf x 6ft = 120 pounds per lineal foot

ceiling dead load:

10 psf x 6ft = 60 pounds per lineal foot

total load:

= 1020 pounds per lineal foot

pic4a
Figure 4

Header Example #3

Again, this house has the same width dimension, but it has 2 levels. Loads are contributed to the lower header by the roof, upper walls and 2nd floor system. The Architectural Graphic Standards lists the weight of an exterior 2×6 wall as 16 pounds per ft2. So an 8-foot tall wall weighs 8 ft x 16 pounds/ft2 = 128 pounds per lineal foot. The loads delivered to the header are:

Conditions:

live load (snow):

50 psf x 14ft = 700 pounds per lineal foot

roof dead load:

15 psf x 14ft = 210 pounds per lineal foot

upper level wall:

= 128 pounds per lineal foot

2nd floor live load:

30 psf x 6 ft = 180 pounds per lineal foot

2nd floor dead load:

10 psf x 6 ft = 60 pounds per lineal foot

total load:

=1278 pounds per lineal foot

Beams

Ridge Beam Example

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Figure 5 – This figure illustrates 2 structural elements: a structural ridge beam and a center girder. Both have a tributary area of 12’0″. The load per foot of beam is determined the same way as for headers.

Ridge Beam Conditions

live load (snow):

50 psf x 12 ft = 600 pounds per lineal foot

roof dead load:

10 psf x 12 ft = 120 pounds per lineal foot

total load:

= 720 pounds per lineal foot

Girder Example

The center beam carries half of the floor load, the partition load and half of the second floor load. Live and dead loads are given in the building code. The weight of the partition is listed in the Architectural Graphic Standards as 10 pounds per square foot.

B) First Floor Girder Conditions

1st floor live load:

40 psf x 12 ft = 480 pounds per lineal foot

1st floor dead load:

10 psf x 12 ft = 120 pounds per lineal foot

8-foot tall partition:

= 80 pounds per lineal foot

2nd floor live load:

30 psf x 12 ft =360 pounds per lineal foot

2nd floor dead load:

10 psf x 12 ft =120 pounds per lineal foot

total load:

=1160 pounds per lineal foot

In Summary

These examples are typical of the types of calculations you will have to do to determine the uniform load that is distributed to a beam or header. You must establish how much of a load each lineal foot of header or beam receives. The next step is to use the technical literature from any of the companies that make engineered wood components to determine span and beam size. They all correlate allowable spans to load per foot of beam. Span listings are based on allowable deflection, live load and dead load, which are all listed in your building code book. In part 2 “Sizing Engineered Headers and Beams” we compare cost and performance of some engineered wood products to sawn lumber.

All illustrations are courtesy of the Journal of Light Construction.

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Last updated: May 8, 2009