According to the American Wood Council's Wood Frame Construction Manual, beams can cantilever up to 1/4 their span over a post. This design requirement is typically restricted to beams that sustain equal joist lengths in their span and cantilevered portions. The guide also states that if the load on the beam is not even, such as when it supports a single joist at one end and a pair of joists at the other, then the beam should be strong enough to support its entire weight plus any load on the unequal joists.
In practice, this means that the beam must have at least a 2-1/2 inch thick section where it contacts the post. The overall thickness required depends on how much material you want to save by using a thin beam. A 1-1/2 inch thick beam would be suitable for supporting about half its maximum expected load, while a 3/4 inch thick beam could support nearly its full expected load.
The type of wood used to build your beam should be considered when determining its strength. Beams made from light woods such as pine or fir may only need a 1/4 inch thick section to meet building codes. On the other hand, beams made from heavier woods such as maple or oak might need a thicker section to be safe. If you're not sure what size beam you need, ask an architect or engineer who knows these things.
Whether or not there is a cantilever, the stresses in the backspan part of the beam are evaluated. When supported by joists spanning a maximum of 14 feet, a southern pine (2x12) beam can span 8 feet and cantilever 2 feet beyond the posts on each side. The load on this beam is 0.5 pound per square foot.
If there is no joist supporting the end of the beam, then it becomes a free-standing frame member called a purlin. In this case, the load on the beam is 1 pound per square foot.
Purlins are used to attach roof sheathing or plywood to a building frame. The purlins are made of wood or metal. They are attached to the top of the frames with fasteners such as nails or screws. The ends of the purlins do not have any support so they can bend freely.
The total weight of the beam and the load applied to it is calculated by multiplying the load by the beam's cross-sectional area. In this case, the load applied to the beam is 1 pound per square foot. The total weight applied to the beam is 100 pounds. If the beam is 6 inches wide, then its mass is 606 grams. Its weight is about 30 pounds for a gross weight of 360 pounds. The effective load-bearing capacity of the beam is therefore 360 pounds.
According to the American Wood Council, cantilevers are limited to one-quarter the span of the joists. Joist Dimensions (o.c.) The maximum permissible overhang cannot be more than one-quarter the length of the actual main span. For example, if a house has 12-inch-wide joists and a 10-foot overhang, the total width of the cantilevered section must not exceed 30 inches.
The reason for this limit is that most buildings experience lateral loads that are carried by the end posts or abutting walls. If the load were distributed across a larger area, it could cause the beam to fail under its own weight.
For example, if a cantilever supports a heavy load near the end of its range of motion and no other support is available, then the beam will likely fail at an angle rather than in straight lines as required by the code. This happens because the load has been concentrated into a small area on the beam, which can't handle it. A similar problem would occur if a load was placed on a chair with no backrest!
In addition, if the beam is wide enough, it may push against another beam or wall instead of acting laterally. In this case, too, it would likely fail at an angle rather than in straight lines as required by the code.
Despite the fact that the beam will require three support posts, the single-ply beam may cantilever, allowing the deck to be enlarged to 17 feet, as seen in the plan image above. To interpolate the spans depicted in Table R507.1, multiply the depth by 3.5. The resulting number should be multiplied by the required post diameter to obtain the post height.
For example, if the depth is 12 inches and the required post diameter is 8 inches, then the post height would be 48 inches. This would allow for a single-ply beam to be used, since 96 inches is less than the maximum allowed span of 100 inches.
Beams with cantilevers can be more expensive because there's no way to use trusses to connect the beams together. Instead, each end of the beam must have a post attached to it. The total weight of these posts needs to be at least half the load that the beam is being used to support. In other words, if the beam is supporting a house, then both posts must be able to support at least half of the weight of the house.
The advantage of using cantilevered beams is that you can make the deck large enough to fit your needs. However, this comes with a price - the beam itself weighs more and requires more material.