26 Oct. 2002|
Wood truss construction details,finish plywood table top,shelf designs - How to DIY
This article discusses wood roof trusses, including problems with manufacturing, damage, installation, bracing, alterations and partition separation. Links to the woods Truss Council of America Southeastern Truss Manufacturers Association Building constituent Manufacturers Conference and Structural.
Much of this information also applies to open-web floor trusses, although floor trusses have other unique issues not discussed here.
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Determine if homemade trusses are suitable wood truss construction plans for the project you're building. Sir Henry Wood trusses are widely used in single and multi family residen tial institutional agricultural and commercial construction. The earliest trusses in my area were built on-site in the 1950s on small, simple post-war houses (see Figure 1).
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CLR is not effective when the web pattern changes from one truss to the next (see Figure 8).
Wood is not a homogeneous It is a great deal stronger in The plans abbreviation central was created to stave off repetition and. Figure 1: Site-built truss from 1950sWith modern trusses, we’re no longer confined to building a roof with simple rafter spans. Trusses allow us to build larger structures with more complicated shapes for lower cost, using less lumber and at the same time usually eliminating the need for interior bearing walls. Damaged or improperly installed trusses are weak and may not support extreme snow or wind loads. Normally, the sheathing restrains the top chord against buckling, but the sheathing can be missing where intersecting ridge lines are created by one set of trusses resting on top of a lower set at right angles (also called a ‘valley set’). In this configuration, the lower set of trusses should still have sheathing, usually with a hole to provide access between the two parts of the attic.
For simple joints, the plate should be centered, although this isn’t always true for larger joints with multiple members coming together. Missing plates are almost always due to improper handling during construction (see Damage), but if there are no marks in the wood, then the plate was never installed. A girder truss could be a single truss, but more commonly it’s a multi-ply girder made up of several trusses.
As we’ve made houses larger, sometimes the trusses are too tall to be shipped on the highway as a single unit. In these cases, the trusses are shipped in two sets, a lower set with a flat top chord and a smaller triangular set that rests on top of the lower trusses (commonly called piggyback trusses). DamageTrusses are incredibly strong once they’re installed as a system, but individual trusses are surprisingly fragile and construction crews often don’t treat trusses with the respect they deserve. Just like other trusses, piggyback trusses are designed with a specific bearing point, which is almost always at the end (see Figure 11). In high wind zones, the top section of piggyback trusses may need to be strapped down to the lower set; toenailing may not be sufficient. If lifted improperly, trusses will bend sideways like a piece of spaghetti, resulting in plates that are torn out, plates that are buckled and broken web members. Perhaps the crews in my area have just gotten sloppy, but I find some truss damage in about one-forth of newly constructed houses, whereas damaged trusses are fairly rare in older houses (before about 1990). Conventional framing is often mixed with trusses, particularly to create intersecting gables (often called overframing).
At the resulting valley, you’ll want to pay close attention to how the conventional framing rests on the truss system below.
The conventional rafters should rest on a bearing plate to distribute the load to the adjacent trusses. Repair is not as simple as hammering the plate back in because the fingers on the plate are designed to be pressed one time into undisturbed wood.
As previously discussed, any alterations to a truss must be backed up by engineering designs.
Truss members commonly get cut during installation of whole-house fans, drop-down stairs, fireplace chimneys and recessed light fixtures. Truss members also get cut during installation of rooftop vents (see Figure 13) or when installing mechanical and plumbing systems. As a truss is bent sideways, the metal plates on the outside of the curve are stretched or pulled out (as described above), but the plates on the inside of the curve are compressed and can buckle.
If the building configuration gets changed, then the truss must be altered to create a new bearing point.
If you find signs of damage in the accessible parts of the truss, you should also pull aside insulation to look for more damage to the lower chord.
Figure 5: Compression buckling of connector plate on truss that was bent during installation (left). Broken lumber in trusses can range from missing members to cracks that are almost invisible. The bottom chords of most trusses are designed to support only the dead weight of insulation and the finished ceiling. If you see a short piece of lumber added to one side of a top chord member with no visible damage on the other side, it’s likely this ‘scab’ was added to support the end of a piece of sheathing that didn’t quite reach the truss. In areas where the relative humidity changes significantly from summer to winter, the truss lumber will expand and contract as its moisture content changes. As a result, the bottom chord of the truss will rise and fall seasonally as it bows up and flattens back out.
Internal web members of trusses are just like big yardsticks, and long web members that are under compression can buckle. Other times, you have to look at the truss drawings to know if permanent bracing is needed (see Figure 7). In an older house, you won’t necessarily know if bracing was intended as permanent bracing or simply as temporary bracing unless the truss has a tag. Along with engineering guidance for designing a truss, that document also states that designing permanent truss bracing is the responsibility of the architect or engineer who designs the structure. Sometimes the truss drawings will also specify how to install the bracing, but often they will not. Instead, most building designers simply rely on footnotes on the truss drawings, which refer to industry standards for installation. Temporary bracing is needed to keep the trusses from falling over like dominoes during construction.