It should take about fifteen to twenty minutes to complete. It is helpful to be knowledgeable about both forces and moments and how they work and how to use the functions sine and cosine before reading this Instructable. First, the reaction forces at points A and D must be found. These reaction forces are the forces that the two supports at A and D exert on the truss in order to keep it stationary.
Step 1: Step 1: Naming Reaction Forces
These forces must be found first before the internal forces can be found. In order to get the reaction forces at the supports, a moment must be taken about A or D and the forces in the horizontal and vertical directions must add up to zero.
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Now that the external forces on the truss are known, one can solve for the internal forces within the truss. When solving for internal forces, it is easiest to start at one of the supports with the least amount of beams connected to it. Point A or D works for this, but for this example, Point A will be looked at first.
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Draw the forces that are applied at Point C. The external force at C, N, applies here like the force of the support did in Step 3. Since the force is going toward the rest of Beam AC, the beam is in tension.
Since the force is going toward the rest of Beam BC, the beam is in tension. Using this method of using sum of forces in the x-direction and y-direction being equal to 0, the rest of the internal forces can be found.
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The easiest way to do this after starting at Point A is to move from left to right across the truss. The final answers are shown below:. The left support is a roller support, which can only have a vertical y-direction force applied to it because it moves freely horizontally. You should always see a metal connector plate on both sides of a joint. 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. Since wood is a natural product, some knots are inevitable. But large knots weaken the member see Figure 3 , particularly if a metal connector plate is pressed into one.
If in doubt, refer the issue to a licensed structural engineer. Figure 3: Knotholes weaken web members. Plates can be torn off quite easily. Long trusses should be lifted with a spreader bar. 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 Damaged trusses are often patched hastily by construction crews.
I also suggest that they keep a copy of the engineering documents for when they sell the house.
Problem 426 - Fink Truss by Method of Sections
Damaged metal connector plates are one of the most common forms of damage. 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. Figure 4: Metal connector plates loose strength even when pulled only part way out. The normal repair is to install a plywood or OSB gusset plate, nailed in place. These are usually much larger than the metal plate being replaced. The size and nailing pattern will be specified by the engineer. Sometimes an engineer will specify a metal replacement plate with nails.
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.
This damage is not as obvious see Figure 5. 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. Several times I have passed by cracked lumber without noticing it, only to find it on my way back out of the attic because it moved when I put my hand on it to steady myself.
Take your time and look closely. Metal banding used to bundle trusses for shipping can also cut deeply into the lumber. Damaged lumber is usually repaired by splicing new lumber along the side sistering , but may also involve gusset plate repairs. Inspectors sometimes mistake this for truss damage. Bracing Buckling is what happens when you push on the end of a yardstick and it bows sideways.
Internal web members of trusses are just like big yardsticks, and long web members that are under compression can buckle. To prevent this, designers call for bracing on some web members. Sometimes the need for permanent bracing is indicated by a stamp or tag placed directly on the web member see Figure 6. Other times, you have to look at the truss drawings to know if permanent bracing is needed see Figure 7.
Figure 7 Who should design the bracing? 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. The truss designer will indicate on the drawings which web members need to be braced. 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. BCSI discusses many things, including both temporary and permanent bracing.
Temporary bracing is needed to keep the trusses from falling over like dominoes during construction. Bracing can be either continuous lateral restraint CLR or individual web member reinforcement such as T-reinforcement. Less obvious issues are discussed below. Note that some of these criteria are more stringent in BCSI than in the previous HIB91; the older the building, the less likely it will conform to modern standards. Bracing lumber should be 2"x4" stress-grade lumber unless otherwise specified by the designer.
Each connection should have at least two nails.
CLR must be connected to a fixed point in the building such as a shear wall or roof plane or it must be diagonally braced. This includes the top chords of the lower set of trusses in a piggyback configuration. CLR is not effective when the web pattern changes from one truss to the next see Figure 8. T-reinforcement should be used instead. T-reinforcement should be 90 percent of the length of the web member and nailed at 6" on center unless specified otherwise by the designer.
Figure 8: CLR is not effective when applied to trusses that do not have similar web patterns. Other issues related to bracing include: Bracing needs to be tight to the web member to be effective. CLR should be roughly centered in the span see Figure 9. Note that some web members may require more than one CLR.
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