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NOTE: This is an unformatted excerpt from our online fire protection training library.

Plan Reading and Symbols

Basic Terms

Beams are horizontal support units, typically holding up the roof or a floor of a building. Beams can be constructed of wood, steel or reinforced concrete. If a beam is concrete, it must be reinforced with steel tendons or members, as concrete has little tensile strength. That is to say, concrete in a horizontal plane has little ability to support any weight other than its own, and needs the steel to support the weight applied in a horizontal plane.

Joists are also horizontal support members and can be considered beams, but are typically lighter in construction than a beam. Thus, they cannot usually carry the same load that a beam can, and it will usually take more joist members to carry the load. Joists are often used to more evenly distribute a load, and are used in conjunction with beams to pass the load of the building to the columns. Joists can be solid, as in the typical floor joist in a house, or can be open web. Open web joists are typically called "bar joists" due to their construction using rolled steel forms between two horizontal members. The proper construction term is "parallel chord truss."

Trusses are horizontal support members usually, but have been used in the vertical position also in some structures. A truss will usually have an upper and lower "chord" and use the principle of triangles for strength. The triangles will be formed between the chords of the truss, and the load is shared by the members of the triangle. The triangle shape is one of the strongest, most stable support shapes, so a truss is typically strong, given its lightweight components. In many cases, a truss is equivalent in strength to a solid member, such as a solid joist, even though it is much lighter in construction. The more triangulation, the more strength. It is not uncommon to find three-dimensional trusses in some designs.

Columns are the basic vertical support member, and can also be constructed of steel, concrete or wood. In the vertical plane, compression of the column has to be resisted, and the purpose of the column is to pass the weight of the structure above to a solid foundation below, such as earth through or a footing.

Girders are beams of any construction that support other beams. It is not uncommon to see a roof assembly with beams under the roof, supporting the roof, but being supported by a second set of beams that run perpendicular to the beams directly beneath the roof. The lower set of beams, the "girders," are supported by the columns.

Purlins are also beams that are lightweight, usually channel-shaped. These are set at right angles to trusses or beams, directly under the roof deck in a lightweight steel building. They will be spaced relatively close together, perhaps as close as five feet on center, and supported by the beams.
NOTE: This is an unformatted excerpt from our online fire protection training library.

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