Joists are often doubled or tripled, placed side by side, where conditions warrant, such as where wall partitions require support. Steel joists can take on various shapes, resembling the Roman capital letters “C”, “I”, “L” and “S”. Joists must exhibit the strength to support the anticipated load beams and columns pdf a long period of time. In many countries, the fabrication and installation of all framing members including joists must meet building code standards.
Considering the cross section of a typical joist, the overall depth of the joist is critical in establishing a safe and stable floor or ceiling system. Many steel joist manufacturers supply load tables in order to allow designers to select the proper joist sizes for their projects. Standard dimensional lumber joists have their limitations due to the limits of what farmed lumber can provide. A common saying regarding structural design is that “deeper is cheaper”, referring to the more cost-effective design of a given structure by using deeper but more expensive joists, because fewer joists are needed and longer spans are achieved, which more than makes up for the added cost of deeper joists.
A ceiling joist may be installed flush with the bottom of the beam or sometimes below the beam. Jack Sobon called an “inverted sill” or with a “plank sill”. These joists land on a beam. Joists can have different joints on either ends such as being tenoned on one end and lodged on the other end.
A reduction in the under-side of cogged joist-ends may be square, sloped or curved. Typically joists do not tie the beams together, but sometimes they are pinned or designed to hold under tension. Trimmers take the name of the feature such as hearth trimmer, stair trimmer, etc. The outermost joist in half timber construction may be of a more durable species than the interior joists. This page was last edited on 29 December 2017, at 14:15. This I-beam is used to support the first floor of a house. The web resists shear forces, while the flanges resist most of the bending moment experienced by the beam.
American bridge and skyscraper work of the mid-twentieth century. These sections have parallel flanges, as opposed to the varying thickness of RSJ flanges which are seldom now rolled in the UK. Parallel flanges are easier to connect to and do away with the need for tapering washers. UCs have equal or near-equal width and depth and are more suited to being oriented vertically to carry axial load such as columns in multi-storey construction, while UBs are significantly deeper than they are wide are more suited to carrying bending load such as beam elements in floors. However, there has been some concern as to their rapid loss of strength in a fire if unprotected. Illustration of an I-beam vibrating in torsion mode.