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They are designed as simple spans supporting equally spaced concentrated loads for a floor or roof system. These concentrated loads are considered to act at the panel points of the Joist Girders. These members have been standardized for depths from 20 to 120 inches (510 to 3,050 mm), and spans to 120 feet (37,000 mm).
For example, in designing a staircase, a dead load factor may be 1.2 times the weight of the structure, and a live load factor may be 1.6 times the maximum expected live load. These two "factored loads" are combined (added) to determine the "required strength" of the staircase.
Shear and Bending moment diagram for a simply supported beam with a concentrated load at mid-span. Shear force and bending moment diagrams are analytical tools used in conjunction with structural analysis to help perform structural design by determining the value of shear forces and bending moments at a given point of a structural element such as a beam.
The term binding joist is sometimes used to describe beams at floor level running perpendicular to the ridge of a gable roof and joined to the intermediate posts. Joists which land on a binding joist are called bridging joists. [4] [5] A large beam in the ceiling of a room carrying joists is a summer beam. A ceiling joist may be installed flush ...
The deflection at any point, , along the span of a center loaded simply supported beam can be calculated using: [1] = for The special case of elastic deflection at the midpoint C of a beam, loaded at its center, supported by two simple supports is then given by: [ 1 ] δ C = F L 3 48 E I {\displaystyle \delta _{C}={\frac {FL^{3}}{48EI}}} where
For example, a "W250x33" beam is approximately 250 millimetres (9.8 in) in depth (height of the I-beam from the outer face of one flange to the outer face of the other flange) and weighs approximately 33 kg/m (22 lb/ft; 67 lb/yd). [8] I-beams are still available in US sizes from many Canadian manufacturers.
Flat roofs may also need to be designed for live loads if people can walk on them. In the United States, building codes specify the loads in pounds per square foot which vary by region. The load and span (distance between supports) defines the size and spacing of the rafters and trusses. [4]
Figure 1: (a) This simple supported beam is shown with a unit load placed a distance x from the left end. Its influence lines for four different functions: (b) the reaction at the left support (denoted A), (c) the reaction at the right support (denoted C), (d) one for shear at a point B along the beam, and (e) one for moment also at point B. Figure 2: The change in Bending Moment in a ...