Built-up Beams and Shear Flow Transverse Shear Built-up Beams and Shear Flow

Built Up Beams Built up beams are made from two or more pieces of material joined together to form a single beam. Such beams can be constructed in a variety of shapes to meet structural needs and provide a larger cross section than ordinarily available. Figure 7-13

FG07_13.TIF Notes: Practice members“built up” from several composite parts in order to achieve a greater resistance to loads.

Box Beam Other examples are a wood box beam, constructed of two planks, which serve as flanges, and two plywood webs. Pieces are joined together with nails, screws, or glue, in a manner that the entire beam acts as a single unit. Box beams are also made from steel, plastic, and composites. Draw picture

Glulam Beam A Glulam beam is made of boards glued together to form a much larger beam than could be cut from a tree. Glulam beams are used in the construction of small buildings. Draw Picture

Plate Girder This is commonly used in bridges and large buildings. The girders consist of three steel plates joined by welding. They can be made in much larger sizes than available with wide-flange or I-beams.

Built Up Beams Built up beams must be designed so that the beam behaves as a single member The design calculations involve two phases Beam design as though made of one piece, looking at both bending and shear stresses Connections between the parts (nails, bolts, welds, or glue) are designed to make sure the beam behaves as a single entity

Built Up Beams Connections must be strong enough to transmit the horizontal shear forces acting between the parts of the beam To find these forces we use Shear Flow. Recall: F3

Shear Flow Shear Flow q is the horizontal shear force per unit distance along the longitudinal axis of the beam. Since the force acts along the distance dx, the shear force per unit distance is equal to:

Shear Flow Replacing dM/dx by the shear force V, and noting the first moment of area Q, we get the Shear Flow Formula.