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Built-up beams and columns are often composed of two or more lumber members fastened together side by side with bolts or nails. Built-up members are often used in situations where the loading is light and dimension lumber is used for secondary framing, thus simplifying the set of materials required for the structure.
Assumptions
The program assumes that there are no butt joints in a built-up member, except at beam supports.
Where possible, built-up beams should be top loaded and built-up columns loaded on the b-face. When side loaded, the load is transferred between plies and the fastening between them can be critical.
Except as noted below, cross-section properties are calculated as the sum of the properties of individual plies. For example, the section modulus of a multiply member Sn = S x n, where S is the section modulus of one ply, and n is the number of plies.
Modification Factors
Lateral Stability Factor
For lateral stability calculations for the lateral stability factor KL in CSA O86 7.5.6.4.4, Sizer offers you the a Design Setting choice of whether to use the full member width or the single ply width as b in the expression for the slenderness ratio and and in the limiting ratio for which the stability factor can be assumed to be 1.0. Research has shown that nailed and bolted beams have at most 30% composite action effect in terms of resisting torsional buckling, so it is recommended to use single ply width unless adhesives are used to laminate the members together.
A load sharing factor KH (= 1.15 for sawn lumber and glulam) is applied to built-up bending members with three or more plies if you specify that in the Beam Input view that it is subject to load sharing. The default setting is to apply the factor, as per O86 6.4.4.3.
Service Condition Factor
The service condition factor for built-up lumber, MSR and MEL materials is based on constituent member thickness rather than gross section thickness, because drying time depends on individual plies rather than gross section
Size Factor
he program always uses the single play width for the size factor Kzb or Kzbg from O86 6.4.5 and 7.5.6.5.1 respectively. This is because size factors are based on the variability of defects in the manufactured sizes of lumber (or lamination end joints in the rare case that more than one glulam member is built-up). When glulam is treated as a built-up system of sawn members when loaded on the weak axis according to O86 7.5.3, the dimensions used are those of a single lamination, whose width may be less than the member width.
Comparison with Part 9 Spans
In Part 9 of the NBC, maximum spans for built-up beams are based on serviceability experience. Therefore, the spans in Part 9 may differ from those calculated using Sizer