Shear Resistance

Shear Resistance Equations

Using O86 6.5.4 for sawn lumber, 7.5.7.3(b) for glulam, and 15.3.3.4 for SCL SIzer determines shear resistance with equations of the form

V_r = 2/3 f F_v AK_zvK_N

• f = 0.9.
• F_v is the specified shear strength f_v multiplied by wet service, treatment, system, and load duration factors.
• A is the cross-sectional area. Under different circumstances, gross cross sectional area and net area after notch removal are used.Refer to Notch Design for more details.
• K_zv is a size factor defined differently for each material.
• K_N is the a notch factor, defined differently for glulam and sawn lumber. CLT and SCL members are not notched. Refer to Notch Design for more details.

For glulam design, this formula applies to columns and optionally to beams less than 2 cubic meters in volume. For beams greater than 2 cubic meters, a formula from 7.5.7.3(a) based on gross loading must be used, and it can be used for smaller beams. Refer to Glulam Shear Design for more details.

For sawn lumber and glulam, modified equations are used for shear design with notches. For tension-side sawn lumber members, the fracture shear check for notches is required in addition to the equation from 6.5.4. Refer to Notch Design for more details.

Loads Near Support

Sizer applies the provisions of O86 6.5.4.2.1 and 7.5.7.2.1, and by adjusting the shear values calculated using the actual loading conditions of the member. For each load combination, the shear value at the support is reduced with the ignored portions of the shear that results from each load within the distance “d” of the edge of the support.

In other words, Sizer takes literally the provision that loads within a distance d of a support "may be permitted to be omitted" . This is slightly different than taking the value of the shear a distance d from the support, as the latter approach does not take into account the contribution to the reaction at the other end of the member of the ignored loads. Sizer measures the distance d from the inner edge of the support.

Because the adjustments are done after the loading analysis, the contribution of each load to be ignored is distributed proportionally only in the two supports enclosing the current span. This provides exact values of the design shear for single span members and exact or slightly conservative values for multi-span members.

Uplift loads within a distance d of the support are not ignored. As per 6.5.4.2.2 and 7.5.7.2.2, no loads are ignored for shear design at notches.

Shear from Biaxial Loading

The O86 gives no guidance on how to deal with bending shear for members loaded on both faces. In Sizer, this currently applies only to beams with an oblique rotation angle. Refer to Oblique Loading for Sizer's design methodology for beams rotated with respect to the load.