Hold-down Effect Factor Jhd

According to CSA O86 11.5.5., a hold-down effect factor, J_hd is to be applied to the shear strength of shear wall segments. Refer to Holddowns and Anchorages for a discussion of the use of anchorages, as opposed to hold-downs, so that this factor applies. A complete description of this factor can be found in Chapter 9 Shearwalls and Diaphragms of the CWC's Wood Design Manual as well as in the Commentary to CSA O86, included in the Wood Design Manual.

For a shear wall segment without hold-downs on its tension side, the hold-down effect factor is calculated as (CSA O86 11.5.5.3):

where:

P_ij = uplift restraint force at the base of the tension end-stud of the segment

V_hd = basic shear resistance of segment (i.e. shear resistance with J_hd = 1)

height, length

H_s = The height of the shear wall as shown in figure 11.5.5.3 and defined in 11.4.1.2. Does not include the depth of the diaphragm joists, or the portion at a gable end above the wall but below the roof, both of which are considered in the calculation of the moment arm for the overturning force R_ij

and,

For segments with hold-downs only at the bottom of tension end-stud of the segment, the factor is calculated as (CSA O86 11.5.5.4.):

where Ptop is the uplift restraint force at top of the tension end-stud.

CSA O86 Figure 11.5.5.3 shows an example of the calculations involved in determining J_hd. The uplift restraint force in the tension end-stud, P is dependent on:

• the lateral force acting at the top of the shear wall segment (F_ij)
• the dead load acting on the top of the segment (q_i)
• the self weight of the wall segment(q_wij)
• the reaction forces of shear wall segments on the storey above the one under consideration (R_i+1,j)

In determining J_hd, the software follows the same procedure as the example shown in CSA O86 Figure 11.5.5.3. However, in the O86 example, the shear wall openings in the lower level line up with those of the upper level. The standard does not explicitly show how to determine P in cases in which the shear wall segments are offset from one storey to the next.

In determining the uplift restraint force, P the hold-down forces from an upper level transferred to a point within a shear wall segment, is not included in the calculation of P. Instead, the force is transferred straight through to the level below the lower level, by means of a vertical load transfer element.

If the hold-down force is transferred to a point above an opening on the lower level, it is distributed by statics to the end studs of the two lower shear wall segments that are adjacent to the opening.