Overturning Forces

1. Moment Arm in Hold-down and Compression Force Calculation

The calculation of the overturning forces T and C from Equation 4.3-7 in 4.3.6.1.2 now incorporates a moment arm b_eff that takes into account the hold-down and compression force offsets from the wall ends, rather then using the segment length b. The use of this "effective" moment arm had been optional in Shearwalls, but is now compulsory.

Following the new SDWPS Example C3.3.4-2, a more accurate positioning of hold-down and compression forces for the calculation of the moment arm has been implemented.

The following details apply to segmented and FTAO walls only; b_eff is not used in the calculation of T and C for perforated walls.

1. Hold-down Settings

   In the Hold-down forces data group of the Hold-down Settings:

   1. Subtract Offset from Moment Arm

      The checkbox offering you a choice of whether to subtract the hold-down offset from the segment length to arrive at the moment arm has been removed. The offset will always be subtracted.

   2. Center of Wall Studs

      The Center of wall studs choice has changed to Calculate for each wall segment . If selected, the calculation described below will be performed, otherwise the program will use the value entered in Hold-down offset at both the compression end and tension end for all walls in the structure.

2. Moment Arm Calculation
   1. Tension End Offset

      At the tension end of the segment, the force is assumed to be at the hold-down anchor bolt, which is the width of the end post stud pack plus the distance from the outside face of the hold-down device bracket to the center of the anchor bolt. e.g. for two studs and a 1.5 center line = 4.5" from the wall end.

   2. Compression End Offset

      The compression force is assumed to be in the center of the end post stud pack.g. for two studs, 1.5" from the wall end

3. Anchor Bolt Center Line in Database

   A field has been added to the Hold-down Database for the anchor bolt center line for each hold-down. Values have been added for all existing hold-downs from the the manufacturers' literature. An input has been added to the Hold-down Database Editor to allow you to enter it for custom hold-downs.

   1. No Anchor Bolt

      For hold-downs that utilize straps or other mechanisms other than anchor bolts, as is indicated by "No anchor bolt" in the Database editor, the center line is zero.

   2. Old Database

      When using the old database without this input, the anchor bolt center line defaults to 1". You can modify it with the Database Editor.

4. Output
   1. Shearline, Wall and Opening Dimensions Table

      In the Shearline, Wall and Opening Dimensions table, the value of beff is shown in the FHS column for

      • wall segments in Segmented or FTAO walls
      • any walls without openings
      • shear lines composed of one segment

      For these elements, the full length of full-height sheathing can be seen in the Length column.

      For walls with openings and shear lines with multiple walls, the full length of full height sheathing is still shown in the FHS column. The b_eff used for overturning calculations can be seen in the indivdual segments in these walls.

      There is an explanation in the legend as to what the FHS column now means.

   2. Hold-down Design Table

      An explanation of the calculation of the b_eff has been added to the Legend to the Hold-down and Compression Force Design table.

      The Note under this table has been modified to indicate where to find the beff value in the Shearline, Wall and Opening Dimensions table.