Hold-down Deflection

Important Note – These are descriptions to changes implemented in WoodWorks Shearwalls for version 9 and may not reflect current behavior.

The fourth term in the deflection equation relates to the displacement of the shearwall anchorage devices and the movement of the wood material at the hold-down location. The following sections give the various components which are added to give vertical hold-down displacement d_a.

1. Elongation and Displacement

   Refers to the elongation in tension of the hold-down brackets or straps plus anchor bolt elongation.

   1. Database value

      For those hold-downs with separate elongation and slippage (see , the hold-down database contains the strength-level elongation that occurs at the maximum capacity. For those hold-downs for combined slippage, the overall strength-level displacement at maximum capacity comes from the database.

   2. Displacement/Elongation at Maximum Capacity

      If this method ( see ) is selected for a particular hold-down, the program uses the database maximum value regardless of the force.

   3. Displacement/Elongation at Actual Force

      If this method is chosen, then the program divides ASD factored hold-down force by the ASD factored capacity, then multiplies this ratio by the strength-level displacement/elongation. (ASD capacities are used because these are needed for hold-down design, however the ratio is the same as the strength-level ratio.)

   4. Additional bolt length

      In some cases, separate elongation of the anchor bolt is added to the database deflection. This happens when

      • the Elongation for connector only (without anchor bolt) hold-down option is selected (see ), and the entire anchor bolt is analyzed separately.
      • This setting is not selected, but the published displacement or elongation is for an anchor bolt which is shorter than the one input in Structure input view for the level the hold-down is on. The elongation for the additional length is calculated. Note that in this case, for double bracket hold-downs, the published length is doubled before being compared to the actual length in the program.

      The elongation of the length L of bolt that is to be analyzed is PL/AE, where A is the bolt cross-sectional area, E is the steel modulus = 29000000 psi and P is the strength level hold-down force at that location.

2. Fastener Slippage

   This value is calculated only for those hold-downs with separate elongation and slippage. It refers to the vertical slippage of the horizontal fasteners that connect to the wall studs.

   1. Bolts

      When bolts are selected as the hold-down fastener type, the slippage displacement is given by NDS 10.3.6 as P_f / (270,000 D^1.5), where P_f is the strength level hold-down force per fastener, and D is the bolt diameter.

   2. Bolt hole tolerance

      For Assembly displacements that include slippage (see 1b, above), any value of bolt hole tolerance entered in the Hold-down Settings that is greater than 1/16” is added to the published displacement, which includes the effect of standard size bolt holes. For separate slippage and elongation, the entire value is added to the calculated slippage.

   3. Nails

      When nails are selected as the hold-down fastener type, the slippage displacement is e_n, from SDPWS Table C4.2.2D using the values for wood structural panels, and P_f is the strength level hold-down force per fastener. For nails greater than 10d, we conservatively use the values for 10d.

   4. P_f

      Note that the value of P_f is arrived at by dividing the uplift force by the fasteners on just one bracket in a double bracket connection. The same force P is transmitted to the fasteners in the other bracket.

   5. Specific Gravity Limitation

      SDPWS Table C4.2.2D indicates that nail slip applies for lumber framing members with specific gravity of 0.5 or greater. As there is no guidance in SDPWS about what to do with materials such as S-P-F with specific gravity less than 0.5, analysis proceeds with the Table C4.2.2D values and a warning note appears under the Hold-down Displacement table in the output.

3. Shrinkage

   Refers to the wood shrinkage that occurs between fabrication and service of the perpendicular-to-grain wood members spanned by the hold-down.

   It is calculated when the hold-down does not include a shrinkage compensating device.

   1. Calculation

      The vertical shrinkage displacement is 0.002 x (% fabrication moisture content – % in-service moisture content) x shrinkage length for that building level from the Structure input view.

   2. Moisture content input

      The fabrication and in-service moisture content are input in the Design Settings. Previously you could input only whether it was greater or less than 19%, for use in nail withdrawal design. Now the actual moisture content is input.

   3. In service Greater then Fabrication

      If for some reason in service moisture content is greater than fabrication, shrinkage is set to zero.

4. Crush

   The wood crush as input in the Hold-down settings is applied to all hold-down locations in the program. Typically ranges from 0.2 – 0.4”

5. Additional Components

   The additional components in the “Other – miscuts/gaps” input of the Hold-down settings are applied to all hold-down locations in the program.