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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.
Bolts
When bolts are selected as the hold-down fastener type, the slippage displacement is given by NDS 11.3.6 as Pf / (270,000 D1.5), where Pf is the strength level hold-down force per fastener, and D is the bolt diameter.
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.
Nails
When nails are selected as the hold-down fastener type in the hold-down database, the slippage displacement is en, from SDPWS Table C4.2.2D using the values for wood structural panels, and Pf is the strength level hold-down force per fastener. For nails greater than 10d, we conservatively use the values for 10d.
Pf
Note that the value of Pf 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.
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.