Lag Screw Withdrawal Design Value (NDS 12.2.1)
The lag's axial capacity from the NDS equation, not a table.
Example
You enter
- Specific gravity G (0.50 DF-L, 0.42 SPF) 0.5
- Lag shank diameter D (in) 0.5
- Thread penetration (in) 4
- Load-duration factor CD 1
- Into end grain? (Ceg = 0.75) no
You get
- Reference withdrawal W 378 lb/in
- Withdrawal capacity 1514 lb
Details, formula, and sources
W = 1,800 G^(3/2) D^(3/4) lb/in of thread penetration. A 1/2 in lag in DF-L over 4 in of thread holds 1,514 lb; step to a 5/8 in lag and it rises only 18% (the D^(3/4) law) - more or deeper lags beat one fat lag. Withdrawal (axial) value only, not the lateral yield-limit connection or the head bearing; end-grain installs take a 0.75 factor. A design aid, not the engineer of record's stamped design.
W = 1,800 G^(3/2) D^(3/4) (lb/in of thread); Ceg = end grain ? 0.75 : 1.0; Z_w = W x p_thread x CD x Ceg.
The NDS 2018 12.2.1 reference lag-screw withdrawal design value and the end-grain factor, by name; the 1,800 empirical constant is named.
The NDS is free to view at awc.org (ANSI/AWC NDS); the 12.2.1 lag-withdrawal equation is in the published standard.
Estimate. AHJ and licensed professional govern.
Field names used by the API: g, d_in, p_thread_in, cd, end_grain, w_lbin, z_w
- Withdrawal value W = 1,800 G^(3/2) D^(3/4) per inch of thread penetrationNDS 2018 12.2.1
- End-grain factor Ceg = 0.75 for a lag installed into end grainNDS 2018 12.2.1
- Axial only the withdrawal value, not the lateral connection or the head bearingscope of this tile