Continuous rod tie-down
One rod run at one end of a stacked wood shear wall, restrained by a bearing plate at every level, story by story from the top: overturning and uplift, rod tension, bearing plates, compression posts, shrinkage and take-up device travel, the vertical chord deformation, wall deflection and story drift. It follows WoodWorks' Five-Story Light-Frame Wood Over Podium example; that wall is the default input.
Inputs of the rod tie-down #
- Wall length b, lateral load, SDS. Seismic takes the vertical effect 0.2 SDS D from ASCE 7-22 12.4.2.2, so uplift uses (0.6 − 0.14 SDS)D + 0.7E (allowable) or (0.9 − 0.2 SDS)D + E (strength). Wind uses 0.6D + 0.6W or 0.9D + W. The load factor on L with the lateral load is yours (ASCE 7-22 2.3.1 exception).
- Stories, top first, up to 12. F, dead wD and live wL are what is added at the top of that story on this wall; wD includes the story's own wall. Lever arm d runs from the rod to the centre of the posts at the other end, per story, because the posts change. Chord length is the wall tributary to the posts for gravity. Rod length is restraint to restraint, empty for the story height. Wood across grain is the wood in the shrinkage zone at that level, for example 4.5 in. for a double 2x4 top plate and a 2x4 bottom plate, 15.75 in. for platform framing with 2x12 joists. Ga comes from SDPWS Tables 4.3A to 4.3D for the nailing you use.
- Posts and wood plates. Species, grade and post size; the posts do not act compositely. The plate's species sets Fc⊥ under the plates and posts.
- Bearing plates. Fy, and the hole sizes over the rod: WoodWorks uses 3/16 in. in the wood so the rod moves freely and 1/16 in. in the steel. Cb applies unless a plate is within 3 in. of the end of the wood member.
- Take-up device. On by default, as WoodWorks recommends. Travel, slack and elongation come from the evaluation report of the device you choose: ILX has not read ICC-ES AC316 or any device's report. The prefilled slack and elongation are the example's; the travel is a placeholder to replace.
- Shrinkage. S = C D (Mi − Mf) plus construction gaps, with C, the moisture contents and the gaps as inputs. The fuller formula of the Wood Handbook is not offered: WoodWorks shows it within 0.1 % of the simple one on its plates.
- Forces for deformation. WoodWorks departs from ASCE 7-22 12.8.6.1, whose 1.0D + 0.5L gives less uplift, and takes the strength uplift combination instead. That is the default; 1.0D + 1.0 lateral or your own factors are the alternatives.
- Wood-on-wood crushing factor. 1.75 by default, as the example. WoodWorks calls this the engineer's judgment: the SDPWS Commentary example leaves it out, and a factor of 1.0 gives less drift.
- Cd, Ie, allowable drift, local limit on rod elongation. ASCE 7-22 Tables 12.2-1, 1.5-2, 12.12-1. The limit on rod elongation is a note only: some jurisdictions set one.
Auto-design varies the rod size and the bearing plate length and thickness at each story, smallest steel first, and larger rods if drift asks for them. Posts, lever arms, plate width and grades stay yours.
Checks performed #
| Check | Basis |
|---|---|
| Rod tension, each story | AISC 360-22 J3, Table J3.2: 0.75 Fu on the gross area, φ 0.75, Ω 2.00 |
| Plate bearing on the wood, each story, under the difference in tension from the story above | NDS 2024 3.10.2, 3.10.4, net of the hole |
| Plate bending, each story, as a cantilever from the rod on the net width, the worse direction | AISC 360-22 F11.1 |
| Compression posts, each story, every combination with its CD or λ | NDS 2024 3.6, 3.7, 3.10; ASCE 7-22 2.3, 2.4 |
| Take-up device travel against the accumulated movement at the top level | Device report; IBC 2304.3.3 |
| Story drift, each story: rod elongation, plate crushing, slack and device elongation (or shrinkage), post crushing on the plates and sheathing | SDPWS 2021 Eq. 4.3-1, ASCE 7-22 12.8.6, Table 12.12-1, the curve of WoodWorks p. 43 |
| Rod elongation against the local limit, a note | Local requirement |
Limitations of the rod tie-down #
- Every level has a bearing plate. A run that skips levels is not covered.
- Not designed: sheathing and edge nailing (use the wood shear wall), the anchorage at the base (use concrete anchors), couplers, which must develop the rod, the podium and its overstrength under discontinuous walls, the other end's different chord, multi-story wall effects.
- Tension is on the gross area with Fnt, as WoodWorks does. Rod elongation uses the threaded tensile area for every rod length, also conservative where a rod is not threaded through.
- Dead load resisting overturning is w b² / 2 on the wall length b, not the lever arm d, as the example.
- Compression for crushing is the lateral force only, as the example says; gravity preload is left out. WoodWorks calls this somewhat unconservative for the crushing itself and offsets it elsewhere.
- Deformation curves are limit-state values, not adjusted for load duration.
Verification examples #
Verification status: 186 values compared in 13 examples (11 published, 2 hand calculations from the cited equations): 185 within tolerance, 1 recorded deviations. Code basis: partial. Only some provisions were read in the target edition; the rest rests on older editions, examples or summaries.
WoodWorks DE-2025, 2nd Ed., pp. 14-15, 31-50: Tables 2.1, 3.6, 3.8 to 3.19 for its 29 ft wall, allowable stress and the drift forces. One value differs, plainly: Table 3.17B prints 0.466 in. at the 4th level, where its own columns give 0.446 in. and its accumulated column follows 0.446 in. On p. 15 the prose says 0.063 in. per floor of shrinkage where its own p. 14 and Table 2.1 use 0.090 in.; ILX follows the table.
The strength side has no published example: hand calculations of the top story for strength design and of wind for allowable stress. The comparison with a device's report, the checks of a real device's travel, and anything on AC316 are not done.