Also in: Disaster Response and Recovery
OSHA Appendix C Timber Trench Shoring Lookup
OSHA's Appendix C timber tables answer the question a utility repair or a trench rescue asks first.
Example
You enter
- Soil type (Appendix A) A
- Trench depth (ft, 5 to 20) 13
- Trench width (ft, up to 15) 5
- Timber table set oak
- Crossbrace horizontal spacing all
- Any (d)(2)(ii) limitation present (heavy surcharge, equipment over 20,000 lb, load on crossbraces, partly sloped trench) no
You get
- Design pressure (psf) 397
- Arrangement count 4
- First crossbrace load (lb) 9528
Checked against published worked examples from: OSHA (29 CFR 1926 Subpart P).
Details, formula, and sources
ScopeWhat size crossbraces, wales, and uprights, at what spacing. All six federal tables are transcribed: actual-size mixed oak (C-1.1 to C-1.3) and nominal Douglas fir (C-2.1 to C-2.3). Enter the soil type, depth, width, and the crossbrace spacing the crew chooses, or show all, and the calculator lists every acceptable arrangement. OSHA's own Example 1, Type A soil 13 ft deep and 5 ft wide, returns all four: 4x4 crossbraces at 6 by 4 ft with no wales and 3x8 uprights at 6 ft (skip shoring); 4x6 at 8 ft with 8x8 wales and 2x6 uprights at 4 ft; 6x6 at 10 ft with 8x10 wales and 2x6 at 5 ft; and 6x6 at 12 ft with 10x10 wales and 3x8 at 6 ft. The design pressure printed on each table is 25, 45, or 80 psf per foot of depth plus 72 psf for a 2 ft surcharge -- 397, 657, and 1,112 psf at 13 ft for Types A, B, and C -- and the first crossbrace carries 397 x 6 x 4 = 9,528 lb. In Type B the uprights close from 6 ft to 2 ft apart; in Type C every depth needs close sheeting. The limitations are the part that must not be skipped: adjacent loads above a 2 ft soil surcharge, more than 240 lb on a crossbrace, equipment over 20,000 lb, or a partly sloped trench put the tables out of reach, and in disaster work one often is. Deeper than 20 ft or wider than 15 ft is refused. It does not classify soil or cover hydraulic shoring, shields, or sloping. 29 CFR 1926 Subpart P and the competent person govern.
Formula or tablelookup: soil type (A, B, C) x depth band (5-10, 10-15, 15-20 ft) x crossbrace horizontal spacing (up to 6, 8, 10, 12 ft) x width column (up to 4, 6, 9, 12, 15 ft) -> crossbrace size and vertical spacing, wale size and vertical spacing or not required, upright size and maximum horizontal spacing or close sheeting; Pa = 25 H + 72 (A), 45 H + 72 (B), 80 H + 72 (C) psf; crossbrace load = Pa x horizontal spacing x vertical spacing.
Source and edition29 CFR 1926 Subpart P, Appendix C (Timber Shoring for Trenches), Tables C-1.1 to C-1.3 (actual-size mixed oak, 850 psi bending) and C-2.1 to C-2.3 (nominal S4S Douglas fir, 1,500 psi), paragraphs (d)(2) limitations, (f) examples, and (g) notes. Federal regulation, public domain, transcribed and checked against the GovInfo CFR and osha.gov.
Free accessThe regulation is free at osha.gov and govinfo.gov; the six tables are reproduced in full because federal regulation is in the public domain.
What governsMath aid for personal verification. Stop work and consult the qualified person on site if any number does not match the field condition.
API field namessoil_type, depth_ft, width_ft, table_set, crossbrace_spacing, limitation_present, design_pressure_psf, arrangement_count, first_crossbrace_load_lb
- Depth over 20 ft or width over 15 ft is out of scope Appendix C does not apply; a registered professional engineer designs the system29 CFR 1926.652(c) and Appendix C
- Surcharge is a 2 ft soil equivalent Pa = k x H + 72 psf, with k = 25, 45, or 80 for Types A, B, and C29 CFR 1926 Subpart P, Appendix C tables
- Saturated or submerged conditions need tight sheeting tongue-and-groove planks at least 3 in thick, sheet piling, or similar29 CFR 1926 Subpart P, Appendix C, Note 2