Subpart P
You can use the timber shoring tables in Appendix C only for trenches that do not exceed 20 feet (6.1 m) in depth and when the shoring design is being performed in accordance with 1926.652(c)(1). See Appendix C to Subpart P of Part 1926 for the scope and application.
Yes — you must determine the soil type using the soil classification method in Appendix A to Subpart P before using the timber shoring tables. Appendix C states that the data in its tables apply only when the soil type or types have been first determined using Appendix A, so follow 1926 Subpart P App C and the soil-classification method in 1926.652 when choosing member sizes.
No — stable rock is exempt from shoring requirements, and Appendix C provides no table data for stable rock. Appendix C explicitly states that "stable rock is exempt from shoring requirements," so you do not use the timber tables for that condition; see Appendix C to Subpart P of Part 1926 and 1926.652 for context.
The timber member dimensions in the Appendix C tables refer to actual dimensions, not nominal dimensions. Appendix C states the sizes listed in Tables C–1.1 through C–1.3 refer to actual dimensions, so select members based on their actual measured size; see Appendix C to Subpart P of Part 1926 and 1926.652(c)
You may use nominal-size timber by following the data in Tables C–2.1 through C–2.3 or by complying with the allowance in [1926.652(c)(3)]. Appendix C directs employers wanting to use nominal sizes to Tables C–2.1 through C–2.3 or other acceptable data sources such as The Corps of Engineers or the Bureau of Reclamation; see Appendix C to Subpart P of Part 1926 and 1926.652(c)(3).
The table-listed members are inadequate when certain conditions exist — for example: adjacent structure or stored-material loads exceeding a 2-foot soil surcharge; vertical loads on crossbraces exceeding a 240-pound gravity load on a one-foot center section; equipment surcharge loads over 20,000 pounds; or when only the lower portion of a trench is shored unless specific sloping or depth-based selection criteria are met. Appendix C lists these limitation conditions and directs that under such conditions you must design an alternate system in accordance with 1926.652(c) or use another type of protective system; see Appendix C to Subpart P of Part 1926 for the full list.
"Adjacent" means the area within a horizontal distance from the edge of the trench equal to the trench depth. Appendix C defines the term "adjacent" this way when discussing surcharge and structural loads, so evaluate potential surcharge within that horizontal distance; see Appendix C to Subpart P of Part 1926 and 1926.652 for related requirements.
If trench conditions are saturated or submerged, you must use tight sheeting (special tongue-and-groove planks, steel sheet piling, or similar tight wall construction) to resist lateral water pressure and prevent loss of backfill. Appendix C Note 2 specifically requires tight sheeting when conditions are saturated or submerged; see Appendix C to Subpart P of Part 1926 and 1926.652(c).
Use tight sheeting when water must be resisted or retained (e.g., saturated/submerged conditions); close sheeting places planks side-by-side with minimal gaps and may be acceptable where water pressure is not an issue. Appendix C Note 2 defines tight sheeting and contrasts it with close sheeting and requires tight sheeting for water conditions; see Appendix C to Subpart P of Part 1926 and 1926.652 for guidance.
You select member sizes by first determining soil type (Appendix A), then choosing the table for that soil type, and finally matching the trench width, depth, and chosen horizontal spacing of crossbraces to read the required crossbrace, wale, and upright sizes and spacings from the table. Appendix C explains this selection process and the need to choose horizontal crossbrace spacing before reading member sizes; see Appendix C to Subpart P of Part 1926 and 1926.652(c).
All spacing indicated in Appendix C tables is measured center-to-center. Appendix C Note 3 states that all spacing is measured center to center, so apply that measurement method when laying out crossbraces, wales, and uprights; see Appendix C to Subpart P of Part 1926 for details.
Wales must be installed with their greater (larger) dimension horizontal. Appendix C Note 4 requires installation of wales with the greater dimension horizontal, so select and place wales accordingly; see Appendix C to Subpart P of Part 1926 and 1926.652 for context.
If the vertical distance from the center of the lowest crossbrace to the bottom of the trench exceeds 2.5 feet, uprights must be firmly embedded or a mudsill used; embedded uprights limit that vertical distance to 36 inches, while mudsills may be used up to 42 inches. Appendix C Note 5 explains these requirements and distances, so ensure upright embedment or mudsill placement meets these maximums; see Appendix C to Subpart P of Part 1926 and 1926.652(c).
Yes — trench jacks may be used in lieu of or in combination with timber crossbraces. Appendix C Note 6 explicitly permits trench jacks as an alternative or supplement to crossbraces; see Appendix C to Subpart P of Part 1926 and 1926.652(c) for design requirements.
When crossbrace vertical spacing is four feet, place the top crossbrace no more than two feet below the top of the trench; when spacing is five feet, place the top crossbrace no more than 2.5 feet below the trench top. Appendix C Note 7 provides these placement limits, so position the top crossbrace accordingly; see Appendix C to Subpart P of Part 1926 and 1926.652(c).
Skip shoring is an arrangement where crossbraces are spaced horizontally and some uprights are omitted (as in Arrangement B1 example), and Appendix C shows that skip shoring is an acceptable configuration when selected directly from the tables for the given soil, depth, width, and brace spacing. Appendix C's Example 1 describes skip shoring arrangements, so you may use skip shoring only as one of the acceptable table-listed arrangements for the applicable conditions; see Appendix C to Subpart P of Part 1926 and 1926.652(c)(1).
For trench sections over 15 feet in depth, Appendix C requires using the data in Table C–1.3; Appendix C Example 4 explains that member sizes and spacing for depths over 15 feet are determined from Table C–1.3, so follow that table and 1926.652(c) when designing shoring for deeper sections; see Appendix C to Subpart P of Part 1926 for details.
If vertical loads on a crossbrace exceed a 240-pound gravity load distributed on a one-foot center section of the crossbrace, the table-specified members are not considered adequate and an alternative design per 1926.652(c) is required. Appendix C lists this 240-pound limit as a condition that invalidates the table data; see Appendix C to Subpart P of Part 1926 for the limitation.
You may combine shoring for the lower portion of a trench with sloping or benching for the remaining portion only if the sloped portion is sloped at an angle less steep than three horizontal to one vertical, or if members are selected from the tables for a depth measured from the top of the overall trench (not from the toe of the slope). Appendix C specifies these conditions and requires compliance with 1926.652(c); see Appendix C to Subpart P of Part 1926.
If a surcharge is imposed by equipment weighing more than 20,000 pounds, the table-specified timber members are not considered adequate and you must design an alternate protective system in accordance with [1926.652(c)]. Appendix C lists the 20,000-pound equipment surcharge as a disqualifying condition for the table data; see Appendix C to Subpart P of Part 1926 for details.
If member sizes or spacings other than those shown in the tables are needed, you must determine appropriate sizes and spacings by designing the protective system as specified in 1926.652(c), since Appendix C Note 1 directs that member sizes at spacings other than indicated be determined by that design procedure; see Appendix C to Subpart P of Part 1926.
The example arrangements in Appendix C illustrate how to read the tables and select member sizes and spacing for a given soil type, trench depth, and width — they provide practical, acceptable configurations (including skip shoring) that meet the table criteria so you can follow the same selection steps for your trench. Appendix C includes Examples 1–4 to demonstrate table use and references 1926.652(c)(1) for design alignment; see Appendix C to Subpart P of Part 1926 for the examples.
Yes. You may use the tabulated timber shoring requirements in Appendix C as a prescriptive method to install timber shoring when the site conditions and soil classification match the assumptions in those tables.
(Reference: 1926.652(c) and Appendix C tables.)
You must use an engineered design for trenches over 20 feet; the Appendix C tables direct you to "See Note 1" for depths over 20 ft, which requires a professional design.
(Reference: 1926.652(c)(3) and Appendix C.)
The Appendix C tables include an assumed surcharge pressure Pa for each soil type; use those values when applying the tables.
These Pa formulas and surcharge assumptions are shown in the headers of the timber shoring tables in Appendix C. Always confirm that your site surcharge conditions match the table assumptions; otherwise get an engineered design under 1926.652(c)(3).
(Reference: Appendix C timber tables and 1926.652(c)(3).)
Yes. Appendix C explicitly allows manufactured members of equivalent strength to be substituted for the wood members given in the tables.
(Reference: Appendix C and the prescription authority in 1926.652(c).)
The tables include footnotes specifying minimum bending strengths or species assumptions for the timber used in those tables.
Always use the table that matches the lumber specification you have available, or substitute manufactured members of equivalent strength. See the footnotes in Appendix C and confirm applicability under 1926.652(c).
(Reference: Appendix C table footnotes.)
Appendix C tables specify the minimum sizes and spacings for the primary timber shoring components: cross braces, wales, and uprights, including both horizontal (maximum allowable horizontal spacing) and vertical spacings.
(Reference: Appendix C timber tables and the authority in 1926.652(c).)
Yes. You must confirm the soil type (A, B, or C) and ensure site conditions match the assumptions for the table you use before applying Appendix C tabulated sizes.
(Reference: 1926.652(c) and Appendix C.)
A table cell that reads "Not Req'd" or is left blank for maximum allowable horizontal spacing means that, for that specific combination of trench depth, trench width, and soil type, horizontal spacing limits are not required by the table (i.e., the table does not impose a horizontal spacing restriction for that configuration).
(Reference: Appendix C and 1926.652(c).)
As trench width increases the Appendix C tables generally call for larger timber members (bigger uprights and wales) and/or reduced spacings to resist the greater soil pressures.
(Reference: Appendix C timber tables and the prescriptive option in 1926.652(c).)
Yes. The Appendix C tables assume a specific surcharge condition (the tables indicate a 2‑ft surcharge and include Pa formulas that incorporate that surcharge). If your actual surcharge differs, you cannot rely on the table without verifying that the different surcharge will not increase loads beyond the table assumptions.
(Reference: Appendix C and 1926.652(c).)
Yes — but only if your design is an engineered protective system prepared by a registered professional engineer or otherwise complies with the standard's requirements.
(Reference: 1926.652(c)(3) and Appendix C.)
"S4S" stands for "surfaced four sides," meaning the lumber dimension given in that table is for dressed (planed) stock that has been surfaced on all four faces.
(Reference: Appendix C and the prescriptive provisions in 1926.652(c).)
The tables commonly call for vertical spacings of about 4 to 6 feet between horizontal members (wales or cross braces), depending on soil type, trench depth, and trench width.
(Reference: Appendix C timber tables.)
Timber requirements for Soil Type C are more conservative (larger members and/or closer spacing) than for Soil Type A because Soil Type C is less stable and exerts higher loading on the shoring.
(Reference: Appendix C and 1926.652(c).)
The Appendix C tables are an optional prescriptive method employers may use to meet the protective system requirements; they are not the only way to comply.
(Reference: 1926.652(c) and Appendix C.)
Select the table column that corresponds to your measured trench width (the tables are organized by discrete width ranges); then use the row for your trench depth and the column for your soil type to read the required member sizes and spacings.
(Reference: Appendix C tables and 1926.652(c).)