Also in: Disaster Response and Recovery
US&R Box Crib Capacity and Height Limit
A box crib's capacity is not the wood's column strength; it is the bearing where one layer crosses the next.
Example
You enter
- Crib timber 6x6
- Layup 3x3
- Species group syp_hf_spf
- Allowable cross-grain bearing (psi; SOG 500) 500
- Shortest crib width (ft) 4
- Bearing condition all
- Load on the crib (lb, 0 to skip) 0
You get
- Contact area (sq in) 30.25
- Contact points 9
- Base capacity (lb) 136125
- Crib capacity (lb) 115706
- Ratio height (ft) 12
- Max height (ft) 6
Checked against published worked examples from: USACE (US&R Shoring Operations Guide).
Details, formula, and sources
ScopeThe US&R Shoring Operations Guide writes it L = A x N x P: the area of one crossing, times the crossings per layer (4 for a 2x2 layup, 9 for 3x3), times 500 psi across the grain for No. 1 and better Douglas Fir, with the same 0.85 and 0.75 species factors as vertical shores. A 3x3 crib of 6x6 Spruce-Pine-Fir is 30.25 x 9 x 500 = 136,125 lb, or 115,706 lb after the species factor -- the guide rounds the Douglas Fir figure to 135,000. A 2x2 of 4x4 Douglas Fir is 24,500 lb against the guide's 24,000. The crushing failure is slow and audible, which is why cribbing is trusted. Height is a stability limit that tightens as the crib loses contact: three times the narrowest width with every point bearing, two for lift-and-crib work, one and a half on two of four corners, one on a single corner, and a practical ceiling of about 4 ft for 4x4 and 6 ft for 6x6 cribbing. A 4 ft wide 6x6 crib is limited to 6 ft by the practical ceiling, not the 12 ft ratio; on one bearing corner it falls to 4 ft, and a crib that must be taller is widened, not stacked. The 2006 Field Operations Guide lists 625 psi rather than 500, so the stress is an input with the 2021 value as the default. It does not check the surface under the crib, wedges and shims, lateral stability, or plastic cribbing. The US&R guides, the US&R Structures Specialist, and the incident's structural engineer govern.
Formula or tableL = A x N x P x species factor: A = actual member width squared (12.25 sq in for 4x4, 30.25 for 6x6), N = 4 (2x2) or 9 (3x3) when the load bears on every crossing, 2 when it bears on two corners and 1 on one corner (the crossings under the load, applied here on the formula's own terms), P = 500 psi (entered; SOG value); maximum height = min(ratio x shortest width, practical ceiling), ratio 3 (all bearing), 2 (lifting), 1.5 (two of four corners), 1 (one corner); practical ceiling 4 ft (4x4), 6 ft (6x6).
Source and editionUSACE US&R Shoring Operations Guide, Ed. 5.0 (2021), Additional Information -- Cribbing (L = A x N x P, 500 psi for No. 1 and better Douglas Fir, 85% and 75% species factors, 4 ft and 6 ft recommended maxima) and Rescue Specialist Reference Data (height-to-width ratios, 15 degree maximum slope), cited by figure.
Free accessA product of three numbers and published ratios; the SOG is a free USACE publication.
What governsEstimate. AHJ and licensed structural engineer govern.
API field namestimber_size, layup, species, bearing_stress_psi, crib_width_ft, bearing_condition, crib_load_lb, contact_area_sqin, contact_points, base_capacity_lb, crib_capacity_lb, ratio_height_ft, max_height_ft
- Capacity is bearing at the crossings L = A x N x P with P = 500 psi for No. 1 and better Douglas FirUSACE US&R Shoring Operations Guide (2021)
- Height ratio follows contact 3:1 all bearing, 2:1 lifting, 1.5:1 two of four corners, 1:1 one cornerUSACE US&R Shoring Operations Guide (2021)
- Practical ceilings about 4 ft for 4x4 and 6 ft for 6x6 cribbingUSACE US&R Shoring Operations Guide (2021)