Cribbing Plans & Elevations
Cribbing plans are for temporary shoring to support a building.
Many coastal towns as part of their flood hazard mitigation plans are requiring house lifters and contractors to provide cribbing plans and elevations sealed by a registered design professional to ensure that the temporary shoring (known as cribbing) that supports the steel beams used to lift the house will be strong enough to support the weight of the house. Based on the house lifter’s sketch of how he wants to support the floor and walls during the lift, the firm of Lawrence S. Schreiber, Architect will create a scale drawing showing the plan and elevation of the cribbing. We will detail the size of the cribbing members and the overall size and placement of each support. All of our calculations are based on ASCE-7, and FEMA’s current guidelines on temporary shoring. We can usually create these plans in a day or two if we have the required information provided to us.
Some key points about cribbing are:
- Multi member lay-up of 4×4 to 8×8 lumber in two or three member per layer configuration.
- Capacity is determined by perpendicular to grain load on sum of all bearing surfaces.
- Stability is dependent on height to width of crib and should not exceed 3 to 1. (Need to overlap corners a minimum of 4” to guard against splitting off corners of individual pieces that can negatively impact overall stability.)
- Cribs used by contractors (or in short-term emergencies) often rely only on the friction between bearings for lateral strength, not sufficient for aftershocks.
- Individual pieces may be notched like Lincoln logs, to provide lateral resistance in addition to the friction between pieces. Metal clips may also be used to improve lateral strength, as well as diagonal braces between pairs of cribs.
- Failure is slow, noisy crushing of softer spring wood fibers, which make system very desirable for unknown loading of US&R work. In order to assure this desirable failure mode, the crib corners must be made by overlapping the individual pieces by four inches as previously mentioned.
- Solid levels can be placed within the crib to support a jack or spread the load at the ground level.
- Heavily loaded cribbing can crush so that it will loose from 10% to 20% of it’s height. (This is a good thing as far as providing warning of overload, but may present problems regarding stability and the need to keep the system tight.)
- Shrinkage of green lumber will cause crib to shorten and they should be checked daily for tightness.