Building Moving and Demolition (C-21) · Lesson 2 of 6

Jacks, Cribbing, and Ground Bearing Under a Lifted House

"Jacks, Cribbing, and Ground Bearing Under a Lifted House" is lesson 2 of 6 in the Building Moving and Demolition (C-21) Learning Hub. It opens with "Jacks Lift, Cribbing Holds" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Jacks Lift, Cribbing Holds
  2. Turn the House Weight into a Point Load
  3. The Ground Under the Pad Usually Governs
  4. Check the Wood at the Crossings
  5. Take the Lesser Value, Then Count the Stacks

The worked example in the course applies this lesson to a realistic job in 7 steps, from "Ground capacity per pad" through "Stacks needed".

What you'll be able to do

  • Explain the main idea of "Jacks, Cribbing, and Ground Bearing Under a Lifted House" in plain language you can use on the exam.
  • Apply this step with confidence: "The Ground Under the Pad Usually Governs".
  • Apply this step with confidence: "Check the Wood at the Crossings".
  • Apply this step with confidence: "Take the Lesser Value, Then Count the Stacks".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Cribbing and Jacks: What Governs Each StackONE LIFTING POINT (SIDE VIEW)HOUSE BEAMbox crib,4x4 membersJACKCLAY / SILT: 1,500 psf(CBC Table 1806.2, Class 5)timber pad 3 ft x 3 ft = 9 sfNo one under a jack-supported load until it iseffectively blocked or cribbed (Title 8 Sec. 3562(e))CAPACITY AND COUNTSoil under pad9 sf x 1,500 psf13,500 lbWood crossings4 x 12.25 = 49 sq inWood crush limit49 x 500 psi24,500 lbGoverns: lesser valuesoil13,500 lbStacks needed70,937 / 13,500 = 5.256 stacksLoad per stack70,937 / 611,823 lbJack rating10-ton = 20,000 lbOKLifts at 6 in stroke42 in / 6 in7 lifts6 stacks at 11,823 lb each: soil governs, so size the pads firstWood crush stress of 500 psi is the engineer-supplied example value; jack rated load is marked on the jack (Sec. 3562(a)).Soil value is a ceiling: Table 1806.2 and Sec. 1806.2. Mud, organic silt and undocumented fill have no presumptive value.CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Jacks Lift, Cribbing Holds

Cal/OSHA General Industry Safety Orders Section 3562(e) says employees shall not enter the zone beneath a jack-supported load unless it has been effectively blocked or cribbed. A jack raises the house; the cribbing is what you trust to carry it while anyone works underneath. Section 3562(b) adds that jacks are designed so their maximum safe extension cannot be exceeded, so a tall lift is done in stages: lift, crib, lift again.

Step 2: Turn the House Weight into a Point Load

Divide the moving weight (from your takeoff) by the number of lifting points to get the load each jack and stack must carry. Section 3562(a) requires the rated load to be marked on the jack, so you compare the point load to the marked rating and pick a jack rated above it. The house weight itself is the number from the previous lesson; if that number is wrong, every point load is wrong.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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