What is the maximum permitted tolerance between jacking points when raising a slab during lift-slab operations?

What is the maximum permitted tolerance between jacking points when raising a slab during lift-slab operations?

• A. 1/4 inch
• B. 1/2 inch
• C. 3/4 inch
• D. 1 inch

Answer: (B)

In lift-slab work, the slab must be raised as a nearly level unit to avoid uneven bending, which can create dangerous stresses, cracks, or instability. The allowable difference in height between jacking points is half an inch. This small margin lets you account for minor measurement or adjustment variances while preventing a tilt that could compromise the formwork, temporary supports, or the slab itself. Before each lift increment, check elevations at all jacking points with a level or laser, and ensure the difference is 0.5 inches or less. If any pair exceeds this, lower or re-level the points (adding shims or adjusting jacks) until the tolerance is met, then continue. Keeping within this limit helps minimize differential settlement and the risk of tilt or failure during the raising process. Choosing a larger tolerance would permit more tilt and increase safety hazards, while a much smaller tolerance would be impractical in field conditions.

In lift-slab work, the slab must be raised as a nearly level unit to avoid uneven bending, which can create dangerous stresses, cracks, or instability. The allowable difference in height between jacking points is half an inch. This small margin lets you account for minor measurement or adjustment variances while preventing a tilt that could compromise the formwork, temporary supports, or the slab itself.

Before each lift increment, check elevations at all jacking points with a level or laser, and ensure the difference is 0.5 inches or less. If any pair exceeds this, lower or re-level the points (adding shims or adjusting jacks) until the tolerance is met, then continue. Keeping within this limit helps minimize differential settlement and the risk of tilt or failure during the raising process.

Choosing a larger tolerance would permit more tilt and increase safety hazards, while a much smaller tolerance would be impractical in field conditions.