Direct Answer
Plumbing and squaring is the process of measuring and adjusting the structural steel frame to bring every column to plumb (vertical) and the overall frame to square (right angles maintained at the grid). It is performed before connections are permanently tightened and before anchor bolts are fully grouted. An out-of-plumb or out-of-square frame creates compounding errors at every subsequent level.
Full Explanation
Columns are checked for plumb using optical plummet instruments, total stations, or — on smaller structures — plumb bobs. The frame is checked for square using diagonal measurements or a total station survey. AISC Code of Standard Practice Section 7.13 defines fabrication and erection tolerances: columns must be within 1:500 of true plumb. If a column is out of tolerance, it is adjusted using turnbuckles on the temporary bracing or by adjusting shims under the base plate before the grout sets. Plumbing and squaring is typically done tier by tier as erection progresses.
What This Means for Your Shop
As a fabricator, your responsibilities in plumbing and squaring include: ensuring base plate grout pockets are properly sized, providing shim sets with the base plates, and confirming that your column lengths were fabricated to the tolerances specified. A column that is ±3/4" long compounds with a 5-story building to create significant out-of-plumb conditions.
Common Mistakes
Permanently tightening anchor bolt nuts before the plumb-and-square check is complete. Once anchor bolts are fully tightened and grouted, adjusting columns becomes extremely difficult.
Recommended Process
Include a mandatory plumb-and-square check milestone in the erection schedule after each tier of columns is set. Document survey results. Do not release tier for permanent connection tightening until the plumb-and-square survey is accepted in writing.