Out-of-Plumb Column Measured Under Solar Gradient
During erection of a four-story steel frame, the surveyor reports that a second-tier column is out of plumb 1 in. over its 32 ft unbraced length, measured at 7:30 a.m. The column faces east and had direct sun on that face for about two hours before the reading. The column is part of a moment frame; the base plate sits on 1-1/2 in. of grout with leveling nuts still in place. The top of the column is connected with snug-tight bolts and the tier above has not been erected. The erector proposes three responses: (a) re-survey, (b) 'compensate for it in the next tier,' and (c) 'slot the anchor holes at the base and pull it back.' The contractor asks the inspector whether the column must come out.
1. Evaluate the reported deviation against the erection tolerance and assess whether the reading is reliable.
Read the worked answer
Tolerance: plumbness of columns is conventionally limited to 1:500 of the height.
- Reported: 1.0 in. / (32 ft × 12 in./ft = 384 in.) = 1:384, which exceeds 1:500 (allowable ≈ 0.77 in. over 32 ft).
But the reading is suspect. A column with one face in direct sun develops a temperature differential across the section and bows, producing an apparent out-of-plumb that is not a fabrication or erection error. Two hours of morning sun on a single face is a classic false reading. The survey should be repeated when the member is at uniform temperature - early morning before sun, after the sun has moved off, or on an overcast day - with the target placement and instrument setup checked. The measurement should also reflect the as-erected condition before the frame is fully bolted and before dead load of subsequent tiers.
So: the number as reported fails the tolerance, but it is not yet a finding. Re-measure first; do not direct corrective work based on a thermally distorted reading.
2. Assuming a re-survey under uniform temperature confirms a deviation beyond 1:500, evaluate the erector's three proposed responses.
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- (a) Re-survey - correct first step, and it should be done regardless of what the number shows, with the instrument, targets and temperature documented.
- (b) Compensate in the next tier - not the erector's call. Inducing an opposite lean above to bring the top of the frame back to line changes the geometry of a moment frame, alters the as-built eccentricity and can affect connection fit-up and the design's second-order assumptions. It is only permissible if the EOR accepts it in writing as a correction plan, with the residual out-of-plumb at each level quantified.
- (c) Slot the anchor holes - a connection detail change to a seismic-force-resisting base. Anchor rod holes, edge distances and base plate capacity are design items; slotting, enlarging or flame-cutting holes in the field without EOR approval is a nonconformance, not a repair.
The proper path: confirmed deviation → written notice with survey data → EOR evaluates the structural effect and issues a specific correction procedure (for example, adjusting leveling nuts and realigning within the connection's available clearance) → erector performs it → inspector re-surveys and documents the final condition. The inspector does not direct the method and does not approve it.
3. What documentation should accompany this issue, and what is the inspector's role if the EOR accepts the as-built condition?
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Documentation:
- Original and re-survey data: date, time, ambient and member temperature conditions, instrument and surveyor, points measured, and the computed ratio (not just inches).
- Sketch or diagram of the column, tier, and the direction of the deviation relative to the frame.
- A numbered nonconformance with the tolerance cited and the disposition trail.
- The EOR's written acceptance or repair directive, including any limits on subsequent tiers (for example, a maximum cumulative drift at the top of the frame).
- Final verification survey after correction, with sign-off.
If the EOR accepts the as-built condition, the inspector's role is to record that acceptance and verify that the work matches the accepted condition - not to independently bless the deviation. The inspector also verifies that whatever correction or compensation was directed is actually executed and that subsequent tiers are checked against the stated cumulative limits. If the EOR's acceptance depends on a condition (for example, limitation on the next tier's plumbness), the inspector must verify that condition at the next survey point.