When Field Dimensions Are Required
Field dimensions are required when the design geometry depends on a physical condition that cannot be reliably inferred from drawings alone. Common triggers:
- Anchor bolt actual positions for column base plates in renovation or alteration work
- Existing beam or column dimensions in renovation projects where contract drawings may be inaccurate
- Slab edge or wall face locations where new steel must bear or connect with tight tolerance
- Connection points on existing structure where new steel frames in and the as-built location differs from drawings
How Field Dimensions Are Gathered and Communicated
Field dimensions should be taken by a qualified person — an ironworker superintendent, a field engineer, or a survey crew — and reported in a format that is unambiguous: sketch showing what was measured, from what reference point, and the measured value. Verbal field dimension reports or approximate 'close enough' descriptions are not acceptable inputs for shop drawing production. The measured value must be tied to the model's coordinate system or to a reference on the drawing.
Incorporating Field Dimensions into the Model
When field dimensions are received, the detailer compares them to the model geometry. If the difference is within allowable tolerance, the model is adjusted to match. If the difference requires a design change — a connection plate must move, a member must be shortened, a shim must be designed — the EOR must be notified before the drawing is revised. The revised drawing should document that the dimension was field-verified and by whom.
Tolerance Limits and When Structural Input Is Required
AISC Code of Standard Practice Chapter 7 defines column plumbness tolerances (1/500 of the column height, max 1" in 200'), beam sweep tolerances, and base plate leveling requirements. If a field dimension reveals that an existing condition is outside these tolerances, the EOR must be involved before detailing proceeds — not as a formality, but because out-of-tolerance conditions may affect structural behavior. Base plates that cannot be leveled within tolerance require engineered shim stacks or grout design. Beams that are out of sweep by more than the AISC limit may need to be straightened or replaced.
- Column plumbness — 1:500 maximum, not to exceed 1" in 200' per AISC Code of Standard Practice 7.12
- Beam sweep — 1/8" per 10 feet of length, cumulatively
- Anchor bolt out-of-tolerance — tolerance per AISC Table 7-5; beyond tolerance requires EOR review
- Base plate elevation — +3/8" / -1/4" from plan elevation is typical for leveling nut adjustment
- Connection alignment — bolt hole patterns must be within 1/16" at time of assembly
Field Dimension Workflow for Renovation Projects
Renovation projects present the most intensive field dimension requirements because the existing structure — even when original drawings exist — rarely matches the as-built condition. A systematic workflow for renovation field dimensions: (1) survey the affected structural elements before detailing begins, using a licensed surveyor or a qualified field engineer; (2) record all dimensions in a standardized format tied to gridline coordinates; (3) compare surveyed dimensions against original structural drawings to identify discrepancies; (4) build the detailing model from surveyed dimensions, not from original drawings; (5) flag any discrepancy that affects the new structural design as an RFI to the EOR before detailing proceeds. This workflow prevents the most expensive renovation scenario: fabricating to original drawings that don't reflect the actual structure.
Communicating Field Dimensions to the Detailing Team
Field dimensions are only useful if they are communicated in a format the detailer can consume without interpretation. A photograph of a tape measure on a beam flange is not a dimension report. A sketch showing the measured dimension in relation to a column centerline, with a note about the reference datum, is a usable input. The best format is a dimensioned field sketch submitted via the project management platform with the submitter's name, date, and the member or condition being measured. When field dimensions come from multiple sources, the detailing PM should consolidate them into a single field dimension log before incorporating them into the model.