Steel Detailing — Shop Drawings, Bolt Gages, and Fabrication Standards

Steel detailing is the process of creating shop drawings (fabrication drawings) from the engineer's design drawings. Good detailing ensures that every member can be fabricated and erected without ambiguity. This reference covers the dimensions, conventions, and tolerances from AISC 303-22 (Code of Standard Practice), the AISC Steel Construction Manual 15th Edition, and AISC Design Guide 21 (Welded Connections).

W-shape bolt gages from the AISC Manual

Bolt gages are the standard transverse spacings for bolt holes in the flanges of W-shapes. They are tabulated in AISC Manual Part 1 (Dimensions and Properties) and depend on the flange width.

Worked example — selecting bolt gage for W16x50

Given: W16x50 (b_f = 7.07 in., t_f = 0.630 in., k_1 = 13/16 in.). Need to place 3/4 in. A325 bolts in the flanges for a moment end plate.

Step 1 — AISC Manual gage: For b_f = 7.07 in., the standard gage is g = 5.5 in. (single gage line).

Step 2 — Verify edge distance: Each side of the gage line to the flange edge = (b_f - g) / 2 = (7.07 - 5.5) / 2 = 0.785 in.

Minimum edge distance for 3/4 in. bolt at rolled edge = 7/8 in. (AISC Table J3.4). Since 0.785 in. < 7/8 in. = 0.875 in., the standard gage of 5.5 in. does not work for 3/4 in. bolts on a W16x50.

Step 3 — Adjust gage: Maximum gage = b*f - 2 * (minimum edge distance) = 7.07 - 2 _ 0.875 = 5.32 in. Use g = 5.25 in. or consider using 2 rows with smaller gage.

Alternatively, with 5/8 in. bolts: minimum edge distance = 3/4 in., max gage = 7.07 - 2 * 0.75 = 5.57 in. The standard 5.5 in. gage works.

This example illustrates why detailers must always verify the edge distance — the AISC Manual gage values assume a typical bolt size and may not work for all combinations.

Camber conventions per AISC 303

Camber is specified by the engineer on the design drawings. AISC 303 Section 6.4 defines the standard conventions:

• Minimum practical camber: 3/4 in. (19 mm) for W-shapes up to 36 in. deep. Smaller values are difficult for the fabricator to achieve and measure.
• Camber tolerance: +/- 0 (under) to + 1/2 in. (over) for beams up to 50 ft. For beams over 50 ft, the tolerance is 1/2 in. + 1/8 in. for each 10 ft over 50 ft.
• Camber notation: Shown as an upward arc on the beam elevation with the camber value in inches. Example: "W24x76 x 30'-0, Camber = 1-1/2 in."

Camber is applied by cold bending in a press. The beam is placed web-horizontal in a hydraulic gag press and deflected beyond the target camber to account for spring-back.

Mill tolerances per ASTM A6

Hot-rolled shapes from the mill have dimensional tolerances that the detailer must account for:

Shop drawing abbreviations

Common abbreviations used on steel shop drawings per AISC 303 and standard practice:

Code comparison for detailing standards

EN 1090-2 introduces four execution classes (EXC1 through EXC4) with increasing inspection and tolerance requirements. Most building structures fall under EXC2, while bridges and high-consequence structures require EXC3 or EXC4.

Key clause references

• AISC 303-22 Section 6 — Fabrication tolerances and practices
• AISC 303-22 Section 4.4 — Shop drawing review timeline
• AISC Manual Part 1 — Bolt gage dimensions for W-shapes
• ASTM A6/A6M — General requirements for structural steel shapes (mill tolerances)
• AISC 360-22 Table J3.4 — Minimum edge distances
• EN 1090-2 Annex D — Geometric tolerances for structures

Topic-specific pitfalls

• Assuming bolt gages from the AISC Manual work for all bolt sizes — as shown in the worked example, the tabulated gage may violate minimum edge distance for larger bolts. Always verify.
• Not coordinating beam camber with concrete floor tolerance — if a cambered beam has insufficient dead load to flatten the camber, the top-of-steel will be higher than planned, reducing concrete slab depth and potentially exposing shear studs. Coordinate camber with the concrete pour tolerance.
• Specifying tolerances tighter than AISC 303 without a cost discussion — tighter tolerances (e.g., +/- 1/16 in. on member length) require additional shop time and inspection, increasing fabrication cost by 10-30%. Only specify tighter tolerances when structurally required.
• Omitting the workpoint location on connection details — AISC 303 defines the workpoint as the intersection of member centerlines. When the workpoint is offset (e.g., column face workpoint for a shear tab), it must be clearly shown on the detail to avoid dimensioning errors in the shop.

Steel detailing standards — AISC Code of Standard Practice

The AISC Code of Standard Practice (COSP) for Steel Buildings and Bridges (AISC 303-22) is the governing document for the relationship between the structural engineer, the fabricator, and the erector. It establishes standard practices for the preparation of shop drawings, fabrication tolerances, erection procedures, and quality expectations. The COSP is referenced in the general notes of virtually every US structural steel project.

Key sections of AISC 303-22 relevant to steel detailers:

A critical point in AISC 303 Section 4.4: the engineer of record has 14 calendar days to review shop drawings. If no response is received within 14 days, the drawings are deemed approved and the fabricator may proceed. This provision exists to prevent project delays from late reviews but places a heavy burden on the engineer's review workflow.

Common detail types

Beam-to-column connections

Beam-to-column connections are the most common detail type in structural steel buildings. The detail must show the connection type, all bolt sizes and grades, weld sizes and electrodes, plate material grades, and all relevant dimensions.

Beam-to-beam connections

Beam-to-beam (beam-to-girder) connections occur when a secondary beam frames into the web of a primary girder. The detail must address the framing depth relationship — if the beams are the same depth, a top-flange cope is required on the incoming beam.

Base plate connections

Column base plates transfer axial load, shear, and moment from the column to the concrete foundation. The detail must show the plate size, anchor bolt layout, grout thickness, and any shear lugs or embedded plates.

Brace connections

Brace connections (diagonal bracing, chevron braces, X-bracing) require careful dimensioning of the gusset plate geometry. The detail must show the brace-to-gusset connection, the gusset-to-beam interface, the gusset-to-column interface, and the work point location.

Drawing conventions — bolt and weld symbols

Bolt symbols on shop drawings

Bolts are represented on shop drawings using standard symbols that convey the bolt type, diameter, and installation requirements:

Bolt hole types are also shown with specific symbols. Standard round holes are the default. Oversized holes are shown with a larger circle and noted "OVS." Short-slotted holes are elongated in one direction and noted "SSL" with the slot direction. Long-slotted holes are noted "LSL" with the slot direction.

Weld symbols per AWS D1.1

Weld symbols follow the AWS D1.1 standard. The weld symbol is placed below the reference line for near-side welds and above the reference line for far-side welds. Key elements of the weld symbol:

Common weld callouts on shop drawings:

• 5/16 FW both sides 8" long — two 5/16 in. fillet welds, 8 in. long, one on each side
• 1/4 FW NS, 3/16 FW FS — different weld sizes on near side and far side
• CJP (TC-U4a) — complete joint penetration groove weld with specific joint detail
• PJP 1/2" depth — partial joint penetration weld with 1/2 in. effective throat
• 5/16 FW (cont.) — continuous 5/16 in. fillet weld along the full joint length

Fabrication and erection tolerances

The following table summarizes the critical tolerances for fabricated steel members per AISC 303-22 Section 6 and ASTM A6. These tolerances must be shown on shop drawings or referenced in the project specifications.

For erection tolerances, the most critical dimension is column plumbness. A multi-story building can accumulate out-of-plumb offsets that cause alignment problems at upper floors. AISC 303 Section 7.13 limits the cumulative out-of-plumb to 1/500 of the height above the base, measured at each floor level.

BIM and CNC workflow overview

Modern steel detailing is increasingly performed in 3D Building Information Modeling (BIM) environments. The BIM-to-fabrication workflow connects the structural design model directly to the fabrication shop floor:

BIM detailing workflow steps

1. Structural design model — the engineer creates the design model (in Revit, Tekla, or RAM) showing member sizes, connection forces, and design intent. This model is typically LOD 300 (Level of Development) — member sizes and locations are defined, but connection details are schematic.

2. Detailing model — the steel detailer imports the design model into a detailing platform (Tekla Structures, SDS/2, Advance Steel) and adds complete connection details: bolt layouts, weld sizes, plate dimensions, cope details, and stiffeners. The model reaches LOD 400 — every piece of steel, every bolt, and every weld is modeled.

3. CNC data extraction — the detailing software generates CNC (Computer Numerical Control) files for the fabrication shop. These include drill line programs (hole locations and sizes), plasma cutting profiles (plate outlines, copes, gusset shapes), and robotic welding instructions for repetitive connections.

4. Shop drawing generation — the software automatically generates shop drawings from the 3D model, including piece marks, assembly drawings, and bills of material. Each piece is assigned a unique piece mark (e.g., "B24" for beam 24, "C7" for column 7) that travels with the piece from fabrication to erection.

5. Erection sequencing — the detailing model also produces erection drawings showing the sequence of steel placement, crane locations, and temporary bracing requirements.

BIM software comparison

CNC file formats

The DSTV/NC1 format is the universal standard for steel fabrication CNC data. It contains instructions for drilling (hole diameter, depth, location), cutting (start/stop points, kerf compensation), and marking (part numbers, reference lines). Modern CNC drill lines can process a W24x94 beam with 30+ bolt holes in under 5 minutes, a task that would take 30-45 minutes manually.

Run this calculation

• Section Properties Database
• Beam Calculator

Related references

• How to Verify Calculations
• bolt hole sizes
• weld inspection and NDT
• structural capacity calculator
• bolt capacity calculator
• weld capacity calculator
• Steel Connection Detailing

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable standard and project specification before use. The site operator disclaims liability for any loss arising from the use of this information.