A steel Bill of Materials is the master list of every member on a project: AISC shape (W18x35), grade (A992), length (28'-6"), quantity, unit weight (35 lb/ft), and total tonnage, and it is the single document that drives purchasing, shop labor, coating, and freight [S2].
For structural fabricators, an Advanced BOM or Preliminary Material List generated from the 3D detailing model now allows procurement to start before detailing is fully closed, cutting weeks off lead time on standard A992 and A36 members [S1].
BOM structure: the eight columns every steel estimate needs
A working steel BOM carries at minimum eight columns: Mark, Shape, Grade, Length, Quantity, Unit Weight (lb/ft), Piece Weight, and Total Weight, with Notes reserved for camber, coating, and special fabrication [S2]. A typical 28'-6" W18x35 column weighs 998 lb per piece, and 12 pieces push the line to 11,970 lb [S2]. Optional columns (Location, Connection type, Coating, Delivery sequence) feed downstream labor and erection planning, and they are what separates a procurement-only list from an estimate-grade BOM [S2].
The Mark column is the contract between the shop floor and the field: a unique identifier such as B1, B2, C1, or BR1 stamped on every piece so it can be traced from the model, through the shop, and onto the correct grid line on site [S2]. Grouping rows by member type (columns first, then beams and girders, bracing, miscellaneous) and sorting by shape and size inside each group is the simplest way to consolidate purchasing, for example all W18x35 stock rolled up to 4,200 linear feet in one block [S2].
Material grades and shapes that appear in the BOM
ASTM A992 is the default grade for hot-rolled wide-flange beams and columns in modern U.S. commercial building work, while ASTM A36 governs plates, angles, and channels in the secondary-steel bucket [S2]. AISC shape designations such as W18x35, W12x26, and HSS sections carry their own unit-weight values (35 lb/ft, 26 lb/ft) that convert directly into piece weight once cut length is locked [S2]. Secondary steel (lintels, base plates, stiffeners, gusset plates) routinely adds 10–20% to the tonnage a junior estimator captures from the primary framing plan, and omitting it is the most common source of under-bidding [S2].
For project work in the construction machinery and equipment and structural categories, the BOM is usually built around a small library of AISC shapes plus a long tail of plates, bolts, and welds, so the spreadsheet structure has to scale to both at once. Designers using Tekla PowerFab or similar 3D detailing environments can push an Advanced BOM straight from the model into procurement, which is how the Preliminary Material List workflow reduces weeks of pre-order slack on A992 wide-flange [S1].
Step-by-step build: foundation up, grid by grid, then cross-check
The disciplined build order is: set up the template and column groups; work the framing plans from the foundation up, floor by floor, starting with columns, then beams grid by grid; mark every recorded member on a printed or PDF plan; record lengths from schedules or by measuring between grid lines; then cross-check the take-off against the engineer-supplied column and beam schedules [S2]. A discrepancy where the take-off count exceeds the schedule usually means a double-count; a shortfall usually means a missed sheet or grid area; a size mismatch almost always points to an unrevised addendum [S2].
Resolving every discrepancy before the BOM is released is the single highest-value QC step, and it is the step most often skipped under bid pressure [S2]. Highlighting each member on the plan as it is recorded is non-negotiable for accuracy because the human eye will otherwise double-count identical W12x26 runs on parallel grid lines [S2].
BOM types: EBOM, MBOM, and the procurement list
Manufacturing distinguishes three BOM views: the Engineering BOM (EBOM) as designed in CAD, the Manufacturing BOM (MBOM) as it will actually be built, and a procurement-focused list that collapses the EBOM into buyable line items with grades and quantities [S3][S5]. For a steel project the procurement list is closest to the Advanced BOM exported from the detailing model, while the MBOM adds shop operations (cuts, copes, holes, welds) and the EBOM stays in the engineer's model [S1][S3].
The BOM also differs from a Bill of Quantities: a BOM lists every raw material, sub-assembly, intermediate assembly, sub-component, and part needed to manufacture a product, while a Bill of Quantities catalogs materials needed to execute construction activities [S3][S5]. On a structural job that distinction maps to the engineer's take-off (BoQ-style) versus the fabricator's shop release (BOM-style), and mixing them up is a recurring source of contract disputes [S5].
Comparison: procurement BOM vs fabrication BOM vs erection BOM
On a single project, three BOM views coexist and they do not overlap cleanly: a procurement BOM groups by shape and grade for mill orders, a fabrication BOM groups by shop operation and delivery sequence, and an erection BOM groups by grid and floor level [S1][S2]. A typical member, say 12 pieces of W18x35 A992 at 28'-6", appears as one procurement line (12 pcs, 11,970 lb), several fabrication lines (cut, cope, drill, weld), and one erection line per piece on the field sheet [S2].
Against four decision criteria, the three views rank as follows: cost accuracy is highest in the procurement BOM because it uses mill-grade unit weights; shop-floor usability is highest in the fabrication BOM because it carries operation codes; field traceability is highest in the erection BOM because each line carries a grid coordinate; and the Advanced BOM exported from the 3D model is the only view that is consistent across all three when it is properly synchronized [S1][S2]. For estimators, the rule is: build the procurement BOM first, derive the fabrication and erection views from it, and never re-key a number downstream [S2].
Common BOM errors and the signals that catch them
The five most common steel BOM errors are: missing secondary steel (lintels, base plates, stiffeners), unit-weight confusion between A992 and A36 grades, length rounding on grid-line members, double-counting identical W-shapes on parallel grids, and ignoring addenda that change section sizes after the schedule was issued [S2]. Each one shows up as a measurable delta: a tonnage shortfall at PO time, a fabrication labor overrun at shop loading, or a sudden change-order at erection [S2].
The corrective control is mechanical, not judgemental: highlight as you go, cross-check against the column and beam schedules, and re-run the tonnage against historical similar projects before bid submission [S2]. On the materials side, carbon steel grade callouts (A36 for plate and channel) and alloy steel callouts for higher-strength connection elements must be separated because they carry different unit prices and different mill lead times. Tools that automate the cross-check, including Tekla PowerFab's ABM export and the Excel template workflow used by many small-to-mid fabricators, are what makes a BOM auditable in hours rather than days [S1][S2].
Where steel BOMs meet adjacent spec work
For fabricators who also handle miscellaneous metals, the BOM template has to be widened to include connection hardware (shear tabs, moment plates, anchor rods) and finish specs (primer, galvanizing, paint) because each of those carries its own supplier, lead time, and cost line [S2][S6]. Standard joist and steel deck producers (for example, New Millennium) publish blank BOM forms keyed to their product lines so that joist and deck quantities can be merged with the structural-steel take-off in one consolidated order [S4].
For project teams sourcing fasteners and shaft key hardware into the same procurement package, the BOM discipline is the same: a unique Mark, a grade, a quantity, a unit weight, and a delivery sequence, and the Advanced BOM model exported from the 3D detailing environment is the cleanest way to keep all of those views synchronized from the model to the mill PO to the field [S1][S2]. A reasonable next step for any estimator is to compare this disciplined eight-column structure against the current output of their detailing software, and to verify that the procurement BOM, fabrication BOM, and erection BOM are all generated from one source of truth rather than re-keyed spreadsheets.
See also our earlier report, IBC tank selection for warehouse automation: size, material, and integration criteria.