Bill of Materials (BOM) in Steel Detailing: A Complete Guide

A Bill of Materials (BOM) is one of the most important documents produced during structural steel detailing. It provides a structured list of the materials, components, quantities, sizes, weights, and other information required to fabricate and construct a steel structure.

For steel fabricators and contractors, an accurate BOM is much more than a simple material list. It supports material procurement, estimating, fabrication planning, inventory management, shipping, cost control, and project coordination.

With modern 3D steel detailing software such as Tekla Structures, BOM information can be extracted directly from the detailed model, helping reduce manual calculations and improving consistency between the model, drawings, and fabrication information.

What Is a Bill of Materials in Steel Detailing?

A Bill of Materials (BOM) is a detailed list of the materials and components required to manufacture and assemble structural steel elements.

Depending on the project, a steel BOM can include information such as:

• Member marks.
• Assembly marks.
• Part marks.
• Steel profiles.
• Plate dimensions.
• Material grades.
• Thicknesses.
• Lengths.
• Quantities.
• Unit weights.
• Total weights.
• Bolt sizes.
• Bolt quantities.
• Weld information.
• Surface treatment.
• Coatings.
• Miscellaneous steel components.

The exact format varies according to the fabricator, project specifications, client requirements, and fabrication system.

Why Is a BOM Important in Steel Detailing?

Structural steel projects can involve thousands of individual components.

A large commercial building may contain:

• Hundreds of columns.
• Thousands of beams.
• Large quantities of connection plates.
• Thousands of bolts.
• Stiffeners.
• Bracing members.
• Miscellaneous steel.
• Handrails and stairs.

Manually tracking all these components can be extremely difficult.

A properly prepared BOM provides a consolidated overview of the materials required for the project.

It helps project teams answer important questions such as:

What materials are required?

How much steel needs to be purchased?

How many plates are required?

How many bolts are needed?

What is the total fabricated steel weight?

Which materials have already been procured?

What materials are still required?

Key Components of a Steel BOM

A typical structural steel BOM may contain several categories of information.

1. Part Mark

The part mark identifies an individual component within the project.

For example:

• B102.
• C205.
• PL305.
• BR410.

Part marks allow fabrication teams to connect material information with shop drawings and the 3D model.

2. Assembly Mark

An assembly mark identifies a fabricated assembly.

For example, a column assembly might consist of:

• Main column.
• Base plate.
• Cap plate.
• Stiffeners.
• Connection plates.

The assembly mark allows the fabricator to understand which individual components belong together.

3. Profile

The BOM identifies the steel section used for each structural member.

Common profiles include:

• W-shapes.
• HSS.
• Channels.
• Angles.
• I-sections.
• T-sections.
• Rectangular hollow sections.
• Circular hollow sections.

The profile information is important for procurement and fabrication.

4. Material Grade

The material grade identifies the specified strength and type of steel.

Examples may include:

• ASTM A36.
• ASTM A572 Grade 50.
• ASTM A500.
• EN 10025 grades.
• CSA steel grades.

The applicable material grade depends on the project’s design requirements and governing standards.

5. Dimensions

For plates and fabricated components, the BOM may include:

• Length.
• Width.
• Thickness.
• Diameter.
• Area.

For rolled sections, it may include:

• Profile.
• Length.
• Weight per unit length.

6. Quantity

Quantity indicates how many identical components are required.

For example:

Part	Description	Quantity
B101	W18×35 beam	12
PL201	10 mm plate	24
BR301	HSS brace	8
BL401	3/4 in bolt	96

Quantity information supports purchasing, fabrication planning, and inventory management.

Steel Weight in a BOM

Steel weight is one of the most important pieces of information in structural steel projects.

The BOM may provide:

• Individual part weight.
• Assembly weight.
• Total member weight.
• Total plate weight.
• Total project steel weight.

For rolled steel sections, weight can often be calculated using:

Weight = Length × Unit Weight

For example, if a steel section weighs 50 kg/m and the required length is 8 m:

Weight = 50 × 8 = 400 kg

In real projects, the software may calculate weights based on the actual modeled geometry.

This is particularly useful when the model contains:

• Cuts.
• Holes.
• Coping.
• Notches.
• Plates.
• Connection components.

 

Types of BOMs in Steel Detailing

There is not necessarily one single BOM for every project.

Different BOMs can serve different purposes.

1. Material BOM

A material BOM provides an overview of raw materials required for fabrication.

It can include:

• Steel profiles.
• Plates.
• Pipes.
• Angles.
• Channels.
• Bolts.
• Other components.

This type of BOM is useful for procurement.

2. Fabrication BOM

A fabrication BOM focuses on components required to manufacture assemblies.

It may contain:

• Part marks.
• Profiles.
• Plate sizes.
• Thicknesses.
• Quantities.
• Material grades.
• Assembly information.

Fabricators can use this information to organize shop production.

3. Assembly BOM

An assembly BOM lists the individual components belonging to an assembly.

For example:

Column Assembly C105

• Main column.
• Base plate.
• Cap plate.
• Four stiffeners.
• Connection plates.
• Bolts.

This helps fabrication teams understand how the assembly is constructed.

4. Erection BOM

An erection-oriented material list may organize steel according to erection requirements.

It can help site teams identify:

• Member marks.
• Assembly marks.
• Quantities.
• Weights.
• Locations.

This can assist with transportation and erection planning.

5. Bolt List

Bolts may be reported separately.

A bolt list can include:

• Bolt diameter.
• Bolt length.
• Bolt grade.
• Bolt type.
• Quantity.
• Connection or assembly reference.

Separating bolts from primary steel can simplify procurement.

BOM vs. Material Takeoff

BOM and material takeoff are closely related but are not always identical.

A material takeoff generally focuses on determining the quantities of materials required for a project.

A BOM is typically a structured list of components and materials associated with fabrication or assembly.

For example:

Material Takeoff

125 tons of structural steel required.

BOM

W18×35 — 12 members — 96 m total — 4,800 kg
W24×55 — 18 members — 144 m total — 10,500 kg
PL 12×300×600 — 40 pieces
3/4 in bolts — 1,250 pieces

The BOM provides a much more granular breakdown.

How Is a BOM Created in Steel Detailing?

The BOM creation process generally begins with the structural design information.

Step 1: Review Design Drawings

The detailer reviews:

• Structural plans.
• Elevations.
• Sections.
• Connection details.
• Design specifications.
• Architectural information.

Step 2: Build the 3D Model

The steel detailer creates the structural steel model using detailing software.

Typical objects include:

• Beams.
• Columns.
• Braces.
• Plates.
• Connections.
• Bolts.
• Welds.

Step 3: Add Fabrication Information

The model is developed to fabrication level.

This may include:

• Cuts.
• Holes.
• Coping.
• Plates.
• Stiffeners.
• Bolts.
• Welds.
• Connection components.

Step 4: Assign Material Information

Each component should have the appropriate:

• Profile.
• Material grade.
• Thickness.
• Size.
• Length.
• Other required properties.

Step 5: Number the Model

Parts and assemblies are assigned marks.

This allows the BOM to be connected to the corresponding drawings and model objects.

Step 6: Generate the BOM

Once the model has been checked, reports can be generated using the detailing software.

The BOM can then be exported or formatted according to project requirements.

BOM in Tekla Structures

For modern steel detailers, software such as Tekla Structures can automate much of the BOM generation process.

Because the BOM is generated from the detailed 3D model, information can be extracted for:

• Parts.
• Assemblies.
• Profiles.
• Plates.
• Bolts.
• Materials.
• Quantities.
• Lengths.
• Weights.

The detailer can create different report templates depending on the project requirements.

This creates an important relationship:

3D Model → Numbering → Drawings → BOM → Fabrication

If the model is accurate, the resulting BOM can provide a reliable representation of the modeled materials.

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Example of a Steel BOM

A simplified structural steel BOM might look like this:

Mark	Description	Material	Qty.	Length	Unit Weight	Total Weight
C101	W14×48 Column	A992	6	9 m	71 kg/m	3,834 kg
B201	W18×35 Beam	A992	12	8 m	52 kg/m	4,992 kg
BR301	HSS 6×6×3/8 Brace	A500	8	6 m	—	—
PL401	Base Plate	A36	6	—	—	540 kg
PL402	Connection Plate	A36	24	—	—	310 kg

The actual BOM format will depend on project standards and fabrication requirements.

How BOMs Help Steel Fabricators

A reliable BOM provides significant benefits to fabrication shops.

Material Procurement

Fabricators can determine how much steel needs to be purchased.

Inventory Management

The BOM can be compared against existing stock.

Production Planning

Components can be organized according to fabrication requirements.

Cost Estimation

Steel quantities and weights can support project cost calculations.

Shipping

Assembly weights and dimensions can assist with transportation planning.

Waste Management

Material quantities can be analyzed to improve cutting and nesting strategies.

BOM and CNC Fabrication

The BOM is only one part of a modern digital fabrication workflow.

A detailed steel model can also provide information for CNC fabrication.

For example:

Tekla Model

↓

BOM

↓

Shop Drawings

↓

NC/CNC Data

↓

Fabrication

This reduces the amount of information that must be manually re-entered between different stages.

BOM Accuracy: Why Model Quality Matters

The quality of a BOM depends heavily on the quality of the underlying model.

If the model contains:

• Incorrect member sizes.
• Missing plates.
• Incorrect quantities.
• Wrong material grades.
• Duplicate components.
• Incorrect lengths.
• Missing bolts.

then the BOM may also contain incorrect information.

Therefore:

A BOM is only as reliable as the data used to generate it.

This is why model checking is a critical part of steel detailing.

Common BOM Errors in Steel Detailing

Several mistakes can affect BOM accuracy.

Incorrect Member Lengths

Incorrect modeling can produce inaccurate material quantities.

Wrong Material Grades

Using the wrong grade can create procurement problems.

Duplicate Components

Duplicate modeled elements can inflate quantities.

Missing Components

Unmodeled plates, stiffeners, or connection components may be absent from the BOM.

Incorrect Numbering

Poor numbering practices can make it difficult to connect the BOM with shop drawings.

Outdated Reports

A BOM generated before the latest model revision may no longer reflect the current design.

Best Practices for Preparing a Steel BOM

1. Generate the BOM From the Final Checked Model

Avoid generating procurement information from an incomplete model.

2. Use Consistent Naming

Maintain consistent part and assembly marks.

3. Separate Material Categories

Where appropriate, separate:

• Main steel.
• Plates.
• Bolts.
• Miscellaneous steel.
• Other purchased components.

4. Include Revision Information

The BOM should clearly identify the applicable project revision.

5. Cross-Check With Drawings

Verify that BOM quantities and marks correspond to the issued drawings.

6. Check Weight Calculations

Review total weights against expected project quantities.

7. Coordinate With the Fabricator

The BOM format should match the fabricator’s procurement and production workflow.

BOM and Steel Detailing: From Drawings to Fabrication

The modern steel detailing process can be visualized as a connected information chain:

Structural Design

↓

Steel Detailing

↓

3D BIM Model

↓

Model Checking

↓

Numbering

↓

Shop Drawings

↓

Bill of Materials

↓

CNC/Fabrication Data

↓

Fabrication

↓

Transportation

↓

Erection

A properly structured BOM helps maintain continuity throughout this process.

The Future of BOMs in Steel Detailing

The role of the BOM is evolving as the construction industry moves toward digital fabrication.

Future steel workflows are increasingly likely to integrate:

• BIM.
• Cloud collaboration.
• Automated quantity extraction.
• Digital fabrication.
• CNC machinery.
• Robotic welding.
• Automated material tracking.
• RFID/barcode systems.
• Digital twins.
• AI-assisted quality control.

Instead of treating the BOM as an isolated spreadsheet, companies can connect material information directly to the digital model and fabrication process.

This creates a more traceable workflow in which individual steel components can potentially be tracked from procurement through fabrication and erection.

Conclusion

A Bill of Materials (BOM) is a fundamental part of modern steel detailing. It converts information from the detailed steel model into an organized list of materials and components that can support procurement, fabrication, estimating, logistics, and construction.

For steel detailers, creating an accurate BOM requires more than generating a report. It requires a properly modeled and checked structure, correct material information, consistent numbering, and an understanding of how the fabricator will use the information.

With modern detailing platforms such as Tekla Structures, much of the BOM generation process can be automated from the 3D model.

The result is a connected workflow:

Accurate Model → Accurate Drawings → Accurate BOM → Better Fabrication

For steel fabrication projects, that connection can reduce manual work, improve material control, and help minimize costly errors and rework.