Blueprint Reading and Structural Drawings

Lecture de Plans et Dessins Structuraux

Introduction

Reading structural plans and drawings is the very foundation of the ironworker (metal fabricator-fitter) trade. Before you cut, drill, assemble, or erect a single piece of steel, you must be able to transform a graphic representation into a three-dimensional physical reality. For the Red Seal exam, this skill is rigorously assessed: you will be asked to interpret welding symbols, calculate developed lengths, locate elevations, and understand the conventions specific to shop and field drawings.

This chapter covers all the essential concepts: types of drawings, scales, conventional lines, welding symbols according to CSA W59, orthographic and isometric views, dimensional tolerances, as well as the classic pitfalls that cause candidates to fail.

Types of Drawings in the Structural Steel Industry

General Arrangement Drawings (Overall Plans)

The general arrangement drawing (or overall plan) shows the complete structure in its context. It indicates the layout of columns, beams, bracing, and floors. You will find reference axes (positioning benchmarks), main dimensions, and general notes. This type of drawing is used to understand the overall logic of the structure, but it does not contain fabrication details.

Shop Drawings

Shop drawings are prepared by the fabricator (often called the "detailer"). They show each individual piece with all the dimensions necessary for its fabrication: lengths, angles, hole positions, types of welds, etc. Each piece receives a unique identification number (e.g., "B-101" for a beam, "C-205" for a column). These numbers are transferred to the erection drawings to enable assembly on site.

Erection Drawings (Field Assembly Plans)

Erection drawings indicate how the fabricated pieces must be assembled on site. They show the erection sequence, field connections (bolted or welded), temporary bracing, and alignment tolerances. The ironworker uses these plans to position elements precisely, often using a theodolite or laser level.

Table of Drawing Types

Scales and Measurement on Plans

Common Scales

Structural drawings are rarely at full scale (1:1). The most frequent scales are:

Golden Rule: never measure directly on a plan with a graduated ruler to deduce a fabrication dimension. Drawings may be reduced or enlarged during printing. Always use the written dimensions (numbers) rather than graphic measurement. If a dimension is missing, consult the detailer or engineer — never guess.

Reading Dimensions

Dimensions on shop drawings are expressed in millimeters (mm) in the metric system. Standard tolerances are generally:

These values may vary depending on project specifications. Always check the general notes on the plan.

Conventional Lines

Understanding line types is essential to avoid confusing a visible edge with a hidden edge, or a centerline with a dimension line.

Frequent Pitfall: on shop drawings, hidden edges are often omitted to simplify reading. If you do not see a dashed line, it does not necessarily mean there is no element behind — it may be a simplification convention.

Orthographic Views

Principle of Orthogonal Projection

Orthogonal projection (or multi-view projection) represents an object through several plane views perpendicular to each other. The system used in Canada is third-angle projection (American method), where the front view is in the center, the right view is to the right, the top view is above, etc.

The three principal views are:

For an ironworker, the ability to mentally move from one view to another is crucial. For example, an I-beam viewed from the front appears as a vertical rectangle; viewed from the top, it appears as a horizontal rectangle with visible flanges; viewed from the profile, it shows the I-shape.

Correspondence Between Views

The views are aligned according to projection lines. A dimension measured on the front view corresponds to the same dimension on the top view (width) or on the right view (height). This correspondence allows you to verify the consistency of dimensions.

Isometric Views

The isometric view is a three-dimensional representation where the three axes (X, Y, Z) make equal angles of 120° with each other. It is used to clarify complex assemblies, particularly bracing connections or frame nodes.

On an isometric drawing, dimensions are not strictly to scale, but relative proportions are preserved. Use this view to understand the geometry, but always return to the orthographic views for exact dimensions.

Welding Symbols According to CSA W59

The CSA W59 standard (Welded Steel Construction) governs the welding symbols used in Canada. These symbols are standardized according to AWS A2.4 (Standard Symbols for Welding, Brazing, and Nondestructive Examination).

Basic Structure of a Welding Symbol

The welding symbol includes:

Arrow Position and Meaning

If the welding symbol is placed below the reference line, the weld is located on the same side as the arrow (arrow side). If it is placed above the reference line, the weld is located on the opposite side of the arrow (other side).

Classic Error: reversing the meaning of above and below the reference line. Remember: the symbol is always drawn on the side of the reference line that corresponds to the side of the weld relative to the arrow.

Common Weld Types

Weld Dimensions

The size of a fillet weld is expressed by the leg length in millimeters. For example, an 8 mm fillet weld means each leg of the triangle measures 8 mm. The weld length is indicated to the right of the symbol. If a pitch is required (intermittent weld), the length and pitch are indicated as follows: 50-100 (50 mm of weld every 100 mm).

Example: A fillet weld symbol with "8" below the reference line and "100-200" to the right means: 8 mm leg fillet weld, arrow side, 100 mm long, spaced 200 mm (center to center).

Field vs. Shop Welds

Welding symbols may be accompanied by a flag at the junction of the arrow and the reference line. This flag indicates a field weld (on site). The absence of a flag means the weld is performed in the shop. This distinction is crucial for work planning.

Bolting Symbols

Bolted connections are represented by specific symbols:

Bolts are designated by their diameter and grade. Common grades are:

On plans, a 3/4" (19 mm) A325 bolt will be noted as "3/4" A325" or "19φ A325". Holes are generally 2 mm larger than the bolt diameter (standard hole).

Elevations and Levels

Elevations (or levels) indicate the height of an element relative to a reference point, generally the finished floor level (noted ±0.00) or sea level. On structural plans, elevations are expressed in meters with three decimals (e.g., 12.450 m).

Elevation symbols include:

Critical Point: always check whether the indicated elevation corresponds to the top or bottom of the element. For a beam, the elevation may be given at the top of steel or at the neutral axis level. Confusion here leads to costly assembly errors.

Tolerances and Adjustments

Fabrication Tolerances

CSA S16 (Design of Steel Structures) and CSA W59 define allowable tolerances. Common fabrication tolerances are:

Erection Tolerances

On site, alignment tolerances are more generous:

These tolerances are cumulative — you must monitor the accumulation of deviations over several storeys.

General Notes and Specifications

Each plan contains general notes that apply to the entire project. These notes may include:

Exam Tip: always read the general notes before answering a question about a plan. Often, the answer to an apparently ambiguous question is found in these notes.

Development Length Calculations

Development (or layout) is the calculation of the material length needed to fabricate a bent piece. For a 90° bend, the developed length is calculated using the bend allowance.

Basic Formula for a 90° Bend

For sheet metal of thickness t and inside bend radius r, the developed length L for a 90° bend is:

L = (π/2) × (r + k × t)

where k is the neutral axis factor (generally 0.33 for mild steel, 0.4 for aluminum).

Example: For a 6 mm thick steel plate, bent 90° with an inside radius of 10 mm:

L = (π/2) × (10 + 0.33 × 6) = 1.5708 × (10 + 1.98) = 1.5708 × 11.98 = 18.82 mm

The total developed length of the piece is the sum of the straight lengths plus the bend length for each bend.

Multiple Bends

For a piece with multiple bends, add the straight lengths and the bend lengths. Watch out for bend deductions that may be used in certain calculation methods. The exact method depends on the fabricator's tables and the type of bending (air bending, bottoming, coining).

Beam Length Calculations

To calculate the length of a beam between two columns, you must account for:

Formula: Beam length = centerline distance − (column 1 width / 2) − (column 2 width / 2) − erection clearance

Example: Centerline distance = 8000 mm, HSS 200×200 columns, erection clearance = 10 mm.

Length = 8000 − 100 − 100 − 10 = 7790 mm

Piece Marking Symbols

Each piece in the structure carries a unique mark number. The marking system generally follows a convention:

The mark is indicated on the shop drawing and transferred to the erection drawing. On site, each piece is identified by a tag or paint marking. Always verify the mark before erecting a piece.

Foundation Plans and Anchor Rods

Foundation plans show the position of anchor rods that connect the steel structure to the concrete foundations. Critical information includes:

Placement Tolerance: anchor rods are generally positioned with a tolerance of ± 3 mm. If the rods are mispositioned, the ironworker must report the problem before drilling additional holes in the base plates.

Detail Drawings: Connections

Connections (assemblies) are the critical points of the structure. Common connection types are:

On detail drawings, each connection is represented with its exact dimensions, the number and position of bolts, and the required welds. The ironworker must verify that the holes of the pieces match the holes of adjacent pieces before erection.

Common Abbreviations

Here are the most frequent abbreviations on structural plans:

Most Frequent Reading Errors

Confusion Between Views

A classic error is interpreting the top view as if it represented the front view. On a general arrangement plan, the top view shows the horizontal layout of elements; the front view (elevation) shows heights. Take the time to identify the view type before reading dimensions.

Misinterpreting Welding Symbols

The fillet weld symbol (triangle) is often confused with the groove weld symbol (semicircle). Remember: the triangle represents a fillet weld, the semicircle represents a groove weld. The position of the symbol relative to the reference line determines the side of the weld.

Neglecting General Notes

General notes contain essential information such as material specifications and tolerances. Never ignore them. On the exam, a question may focus on a specific note from a provided plan.

Measuring Directly on the Plan

Never measure with a ruler on a plan to obtain a dimension. Plans are often reduced or enlarged. Always use the written dimensions.

Pitfalls to Avoid

Summary

Final Exam Tips

Reading plans is a skill that develops through practice. On the Red Seal exam, questions on this topic are designed to test your ability to apply conventions in realistic situations. Stay methodical, check every detail, and leave no ambiguity unresolved.