Reading Welding Symbols & Connection Details on Structural Drawings

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Alright, Reading Welding Symbols & Connection Details on Structural Drawings. Don't let the plain title fool you. Here's the part that actually matters on the job: On structural drawings the weld symbol, the connection detail, and the general notes work together, and the welder has to read all three before deciding what to weld and how big. Learn it well and you stop guessing at weld sizes and start building exactly what the engineer designed.

The intro course taught the symbol itself: reference line, arrow, arrow side versus other side, and the basic weld symbols. This lesson is about reading symbols where they live, on structural and shop drawings, and understanding the connection details they describe.

Where the weld information lives

A field welder works from three layers of documents, and they do not always say the same thing:

• The structural drawings (S-sheets) from the engineer of record. The general notes set the rules for the whole job: the governing code edition (AWS D1.1, AISC 360), the steel grades, the electrode class (usually E70XX), minimum fillet sizes, and often "all welds continuous unless noted." Typical details show standard connections; the framing plans say where they apply.
• The shop drawings from the fabricator's detailer: piece drawings for each member and erection drawings showing where each piece goes and which connections are field welded. Under AISC 303, the Code of Standard Practice, the fabricator prepares these and the engineer reviews them.
• The WPS the contractor's quality program assigns to each joint type. When the layers disagree, the welder does not pick one. The foreman writes an RFI and the engineer answers it.

The symbol beyond the basics

Structural symbols carry more than a weld type and a side. Read every element:

• Size and effective throat — a fillet size sits to the left of the fillet symbol. For a partial-joint-penetration groove weld, the groove depth sits to the left of the groove symbol and the required effective throat (E) appears in parentheses.
• Length and pitch — a number to the right of the weld symbol is the length; a second number after a hyphen is the pitch, center to center. "4-12" means 4 in. segments on 12 in. centers. Symbols on both sides, aligned, are chain intermittent; offset, they are staggered intermittent.
• Weld-all-around — a circle at the junction of arrow and reference line. The weld continues around every edge of the joint, which is not the same as "both sides."
• Field weld flag — a small flag at the same junction. Shop welds have no flag.
• Backing — a rectangle opposite the groove symbol; an "R" in it means the backing is removed after welding.
• The tail — "CJP," "TYP," a note number, a WPS number, or a prequalified joint designation such as B-U4a, D1.1 shorthand for joint type, thickness range, groove shape, and process.

Fillet welds, sizes and lengths

Fillet welds are the workhorse, and the rules around them matter:

• Minimum size is set by the thickness of the parts joined: 1/8 in. for the thinnest material, then 3/16 in., 1/4 in., and 5/16 in. as thickness climbs past 1/4, 1/2, and 3/4 in. D1.1 and AISC differ slightly on which part governs, so follow the one your project specifies.
• Maximum size along an edge is the full thickness for material up to 1/4 in., and the thickness minus 1/16 in. for thicker material, so the inspector can see the edge.
• Minimum effective length is four times the weld size, and intermittent segments must be at least 1-1/2 in. long. The size on the symbol is the minimum the engineer needs. Welding larger wastes consumables, adds heat and distortion, and earns no credit.

Groove welds, CJP, PJP, backing and access holes

A complete-joint-penetration (CJP) groove weld fuses the full thickness and needs either steel backing or a backgouged root welded from the second side. A partial-joint-penetration (PJP) weld is designed for its effective throat only. Column splices are often PJP; moment-connection flanges and heavy splices are CJP. Backing bars under CJP welds must be continuous and tight to the plate. Where a beam flange is CJP-welded to a column, the shop cuts weld access holes in the web to the geometry AISC 360 and D1.1 require, with smooth ground surfaces. A welder never torch-cuts an access hole freehand in the field; a badly shaped hole is a crack starter.

Common connection details you will field weld

• Shear tabs — a single plate shop-welded to the column, usually field-bolted to the beam web.
• Moment connections — beam flanges CJP-welded to the column flange in the field, web bolted or welded. These carry the most inspection and the most rules.
• Column splices — PJP or CJP flange and web welds, made after the tier is plumbed.
• Stiffeners, continuity plates, and brace gussets — fillet welds to flanges and webs, often double-sided.

Going Deeper (Intermediate)

Read the hierarchy: general note, then typical detail, then specific detail, then shop drawing. A shop drawing that reduces a weld shown on the structural drawings is a flag, not a convenience. "Weld per D1.1" in the tail does not tell you the size; the size comes from the detail or the minimum-size rule, and a welder who cannot find it asks. The arrow on a bevel symbol breaks toward the member that gets the bevel.

Advanced / Pro-Level

Seismic moment connections are built from AISC 358 prequalified types, and AWS D1.8 adds rules the symbol alone does not show: bottom-flange backing removed and the root backgouged and reinforced with a fillet, weld tabs removed and ground smooth, and demand-critical welds identified on the drawings. Pros know the symbol standard itself, AWS A2.4, changes between editions; when a symbol looks odd, they ask rather than interpret.

Practice Challenge

A stiffener detail shows a fillet symbol with "5/16" to the left, "4-12" to the right on both sides of the reference line, the symbols offset from each other, and a flag at the junction. The stiffener is 1/2 in. plate welded to a 3/4 in. flange. What do you weld? (Answer: 5/16 in. fillet welds on both sides of the stiffener, each segment 4 in. long on 12 in. centers, staggered so the segments on one side fall opposite the gaps on the other, made in the field. The 5/16 in. size satisfies the minimum for the 3/4 in. material and the 4 in. segments exceed the 1-1/2 in. intermittent minimum.)

In Practice

A welder who reads the general notes once and checks the shop drawing for each connection builds the frame the engineer designed. A welder who sizes welds by eye either overwelds everything, bending plates and burning hours, or underwelds the one connection the inspector tests.

Common Mistakes to Avoid

• Reading only the symbol and never the general notes or the detail it points to
• Confusing weld-all-around with both-sides
• Treating the size on the symbol as a target to exceed instead of a minimum to meet
• Resolving a conflict between the shop drawing and the structural drawing on your own instead of through an RFI

Takeaway: On structural drawings the weld symbol, the connection detail, and the general notes work together, and the welder has to read all three before deciding what to weld and how big.

Educational content only — not engineering, design, inspection, or safety advice, and not a substitute for hands-on training, a qualified welding procedure, or a licensed professional. Weld to the WPS and the drawings, follow AWS D1.1, AISC, and OSHA requirements as adopted and applied on your project, and follow the manufacturer's instructions for your equipment and consumables.