Steel Drawing Symbols and Abbreviations
What the marks on a shop drawing actually mean — shape designations, weld symbols, bolt callouts and the abbreviations that trip people up.
A structural steel drawing has to fit an entire assembly onto one sheet, so almost everything on it is shorthand. That shorthand is standardised, which is good, but it is only obvious once someone has explained it. This is that explanation.
Nothing here is design guidance. It is a decoder for reading what is already on the sheet.
Shape designations
A member callout carries the shape family, then its defining numbers.
| Callout | Family | How to read it |
|---|---|---|
W21X50 | Wide flange | Nominally 21 in deep, 50 lb per foot |
C10X15.3 | Channel | 10 in deep, 15.3 lb per foot |
MC12X50 | Miscellaneous channel | 12 in deep, 50 lb per foot |
L4X4X3/8 | Angle | 4 in legs, 3/8 in thick |
HSS6X6X3/8 | Hollow structural section | 6 × 6 in outside, 3/8 in nominal wall |
WT6X25 | Tee cut from a W | Half of a W12X50 |
PL1/2X12 | Plate | 1/2 in thick, 12 in wide |
For W, C, MC and HP shapes the second number is the weight per foot, which is why a takeoff can be done straight off the callouts. Angles and HSS work differently — those numbers are dimensions, and the weight has to be looked up.
The catch with W shapes: nominal depth is not actual depth. Every member in a W14 group is called 14 inches, but a W14X90 and a W14X48 measure differently, because heavier members in a group are rolled with thicker flanges. Use the detailing dimension for fit-up, never the nominal.
Weld symbols
A weld symbol is an arrow, a horizontal reference line, and information placed above or below that line.
The rule that matters most: below the line means the arrow side, above the line means the other side. Get that backwards and the weld ends up on the wrong face.
| Mark | Meaning |
|---|---|
| Triangle | Fillet weld. The number before it is the leg size |
| Square or V shape | Groove weld — square, bevel, V, U or J prep |
CJP | Complete joint penetration — full thickness |
PJP | Partial joint penetration, to a stated depth |
| Circle at the elbow | Weld all around |
| Flag at the elbow | Field weld, not shop |
| Tail on the line | A reference — a WPS number, a spec, or a note |
So a 1/4 before a triangle below the line means a quarter-inch fillet on the arrow side. The same triangle above the line puts it on the far side. Both, and you weld both faces.
Bolt callouts
Bolts are called out by diameter, grade and the condition they are installed to.
| Mark | Meaning |
|---|---|
3/4"Ø A325 | Three-quarter inch high-strength bolt |
A490 | Higher-strength alloy bolt |
A307 | Low-carbon — secondary and non-structural only |
ST | Snug tight |
PT | Pretensioned |
SC | Slip critical |
STD / SSL / LSL | Standard, short-slotted or long-slotted hole |
Two things worth knowing here.
A325 and A490 are now ASTM F3125. The two specifications were consolidated in 2015 into F3125 Grade A325 and Grade A490. The bolts did not change; the specification number did. Both legacy names remain in daily use on drawings.
A490 bolts are not to be galvanized, because of hydrogen embrittlement risk. If a job needs high-strength bolts in a corrosive environment, that constraint has to be resolved in detailing, not discovered in the field.
The installation condition is a design requirement, not an installer's preference. It belongs on the drawing, and high-strength bolted joints are governed by the RCSC Specification.
Abbreviations that cause trouble
Most abbreviations on a steel drawing are guessable. These are the ones that are not, or that get misread.
| Abbreviation | Meaning |
|---|---|
T/S | Top of steel |
B/O | Bottom of |
EL | Elevation |
CL | Centerline |
WP | Work point |
O/C | On center |
TYP | Typical — applies to all similar conditions |
UNO | Unless noted otherwise |
SIM | Similar — same detail, not necessarily identical |
OPP HD | Opposite hand — mirrored, not a copy |
NS / FS | Near side / far side |
THK | Thick |
GA | Gauge — sheet, not plate |
NTS | Not to scale |
TYP and SIM are not the same thing. Typical means every matching condition gets this detail exactly. Similar means the same idea, adjusted to fit — and the adjustment is the detailer's to work out, which means it is worth a question rather than an assumption.
OPP HD is a mirror, not a duplicate. Fabricating a second piece identical to the first, rather than handed, is one of the more expensive misreads on this list, because it is usually caught at erection.
Where the shorthand stops
Two conventions cause repeat problems in ordering rather than fabrication.
Gauge is sheet language. Above roughly 3/16 in, material is plate and is called out by decimal or fractional thickness. Gauge numbers run backwards, have no unit, and differ between plain steel, galvanized and stainless — the same gauge number is three different thicknesses depending on the material. If a drawing says gauge, confirm which series before ordering.
Nominal weights are not delivered weights. Hot rolled material carries a thickness tolerance under ASTM A6 and mills tend to roll to the high side, so what arrives usually weighs slightly more than what was calculated. Fine for estimating, worth confirming against the mill test report when it matters.
Look it up
We built the reference tools we kept needing:
- Steel gauge thickness chart — every gauge in inches, millimetres and lb/ft², with the separate galvanized and stainless series
- Plate weight calculator — 3/16 in to 6 in, standard mill sizes, weight per plate and per order
- Structural shape weights — 1,004 shapes with weight per foot, depth and area
- Connection reference — what each connection type does and what goes wrong with it
All free, no sign-up.