W18x55 wide flange beam: designation and section geometry
W18x55 is an American standard wide flange beam rolled to ASTM A6 dimensional practice. The name records the nominal depth in inches and the nominal mass per unit length, so the shape is about 18 in deep and weighs 55 lb/ft, which is about 81.9 kg per metre. The designation does not encode the flange width, which is why the imperial name alone can be misleading on drawings: the flange of a W18x55 is only about 7.5 in wide, not 55 in.
Hot rolling from a beam blank produces parallel flange faces and a continuous rolled fillet at the web junction, with no longitudinal weld. This is a good fit for bolted framing because the parallel faces allow standard end plates and shear tabs to seat without tapered washers, and the depth to weight ratio suits long span floor beams where structural depth has to stay inside the ceiling void.
ASTM A992 steel for wide flange beams
ASTM A992 is the specification written for structural steel shapes used in building frames. It combines a 345 MPa minimum yield strength with an upper yield limit and a controlled tensile to yield ratio, plus a maximum carbon equivalent, so that the steel is both strong and reliably weldable. Grade A992 in shapes is microalloyed with niobium and vanadium to reach the strength without excessive carbon.
| Property | ASTM A992 requirement |
|---|---|
| Minimum yield strength | 345 MPa (50 ksi) |
| Maximum yield strength | 450 MPa (65 ksi) |
| Minimum tensile strength | 450 MPa (65 ksi) |
| Tensile to yield ratio | Not less than 1.05 |
| Minimum elongation | 18% in 200 mm gage length; typical results reach 21% |
| Carbon, maximum | 0.23% |
| Carbon equivalent, maximum | 0.45% |
| Microalloying | Vanadium and niobium additions are typical |
The ductility implied by the ratio and elongation limits is the reason A992 is preferred for frames designed for seismic or dynamic action, and the carbon equivalent limit keeps field welding practical without high preheat. A572 Grade 50 is often offered as an alternative for the same shape; it is a valid substitute where the frame does not depend on the tighter A992 ductility controls, but the two specifications should not be used interchangeably without checking the design assumptions.
Nominal dimensions and section properties of W18x55
The properties below are nominal tabulated values. Historically published pages sometimes give the flange width of a W18x55 as 55 inches or a section area of a few hundred square millimetres, both of which are unit errors: 55 is the weight in pounds per foot, and the section area of this shape is about 16.2 square inches. The corrected values are listed here.
| Parameter | Imperial | Metric |
|---|---|---|
| Nominal depth d | 18.11 in | 460.0 mm |
| Flange width bf | 7.535 in | 191.4 mm |
| Flange thickness tf | 0.630 in | 16.0 mm |
| Web thickness tw | 0.390 in | 9.9 mm |
| Section area A | 16.2 in2 | 104.5 cm2 |
| Moment of inertia Ix | 890 in4 | 37,000 cm4 |
| Elastic section modulus Sx | 98.3 in3 | 1,611 cm3 |
| Radius of gyration rx | 7.41 in | 18.8 cm |
With a section modulus of about 1,611 cm3 the nominal yield moment at 345 MPa is roughly 555 kN.m. That is the moment at first yield on the extreme fibre; the plastic moment is higher, while lateral torsional buckling over an unbraced length and shear at the supports will reduce what the beam can actually be designed to carry. A narrow flange relative to depth also means the section is more sensitive to buckling than a heavier W18 rolled from the same group, so bracing spacing deserves attention in the layout.
Fabrication and connection notes
Cutting and drilling: the shape is easily sawn, sheared at plate thicknesses and drilled with standard tooling; thermally cut edges are dressed where coatings or fatigue details require it.
Welding: electrodes of the E70XX class or matching gas shielded wire are appropriate. Carbon equivalent below 0.45% keeps preheat requirements modest, but procedure qualification and preheat selection remain mandatory.
Bolted connections: parallel flanges allow flush end plates and shear tabs. Bolt bearing and block shear checks at the flange and web often govern the connection size rather than the beam itself.
Holes and cope details: standard cope geometry keeps the flange and web continuity intact; reinforcing plates are added where the design moment or shear demands them.
Applications of W18x55 beams rolled from A992
Floor and roof beams in multi storey steel frames, including long spans where depth is limited.
Bridge stringers, floor beams and cross bracing in short and medium span bridges.
Mezzanine and platform framing in warehouses, plants and logistics buildings.
Machine support frames and conveyor structures that see repeated loading.
Tower, pipe rack and port handling structures where the ductility of A992 is valued.
FAQ about W18x55 wide flange beams
Q: Is the flange of a W18x55 really 55 inches wide?
No. The 55 refers to the mass of the beam in pounds per foot. The flange width of the shape is about 7.5 in, or roughly 191 mm, and the depth is about 18.1 in, or 460 mm.
Q: Can A572 Grade 50 be used instead of A992 for this beam?
In many static structures it can, because both grades reach 345 MPa minimum yield. Where the frame relies on the tighter yield ratio and carbon equivalent controls of A992, the substitution needs engineering approval, and the grade marked on the certificate must match the design.
Q: What strength does an A992 W18x55 actually reach?
The specification sets 345 MPa as the minimum yield and 450 MPa as the minimum tensile strength, with an upper yield limit of 450 MPa and a tensile to yield ratio of at least 1.05. Actual mill results sit above the minimums but the design must be based on the specified values.
Q: How is the beam protected against corrosion?
Shop applied paint, hot dip galvanizing after fabrication, or a combination of primer and site topcoat are all common. Galvanizing is often chosen for external frames and bridge components, and vent and drain holes must be detailed so that the coating covers internal surfaces.
Q: What unbraced length is acceptable for a W18x55?
The allowable unbraced length depends on the moment demand, the bracing type and the governing code. As a rule the narrow flange of a light W18 section benefits from closer bracing than a heavier shape, so the framing layout should place secondary beams and braces with the buckling check in mind.