Wide Flange Beams and H Piles: What They Are
Wide flange beams, written as W shapes or W beams, are hot rolled structural sections with parallel flange faces, a thick web and a flange width that is often close to the section depth. The parallel flanges make the sections efficient in bending and easy to connect, and they allow the same profile family to be used as beams, columns, bracing and, in a heavier version, as driven piles.
H piles use the same geometry but a different design logic. Because the pile transfers load to the ground and is driven through soft strata, the section is chosen for axial capacity, tip and skin friction behaviour and driving resistance, with bending capacity needed mainly to resist handling and lateral loads. Under American practice the grades most often specified for both applications are ASTM A36 and ASTM A572, with ASTM A992 for building frames.
ASTM A36: The Baseline Structural Steel
| Property | Requirement |
|---|---|
| Minimum yield strength | 250 MPa (36 ksi) |
| Tensile strength | 400 to 550 MPa (58 to 80 ksi) |
| Elongation | 20 % minimum in the tensile test |
| Typical characteristics | excellent weldability, good forming and machining, stable supply |
A36 is a carbon structural steel rather than a high strength low alloy steel, and its value lies in cost, availability and ease of fabrication rather than in high strength. It is the appropriate grade for building frames, platforms, walkways, base plates, secondary framing and light industrial structures where the design stress is comfortably below the strength of the material.
ASTM A572: High Strength Low Alloy Sections
| Grade | Minimum yield strength (MPa) | Minimum yield strength (ksi) |
|---|---|---|
| Grade 42 | 290 | 42 |
| Grade 50 | 345 | 50 |
| Grade 55 | 380 | 55 |
| Grade 60 | 415 | 60 |
| Grade 65 | 450 | 65 |
A572 is a columbium vanadium high strength low alloy steel. The micro alloying additions raise the yield strength without the carbon increase that would damage weldability, so the designer can reduce section size and steel mass for the same load. Grade 50 is the workhorse of the series and the grade most frequently rolled as W shapes, while the higher grades are used for long span bridges, crane girders, heavy truck frames and other structures where the dead load of the steel itself matters.
Typical W Beam and H Pile Sizes
| Section | Nominal mass (kg/m) | Depth (mm) | Flange width (mm) | Web thickness (mm) | Flange thickness (mm) |
|---|---|---|---|---|---|
| W4x4x13 | 19.3 | 106 | 103 | 7.1 | 8.8 |
| W4x4x16.3 | 24.0 | 107 | 100 | 7.9 | 12.0 |
| W5x5x16 | 23.8 | 127 | 127 | 6.1 | 9.1 |
| W6x4x8 | 11.9 | 148 | 100 | 4.0 | 4.7 |
| W6x4x12 | 18.0 | 153 | 102 | 5.8 | 7.1 |
| W6x6x15 | 22.5 | 152 | 152 | 5.8 | 6.6 |
| W8x4x10 | 15.0 | 200 | 100 | 4.3 | 5.2 |
| W8x4x15 | 22.5 | 206 | 102 | 6.2 | 8.0 |
These are nominal dimensions for light to medium sections and are quoted to show the range that can be rolled; the actual drawing values should always be confirmed for a particular order, because the depth, flange width and thickness combination changes the section properties that the design depends on.
Engineering Advantages and Selection Guide
Wide flange sections offer high bending resistance because a large part of the material sits in the flanges, efficient load transfer, simpler bolting and welding than tapered sections, and equal suitability for beam, column and pile duties. H piles made from W sections are particularly effective in deep foundations, where the straight profile allows easy splicing and where the section can carry both vertical load and bending from lateral soil movement.
| Aspect | ASTM A36 | ASTM A572 |
|---|---|---|
| Minimum yield strength | 250 MPa | 345 to 450 MPa depending on grade |
| Weight optimisation | standard | excellent, smaller sections for the same load |
| Fabrication | excellent | very good, qualified welding procedures required |
| Relative cost | lower | slightly higher per tonne |
| Typical use | general construction and secondary steel | heavy duty, long span and high rise structures |
Frequently Asked Questions
Q: What is the yield strength of ASTM A36 beams?
A: The specified minimum yield strength is 250 MPa (36 ksi) with a tensile strength of 400 to 550 MPa (58 to 80 ksi).
Q: What is the difference between A36 and A572?
A: A36 is a plain carbon structural steel with a minimum yield strength of 250 MPa, while A572 is a columbium vanadium high strength low alloy steel with minimum yield strengths from 290 MPa to 450 MPa, allowing lighter sections for the same load.
Q: Which A572 grade is the most common for W beams?
A: Grade 50, with a minimum yield strength of 345 MPa, is the grade most widely rolled and specified for structural sections.
Q: Can the same section be used as an H pile?
A: Yes. Wide flange profiles are used as H piles, and the design then considers axial capacity, driving stresses, splicing and the bending demand from lateral loading rather than bending alone.
Q: What section property governs beam selection?
A: Bending strength is governed by the section modulus and stiffness by the moment of inertia; shear, deflection, lateral torsional buckling and connection design must also be checked before the section is confirmed.
Q: What tolerances apply to W shapes?
A: Dimensions, mass, straightness and camber tolerances are set out in ASTM A6, while the mechanical requirements come from the grade standard such as A36, A572 or A992.