----- | -------- | -------- | ---------------------------------------------------- | | A36 | 36 | 58 | General structural plates, base plates, shear tabs | | A572 Gr 50 | 50 | 65 | Higher-strength plates, gussets, heavy splice plates | | A572 Gr 55 | 55 | 70 | Bridge gusset plates | | A588 | 50 | 70 | Weathering steel plates (unpainted, exposed) | | A514 Gr 100 | 100 | 110-130 | Quenched and tempered, crane runway plates |

A36 is the default for plates up to 2 in. thick. A572 Gr 50 is specified when the higher strength allows thinner plate or for material consistency with W-shapes.

Minimum Plate Thickness for Practical Applications

Worked Example -- Tension Splice Plate Design

Problem: Design flange splice plates for a W12x65 column splice. Pu_flange = 210 kip (tension from column uplift). Use A572 Gr 50 plate. 7/8 in. A325-N bolts.

Step 1 -- Required plate area: Ag_req = Pu / (phi x Fy) = 210 / (0.90 x 50) = 210 / 45 = 4.67 in^2

Step 2 -- Trial plate: Two plates, one on each face of the flange. Each plate carries 105 kip. Try PL 5/8 x 8 (Ag = 5.0 in^2 each, total = 10.0 in^2).

Step 3 -- Net section check (rupture): Four 15/16 in. holes per plate. An = (8 - 2 x 0.9375) x 0.625 = (8 - 1.875) x 0.625 = 3.828 in^2 per plate.

phi Rn_rupture = 0.75 x 65 x 3.828 = 186.6 kip per plate > 105 kip. OK.

Step 4 -- Bolt bearing on plate: Lc per bolt = 2.0625 in. Tearout: 1.2 x 2.0625 x 0.625 x 65 = 100.5 kip. Bearing: 2.4 x 0.875 x 0.625 x 65 = 85.3 kip. Bearing controls. phi Rn = 0.75 x 85.3 = 64.0 kip per bolt. 4 bolts per plate: 256 kip >> 105 kip.

Step 5 -- Block shear: Ant = 0.996 in^2, Anv = 5.098 in^2, Agv = 6.563 in^2. Rn = 0.60 x 65 x 5.098 + 1.0 x 65 x 0.996 = 263.5 kip. Cap = 0.60 x 50 x 6.563 + 1.0 x 65 x 0.996 = 261.6 kip. phi Rn = 0.75 x 261.6 = 196.2 kip > 105 kip. OK.

Final plate: 2 PL 5/8 x 8 x 1'-4-1/2 (A572 Gr 50).

Gusset Plate Design — Whitmore Section Method (AISC DG29)

Gusset plates at brace connections carry both axial force from the brace and moments from frame action. The Whitmore section method, described in AISC Design Guide 29 (Vertical Bracing Connections), determines the effective width of the gusset plate for compression and tension checks. The effective width is defined by projecting 30-degree lines from the ends of the brace connection to the last row of bolts or the weld line, then intersecting with a line perpendicular to the brace axis at the end of the connection.

For a brace connected with 4 bolts in two rows at 3-inch pitch and 4-inch gauge, with the last bolt row 6 inches from the gusset-to-beam interface:

Whitmore width = (brace width) + 2 × L_weld × tan(30 degrees)
              = (gauge × 2) + 2 × (bolt pitch × (rows - 1)) × 0.577
              = 8 + 2 × 3 × 0.577 = 8 + 3.46 = 11.46 inches

The gusset plate is then checked for:

1. Tension yielding across the Whitmore width: phi × Rn = 0.90 × Fy × Whitmore_width × t
2. Compression buckling between the last row of bolts and the gusset-to-beam interface, using an effective length factor K = 1.2 for the gusset free edge length
3. Block shear at the beam and column interfaces (two separate block shear checks)
4. Weld capacity connecting the gusset to the beam and column

For seismic applications per AISC 341, the gusset plate is detailed with a 2t offset from the beam and column faces to accommodate frame drift without binding, and the plate must resist the expected yield strength of the brace in tension per the capacity design requirements.

Frequently Asked Questions

What is the difference between block shear rupture and net section fracture?

Block shear involves the simultaneous failure of shear on one plane and tension on a perpendicular plane, typically at the edge of a bolt group. Net section fracture involves tension failure through a row of bolt holes at a single cross-section. Block shear is checked at the end connection (where the bolt group terminates), while net section fracture is checked at any cross-section through holes within the plate. Block shear often governs for short connections with small edge distances; net section fracture governs for long connections where multiple holes are in a single tension line.

When should I specify A572 Gr 50 plate instead of A36?

Specify A572 Gr 50 when the higher yield strength (50 ksi vs 36 ksi) allows a thinner plate, when matching the material of W-shapes (A992/A572 Gr 50), or when plate weight is critical. A36 is typically 5-10% less expensive per pound but may require a thicker plate. For material consistency, many fabricators stock A572 Gr 50 plate exclusively. For baseline designs, try A36 first — if it requires a plate over 1 inch thick, switching to A572 Gr 50 may reduce thickness by approximately 28% (50/36 = 1.39 times strength, accounting for net section).

How do I check plate buckling in compression?

For a rectangular plate loaded in uniaxial compression, the buckling check uses the plate slenderness b/t (width-to-thickness ratio). Per AISC 360 Section E3 for flexural buckling of plates, use r = t / sqrt(12) for the weak-axis radius of gyration and K = 1.2 for gusset plates (per AISC DG29). The effective length is taken as the average of the three lengths from the Whitmore section to the restraint points (beam, column, and brace). For stiffened plates (edges welded to stiffeners), K may be reduced to 0.65.

Can I use stainless steel plate for structural connections?

Stainless steel plates (ASTM A240, typically 304/304L or 316/316L) may be used for corrosion resistance or architectural exposure. The design follows AISC 370 (Specification for Structural Stainless Steel Buildings) or the AISC Design Guide 27. Key differences: (1) Fy and Fu values differ from carbon steel (304: Fy = 30 ksi, Fu = 75 ksi), (2) the stress-strain curve is rounded (no defined yield plateau), (3) the stiffness reduction at elevated temperatures is less than carbon steel, and (4) galvanic corrosion must be considered when stainless plate contacts carbon steel fasteners.

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Related References

• Beam Capacity Calculator
• Steel Connection Design Guide
• Gusset Plate Connection
• Bolt Hole Reference
• Steel Grades Reference
• How to Verify Calculations

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All designs must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) for the specific project. The site operator disclaims liability for any loss arising from the use of this information. Results are PRELIMINARY -- NOT FOR CONSTRUCTION.