Stiffener Design — Bearing, Transverse & Longitudinal Stiffeners

Transverse stiffener sizing, bearing stiffener design, and longitudinal stiffeners for plate girders. AISC 360 Chapters G and J provisions, worked examples, and cross-code comparison.

Why stiffeners are needed

Stiffeners are plates welded to the web of a beam or girder to prevent local web failures. Without stiffeners, deep thin webs fail by three mechanisms:

• Web local yielding — the web crushes directly under a concentrated load because the bearing area is insufficient. Governed by AISC 360 Section J10.2.
• Web crippling — the web buckles locally in a region adjacent to the loaded flange. Governed by AISC 360 Section J10.3.
• Web sidesway buckling — the compression flange buckles laterally due to concentrated load applied to one flange. Governed by AISC 360 Section J10.4.

Additionally, deep girder webs (d/tw > 50-60) may require transverse stiffeners to develop the full post-buckling shear strength (tension field action) per AISC 360 Section G2.2, and longitudinal stiffeners to increase the web bending capacity in very slender plate girders.

Types of stiffeners

Bearing stiffeners

Bearing stiffeners are fitted to the top and bottom flanges at points of concentrated load (supports, point loads from columns or beams). They act as short columns transferring the load from the top flange through the web to the bottom flange. AISC 360 Section J10.8 requires bearing stiffeners when the web fails any of the J10.2 through J10.5 checks.

Design the bearing stiffener as a column with an effective cross-section consisting of the stiffener plates plus a strip of web (25tw on each side of the stiffener per AISC, or 15tw per AS 4100). The column effective length is taken as 0.75h (where h is the clear web depth) because the flanges provide rotational restraint.

Transverse stiffeners

Transverse stiffeners are welded to the web (but not necessarily to the flanges) at regular intervals to subdivide the web into panels for shear resistance. They allow the engineer to take advantage of tension field action — the post-buckling shear capacity where the web acts like a diagonal tension brace after initial web buckling.

AISC 360 Section G2.2 permits tension field action when intermediate stiffeners are provided. Without stiffeners, the web shear capacity is limited to the buckling strength alone. With stiffeners, the capacity can be 1.5 to 2 times the buckling strength.

Longitudinal stiffeners

Longitudinal stiffeners run parallel to the flanges along the length of the girder. They divide the web into two panels for bending compression buckling. Rarely needed for standard rolled sections, but used in deep plate girders (d > 1500 mm) where the web depth-to-thickness ratio exceeds the slender limit.

Worked example — bearing stiffener at beam support

Beam: W24x62 (A992), d = 23.74 in, tw = 0.430 in, tf = 0.590 in, h/tw = 50.0. Support reaction Ru = 120 kips. Bearing length N = 3.5 in (width of support bearing plate).

Check web local yielding (AISC 360 Eq. J10-2, at support): phi*Rn = phi x (2.5k + N) x Fy x tw = 1.0 x (2.5 x 1.34 + 3.5) x 50 x 0.430 = 147 kips > 120 kips. OK.

Check web crippling (AISC 360 Eq. J10-4, at support with N/d <= 0.2): phi*Rn = 0.75 x 0.40 x tw^2 x [1 + 3(N/d)(tw/tf)^1.5] x sqrt(E x Fy x tf / tw) = 0.75 x 0.40 x 0.430^2 x [1 + 3(3.5/23.74)(0.430/0.590)^1.5] x sqrt(29000 x 50 x 0.590/0.430) = 0.75 x 0.0739 x 1.316 x 141.8 = 10.3 kips. This is far less than 120 kips — web crippling fails.

Bearing stiffeners required. Try 2 plates, one each side of web, 5 in wide x 1/2 in thick, ASTM A36 (Fy = 36 ksi).

Stiffener effective section: 2 x (5.0 x 0.50) + web strip (25 x 0.430 x 2 x 0.430) = 5.0 + 9.25 = 14.25 in^2. Wait — let me recalculate: web strip = 25 x tw on each side = 25 x 0.430 = 10.75 in each side, total web strip width = 21.5 in, web area = 21.5 x 0.430 = 9.25 in^2. Stiffener area = 2 x 5.0 x 0.50 = 5.0 in^2. Total A = 14.25 in^2.

Effective column height = 0.75 x (d - 2tf) = 0.75 x (23.74 - 1.18) = 16.9 in. I of composite section about the web axis = (0.430 x 21.5^3 / 12) + 2 x (0.50 x 5.0^3 / 12) + 2 x (0.50 x 5.0 x (0.430/2 + 2.5)^2) = 356 + 10.4 + 37.4 = 404 in^4. r = sqrt(404/14.25) = 5.32 in.

KL/r = 16.9 / 5.32 = 3.2. Very stocky — Fcr is essentially Fy. phi*Pn = 0.90 x 36 x 14.25 = 462 kips >> 120 kips. The 5 x 1/2 stiffeners are more than adequate.

Code comparison — stiffener requirements

Common pitfalls

• Welding transverse stiffeners to the tension flange in fatigue-sensitive locations. Welding a stiffener to the tension flange creates a fatigue Category C detail (AISC 360 Appendix 3, Table A-3.1). For crane girders and bridge beams, stop the stiffener 4tw to 6tw short of the tension flange.
• Omitting the bearing check at interior point loads. Engineers often check supports for web crippling but forget that any column or beam bearing on the girder flange applies the same concentrated load. Every point load location needs the J10.2-J10.5 checks.
• Using too-thin stiffener plates. Stiffener outstands must satisfy the compactness limit to prevent local buckling before the stiffener reaches its design load. The thickness limit is b_st x sqrt(Fy/E) / 0.56 per AISC.
• Not fitting bearing stiffeners to both flanges. Bearing stiffeners must be in contact with (or welded to) both flanges to transfer the full reaction. A stiffener welded to the web but not bearing on the flanges is a transverse stiffener, not a bearing stiffener, and does not resist concentrated loads.

Run this calculation

• Section Properties Database
• Beam Calculator

Related references

• Plate Girder Design
• Coped Beam End Design
• Lateral-Torsional Buckling
• Steel Crane Girder
• How to Verify Calculations
• steel beam capacity calculator
• weld capacity calculator

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable standard and project specification before use. The site operator disclaims liability for any loss arising from the use of this information.