Steel Plate Girder Design: Welded I-Girders, Web Stiffeners & Depth Ratios

Blue-gray industrial tone—a large welded I-shaped plate girder inside a fabrication shop, deep gray web plate, paired transverse stiffeners neatly spaced on both sides, wide flange plates with visible weld beads, an overhead crane hook lifting the girder, steel truss roof of the workshop in the background, no text.

A rolled W-shape comes from a mill in a fixed depth. A steel plate girder design is built from three separate plates—two flanges and one web—welded together in the shop. When you need a 3 m deep girder for a 30 m span, or a flange that tapers over the support, no rolled section exists; you weld it. Steel plate girder design is about three decisions: web slenderness and stiffener spacing, flange plate sizing for moment, and depth-span ratio optimization for deflection and weight.

This article treats the built-up beam itself—web, flanges, stiffeners, and welds. Our steel building floor system piece covers slab-deck selection and floor-system layout. A steel plate girder design goes deeper: it is a member-level welding problem, not a floor-system choice.

Why Plate Girders Beat Rolled Beams for Long Spans

Rolled W-shapes top out around 1 m (36 in) deep and roughly 500 kg/m (335 lb/ft) per section. A plate girder starts where the mill stops: depths from 1.5 to 5 m (5 to 16 ft), webs up to 50 mm (2 in) thick, and spans from 20 to 100 m (65 to 330 ft). Because the section is custom-welded, the flange plates can be thicker at mid-span where moment is high and thinner near supports where moment drops; the web can be tall and thin, with stiffeners carrying the shear buckling problem.

The trade-off is fabrication effort. Every meter of girder carries continuous flange-to-web fillet welds, transverse stiffener welds on both sides, and often full-penetration flange splices. Weld distortion must be controlled during fabrication, and the girder must be shipped in segments because road width limits are around 3.5 m (12 ft). Field splices—usually high-strength bolted—then reconnect the segments on site.

Typical applications include bridge main girders, industrial crane beams, long-span roof girders, and conveyor trestles. For long-span framing parallels, see long span steel structure; for composite action that pairs a plate girder with a concrete slab, read steel composite beam design deep dive.

A plate girder carries distributed floor loads along its length. A different member built on the same welded I-section—but loaded by concentrated column point loads landing directly on top of it—is a transfer girder. Our steel transfer girder transfer beam design guide covers the load-detour problem: upper tower columns land at 5,000–15,000 kN on top of the girder and must be redirected sideways to lower podium columns in a different grid, requiring paired bearing stiffeners at every upper-column bearing point, a box section when offset loads create torsion, and deflection limited to L/500–L/1000 so the masonry above does not crack.

Web Panel, Flange Plates & Weld Design

The web is the heart of a steel plate girder design. Typical web slenderness ratios h/t_w run 200–300, far above the roughly 100 limit that rolled beams naturally stay under. That thin web will shear-buckle at service loads unless stiffened. AISC 360 Chapter G sets the slenderness limits: unstiffened webs stay below 9.36√(E/Fy), transversely stiffened webs up to 1.40√(E/Fy), and webs with both transverse and longitudinal stiffeners up to 1.92√(E/Fy). For Q355 steel (Fy ≈ 355 MPa / 51.5 ksi), those translate to practical h/t_w ceilings of roughly 80, 120, and 165 respectively—meaning a 2 m deep web needs a thickness of at least 12–16 mm to stay in the stiffened range.

The flange plates are sized primarily for bending moment. Flange width-to-thickness ratio b_f/(2t_f) must stay within 0.38√(E/Fy) for plastic design or 0.56√(E/Fy) for compact sections. Because moment is highest at mid-span, designers often use variable flange thickness: a 50 mm flange at mid-span tapering down to 25 mm near the support, saving 15–25% of flange weight. The flange-to-web weld is sized by shear flow—usually a double-sided fillet weld with leg size around 6–10 mm (1/4–3/8 in).

Weld distortion is the fabrication risk. Long continuous flange-to-web welds introduce angular distortion and camber deviation; shops use back-step welding and skip welding to spread heat input, then straighten the girder mechanically or by flame. For local buckling principles that underpin these slenderness limits, see steel member local stability; for weld heat-input control, read steel welding distortion control.

Table 1: Plate Girder Web & Flange Slenderness Limits (Q355 / A992 Steel)

Element	Slenderness Limit	Formula (AISC 360)	Typical Value	Notes
Unstiffened web (shear)	h/t_w ≤ 9.36√(E/Fy)	G2	≤ ~80	Short-span girders only
Transversely stiffened web	h/t_w ≤ 1.40√(E/Fy)	G3	≤ ~120	Most common
Transverse + longitudinal web	h/t_w ≤ 1.92√(E/Fy)	G3	≤ ~165	Deep, thin webs
Flange outstand (plastic)	b_f/(2t_f) ≤ 0.38√(E/Fy)	B4	≤ ~9.4	Plastic hinge allowed
Flange outstand (compact)	b_f/(2t_f) ≤ 0.56√(E/Fy)	B4	≤ ~13.8	Elastic design

Limits assume Fy = 355 MPa (51.5 ksi); recalculate for Q235 or A36 steel.

Depth-Span Ratio & Optimal Weight

The single most important economic decision in a steel plate girder design is the girder depth. The economical depth-to-span ratio is 1/12 to 1/18 for simply supported girders and 1/10 to 1/15 for continuous girders. For a 30 m (100 ft) simply supported span, that means a girder depth of 1.7–2.5 m (5.6–8.2 ft).

The weight optimization logic is straightforward. Bending moment is carried almost entirely by the flanges; the web carries shear. As the girder gets deeper, the lever arm between flanges grows, so the required flange area drops. But the web area grows linearly with depth. Total steel weight therefore has a minimum at the depth where the saved flange steel equals the added web steel—typically around L/15.

Deflection is the other constraint. Live-load deflection limits are usually L/500 to L/600 for roof and floor girders. Because moment of inertia I_x grows roughly with depth squared, adding depth is the cheapest way to stiffen the girder. That is why deeper girders are often chosen even when they are slightly heavier—the deflection requirement dominates. For deflection serviceability principles, see steel structure deflection control.

Where a plate girder composites with a concrete slab, long-term creep and shrinkage add a time-dependent deflection that a bare-steel check misses—see our guide on composite beam creep, shrinkage and deflection for the age-adjusted stiffness and camber logic.

Table 2: Plate Girder Depth-Span Ratio Reference

Span (m / ft)	Optimal Depth (m / ft)	Depth / Span	Typical Application
20 / 65	1.3–1.7 / 4.3–5.6	L/12–L/15	Industrial roof girder
30 / 100	1.7–2.5 / 5.6–8.2	L/12–L/18	Warehouse crane runway
45 / 150	2.5–3.5 / 8.2–11.5	L/13–L/18	Bridge main girder
60 / 200	3.5–4.5 / 11.5–14.8	L/13–L/17	Long-span roof truss alternative
90 / 300	4.5–5.0 / 14.8–16.4	L/18–L/20	Deep deck bridge; check transport

Beyond 60 m, continuous girders and haunched profiles usually beat simple prismatic sections.

A plate girder keeps one constant depth from support to support even though the moment diagram varies sharply along the span. For 24–36 m portal frames where the negative moment peaks at the eaves and drops toward midspan, a tapered haunched beam design tracks that envelope directly: the rafter depth grows linearly from ridge to eaves, and a local haunch deepens the column-to-rafter joint to carry the peak moment—typically 10–20% less steel than an equivalent constant-depth plate girder in the 24–36 m band.

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Transverse & Longitudinal Stiffeners

Once the web is thin, stiffeners are not optional—they are the mechanism that keeps the web from buckling in shear. Transverse stiffeners are vertical plates (or double angles) welded to both sides of the web, spaced at a ratio a/h = 1.0 to 2.0. They divide the tall web into smaller panels, each of which can carry higher shear before buckling. At every support, a bearing stiffener is required to transfer the reaction into the web without local crippling.

When h/t_w exceeds roughly 260, transverse stiffeners alone are not enough. A longitudinal stiffener is then added, running horizontally along the web at about h/5 below the compression flange. It effectively halves the web slenderness in the compression zone. At every intersection between longitudinal and transverse stiffeners, the longitudinal stiffener is clipped so that three welds do not meet at one point—a classic fatigue crack initiation detail.

Those clipped stiffener ends sit deep in AISC fatigue detail Category E; our guide on steel structure fatigue detail Category E explains the allowable stress range, the weld-toe grinding and the run-out geometry that decide whether such a detail survives heavy-duty crane cycles.

Stiffener proportions follow AISC 360 G2: the outstanding width b_st must be at least (h/30) + 40 mm (h/120 + 1.6 in), with thickness proportional to width. For connection design that ties stiffeners to flanges and web, see steel structure connection design; for field splice bolts, read steel high strength bolt connection deep dive.

The bearing stiffener at every support is a special case worth its own design check. Our bearing stiffener at beam support guide works through AISC J10.4: the pair is designed as a compression strut including a 12·t_w-wide web strip, checked against outstanding-leg slenderness λ_r = 0.56√(E/F_y), and cut to bear snug against the loaded flange—not just tack-welded—so the reaction goes into the stiffener by direct bearing.

Where the web must carry MEP ductwork as well as shear, the problem flips from stiffening a thin web to perforating a deeper one—cellular beam circular web openings drill clean round holes at 0.5–0.7 beam depth through the parent section, run ductwork through the beam depth instead of below it, and rely on collar reinforcement and Vierendeel tee checks rather than transverse stiffeners to keep the perforated section elastic.

Table 3: Plate Girder Stiffener Schedule

Stiffener Type	Spacing (a/h)	Size Requirement	Location
Transverse (intermediate)	1.0–2.0	b_st ≥ h/30 + 40 mm (1.6 in)	Both sides of web
Bearing (support)	At each reaction	Area ≥ reaction / Fy	Both sides, tight to flange
Longitudinal	Continuous	b_st ≥ 2 × transverse outstanding	h/5 below compression flange
Diagonal (rare)	1.5–2.5	Same as transverse	Tension-field girders
Intermediate + longitudinal	1.5–2.0	Clip at intersections	Deep webs h/t_w > 260

All stiffener welds are double-sided fillet; minimum leg size 6 mm (1/4 in).

AISC 360 Provisions & Tension Field Action

AISC 360-16 Chapter G governs plate girder shear design. Section G2 covers unstiffened webs; G3 covers stiffened webs and introduces tension field action; G4 covers bearing stiffener design. The key design choice is whether to use post-buckling strength or ignore it.

When a thin web shear-buckles, it does not fail outright. A diagonal tension field forms between the stiffeners, like a diagonal truss inside the web, and continues to carry load. Designing for tension field action allows fewer stiffeners and a lighter web—but it imposes additional bending moments on the flanges at the tension-field anchor points, which must be checked. Many practical designs choose the non-tension-field (conservative) approach to simplify the flange check, especially for bridges and crane beams where fatigue matters.

For second-order moment magnification that interacts with girder deflection, see steel structure second order analysis. Per the AISC 360 Specification—Plate Girder Provisions (Chapter G), these shear and stiffener checks are mandatory built-up member requirements.

Cost & Fabrication Considerations

Fabricated plate girders run $4,000–$8,000 per tonne ($1.8–$3.6 per pound), above rolled beams because of welding, fit-up, and distortion control. Tapered or haunched girders add 20–30% for the extra cutting, fitting, and variable-thusion flange butt welds.

Transport drives a major design decision. Shop-fabricated girders are cut into segments no wider than 3.5 m (12 ft) and shipped by truck or barge. Field splices are typically high-strength bolted end-plates, matching holes drilled in the shop. Camber is built in during fabrication—opposite to the dead-load deflection—so the girder finishes level under permanent load. For erection sequencing, see steel building installation contractor; for shipping constraints, read steel shipping logistics.

Conclusion

A steel plate girder design is a welded cross section—thin web, thick flanges, stiffeners—optimized by a depth-to-span ratio of 1/12 to 1/18. Because the web is deliberately thin, stiffeners are a design requirement, not a detail: transverse stiffeners spaced a/h = 1.0–2.0 prevent shear buckling, and a longitudinal stiffener is added when h/t_w exceeds about 260. Weld distortion must be controlled in the shop; once the girder leaves the factory, correcting it in the field is impractical. Tell our engineers your span, loading, and deflection limit, and we will return with a stiffener layout and segment plan.

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About the Author

Senior Structural Engineer

With over 20 years of hands-on experience in steel structure design and prefabricated building engineering, our in-house senior structural engineer has personally contributed to more than 500 steel building projects—including warehouses, industrial factories, aircraft hangars, agricultural buildings, and commercial structures. The focus is on translating design codes such as AISC 360, ASCE 7, and Eurocode 3 into buildable, cost-effective steel solutions that balance structural performance, fabrication efficiency, and total project cost.

Learn more about our engineering team

Case Example

An industrial crane-runway girder shows why plate girders are welded, not rolled. The span was 32 m (105 ft), beyond any economical rolled section, so the shop built a 2.1 m (7 ft) deep welded I-girder of about 14 t (15 tons) each, in an anonymized northern European plant. The web ran slender, h/t_w near 145, so transverse stiffeners at a/h of 1.5 carried the shear-buckling problem, and flanges tapered from 50 mm at mid-span to 25 mm near supports. Back-step and skip welding controlled distortion; after straightening, built-in camber held within 8 mm (5/16 in). The girder shipped in two road-width segments and the field bolted splice aligned on the first attempt. Long-span framing parallels are in long span steel structure; the heat-input control behind straightness is in steel welding distortion control.

Frequently Asked Questions

Q1: What is the difference between a plate girder and a rolled beam?

A rolled W-shape comes from a mill in a fixed depth (max about 1 m / 36 in). A plate girder is built from separate flange plates and a web plate welded together, allowing depths from 1.5–5 m (5–16 ft) and spans up to 100 m (330 ft). It is custom-tailored, not off-the-shelf.

Q2: What depth-to-span ratio should a plate girder use?

The economical depth-span ratio is 1/12–1/18 for simply supported girders and 1/10–1/15 for continuous girders. For a 30 m (100 ft) span, expect a girder depth of 1.7–2.5 m (5.6–8.2 ft). Deeper girders reduce flange weight but add web area and deflection.

Q3: Why do plate girders need web stiffeners?

Plate girder webs are thin (h/t_w = 200–300, far higher than rolled beams). Without stiffeners, the web would shear-buckle at service loads. Transverse stiffeners (spaced a/h = 1.0–2.0) prevent this; longitudinal stiffeners are added when h/t_w exceeds about 260.

Q4: How much does a steel plate girder cost?

Fabricated plate girders run $4,000–$8,000 per tonne ($1.8–$3.6 per pound)—above rolled beams due to welding and fabrication. Tapered or haunched girders add 20–30%. Girders are shipped in segments (width limited to about 3.5 m / 12 ft) and field-spliced with high-strength bolts.

Q5: What is tension field action in a plate girder?

After a thin web shear-buckles, a diagonal tension field forms between stiffeners and continues to carry load. Designing for this post-buckling strength (AISC 360 G3) allows fewer stiffeners but adds flange bending moments that must be checked. Many bridges and crane beams use the conservative non-tension-field approach for fatigue reasons.

Reference Links

• AISC 360 — Specification for Structural Steel Buildings, Chapter G — shear buckling, stiffener proportions, and tension field provisions for built-up plate girders.
• AASHTO LRFD Bridge Design Specifications — plate girder bridge provisions for highway structures.