Steel Composite Beam Design: Shear Connectors, Concrete Flange & Shoring

A steel I-beam with a row of headed shear studs welded to its top flange, poured into a reinforced concrete slab that forms a composite T-section.

A plain steel beam holds a concrete slab only by friction. A composite beam nails the slab to the steel beam with welded studs, so the two materials act as one T-section—the concrete resists compression and the steel resists tension. You get a deeper, stiffer floor with less steel. Steel composite beam design is the engineering of that composite action: how many shear connectors to weld, how wide the effective concrete flange runs, and whether you prop the beam during the concrete pour.

This article walks through why composite action beats a non-composite section, how headed studs are sized and spaced, when propped versus unpropped construction wins, and how creep and shrinkage shape long-term deflection. A broad floor-system survey (our steel building floor system article) covers deck types and secondary beam selection. This piece goes one level deeper—it makes the slab and the beam work together, not just sit on top of each other.

Composite Action — How the Two Materials Work Together

In a non-composite beam, the concrete slab is just dead load. The steel section alone resists all bending, with its top flange in compression and its bottom flange in tension. The slab does nothing structurally except sit there.

In a composite beam, welded headed studs lock the slab to the steel beam. The studs prevent horizontal slip at the steel-concrete interface, so the slab and the beam bend as a single T-section. The wide concrete flange sits in compression, where concrete is strongest, and the steel beam sits in tension, where steel is strongest. Each material does what it does best, and the section leverages both.

The payoff is measurable. Compared with an equivalent non-composite beam, a properly detailed steel composite beam design typically trims steel weight by 20–35% while increasing stiffness 2–3 times for the same span. That means a smaller beam depth under the floor, more clear headroom, and longer spans between columns—exactly what offices, hotels, and multi-story buildings want. For how this fits a full building program, see multi-story steel building.

The concrete flange does not count at full width. AISC 360 Specification Chapter I limits the effective flange width to a fraction of the span on each side of the beam, so the designer uses a conservatively wide but realistic compression block. Slab thickness on a metal-deck composite floor usually runs 100–150 mm (4–6 in) total, measured from the top of the deck to the slab surface. Deflection limits for the resulting floor are covered in steel structure deflection control.

Shear Connectors (Headed Studs)

The studs are the entire point of the system. If they slip, the composite action collapses and the beam behaves non-composite. The shear connector is a headed stud—a cylindrical steel pin welded through the metal deck onto the beam's top flange, with a head that anchors into the concrete.

Typical studs run 13–22 mm (1/2–7/8 in) in diameter and 65–125 mm (2.5–5 in) tall. The material is a weldable mild steel (commonly ML15Al or A108-equivalent), chosen for both tensile strength and reliable arc welding. The factored shear capacity of a single stud is computed per AISC 360, based on stud area, tensile strength, and concrete strength. The number of studs per beam is then set by the horizontal shear flow at the interface—more studs near the supports where shear flow is highest, fewer toward mid-span.

Spacing rules are strict. Studs must be at least 5 stud diameters apart along the beam and no more than roughly 300 mm (12 in) on center in the critical zones. They are welded by a dedicated stud-welding gun that burns through the metal deck directly onto the beam flange in a single short arc, leaving a secure, inspected connection. For how these welds fit the broader joining logic, see steel structure connection design and bolted vs welded steel connection; for the shop process itself, read steel building welding process.

Table 1: Typical Shear Connector Stud Schedule

Stud Dia (mm/in)	Height (mm/in)	Nominal Shear Capacity (kN/lbf)	Min Spacing	Max Spacing
13 / 1/2 in	65 / 2.5 in	~55 / ~12,400	5× dia (~65 mm)	300 mm / 12 in
16 / 5/8 in	90 / 3.5 in	~85 / ~19,100	5× dia (~80 mm)	300 mm / 12 in
19 / 3/4 in	100 / 4 in	~120 / ~27,000	5× dia (~95 mm)	300 mm / 12 in
22 / 7/8 in	125 / 5 in	~165 / ~37,000	5× dia (~110 mm)	300 mm / 12 in

Capacities are typical nominal values; final values per AISC 360 Eq. I8-1 depend on concrete strength and deck orientation. Consult our engineers for your mix design.

Propped vs Unpropped Construction

A composite beam only becomes composite once the concrete hardens. While the wet concrete is being poured, the slab has no strength of its own, so the bare steel beam must carry the wet load plus construction live load. How you handle that stage defines propped versus unpropped construction.

Unpropped construction shores nothing. The bare steel beam carries the wet concrete and the construction load as a non-composite section during pouring. It is cheaper on shoring, but the beam must be sized up for that construction stage—often the controlling case. The composite section then carries the finished live load.

Propped construction installs temporary shores under the beam before pouring. The shores carry the wet concrete, the beam stays small, and the shores are removed only after the slab gains strength. Now the composite T-section carries the load. The trade-off is extra shoring cost and a slightly longer schedule, but a smaller steel section.

Table 2: Propped vs Unpropped Comparison

Method	Steel Section	Shoring Cost	Schedule Impact	Notes
Unpropped	Larger (sized for wet load)	None	Faster	Beam checked as non-composite first
Propped	Smaller (composite governs)	Added	+3–7 days for shores	Steel savings vs shoring cost decides

The decision is economic: if the steel saved by going propped exceeds the shoring rental and delay, go propped. Otherwise unpropped. For the connections that carry this load to the foundation, see steel column base plate design; for the lateral system that keeps the whole frame upright, read steel building bracing system.

Sizing the Slab and the Studs as One T-Beam?

We design composite floors so the concrete flange and steel beam act together—stud spacing, propped or unpropped, and long-term creep deflection all checked. Tell us your span and floor load.

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Deflection, Creep & Shrinkage

Short-term deflection is checked on the composite section for live load and superimposed dead load. But concrete does not behave elastically forever. Under sustained load, concrete creeps—it continues to deform slowly over years—so the effective stiffness of the concrete flange drops and long-term deflection grows to roughly 1.5–2.0× the short-term value. On top of that, slab shrinkage pulls the concrete flange shorter over time, putting tension into the slab and adding a small additional downward camber demand on the steel beam.

These long-term effects must be folded into the deflection check. Typical serviceability limits are L/360 for live load deflection and L/240 for total load, per ASCE 7 Minimum Design Loads. Camber is often pre-fabricated into the beam to offset the anticipated long-term deflection so the finished floor stays flat under occupancy. Footfall vibration is a separate serviceability check—see steel floor vibration serviceability. Budgeting this creep allowance up front is one of the details that separates a durable steel composite beam design from one that sags within a decade.

For the worked numbers—age-adjusted effective modulus, how camber is scheduled against creep and shrinkage, and the L/240 long-term check—see our deep guide on steel composite beam creep, shrinkage and long-term deflection.

Table 3: Composite Beam Deflection Schedule

Load Case	Deflection Limit	Long-Term Factor	Notes
Live load (occupancy)	L/360	1.0 (instantaneous)	Check on short-term composite stiffness
Total dead + live	L/240	1.5–2.0 (creep)	Creep reduces concrete flange stiffness
Shrinkage (added)	Per design	Additive	Adds tensile force on steel beam
Ponding / construction wet load	L/180	1.0 (stage)	Checked as unpropped non-composite

For the stability context around these beams, see steel structure overall stability and steel structure second order analysis.

Fire, Connections & Detailing

Fire behavior is one of the quiet advantages of a steel composite beam design. The concrete slab on the top flange acts as a heat sink, keeping the top flange of the steel beam cooler. The exposed bottom flange still needs fireproofing, but the protected top flange and the slab can reduce the required coating compared with an unprotected steel beam. At high temperatures, stud shear capacity drops, so the fire check must confirm the remaining composite action at the design fire exposure. For the broader framework, see steel structure fire resistance design and coating choice in steel fireproof coating selection.

Detailing has its own traps. The top flange must leave room for rebar to pass through slab pockets and around the headed studs. If the beam frames rigidly to a column, the end connection is designed for moment while the slab still needs to run continuously over the top flange. These details are where a composite floor either performs or leaks.

When the architect needs ductwork to run through the beam rather than below it, the same framing logic pairs with a castellated beam web opening design: the expanded web depth provides opening space for MEP runs, but the post and tee-section at each opening must be checked for Vierendeel bending and local web buckling—composite action still works through the opening if the slab casts over the full top flange.

Composite action in a beam bends a T-section; the same steel-and-concrete synergy in compression produces a fundamentally different member. Our steel concrete filled steel tube column design guide covers the compression-side equivalent: a steel tube filled with concrete where the shell confines the core in triaxial compression and the core prevents inward local buckling—yielding roughly 1.5–2.0× the axial capacity of a bare steel tube at the same section, with natural 30–60 minute fire resistance from the concrete heat sink and good seismic ductility from the confined core.

When to Use Composite Beams

Composite beams pay off at 6–12 m (20–40 ft) spans with high floor live loads—offices, hotels, and multi-story commercial floors—where a smaller beam depth and a longer unobstructed span are worth real money. Compared with a non-composite steel beam, they save 20–35% of steel; compared with a solid concrete slab, they cut self-weight by 30–40%.

For short spans or light-load factory floors, the steel saving is small and the stud welding adds cost, so a non-composite beam is usually cheaper. Adding a mezzanine or second floor is a classic composite-beam application—see steel building expansion add second floor. Load combinations for both stages are covered in steel structure load combination.

A typical case: an 8 m (26 ft) composite floor in a five-story office. A W18×35 beam supported a 130 mm (5 in) metal-deck slab. 19 mm (3/4 in) headed studs, 100 mm (4 in) tall, were spaced at 300 mm (12 in) near the supports and 450 mm (18 in) at mid-span—about 42 studs per beam. Unpropped construction was chosen, so the W18×35 was first checked for wet concrete as a non-composite section, then as a composite T-beam for live load. Long-term creep deflection at five years was estimated at 18 mm (0.7 in), comfortably under the L/360 limit of 22 mm (0.9 in).

Conclusion

Steel composite beam design turns a beam and a slab into one efficient T-section by welding headed studs at the interface. The stud count is set by horizontal shear flow; propped or unpropped decides how the beam survives the pour; and creep and shrinkage decide whether the floor stays flat for 50 years. Get the studs right, never skip the unpropped construction check, and budget for long-term deflection. That is what makes a composite floor perform.

The Slab and the Beam Working as One T-Section.

We design composite floors around the stud shear connection—stud count from interface shear, propped or unpropped from your schedule, and creep deflection checked for the long term. Tell us your span and floor load.

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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

A 5-story mixed-use office building in the Pacific Northwest, 22,000 m² (237,000 sq ft) on 10.5 m × 8.4 m (34 ft × 27 ft) bays, switched from non-composite wide-flange beams to composite design during design development. The main challenge was controlling live-load deflection under L/360 limits for partitioned office space, while keeping structural depth below a 600 mm (24 in) ceiling plenum. The solution was 240 mm (9.5 in) metal-deck slabs on unshored W18 × 50 beams with 19 mm (3/4 in) headed studs at 300 mm (12 in) spacing, cambered to offset 80% of long-term creep deflection. Effective flange width was checked per AISC 360 Chapter I. Composite action cut the required beam weight by about 22% versus non-composite, saving roughly 28 t (31 US tons) of structural steel and $190,000 in material cost, while adding 150 mm (6 in) of additional clear height on each floor. The design follows the shear-connector logic in steel building floor system and the open-web optimization in steel castellated beam web opening design.

Frequently Asked Questions

Q1: What is a composite steel beam?

A composite beam joins a steel beam and a concrete slab into one T-shaped section using welded headed shear studs. The concrete flange resists compression and the steel beam resists tension. Compared with a non-composite beam, it saves 20–35% of steel and increases stiffness 2–3 times for the same span.

Q2: How are shear connectors sized?

Headed studs are typically 13–22 mm (1/2–7/8 in) diameter and 65–125 mm (2.5–5 in) tall. Their shear capacity follows AISC 360, and the number of studs per beam is calculated from the horizontal shear flow at the steel-concrete interface. Spacing must be at least 5 stud diameters apart and within roughly 300 mm (12 in) on center in critical zones.

Q3: Propped or unpropped—which should I choose?

Unpropped (shoring-free) means the bare steel beam carries the wet concrete load, so it must be sized for construction first. Propped beams use temporary shores during pouring, letting you use a smaller steel section, but adding shoring cost and schedule. Choose propped when the steel savings exceed the shoring cost.

Q4: Does creep affect composite beams?

Yes. Concrete creeps over time, so the effective stiffness of the concrete flange drops and long-term deflection grows by 1.5–2.0× the short-term value. Slab shrinkage also adds a tensile force on the steel beam. Both must be checked against the L/240–L/360 deflection limits, often with factory camber.

Q5: When are composite beams worth it?

They pay off at 6–12 m (20–40 ft) spans with high live loads—offices, hotels, multi-story floors—where a smaller beam depth and longer span are valuable. For short spans or light factory floors, a non-composite beam is usually cheaper.

Reference Links

• AISC 360 — Specification for Structural Steel Buildings — Chapter I governs headed stud shear connectors and effective flange width.
• ASCE 7 — Minimum Design Loads and Associated Criteria — serviceability deflection limits L/360 and L/240 used for composite floors.