Imagine a 35-story high-rise: the first 5 floors are a shopping mall with 8-meter-wide open spaces, and the upper 30 floors are residential apartments with smaller, regular rooms. The “transfer floor” between the 5th and 6th floors is the unsung hero here—it has to carry the entire weight of the upper apartments (tens of thousands of tons) while spanning the mall’s wide gaps. For decades, builders struggled with this: pure reinforced concrete beams were too heavy (needing thicker columns that blocked mall entrances), and pure steel I-beams were too expensive (costing 30% more than concrete options).
That’s why I-beam and concrete composite beams have become the go-to solution. They combine the best of both materials: the I-beam handles tensile forces (pulling) from the span, while the concrete slab on top handles compressive forces (pushing) from the upper structure. The result? A beam that’s 20% lighter than pure concrete, 25% cheaper than pure steel, and strong enough to span 10+ meters—perfect for transfer floors. This article breaks down the step-by-step construction technology of these composite beams, with real stories from high-rise projects that’ve used them to save time and money.
Why I-Beam-Concrete Composite Beams Are Perfect for Transfer Floors
Before diving into construction steps, let’s clear up why this combo works better than traditional options for transfer floors:
Load-Bearing Efficiency: Transfer floors carry “irregular loads”—the upper residential columns don’t line up with the mall’s columns below. Composite beams spread this load evenly: the I-beam’s flanges (the top and bottom “wings”) resist bending, while the concrete slab distributes weight across the beam. A 2022 project in Shanghai used 600×200×11×17mm I-beams with C40 concrete; the composite beams handled 35 kN/m² of load—15% more than pure concrete beams of the same size.
Space-Saving: Pure concrete beams for 8-meter spans need to be 800mm thick, which eats into ceiling height (bad for malls). Composite beams? The I-beam is only 600mm tall, and the concrete slab (150mm thick) doubles as the floor—saving 250mm of vertical space. A mall in Guangzhou used this to add a mezzanine level, increasing retail space by 15%.
Cost & Speed: Composite beams use less steel than pure steel beams (cutting material costs by 20%) and less concrete than pure concrete beams (speeding up pouring by 30%). A 30-story residential project in Shenzhen cut transfer floor construction time from 21 days to 14 days using composite beams.
Step-by-Step Construction Technology of Composite Beams
The success of composite beams depends on strict, sequential construction—skip a step, and the beam won’t perform as designed. Here’s the proven process, used by top high-rise builders:
1. Pre-Construction Preparation: Get the Basics Right
Rushing this step leads to misaligned beams or weak concrete bonds. Do these three things first:
Material Selection:
I-Beam: Use Q355B steel (common for high-rise construction)—it has a tensile strength of 510 MPa, enough for most transfer floor loads. For spans over 10 meters, choose heavier sections (e.g., H700×300×13×24mm) to avoid bending.
Concrete: C40-C50 strength grade works best—C40 has a compressive strength of 40 MPa, matching the I-beam’s tensile strength. Add a water-reducing admixture (1–2% of cement weight) to make concrete flow better around the I-beam.
Shear Connectors: These are the “glue” between I-beam and concrete—use 16mm diameter headed studs (height 100mm). They weld to the I-beam’s top flange and lock into the concrete, ensuring both materials work together.
Deepen the Design: Use BIM (Building Information Modeling) to map the composite beams’ layout. Check for conflicts with pipes or electrical ducts—transfer floors have lots of utilities, and you don’t want to cut into the beam later. A Beijing project used BIM to reposition 3 beams, avoiding 12 utility conflicts that would’ve delayed construction by a week.
Site Prep: Level the transfer floor’s lower structure (usually a reinforced concrete slab) and mark beam positions with laser levels. The marks need to be accurate to ±3mm—even a small misalignment can make the beam carry uneven load.
2. I-Beam Installation: Precision Is Non-Negotiable
The I-beam is the “skeleton” of the composite beam—install it wrong, and the whole system fails. Follow these steps:
Lifting & Positioning: Use a 25-ton crane to lift the I-beams (most are 6–10 meters long, weighing 500–800 kg). Have 2 workers guide the beam into place using ropes—don’t let it swing into columns. Once in position, temporarily support it with steel props (height adjustable, load capacity 10 kN). The props should be spaced every 2 meters to prevent the beam from sagging.
Welding Shear Connectors: Before fixing the I-beam permanently, weld the headed studs to its top flange. The studs need to be spaced 150–200mm apart (closer spacing for longer spans). Use a semi-automatic stud welding machine—manual welding often leads to weak bonds. Test 5% of the studs by pulling them with a hydraulic tool—they should withstand 10 kN of force without breaking.
Final Fixing: Bolt the I-beam’s ends to the transfer floor’s columns using 20mm diameter high-strength bolts (grade 8.8). Tighten the bolts to 600 N·m of torque (use a torque wrench)—loose bolts cause the beam to shift under load. A Shenzhen project skipped torque checking; 3 bolts came loose during concrete pouring, forcing a 2-day delay to reattach the beam.
3. Reinforcement Binding & Formwork Support
Next, add steel reinforcement to the concrete slab and build formwork to hold the concrete while it cures:
Reinforcement: Place two layers of Φ12mm rebars (spacing 150mm) in the concrete slab—one near the top (resists shrinkage) and one near the bottom (bonds with the I-beam). Tie the rebars to the shear connectors with wire—this keeps them from moving during pouring. For spans over 8 meters, add Φ14mm stirrups (spacing 200mm) around the I-beam to resist shear forces.
Formwork: Use 18mm thick plywood for the formwork, supported by steel joists (50×100mm) spaced 600mm apart. The formwork must be level and tight—no gaps, or concrete will leak out. A Shanghai project used plastic tape to seal gaps; this reduced concrete waste by 5% (saving $2.000 on the transfer floor).
Load Test: Before pouring concrete, test the formwork by placing 500kg sandbags on it (simulating concrete weight). Let it sit for 24 hours—if the formwork sags more than 2mm, add extra props. This step prevented a formwork collapse on a Guangzhou project in 2023.
4. Concrete Pouring & Curing: The “Make-or-Break” Step
Concrete needs to be poured and cured correctly to bond with the I-beam—poor curing leads to cracks that weaken the composite beam.
Pouring Sequence: Start pouring from the middle of the beam and work toward the ends—this avoids air bubbles. Use a concrete pump with a 120mm diameter hose (small enough to fit between rebars). Pour at a rate of 0.5 m³ per minute—too fast, and concrete pushes rebars out of place; too slow, and the first batch sets before the next, creating cold joints.
Vibration: Use a 50mm diameter internal vibrator to compact the concrete—insert it every 300mm along the beam, holding it for 10–15 seconds (until no more bubbles rise). Pay extra attention to the area around shear connectors—this is where air gets trapped most easily.
Curing: Cover the concrete with a wet blanket immediately after pouring. Keep it moist for 7 days (use a sprinkler twice a day if it’s hot). Curing helps the concrete reach full strength—skipping it cuts strength by 30%. A Beijing project cured for only 3 days; the concrete developed hairline cracks, forcing them to apply a repair mortar (costing $3.500).
5. Post-Construction Quality Inspection
Don’t call the job done until you’ve checked these key points:
Visual Inspection: Look for cracks, concrete spalling (flaking), or loose shear connectors. Small cracks (width <0.2mm) are okay—fill them with epoxy. Larger cracks mean the beam needs to be assessed by a structural engineer.
Non-Destructive Testing: Use ultrasonic testing (UT) to check the concrete-I-beam bond—UT waves travel differently through good and bad bonds. A Shenzhen project found 2 beams with poor bonding (due to dirty I-beam surfaces); they injected epoxy to fix it.
Load Testing: For critical beams (spanning over 10 meters), do a static load test—apply 1.2 times the design load (using water tanks) and measure deflection. The deflection should be less than 1/250 of the span (e.g., 40mm for a 10-meter beam).
Real-World Case: A 40-Story High-Rise in Shanghai
Let’s see how this technology worked on a real project: a 40-story mixed-use building (1–6 floors mall, 7–40 floors residential) in Shanghai, 2022.
Challenge: The transfer floor needed to span 9-meter gaps between mall columns, carrying 40 kN/m² of load from the upper apartments.
Solution: Used Q355B I-beams (H600×200×11×17mm) with C45 concrete, 16mm headed studs (spacing 180mm).
Results:
Construction time: 15 days (vs. 22 days for pure concrete beams).
Cost:
180permeter(vs.240 for pure steel beams).
Performance: Load testing showed deflection of 32mm (well under the 36mm limit). No cracks or issues after 2 years of use.
“The composite beams were a no-brainer,” said the project’s construction manager. “We saved time, money, and didn’t have to compromise on space for the mall.”
Conclusion
I-beam and concrete composite beams solve the biggest challenges of high-rise transfer floors: they’re strong enough to carry heavy loads, save space, and cut costs. The key to success is following the step-by-step construction process—from material selection to curing—and never skipping quality checks.
For builders, this technology isn’t just about building faster or cheaper—it’s about building safer. A well-built composite beam will support the high-rise for decades, keeping residents and mall visitors safe. And in a world where high-rise buildings are getting taller and more complex, composite beams are becoming an essential tool in the builder’s toolkit.
At the end of the day, it’s simple: when you need a beam that can do it all—carry load, save space, and stay on budget—I-beam and concrete composite beams are the answer.
