Module 11 · Lesson 11.2
The construction stage, and why deflection needs three calculations
Before the concrete sets, the beam is bare steel carrying wet concrete with nothing holding it sideways. That stage frequently governs — and it leaves a deflection behind that never goes away.
Why this matters
A composite beam is at its weakest on the day the slab is poured. It has none of the composite resistance yet, it is carrying the full wet weight of the concrete plus the people and equipment placing it, and the slab that will eventually restrain its compression flange is a liquid. Every part of that sentence is a check. And the deflection it undergoes at that moment is permanent — the slab sets in the deflected shape, so it never comes back.
By the end of this lesson you should be able to
- Check the construction stage as a separate structure
- Say which of strength and stability governs it, and why
- Explain what propping changes and what it costs
- Calculate deflection in the three stages it actually happens in
What you should already know
- Composite action and the plastic moment (previous lesson)
- Lateral-torsional buckling and the effect of restraint (Module 8)
- Elastic section properties and the transformed-section idea
The construction stage is a different structure
Before the concrete has hardened, the beam has:
- no composite action — the resistance is that of the bare steel section
- no lateral restraint to its compression flange, unless the decking provides it
- the full wet weight of the concrete, plus formwork, plus a construction load allowance
For the worked beam the construction load comes to 14.25 kN/m over 9 m, giving 144 kNm. Against the bare steel section's 595 kNm that is a utilisation of 0.24 — comfortable, and it would be easy to stop there.
But the beam is unrestrained over its full 9 m. Module 8's check gives χLT = 0.277 and Mb,Rd = 165 kNm.
Utilisation 0.88, not 0.24. The construction stage is governed by stability, not by strength, and by a factor of nearly four.
That is the check people miss, and it is missed because the strength number looks so comfortable. With restraint at 3 m centres — which profiled decking fixed as it is laid will usually provide — Mb,Rd rises to 488 kNm and the utilisation falls to 0.30. Whether the decking restrains the beam during the pour is therefore a structural question, not a site preference.
Deflection happens in three stages, on two different sections
This is where composite design most often goes wrong, and the error is always in the same direction.
Stage 1 — wet concrete on bare steel. The beam deflects on its own Ia. The slab sets in that deflected position, so this deflection is locked in and never recovered.
Stage 2 — sustained load on the composite section. Finishes, services and partitions act on the composite section. But they stay on, so the concrete creeps, and the effective modular ratio roughly triples. The section to use is the long-term one.
Stage 3 — imposed load on the composite section. This comes and goes, so the short-term section applies.
For the worked beam:
| Stage | Section | Deflection |
|---|---|---|
| Wet concrete | bare steel, 33 500 cm⁴ | 17.3 mm |
| Sustained | composite long-term, 86 900 cm⁴ | 2.1 mm |
| Imposed | composite short-term, 109 200 cm⁴ | 2.8 mm |
| Total | 22.2 mm — span/405 |
Now do it the quick way: put all 26.25 kN/m on the composite short-term section in one calculation. 9.8 mm.
The staged answer is 127% larger. The quick calculation is not slightly optimistic; it is wrong by more than a factor of two.
And notice where the deflection is: 17.3 of the 22.2 mm happened before the slab had hardened. Any limit written against the imposed-load deflection alone would be met with enormous margin — span/3222 — while the floor is visibly 22 mm down. The two numbers answer different questions, and both are worth having.
Try it
Three stages, two sections, one beam
The 9 m composite beam through its life. Each load acts on the section that existed when it arrived — and the bottom bar is what a single calculation on the composite section would have told you.
Construction
Restraint during the pour
- Bare steel Ia
- 33507 cm⁴
- Composite short-term I
- 109230 cm⁴
- Composite long-term I
- 86910 cm⁴
- Wet concrete stage
- 17.3 mm
- Sustained stage
- 2.1 mm
- Imposed stage
- 2.8 mm
- Total deflection
- 22.2 mm
- Span over total
- span/405
- Span over imposed only
- span/3222
- The single-calculation shortcut
- 9.8 mm
- Shortcut understates by
- 127 %
- Construction moment
- 144 kNm
- Construction strength check
- 0.24
- Construction LTB check
- 0.88
- Which governs construction
- STABILITY
Construction: 0.24 on strength, 0.88 on stability — stability governs, by a factor of 3.6. Deflection totals 22.2 mm against 9.8 from the shortcut.
Things worth trying
- Start at the defaults, unpropped and unrestrained. The construction strength check reads 0.24 and the LTB check 0.88 — a factor of nearly four apart, and it is the second that governs.
- Switch the restraint to decking at 3 m. The LTB utilisation falls to about 0.30. Whether the decking is fixed as it is laid is therefore a structural question, not a site preference.
- Look at the deflection bars with restraint off or on — they do not move. Restraint fixes the stability problem and does nothing for deflection, which is a separate concern.
- The wet-concrete bar is the largest of the three, and it is the one that happens on the bare steel section. It is also permanent: the slab sets in that position.
- Compare the TOTAL bar with the shortcut bar. The shortcut is less than half of it — that is the single commonest error in composite design, and it errs unsafely.
- Now switch to propped. The wet-concrete stage collapses because its load lands on the composite section instead. The total roughly halves.
- Compare span/total with span/imposed-only. They answer different questions and differ by a factor of about eight — quoting the wrong one is its own error.
- Push the construction load up until the LTB check passes 1.00 while the strength check is still under 0.5. That gap is exactly the trap.
Worked example
The same beam, through its whole life
Given
- The 457 × 191 × 74 UB composite beam from the previous lesson, 9.0 m span at 3.0 m centres
- Construction: wet concrete 9.75 kN/m plus formwork and construction load 4.5 kN/m
- Sustained after hardening: 4.5 kN/m. Imposed: 7.5 kN/m
- Unpropped, and the decking is NOT assumed to restrain the beam during the pour
- Ia = 33 500 cm⁴; composite 109 200 cm⁴ short-term, 86 900 cm⁴ long-term
Find
Whether the construction stage is adequate, and the total deflection.
Assumptions
- The modular ratio triples for sustained load — a simplified educational treatment of creep
- The uncracked transformed section is used throughout, which is slightly optimistic in sagging where the neutral axis is inside the slab
- χLT is calibrated and unverified here
The detail that falls between two designers
One more check belongs here, and it belongs here because it is nobody's obvious responsibility.
The studs deliver the whole longitudinal force into the slab at the beam line. It then has to spread sideways to the full effective width — and the only things carrying it across the vertical plane beside the studs are the concrete and whatever reinforcement crosses that plane.
If the reinforcement is not there, the slab splits longitudinally along the top of the beam. When it does, the effective width the entire composite design assumed is never mobilised.
For the worked beam: 3464 kN over the 4.5 m half span is 770 N/mm, which spreads both ways, so each plane carries 385 N/mm. Even taking a flat strut, that needs about 440 mm²/m of transverse reinforcement crossing the plane — more than an A393 mesh provides.
The steel designer assumes the slab reinforcement is a concrete matter. The concrete designer sizes the mesh for the slab spanning between beams, which is a quite different requirement. Neither has checked this, and it is the composite designer's job.
Practice
A bare steel beam with I = 33 500 cm⁴ spans 9.0 m and carries 14.25 kN/m of wet concrete and construction load. What is its deflection, in mm? Take E = 210 000 N/mm².
Practice
The three stages give 17.3, 2.1 and 2.8 mm. A single calculation on the composite section would have given 9.8 mm. By what percentage does the shortcut understate the answer?
Practice
A bare steel beam has MEd = 144 kNm at the construction stage, Mc,Rd = 595 kNm and Mb,Rd = 165 kNm unrestrained. What is the governing utilisation?
Practice
3464 kN of longitudinal force is delivered over a 4.5 m half span. What is the shear flow, in N/mm?
Check yourself
Why does sustained load act on a LESS stiff composite section than imposed load?
Summary
- The construction stage is a separate structure: bare steel, wet concrete, no restraint
- Its strength utilisation was 0.24 and its LTB utilisation 0.88 — stability governs
- Restraint at 3 m from the decking took it back to 0.30
- Propping changes the design and the programme, not just the site method
- Deflection is three calculations on two sections: 17.3 + 2.1 + 2.8 = 22.2 mm
- 17.3 mm of that is locked in before the slab hardens
- A single composite calculation gave 9.8 mm — understating it by 127%
- Transverse reinforcement across the shear plane is nobody's obvious job and is often short
This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint