Structural Steel Design / Steel–concrete composite beams
The construction stage, and why deflection needs three calculations
Before the concrete sets, the steel beam carries the construction loads without composite resistance. Check any restraint provided by decking or temporary works, and track deflection as loads are applied and removed.
Lesson review: expert verification remains open
This lesson has received a full-context editorial review and a check of its technical explanations. Selected arithmetic has been checked separately. This is not independent engineering sign-off.
Corrections checked in this pass
- Steel self-weight, factored construction actions and failure against assumed 165 kNm resistance made explicit. Temporary-load removal produces negative elastic recovery; sustained/transient loads use separate stiffnesses. Per-plane slab shear corrected. Unsupported plastic section and interpolation boundaries are withheld.
Checks needed for project use
- A steel-design reviewer must check the first-generation calculation profiles, second-order analysis assumptions, connection rules and execution references against the applicable standards and amendments.
- Actual temporary restraints, construction combinations, deck/stud detailing, creep/shrinkage, propping and removal chronology, connection ductility/minimum connection. Arbitrary propped chronology deliberately unsupported.
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
Read the lesson register and outstanding review notesWhy this matters
Before hardening, steel and any verified temporary restraints carry the construction actions. Track which loads remain and which are removed: removing a temporary load causes elastic recovery on the section active at that time. Later loading acts on the hardened composite section, with sustained effects treated separately.
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
Construction actions and resistance
For the illustrative9m beam, take wet concrete9.75kN/m, permanent decking0.30kN/m and steel self-weight74×9.81/1000=0.72594kN/m. Thus Gk=10.77594kN/m. Assume temporary construction action Qk=4.20kN/m.
Using explicitly assumed teaching factors1.35Gk+1.50Qk gives20.847519kN/m and MEd=211.081kNm. Against the assumed unrestrained Mb,Rd=165kNm, the comparison is1.279: the proposed unrestrained construction arrangement does not satisfy this check.
If a separately verified restraint arrangement justifies Mb,Rd=488kNm, the same action gives0.433. Decking does not provide that restraint merely by being present: check its fixings, stiffness, strength and load path during the pour. Applicable construction combinations and resistance inputs still require the project basis.
An explicit load history
Use Ia=33 500cm⁴, Ishort=109 200cm⁴ and Ilong=86 900cm⁴ with E=210 000N/mm². For this teaching sequence, the temporary4.2kN/m remains through hardening and is then removed.
| Event | Section | Deflection increment |
|---|---|---|
| Gpre+Qtemp=14.97594kN/m before hardening | bare steel | +18.186mm |
| Remove temporary4.2kN/m after hardening | short-term composite | −1.565mm |
| Later sustained4.5kN/m | long-term composite | +2.107mm |
| Assumed sustained live portion2.25kN/m | long-term composite | +1.053mm |
| Remaining transient live5.25kN/m | short-term composite | +1.956mm |
| Total | 21.737mm |
The live-load split uses an illustrative sustained fraction0.30, not a universal category factor. Removal before hardening would instead recover deflection on Ia. This effective-modulus illustration omits shrinkage, slip and redistribution from creep of staged stresses; it is not a complete serviceability assessment.
Try it
Composite beam load stages
Unpropped 9 m teaching example. Permanent construction load includes wet concrete, decking and steel self-weight. Temporary load is removed explicitly; later live load uses an assumed sustained fraction of 0.30.
Temporary load removed
- Signed total
- 21.737 mm
- ULS construction moment (assumed 1.35G + 1.5Q)
- 211.081 kNm
- Comparison with assumed Mb,Rd = 165 kNm
- 1.279
- Construction-stage status
- Actual restraints and resistance require verification
Signed stage increments sum to 21.74 mm. Removing temporary load produces elastic recovery on the section present at removal. This effective-modulus illustration omits shrinkage, slip and detailed staged creep redistribution; it is not a final deflection verification.
Propped construction needs a defined support and load-transfer sequence; no automatic span-fraction reduction is used.
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
- Before hardening:Gk=wet concrete9.75+permanentdecking0.30+steel0.72594=10.77594kN/m;temporaryQk=4.20kN/m
- After hardening, remove4.20kN/m temporaryload; add4.50kN/m sustained load. Split7.50kN/m live load into assumed2.25sustained and5.25transient
- 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
- Use illustrative ULSfactors1.35G+1.50Q; stated short/long-termI values are model inputs. Shrinkage,slip and staged creep redistribution are omitted.
- 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
For a separate arithmetic comparison, assumed stage increments are17.3,2.1 and2.8mm, against a9.8mm shortcut. By what percentage is their sum larger? These values are not the corrected construction example.
Practice
Using the corrected illustrative construction MEd=211.081kNm and assumed Mb,Rd=165kNm, 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
- With explicit illustrative factors, construction MEd=211.1kNm exceeds the assumed165kNm LTBresistance
- A488kNm resistance would give0.433only if the construction restraints justify it
- Propping changes the design and the programme, not just the site method
- The stated signed load history totals21.737mm
- Removing temporary load afterhardening recovers1.565mm in this model
- A single composite calculation cannot represent the complete load history
- Transverse reinforcement across the shear plane is nobody's obvious job and is often short