Reinforced Concrete Design / Steel–concrete composite construction
Partial connection, and the construction stage
Why 60% of the studs gives 80% of the moment, and why the beam before the concrete sets is a different structure.
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
- Corrected the EC4 concrete block, construction actions, partial-connection applicability and temporary-load recovery.
Checks needed for project use
- A concrete-design reviewer must check the stated first-generation clauses, National Annex choices, detailing rules and factor sets. Numerical agreement alone does not establish code compliance.
- Supply actual steel geometry/class, restraints, lateral-torsional buckling, stud layout and transverse reinforcement for member acceptance.
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
Two things separate composite design from everything else in this course. The connection can be deliberately incomplete, and the relationship between how complete it is and what you get is strikingly non-linear. And there is a stage — before the concrete hardens — where the composite section does not yet exist, and the steel beam alone has to carry the wet concrete without lateral restraint.
By the end of this lesson you should be able to
- Explain why partial shear connection is not proportional
- Say when it is and is not permitted
- Check the construction stage on the bare steel section
- Calculate unpropped deflection as the sum of two stages
What you should already know
- Composite action and stud sizing (previous lesson)
- Lateral-torsional buckling (Structural Analysis Fundamentals, Module 18)
Partial shear connection
The steel section has its own bending resistance. Within the permitted ductile partial-connection range, a conservative linear interpolation gives MRd(η) = Ma +η(MRd,full −Ma).
For the stated example,26 of 42 studs per half span gives η =0.61905 and MRd =851.966 kNm, compared with 1048.561 kNm for full connection. Check the minimum degree of connection and the connector detailing before using this interpolation as a design resistance. Points below the permitted minimum can illustrate the algebra but must not be presented as valid designs.
Try it
How many studs does this beam actually need?
Explore a solid-slab, equal-flange Class 1/2 teaching section with ductile studs. The blue line is limited to the permitted connection range. Section properties are illustrative; member stability and connector detailing remain separate checks.
- Effective width beff
- 2250 mm (shear lag)
- Steel force Na
- 3373 kN
- Concrete force Nc
- 4973 kN
- Neutral axis
- 88 mm, in the slab
- Bare steel Ma
- 533 kNm
- Full connection MRd
- 1049 kNm
- One stud PRd
- 81.7 kN (stud shear)
- Studs for full connection
- 42 per half span
- Degree / minimum connection
- 61.9% / 52.0%
- Indicative single-row pitch
- 173 mm; limits 95–780 mm
- Resistance at this η
- 852 kNm
- Studs the moment demands
- 22 per half span
26 studs give MRd = 852 kNm against MEd = 780 kNm. The moment needs only 22, which is 62% connection against the 52% strictly required.
Check the model limits
- Below the stated minimum degree of connection, the interpolation is not a permitted design resistance.
- If the plastic neutral axis enters the steel, flange and web geometry are needed; this lab withholds the resistance.
- The area, depth and plastic modulus are independent illustrative inputs. Confirm a real section’s properties and classification before design.
- Stud spacing, transverse reinforcement, splitting and member stability require separate checks.
The construction stage
Before the slab gains the required strength, the bare steel carries the wet concrete, formwork and construction loads. Include its self-weight and apply the relevant construction-stage action combinations.
Temporary decking, bracing or another engineered restraint system may restrain the compression flange. Establish the actual restraint before assessing lateral-torsional buckling. A vertical prop changes the bending system but is not automatically a lateral restraint.
For a uniformly loaded beam with a central vertical support and two equal continuous spans, the largest absolute elastic moment is the hogging moment over that support, wL²/32. Prop reactions, settlement, removal sequence and the resulting composite-stage forces also need checking.
Worked example
Partial connection and the construction check
Given
- Same equal-flange steel section and solid slab as the preceding example; span 9.0 m, tributary width 3.0 m
- Full composite resistance 1048.561 kNm; bare steel plastic resistance 532.5 kNm
- Full connection requires 42 studs per half span; provide 26 ductile 19 mm ×100 mm headed studs per half span
- Construction: wet concrete 25 kN/m³, formwork 0.5 kN/m², construction allowance 1.5 kN/m², steel density 78.5 kN/m³
Find
Compare partial shear connection and the construction-stage cross-section demand. Withhold the unpropped-member acceptance until lateral restraint, buckling and deflection inputs are available.
Assumptions
- First-generation EC4 equal-flange minimum-connection expression, with all its ductile-stud applicability conditions
- No profiled-deck stud reduction: this is a solid slab example
- For this illustrative construction combination, steel and formwork are permanent; wet concrete and the construction allowance are variable, following the cited SCI construction example
- γG =1.35 and γQ =1.5; verify the project execution load cases and restraint scheme
Predict first
A composite beam has a bare steel moment of 500 kNm and a full composite moment of 1000 kNm. What moment does 50% shear connection give?
Practice
A beam has Ma = 533 kNm and MRd,full = 1022 kNm. What moment does 62% shear connection give, in kNm?
Practice
A 130 mm slab spans 3.0 m between beams. What is the wet-concrete load on one beam, in kN/m? Take concrete at 25 kN/m³.
Practice
An unpropped beam deflects 18 mm under wet concrete and a further 9 mm under imposed load on the composite section. A single calculation on the composite section gives 14 mm. By what factor does the single calculation understate the truth?
Check yourself
Why are composite beams often propped during construction?
Summary
- MRd(η) = Ma + η(Mfull − Ma) — the studs buy only the DIFFERENCE
- 62% of the studs gave 82% of the moment in the worked example
- Partial connection needs ductile connectors and has a lower limit
- The construction stage is a separate structure on a different section
- It is rarely a bending problem — 30% utilisation is typical
- Check lateral stability using the restraint actually provided during construction
- Unpropped deflection is the SUM of two stages, and the first is locked in