Reinforced Concrete Design / module 15
Steel–concrete composite construction
Two materials made into one member by the connection between them — and a construction stage that is a completely different structure.
What you will understand
- Explain what the shear connection buys, and why it is the whole design
- Determine the effective width of the concrete flange
- Calculate the plastic moment with full shear connection
- Size the shear connectors from equilibrium
- Explain why partial shear connection is usually the sensible answer
- Check the construction stage, and say why deflection must be added in two parts
Lessons
Composite action and the connection
What connecting a slab to a beam actually buys, and how the studs that do it are sized.
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.
Module summary
- The connection is the design: unconnected, the two members bend about their own axes
- Effective width allows for shear lag — beff = Σ min(Le/8, b)
- Plastic design is simple because both materials reach their design strengths
- The neutral axis should fall in the SLAB; in the steel means an inefficient section
- Stud numbers follow from equilibrium: the smaller of the two axial resistances
- Full connection often needs impractically close studs, so partial connection is normal
- Partial connection is far from proportional: 60% of the studs can give 80% of the moment
- The construction stage is a separate structure, and deflection is the SUM of two stages
Teaching basis and review status
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 notesCheck your understanding
Practice
A composite beam spans 10.0 m with beams at 2.4 m centres. What is the effective flange width, in mm?
Check yourself
In a composite beam, where should the plastic neutral axis ideally fall?
Practice
A beam has Ma = 600 kNm and MRd,full = 1100 kNm. What moment does 70% shear connection give, in kNm?
Practice
A steel section of 12,000 mm² in S275 acts compositely. What is Na, in kN?
Check yourself
An unpropped composite beam deflects 20 mm under wet concrete. What happens to that deflection?
Practice
An unpropped beam has a construction moment of 159 kNm and a bare steel capacity of 533 kNm. What is the utilisation?