Structural Steel Design / module 11
Steel–concrete composite beams
How shear connection allows a steel beam and concrete slab to act together, and why construction sequence affects the checks.
What you will understand
- Explain composite action physically, in terms of where the neutral axis moves
- Calculate the plastic moment of a composite section and say which case is efficient
- Say what a shear stud has to do and the two ways it can fail
- Explain why partial shear connection is legitimate and often economic
- Separate the construction stage from the composite stage, and say which governs
- Explain why an unpropped beam's deflection cannot be calculated in one go
Lessons
How composite action works, and what it buys
The slab is already there. Connect it to the beam and the two act as one section — with the concrete doing what it is good at and the steel doing what it is good at.
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.
Module summary
- Composite action took a 457 UB from 595 kNm to 1057 kNm — a factor of 1.78
- And from 33 500 cm⁴ to 109 000 cm⁴ — a factor of 3.26 on stiffness
- The neutral axis moves up into the slab, so nearly all the steel is in tension
- 43 studs per half span for full connection; 60% of them gave 83% of the moment
- A stud fails either in its own shank or by crushing the concrete around it
- The construction stage is a separate structure — and LTB governed it, not strength
- Unpropped deflection is THREE calculations on two different sections
- Doing it in one gave 9.8 mm against a true 22.2 — understated by 127%
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 has Ma = 500 kNm bare steel and MRd = 900 kNm with full connection. With η = 0.5, what is MRd in kNm?
Practice
Beams are at 4.0 m centres on a 10.0 m span. What is beff, in mm?
Practice
A steel section of 12 000 mm² in S355 develops Na in tension. How many 80 kN studs are needed over the half span for full connection?
Practice
A composite section is 3.26 times stiffer than the bare steel beam. If the same load acted on each, by what factor would the deflection fall?
Practice
The short-term modular ratio is 6.4 and creep triples it. What is the long-term modular ratio?
Practice
A beam deflects 20 mm in total over a 9.0 m span. What is its span-to-deflection ratio?
Check yourself
Which check most often governs the construction stage of an unpropped composite beam?
Check yourself
Why must transverse reinforcement cross the plane beside the studs?