Structural Steel Design / module 16
Multi-storey steel buildings
Everything repeats. A decision about a floor zone is made once and paid for on every storey, which is why the most consequential choices in a multi-storey building are made before any member is sized.
What you will understand
- Compare floor systems on floor ZONE rather than structural depth
- Say why services through the structure change the arithmetic entirely
- Quantify what a floor-zone saving is worth once multiplied by the storeys
- Apply the imposed-load reduction and say what it acknowledges
- Compare the lateral stability systems on the terms that decide between them
- Say why a braced bay is nearly always the engineering answer and often not the chosen one
Lessons
Floor systems, and the multiplication
The decision that matters is the floor zone, not the beam depth — and whatever you decide, you will pay for it once per storey.
Lateral stability: comparing four systems
Compare lateral stiffness, connection demands and space requirements using four illustrative stability systems.
Module summary
- Compare floor ZONE, not structural depth: a deeper beam can give a shallower floor
- A 600 mm cellular beam gave an 850 mm zone against a 500 mm downstand's 1150
- 300 mm per floor became 3.0 m over ten storeys — 420 m² of façade
- That paid for 60 tonnes of extra steel with £162k to spare
- The imposed-load reduction took 10% off a ten-storey column's total load
- It applies only to the imposed part, so a mostly-permanent column gains little
- A braced bay is 6.7× stiffer than a moment frame at 0.4× the cost — 17× the efficiency
- And it is often rejected anyway, because it puts a diagonal through the floor plate
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 downstand beam is 450 mm deep with 350 mm of services below and 250 mm of finishes. What is the floor zone, in mm?
Practice
A 200 mm saving per floor on a 15-storey building with a 160 m perimeter saves how much façade, in m²?
Practice
A column carries 6 storeys. With αn = (2 + (n − 2) × 0.7)/n, what is αn?
Practice
A column carries 400 kN permanent and 300 kN imposed unreduced. With αn = 0.80, what is its design load, in kN?
Practice
A storey has αcr = 15. Its bracing is replaced by a system one third as stiff. What is αcr now?
Practice
System A has relative stiffness 2.5 at cost 0.6; system B has 0.6 at cost 1.4. What is A's stiffness per unit cost, relative to B's?
Check yourself
Why should a floor system be compared on floor ZONE rather than structural depth?
Check yourself
In what order should the major decisions on a multi-storey steel building be made?