Reinforced Concrete Design / How reinforced concrete works
Load paths, member types and how a concrete frame works
From a footprint on a floor to a pressure on the ground.
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.
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.
- Two-way slab fourth-power division is an illustrative crossing-strip approximation, not a full plate solution; verify boundary, torsion and stiffness assumptions.
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 any section is designed, someone has to decide how load will travel from where it lands to the ground. That decision fixes the member sizes, the reinforcement, the foundations and much of the cost — and it is made before a single calculation. The load path should be clear before individual members are sized.
By the end of this lesson you should be able to
- Trace a load path through a concrete building
- Distinguish one-way from two-way slab action
- Explain what monolithic construction gives you, and what it costs
- Describe the roles of beams, columns, walls and foundations
What you should already know
- Load paths and structural forms (Structural Analysis Fundamentals, Module 1)
- Support reactions (Structural Analysis Fundamentals, Module 2)
Follow a load from the moment it lands.
Slab. The load arrives on a floor as a pressure, in kN/m². The slab spans to whatever supports it, converting pressure into a line load on those supports.
Beams. They collect the line load from the slab and span between columns, converting it into point loads.
Columns. They gather the beam reactions and carry them vertically, accumulating from the roof down. A column at the bottom of a six-storey building carries six floors' worth.
Foundations. They receive a very large force on a small area and spread it over enough ground to keep the bearing pressure acceptable. This is the only step in the chain that makes the load less concentrated.
Ground. The ground receives the foundation reactions.
At every step the load becomes more concentrated — pressure, to line load, to point load — until the foundation reverses that. Noticing this makes preliminary sizing far easier: you can estimate a column load in your head from floor area and a typical loading, long before any analysis.
How a slab spans depends on its proportions and its supports.
One-way. A slab supported on two opposite sides, or one whose long side is more than about twice its short side, sends nearly all its load the short way. The long-direction bending is small enough to ignore for strength, though reinforcement is still needed in that direction for distribution, shrinkage and temperature.
Two-way. A slab supported on all four sides with a ratio nearer 1:1 sends load both ways, in proportion to the stiffness of each direction. An illustrative model of equal-stiffness simply supported crossing strips gives a load ratio of (Llong/Lshort)⁴. At a span ratio of 1:1.5, that model gives about 5.1. Actual slab distribution also depends on plate action, torsion and support conditions. Module 9 develops the approximation.
Flat slab. A slab supported directly on columns with no beams at all. The load path is shorter and the soffit is flat, which is excellent for services and formwork. The price is a severe local problem where the slab meets the column: punching shear, which Module 9 treats at length.
Predict first
A slab panel is 4 m × 9 m, supported on all four sides. Which way does most of the load travel?
One feature of cast-in-situ concrete shapes everything: it is monolithic. Slab, beam and column are cast together and are continuous through their joints, so the structure is not an assembly of pieces but one object.
That brings real advantages:
- Continuity. A continuous slab or beam carries far less span moment than a series of simply supported ones, because hogging over the supports relieves the sagging in the spans.
- Frame action. Beams and columns share moment at their joints, so the frame can resist horizontal load without any bracing.
- Robustness. Continuity gives alternative load paths. If one element is damaged, load can find another way down — provided the reinforcement is detailed to be continuous through the joints.
- T-beam action. A beam cast with the slab acts with part of the slab as a flange, which is why Module 4 spends time on flanged sections.
And it brings costs:
- The structure is indeterminate, so analysis is harder and settlement, shrinkage and temperature all generate forces.
- Continuity has to be detailed. Reinforcement must be lapped and anchored through joints to make it real. Continuity assumed in analysis but not detailed on the drawing is a fiction.
- Construction is sequential and wet. Precast construction trades some of this away for speed and factory quality — and takes on the connection design that monolithic construction gets for free.
Worked example
Estimating a column load from the load path alone
Given
- A four-storey concrete building on a 6.0 m × 7.5 m column grid
- Each floor: 250 mm slab, plus finishes and partitions, plus imposed load
- Take a combined design load at the ultimate limit state of about 15 kN/m² per floor, including self-weight
- An internal column supports a full grid bay
Find
The design axial load at the base of an internal column, without any analysis.
Practice
An internal column supports a 6.0 m × 7.5 m grid bay. At an ultimate design load of 15.0 kN/m² per floor, what load does one floor deliver to that column, in kN?
Practice
A 250 mm concrete slab has a density of 25 kN/m³. What is its self-weight, in kN/m²?
Practice
Model a 5.0 m by 6.5 m slab as two equal-stiffness, simply supported crossing strips with equal central deflection. In this approximation, load divides as the fourth power of the span ratio. How many times more load does the short span carry than the long span?
Summary
- Slab → beam → column → foundation → ground, becoming more concentrated at each step until the foundation
- One-way where the span ratio exceeds about 2; two-way nearer 1:1
- The crossing-strip approximation gives a fourth-power span effect under distributed load; the actual slab also depends on its supports and plate action
- Flat slabs shorten the load path but create a punching-shear problem at the columns
- Monolithic construction gives continuity, frame action, robustness and T-beam action
- It also makes the structure indeterminate and sensitive to imposed deformations
- Continuity must be detailed to be real