Lessons · Engineering · Concrete works in compression, steel in tension
Concrete takes compression, steel takes tension
Concrete is strong in compression and weak in tension -- roughly a tenth as strong. Reinforced concrete works by putting steel where the tension is and letting concrete do the compression. That is why reinforcement sits near the bottom of a simply supported beam and near the top over a support.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
It explains every bar you will ever see drawn. Bars are not spread evenly for strength; they are placed where the member is being pulled apart, and that place changes along the span.
How to think about it
Before reading any reinforcement drawing, work out which face of the member is in tension at that point. Sagging between supports puts the bottom in tension; hogging over a support puts the top in tension. The steel follows the tension.
Worked example
Concrete is far weaker in tension than compression.Roughly a tenth, and it cracks rather than stretches.
Simply supported beam sags: bottom in tension.Main bars low.
Over a continuous support it hogs: top in tension.Main bars high, which surprises people.
Cover protects the steel from corrosion.Steel too near the face rusts and spalls the concrete.
Your turn
Which face of a simply supported beam is in tension at midspan?
At midspan of a simply supported beam the face is in tension
Solve one, graded on the server
The trap
Assuming bars always sit low. Over a support the member hogs and the tension is at the top, which is where the steel has to be.