Lessons · Engineering · Dead, live and the difference it makes
What is always there, and what comes and goes
A dead load is the weight of the structure itself and anything permanently fixed to it. A live load is everything that moves: people, furniture, stored goods, vehicles. They are kept apart because they are known with different certainty -- you can weigh a slab, you can only estimate a crowd -- and the codes treat the uncertain one more cautiously.
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
A mezzanine was designed for the weight of its own steel and deck and the machines drawn on the layout. Nobody asked what the floor would hold the week the machines were swapped and forty pallets sat there instead. The dead load was exact and the live load was the whole question.
How to think about it
Split every load you are given into permanent and variable before doing any arithmetic, and write the two totals separately. They get different treatment downstream, so combining them early destroys information you will need.
Worked example
Slab self-weight, screed, finishes, fixed services: dead.Weighable, and it does not leave.
Occupants, furniture, stock, snow, wind: live.Estimated from use, and the use can change.
Keep the two totals apart until the combination step.They are multiplied by different numbers there.
A change of use changes the live load, not the dead.Which is why a change of use is a structural question.
Your turn
A floor carries 4.0 kPa of permanent construction and 2.5 kPa of occupancy. Write the live load in kPa.
Permanent 4.0 kPa, occupancy 2.5 kPa: live load is kPa
Solve one, graded on the server
The trap
Adding them into one number early. Every combination rule downstream needs them separately, and recovering the split from a total is guesswork.