Lessons · Engineering · Why loads are combined before they are checked
Loads are combined before anything is checked
A member is not checked against dead load, then live load. It is checked against combinations in which each load is multiplied by a factor that reflects how well it is known and how likely the loads are to peak together. The combination, not the raw load, is what the member must survive.
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
Factors are often read as a safety margin bolted on at the end. They are not: they are applied per load, differently, and the pattern that governs is often not the one with the biggest total -- which is why more than one combination is checked.
How to think about it
Take the factors the problem gives, apply each to its own load, and total. Do that for every combination named, then design for the worst result. Never average combinations and never assume the largest factor gives the largest answer.
Worked example
Given: dead 5 kPa, live 3 kPa.Keep them apart, as the first lesson said.
Combination: 1.35 x dead + 1.5 x live.Each load gets its own factor.
1.35 x 5 = 6.75; 1.5 x 3 = 4.5.Separately, then add.
Design load 11.25 kPa.This, not the unfactored 8, is what the member sees.
Your turn
Dead 4 kPa, live 2 kPa, factors 1.35 and 1.5 as given. Write the combined design load in kPa.
1.35 x 4 + 1.5 x 2 = kPa
Solve one, graded on the server
The trap
Factoring the total instead of each load. One factor on a mixed total gives a different number and is not what any code asks for.