Service loads are the real, unfactored loads a structure carries; factored loads are those same loads multiplied by code load factors for strength design. You collect service loads in a load takedown, then a load combination turns them into the factored value that sizes the member. StructLoads reports both the service and the factored reaction at each support, and the difference between them decides which check you are doing.
What service loads are
Service loads are the actual loads a structure sees in normal use: the real dead load of the slab and finishes, the real live load from occupancy, the real roof and snow loads. They are unfactored, which means they represent expected reality rather than a design margin. You use them whenever you want to predict how the structure will actually behave. ASCE 7’s commentary to Appendix C defines them as the loads acting on the structure at an arbitrary point in time, and ICC’s serviceability design guide notes they may be only a fraction of the nominal loads: its suggested deflection checks use nominal dead plus live load for floors, and dead plus half the snow load for roofs.
What factored loads are
Factored loads are service loads multiplied by load factors greater than one, then combined. They exist so that a member sized on them keeps a calibrated margin against failure even if loads are higher or strength is lower than expected. This is the load side of limit state design: factor the demand up, factor the capacity down, and confirm the first stays below the second.
Service vs factored at a glance
The two values come from the same loads but serve different checks.
| Aspect | Service loads | Factored loads |
|---|---|---|
| Definition | Actual unfactored loads | Service loads times load factors |
| Factor size | One, or near one | Greater than one |
| Used for | Deflection, vibration, cracking, ASD | Strength design (LRFD) |
| Question asked | How will it behave? | Will it stay safe? |
The same beam is therefore checked twice: once at service load for how much it deflects, and once at factored load for whether it has the strength.
How the two connect in practice
A takedown produces service reactions first, because the floor loads you enter are service values. To get the factored design reaction you apply a code combination, for example the ASCE 7 gravity case of 1.2 times dead plus 1.6 times live plus 0.5 times roof. The full set and how the governing one is chosen are covered in ASCE 7 load combinations, and the dead and live split that feeds them is in dead load vs live load.
A worked comparison
Take a column carrying 519 kN dead, 270 kN live and 30 kN roof in service. The simple service total is 819 kN, the number you would use to think about settlement. The factored strength value is 1.2 times 519 plus 1.6 times 270 plus 0.5 times 30, which is 1070 kN, the number that sizes the column and footing. Same loads, two answers, two different questions.
Key takeaways: factored vs service loads
Service loads are reality and drive serviceability; factored loads are reality plus a margin and drive strength. A takedown gives the service reactions, a load combination produces the factored ones, and you check the structure against both. StructLoads shows the service and factored values per support so the distinction is explicit, and a qualified engineer confirms the governing cases in design.
Quick answers
What is the difference between factored and service loads?
Service loads are the actual, unfactored loads the structure carries in use, the real dead, live and roof loads. Factored loads are those service loads multiplied by load factors from the code, such as 1.2 on dead and 1.6 on live. Service loads are used for serviceability and allowable stress design; factored loads are used for strength design with a safety margin.
When do you use service loads instead of factored loads?
Use service loads for serviceability: deflection, vibration, crack widths and settlement all use unfactored loads, because you are predicting actual performance, not guarding against collapse. Service loads are also used directly in allowable stress design. Strength design, sizing a member against failure, uses factored loads instead.
How do you calculate a factored load?
Take each service load, multiply by its load factor from the governing combination, then add them, for example 1.2 times dead plus 1.6 times live plus 0.5 times roof. You evaluate every relevant combination and the largest governs. A takedown gives the service loads; the combination turns them into the factored design load.
Is the factored load always larger than the service load?
For strength design, yes, because the load factors are greater than one, so the factored combination exceeds the service sum. Serviceability combinations use factors at or near one, so those values stay close to the service loads. The point is not size for its own sake; it is a calibrated margin against failure on the strength side.