The two terms come from the design of reinforced concrete. The old way, Working Stress, (similar approach to ASD with steel). The service load is the best estimate of the actual load that a concrete member may be called on to support.
The current way, Ultimate Strength. The design load is the service load increased by specified load factors in order to provide a factor of safety.
Traditional Examples:
Service Load = Dead Load + Live Load
Design Load = 1.4 x Dead Load + 1.7 x Live Load
Reference: "Simplified Design of Reinforced Concrete - Fourth Edition", Harry Parker
The design load of a structure is the probablistic loading of the structure that would put the structure into a failure mode.
The service load is the load a structure would see on a daily basis when used as designed.
Failure does not mean on the ground. Buildings may reach failure, but still stand to allow people to exit safely.
Service loads reflect the daily loading of a structure and are most directly related to the comfort of the user of the structure.
A building designed soley on the basis of Ultimate or Design loading may be acceptable from the standpoint that it will resist failure to an acceptable point, but would be an uninhabitable building. Service loading directs one to deflection criteria amoung other things. A springy floor may be fine in an overall sense for capacity, but if the users of the structure are uncomfortable with a boucing deflecting floor the structure has "failed" from a serviceability standpoint.
Maybe this post is helpful or perhaps an overstatement of the obvious.
I am not trying to define loadings per any particular code, just more broad definitions.
ASCE 7 "Minimum Design Loads for Building and Other Structures". It shows that design loads may be unfactored or factored for ASD or LRFD. ASCE 7 do not mention service load.
ACI 318 specifies service load as load without factors.
