Click Personal Profile after login to opt out. I want to know why working loads are used in case of foundation design while the entire structure is designed on ultimate. In answer to your first query only, I would suggest that foundation design is not really any different from the rest of structural design in regard to the use of 'ultimate' or 'working' loads. Whether we are designing steel or concrete structures, BOTH types of load need to be considered for most structures. But that is not all that they require.
Most codes also specify particular limits that apply under 'serviceability' or 'working' conditions.
When it comes to the foundations, you have a similar situation. The reinforced concrete design can be done with ultimate loads, but the permissible bearing pressure can often depend on the maximum acceptable settlement under actual loading, and therefore common practice is to specify it as a 'working' limit. The actual foundation and supporting soils tend to 'feel' a fairly light structure. Dead loads are usually overestimated as a whole. It must be recognized that ndividual beams, columns and slabs may experience localized heavier dead loads.