I cannot say for Eurocodes or British Standards but in Canada this separation of a large structure into 2 smaller structures is done to lessen the fire separation requirements inside each smaller structure by the installation of a fire wall between the two. It is also useful when you have to different types of occupancies, say residential in one portion and some type of commercial in the next.

Also two different types of structures with structural systems and different stiffnesses, necessitating a seismic joint.

Mike McCann
MMC Engineering

When i first heard about this rule, (dividing a large structure into two smaller ones) i could only think of two possible reasons. 1) In cases where the ground conditions below a structure are different at one end therefore different settlements are expected at one end of the structure. 2) or for seismic safety, for example diving an L shaped structure into two structures,

Fire wall construction can be interesting... it has to remain intact given one or the other 'building' collapses.

Dik

Thermal volume change, concrete shrinkage, and stress risers due to reentrant corners are other reasons for separation.

Hy!
The division of one structure into two smaller structures can be required for more than one motive.
1)It can be required from a seismic point of view. In this case you must observer the plan view of the structure and you must separate it into simple geometric form like squares ( an L plan view structure must be separated into two square models).
2)From a thermic point of view. A metallic structure must be divided if it's longer than 30meters. The elements dilating so you must give them room to expand.

Thank you everyone for your replies, i can now understand that there are various reasons for dividing a structure. However i need specific values, does anyone of you know if the Eurocodes give specific guidance for such requirements?

I remember that the casinos in Atlantic City were usually built as "two" structures - the high rise (built first) as it will lead to more settlement beneath it that the lower storied casino sections. The low rise would be built later (with columns placed relatively near the highrise columns) as the high rise was topping out and then a simple grade beam/slab would be built connecting the two structures. This would handle the settlement issues - Atlantic City had extensive sand deposits for founding on.

Well thats one more reason why build two separate structures. That is a very interesting approach. So the low rise casino was build after the settlement of the high rise occurred right?