Expansion joint is needed in buildings when the length of the building is more. However many young civil engineers may be confused if the joint needs to be there in foundations aswell? Below is a video that will explain the need of this. Also the video covers some other additional points on what the gap of separation should be and some quick methods to cover this separation gap so that the building will not leak. It is also important to note that there can be seismic factors that decides the need of this separation and dimension of this separation joint gap. The video discusses all these points. Also ensure you read the additional blog and extra video given at the end of this section. You may need a separation joint for other reasons too. Some of the reasons are listed below When there is an abrupt change in geometry of the building When there is vertical irregularity When there is a difference in loads in different parts of the building. I have a separate blog for this and you can read it here You can discuss any of your questions and doubts in this forum. If your question is on a completely different topic,please initiate a new post. You can create your log in and then access the forrum space. Join my structural design group here