I applied online after having toured the office with my graduate school class. I was emailed about a phone interview about a month after submitting my application. The phone interview was 30 minutes long, pretty general questions, nothing technical. Then two days later I got another email asking me to come in for an in-person interview.
I came in for the in-person interview a week later. It was a three hour long interview that began by me giving a 30 minute presentation on a technical topic of my choice, then a 30 minute casual 1 on 1 interview with a principle in the structures group, he was really nice and fairly laid back. After that came another man, an associate, who was equally friendly and welcoming. I got a small 8 question test to complete in 30 minutes (it only took about 10 and was really easy). After the test we talked 1 on 1 about my answers and then had our own interview. Finally another associate came in and him and I also had a 1 on 1 interview, however this was mostly him telling me about Arup's culture to make sure I actually thought I would like working there. He emphasized how the work is very diverse, the group is very tight nit and works together a lot.