Would anyone happen to have any details or technical drawings relating to a portal frame construction to make a gable window? Our structural engineer is designing the steel structure but it would be useful to see any past projects/hear your experiences. 

We constructed our frame in 90mm square hollow tube, it is around 3 to 4mm wall thickness.  Then we attached timber to the outside face and the windows sit in the wooden frame and in front of it.  As the frame attached below insulation level its a thermal bridge so needs to be insulated internally

3 hours ago, Mike Wynn said:

a portal frame construction to make a gable window

It should be a routine framed opening within the wall construction.

What is the wall construction?

 

Your SE will have allowed that the wall construction can support vertical load (up as well as down) from the rafters.

In reality the gable wall is often not a portal frame as it isn't spanning from foundation to foundation.

The current wall is of cavity wall construction. The SE has proposed a to re-construct with a gable window taking up most of the wall and block piers to either side. Like you say, it may just be an apex frame of the piers with a beam to vault the ceiling inside. proposed. 150mm lean to rafters will be used from beam to walls in vaulted areas.

1 hour ago, Mike Wynn said:

The SE has proposed

Sounds as if it is straightforward.

The SE will show piers and specify a lintel. Bricklayer forms the window opening which the window supplier measures (don't order by theoretical window dimensions as they can turn out different.  , but you can get a quote from it).