Parts are always inserted using the orientation of the last selection. You can reset the ori for the 'floating' part by pressing 'home' before placement. Or by ctrl+home (on a selection) after placement.
Also while working with angled models like yours you'll probably want to change the grid orientation now and then to match alignment with the thing you want to add stuff to (e.g. one beam parallel to another.). This is done using 'o' on a selection, when done use 'o' again while nothing is selected to go back to the normal grid.
The upcoming updated manual will address this way of working a deeper.
Also while working with angled models like yours you'll probably want to change the grid orientation now and then to match alignment with the thing you want to add stuff to (e.g. one beam parallel to another.). This is done using 'o' on a selection, when done use 'o' again while nothing is selected to go back to the normal grid.
The upcoming updated manual will address this way of working a deeper.