Replacing Wooden Timbers and Floor Boards with concrete understairs

Remove all timber hardcore it to 150mm below damp apply membrane and pure 140mm concrete in to set

You don't say what the configuration of your stairs are. The sides of your stairs are called 'Strings'. If the strings have handrail and spindles attached or don't have any safety rail they are called open strings. A side attached to a wall is the closed string. If both sides are closed (attached to a wall) there's probably little you need to do to support as there attached to the side walls except monitor any movement of the stairs as you take the the floor out from below and support as necessary. If you have any open strings fix timber ties (100 x 25mm 4" x1" sawn) from 1 or 2 steps up to the inside of the string and to the 1st floor joists through the lining around the stairs opening . 3No. 50mm x 5mm screws though the tie into the string at the bottom and min. 2No.100mm x 6mm screws through the tie and lining into the joist . ALWAYS check for pipes and cables, shouldn't be any but I've come across some weirdness over the years. Use of the stairs should be kept to the absolute minimum and if a fitted string is available walk up towards that side. When you have access to the area to be concreted remove all debris that will rot, its a favourite place for some to dump all the renovation rubbish bricks etc can stay. compact if you can and add a layer or hardcore min 100mm or to the house damp proof course and compact then 50 mm sand and compact, cover with polythene damp proof membrane min 1000gauge and up the walls to above the finished floor level (at least 100mm) this should be in one piece folded into or over corners, do not cut. you can cut this off above the floor after your set and dry. hope that helps