Structural Engineer - Before or After Tender

kev1234

I'm preparing my tenders for an extension on a 3 bed semi detached in Dublin. I contacted a Structural Engineer with an eye towards getting them to spec and provide drawings for structural steel, foundations, ceiling joists etc. I was informed that it's 'usually best to do the tendering without such details since the final design for those items depends (almost entirely) on site and property conditions which can only be determined with exploratory work when the builder starts. For most domestic extension projects PC Sums can be used for builder quotes, most experienced builders or QS's working for them can provide accurate figures.'

Does this sound right? Is it normal to spec the foundations and structural elements after construction has begun? I would have thought it should be done pre-tender to ensure their were no omissions from the contractor and then perhaps updated if needed during construction? A second engineer I spoke with said that he would do it for the tender as it's too open ended for the contractor to get away with adding costs later. Should I be getting all of this spec'd for the tender?

Calahonda52

You cannot full design foundations without seeing the site conditions once the trench is dug so either way will lead to possible variations.

Likewise with the steel beam design.
The fundamental issue here is that the SE will need to be on site at the critical points so as he can certify the foundations and the beams, based on what he sees.

This point is often overlooked

Ceiling joists, unless something unusual, are normally not an issue as you want to minimise flexing so normally they are oversized from a design load perspective, as long as graded timber is used.

kev1234

Calahonda52 wrote: »

You cannot full design foundations without seeing the site conditions once the trench is dug so either way will  lead to possible variations.

Likewise with the steel beam design.
The fundamental issue here is that the SE will need to be on site at the critical points so as he can certify the foundations and the beams, based on what he sees.

This point is often overlooked

Ceiling joists, unless something unusual, are normally not an issue as you want to minimise flexing so normally they are oversized from a design load perspective, as long as graded timber is used.

Ok, so should I still get an Engineer to spec these and complete drawings for the tender stage and then also to inspect and certify these elements on site when works are underway? If not then how do I get the tenders detailed enough to make a comparison if there is no details of the foundations/steel requirements?

Calahonda52

Well you can put in an estimate, subject to ground conditions, for the foundations and steel beams based on a chat with your engineer.
foundations are usually 3 times wall width maybe 60cm deep, starting down below the frost line.
You could spec A393 mesh reinforemcent and 25N concrete.

You also need to spec the hardcore: 804 is no longer good enough: must comply with IS 888 2016.

Steel beam will depend on the wall composition