Why not just buy a pre made beam, they're cheap enough. In fact, buying a
pre made one might be cheaper than trying to make your own.
I have to cast 2 beams to make a tee shape to bridge over a well or
similar hole I've discovered under where I need a wall for an
extension. The beams have been designed by a structural engineer to
meet the building inspctors requirements. They are 3500 x 450 x 375 -
so don't think |I could move anything pre-cast.
Then, for your own safety, I'd definitely say, go for something bought in,
rather than making your own, because the building inspector, and most
insurance companies I'm sure, won't accept anything that hasn't been tested
and certified as being able to take the stresses that have been designed.
Another idea you could put the structural engineer, is to have the hole
sufficiently filled with high grade aggregate and soil or a heavy mix
concrete pour. Of course, this totally depends on the size and depth of the
hole, and what the sides of the hole are like in retaining the weight of
being back filled.
The idea of a "T" shaped structure to span a hole must mean the hole is
wider than it is deep, or of equal proportions, but I'd still assume that
shuttering and pouring a block of concrete in the hole where your going to
build, would be adequate to hold the weight of a new wall. If the hole is
for a seasonal spring or burn that runs under the property, then a concrete
pour into an arched shaped timber mould would, I'd have thought, also be
able a better idea than a "T" made from pre-stressed concrete.
A solid arch shape would surely be able to take the weight of the wall on a
new building and the arch would allow the burn to run under. Just have a
look at the Roman bridges and via-ducts that are still standing after all
these years, and they've allowed rivers and roads to run under and over them
without the need to ballast them with "T" shaped beams.
I'm on the outside of this question and not really able to look in, so my
thoughts might be well off mark, but maybe my comments could be used to make
other judgements on the suitability of other means of spanning this gap.
Good luck with it.
On 1 Nov 2003 14:03:23 -0800, firstname.lastname@example.org (BryanB)
Depending on the attitude of your structural engineer and building
inspector, you may have difficulty in proving that your self mixed
concrete is C40.
What C40 means is that the characteristic strength of 100mm cubes
made, cured and testing in the standard way achieve a strength of 40
Depending on the aggregates and cement used, to give you a rough idea,
the required mix would probably need to be:-
Portland cement 375kg/m^3
20 - 10 mm aggregate 690kg/m^3
10 - 5 mm aggregate 345kg/m^3
Water 187 litres
Depending on where you are and if there are any sulfates in the
ground, there may be other mix design restrictions that the building
inspector may want satisfied.
Its not that easy to define a mix to meet the c40 strength, as the strength
of the concrete is affected by how well you mix it and how well to place and
compact (vibrate) the concrete.
You could mix a 1:2:3 mix, but if it is full of air pockets when dry, then
it will be weaker than a 1:5:10 which has been well vibrated.
A 1:3:6 may do if properly mixed and vibrated.
The placement of the re-bars is important too.
Mixed sand/gravel depends on the ratios chosen by the supplier, and it is
difficult to say what ratio they have used.
3/4 to dust is not a term used for concrete - it is used for grading stone
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