Concrete Joists

[Dex_evo]

Hi.
I've just looked under the floor of my new house and it has concrete joists. They look the same as wooden 1's in style and are supported by block work around every 8', they have wooden fillers over them with floor boards layed over that. The house was built in the 50's if this makes any differance.
I was wondering if anyone has experience with these and can advise at all as to their strength??
I will be putting a 350gal tank in the house (84'' x 36'') but it will be running the same way as the joists and will cover 2 maybe 3 of them. Is their any hope of them being strong enough to take the tank without reinforcing them because the foor boards will be a nightmare to lift and there is not enough crawl space to go under the floor.

If they were wooden I wouldn't ask but I've never seen concrete used before and just wondered if anyone knew anything about how strong they will be

Dex...

 

[JoelB]

Concrete has enormous compressive strength (when you put something on top of it). But it's shear strength (when you try to pull it apart or snap it), is way (several times) less.

The joists may be pre-stressed. This is where they are held in a compressed state by the internal reinforcing steel. This technique is often used in building bridges.

Still, I do not know your flooring. If it was me, I'd probably want to cut small access panels in the floor and add extra concrete pads under the joists at key locations using a few bags of premix concrete.

This could be very effective, however, your local builder may be in a better position to serve you.