Casting threaded rod in concrete - pressure load?

[ensamresande]

I am considering embedding threaded rods in a concrete slab (think post shoes), which will only be subjected to load "from above" (i.e. pressure "into" the concrete), but I'm having trouble finding information about what applies for this type of load.

Many post shoes are cast in as they are, but at the same time, some post shoes have, for example, a small plate welded at the bottom. Does this help against compressive force as well or is it mainly against tensile force?

What I really want to know then is, should/can I do something more with the rods than just embed them as they are (like welding a plate at the bottom, placing a nut/square washer against the concrete), or will the nut typically strip the threads before the concrete fails?

 

[Stefan1972]

When casting a foundation post that something will stand on, the post is, of course, dimensioned according to what it is supposed to support. It doesn't matter how the load rests on the post as long as the threaded rod/rods are also dimensioned to carry the load.

If the load is so large that the rod cuts into the post and cracks it or something like that, then the post is improperly dimensioned/improperly constructed.

 

[Lutte]

From the construction steel side, undercasting is always done to not put pressure on the threaded rods and nuts, so I would not let it stand only on the rods if there's nothing to absorb the pressure under the threaded rod.

If it's a bolt group of 2 or more, we always welded it into a "cage" with the correct hole spacing, so if you weld a plate underneath, you will be safe according to me.

Then, what gives first depends on the choice of threaded rod, embedment depth, and concrete quality.

 

[Matti_75]

Concrete withstands compression extremely well so the threaded rod buckles before the concrete as long as you have enough concrete and reinforcement underneath.