Cast in Theaded Rod, tensile capacity and anchorage length

[sybie99]

Good Day,

I have a carport canopy design where the bottom section of the canopy column is concrete and the top is steel. See attached sketch depicting what is required. I need to transfer a 50kNm moment from the steel "star" column on top to the concrete column below. There will be a baseplate to the bottom of the steel column, bolted down onto the top of column, transferring the tensile forces as result of the bending moment is the tricky bit.

The concrete column is 350mm x 350mm, so the lever arm created between the holding down bolts can only be about 250mm. That results in 100kN pull out on the 2 bolts on the one side. Holding down bolts with anchor plates wont work, there is not enough space and column edge could split. Post fixed chemical epoxy anchors can also not take such a tensile load in this situation.

The options left is to machine a thread to the top section of 4 of the column vertical reinforcing bars, thus transmitting the full out force directly to the column rebar. But this machining takes time.

I was thinking, what if I cast 4 threaded rods straight into the concrete column near the top? How does one calculate the required length of this rod to give its full tensile capacity? The concrete codes all give formulas for normal reinforcing but not for a threaded rod.

I read somewhere that for a smooth rod one should double the anchorage length of reinforcing bar of the same diameter, not sure if this is true. The thread will give some "bite" into the concrete similar to a deformed rebar, but can one calculate the anchorage length needed?

In this case I would use 20mm threaded rods which can take a 100kN tensile load, its just what length I need.

 

[dlawrence529]

Can you just use a mechanical anchorage instead of relying on threads? Something like a foundation "J Bolt" for bolting a sill plate to a concrete foundation wall. There should be manufacturer cut sheets that give capacities.

You can use oversize washer sandwhiched between two nut to create a mechanical anchorage. Calculate the capacity of the shear cone created at failure.

 

[azcats]

Lenton makes a wide array of couplers including some that couple rebar to threaded rods. The bars usually still need to be machined however. The one time I used the couplers for an odd project, it was not difficult to find a rebar shop that had the tools necessary.