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Existing Concrete to New Steel Connection

Existing Concrete to New Steel Connection

Existing Concrete to New Steel Connection

(OP)
I've got a situation where we need to connect between an existing concrete pedestal and a new HSS post/column. Pedestal is 24" square with (8) #11 verts and #4 ties. The rebar are terminated with Lenton (rebar to rebar) couplers just below the top of existing concrete.

I need to place a 14" square HSS atop this pedestal that needs to be resist moment.

I can't seem to come up with a manner of connecting these two members that 'works' and is reasonable to construct.

Anyone have any ideas or direction? I feel like I'm missing something simple. Rough elevation sketch attached.

RE: Existing Concrete to New Steel Connection

Maybe you could sleeve over the pedestal and bolt into the side of it?

Nothing stopping you Breaking the pedestal back? Or extending it with more concrete to increase the size? This opens up a few more options possibly?

RE: Existing Concrete to New Steel Connection

(OP)
Good points. Not sure about sleeving over - I'll have to check with client. We can add some height to the pedestal. Perhaps around 24" to 36".

RE: Existing Concrete to New Steel Connection

Don't LENTON make 'Bolt Couplers' that provide a full strength joint between a reinforcing bar and a standard bolt?

But you may have to chip and remove the existing rebar-to-rebar coupler to install the rebar-to-Bolt Coupler.

RE: Existing Concrete to New Steel Connection

I suggest the following:

.1 Add 4-#8 rebars from the couplers upward six or eight inches.
.2 Provide a base plate on the column with holes to match the bar positions.
.3 Place column and base plate on shims leaving 2" for grout.
.4 Place Lenton Terminator on each bar over base plate.
.4 Place grout.
.5 Have a glass of champagne.

BA

RE: Existing Concrete to New Steel Connection

Quote (BAretired)

.4 Place Lenton Terminator on each bar over base plate.

Interesting use of LENTON TERMINATORs as nuts to the base plate. Going to require washers to make up any out-of-level relative to each vertical bar, or precision cuts to the #8 double threaded transition bars.

RE: Existing Concrete to New Steel Connection

Can't you just thread the top end of reinforcing bars, and install them in the couplers?

RE: Existing Concrete to New Steel Connection

I don't think 4#8 is going to work for a 200 k-ft moment. Also, can you couple a #8 to a #11?

RE: Existing Concrete to New Steel Connection

Does the column have to be HSS 14x14? An HSS 12x12 would provide an inch more clearance.

I misread the sketch in my earlier post...thought the bars were #8, so step 1. in my previous post would be add #11 bars, not #8 bars.

I like hokie's suggestion too if there is enough clearance to place the nuts.

BA

RE: Existing Concrete to New Steel Connection

(OP)
Can I just thread rebar? I'm not led to believe that there's a code spec for that. But it would be a fairly ideal solution.

Nominal diameter of a #11 is 1.41". Can I get that cut to a 1 3/8" UNC thread? If so, just use allowable stresses based on yield strength as I would for an anchor bolt?

12" tube would probably work. This is all wind induced moment and a fairly short structure so I don't think deflection will be much of an issue. Although it may look a bit spindly.

I've never worked with these Lenton couplers. Is there enough tolerance to use the terminator as nuts as in BAs suggestion?

Thanks for the help so far guys.

RE: Existing Concrete to New Steel Connection

The fabricators I have run into want to turn the bar down a sized (#11 to a #10) and then cut the threads. Results in a reasonable loss of section, but I have done it a couple of times.

RE: Existing Concrete to New Steel Connection

We frequently thread reinforcing bars in Australia. As dcarr said, you must allow for loss of section. Additionally, with the higher strength rebars used today, the steel on the outside of the bars is actually stronger than the middle. We have published values for threaded bars, but those would not necessarily apply in the US.

RE: Existing Concrete to New Steel Connection

I like BA's solution if the capacity you need is there.

If not, you could couple BA's solution with an external steel sleeve of the concrete column to increase the moment capacity.

Mike McCann, PE, SE (WA)


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