Post Tensioned Tie Rod for Nonvoided Slab Bridge

[OSUCivlEng]

I'm designing an off system (non NHS) prestressed solid slab bridge for a low volume road. It will have six 5' wide slab sections. I have seen details by various DOTs that use solid rods (A449) with nuts and DTIs or 1/2" dia. post tensioned tendons for transverse tensioning of the slabs. Since this bridge is going not going to be built by a contractor I am going with the rods. Is it possible to get a 30' long ASTM A449 rod? It looks like 12' is a standard stocked length.

I have also considered threading a piece of #8 rebar, but it has a much lower yield strength and is not really made to be used as tension rod.

 

[Ingenuity]

What is the expected magnitude of effective prestress to the rods, as a % of yield or UTS?

I am not familiar with ASTM A449 rods (so I can't help you with available production lengths) but if your applied prestress is too small then the expected losses due to concrete shrinkage and creep will reduce you assumed P/A to near zero.

Also, if you are stressing the rods to significant levels of stress, your stress relaxation properties may not be favorable compared to ASTM A722 Grade 150 ksi PT threaded bar.

 

[TehMightyEngineer]

Why not just design each slab to act independently with either a flush joint or shear key? I imagine the thickness increases and any extra rebar will be much less than your design time and the costs involved in installing the PT system.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[OSUCivlEng]

Technically, the slabs are "connected only enough to prevent relative vertical displacement at the interface". Maybe post tensioning was the wrong way to describe it. It's not meant to add additional strength by causing all the slabs to act as one unit.

This is a normal detail for these types of bridges. Per the A449 spec minimum tensile strength is 120 ksi and minimum yield strength is 92 ksi. Normally the rods are 7/8" diameter and are tensioned to 39 kips. That works out to 70% of yield strength. According to ASTM the proof load is 85 ksi which would be 51 kips for a 7/8" diameter rod.

I'm just trying to figure out if it's a good idea to spec a rod that long or what I should spec instead.

Thanks

 

[bridgebuster]

If you can't find 30' A449 bars, what about threaded couplers to splice them, use something like a Dayton Taper-Lock? 1" diameter couple would probably have an OD of 1 1/2 to 1 3/4", assuming a 3" diameter opening through the boxes it should be possible.

Did you call Dywidag? They should have something in the length you need. You'll probably pay a premium for a small order.

There's also 100 ksi stainless rebar that's available.

 

[3Fan]

Our DOT has used 1 1/4" diameter A722 Grade 150 ksi PT bars as Ingenuity mentioned. The bars were around 38' long per phase. A coupler was used to connect the PT bars between phases.

 

[OSUCivlEng]

Looks like A615 Grade 75 or the A722 150ksi bars are the way to go. They are readily available in the length I need.

Thanks for the help. Our state hasn't done many of these types of bridges, so I'm interested in how this turns out.

 

[Ingenuity]

Looks like A615 Grade 75 or the A722 150ksi bars are the way to go.

I would recommend you not use Grade 75 due to the low pre-strain and probable prestress losses that would be sustained.

A722 150ksi bars have a proven record, readily available, and competitively priced.

 

[rapt]

My earlier reply seems to have been lost somewhere.

Agree with Ingenuity, Stress Bar, Dywidag bar or similar due to its reduced relaxation performance.

 

[OSUCivlEng]

Just to close the loop on this, I contacted Portland Bolt. Standard marketplace length for A-449 bars is 24'. Hex coupling nuts are easy to obtain as well, but sleeve nuts would be a custom order.