Rebar Hoops

[05dtaylor]

I'm work for a precast concrete manufacturer. I'm drawing up some production plans for full-depth post-tension deck panels. The post-tensioning runs through 3" Dia PVC Ducts(3.5" O.D.) that will be cast-in of course. At the End Panels the engineer has specified that there be these #4 rebar hoops that encircle the ducts (as jacking reinforcement I'm assuming). I through together a drawing real quick to show my issue (Conceptual view doesn't show the 4-#5 Bars that run along the PVC inside of the ring but elevation view does). You'll see the interference issues that will happen. Any ideas/solutions that I should go to the engineer with? Hopefully I am clear enough.

Thanks for any help

 

[dik]

Why not just use a circular bent bar about 6" dia that laps half a circumference. The vertical bars don't likely do anything, but get in the way... and pose a problem for placing.

Dik

 

[hokie66]

The reinforcement around the ducts is anti-burst reinforcement. Why is the duct so large and why is it PVC rather than standard corrugated duct? I like using a spiral for the ant-burst rather than individual ties. The anti-burst has to go around the tensioning anchorages, not just around the duct.

Dik, what vertical bars? He said it is a deck panel.

 

[rapt]

05dtaylor,

- use a spirial!

-why atre you using such a large duct? How many strands are in it?

- What is the depth of the concrete panel? The detailing of the bursting reinforceemnt is dependent on the overall concrete dimensions and the stressing anchorage dimensions. It is hard to comment fully without knowing anything about it.

 

[05dtaylor]

I didn't design the panel system, I just work for the Precaster who will make it. Engineers provide us with rough drawings and specs and then we need to provide then with detailed production drawings for approval.

Bridge deck is 15' wide. The panels are 8" thick. They want 1.5" of cover for the reinforcing which leaves 5" of space. I don't know why they want 3" PVC, but that's what they want. The Post-Tensioning is 1.25"Dia Grade 150 Rods jacked to 35 tons with an anchorage plate that is 6"x6"x1.25"

 

[hokie66]

So are these transverse post-tensioning rods? How wide are the individual panels? Are the panels pretensioned in the long direction? 8" is very thin for bridge deck panels, so no wonder you have congestion issues.

 

[05dtaylor]

This is my first post-tension project by the way.

To clarify I think, there are no integrated tensioning anchorages. Its just panel w/pvc duct through it. Two end panels have this hoop reinforcing around the duct (specified 4 hoops per PVC end so 8 per panel, and 16 total). The tensioning rod will just go through the panels,then the 6"x6" plates then there will be a nut. That is it. (The approach slabs we will make, which butt up againts the end panels, will have pockets for the bolt ends to fit into)

 

[05dtaylor]

Bridge is 85' long. The two post-tension rods run the entire 85'. No pretensioning in the panels

 

[05dtaylor]

The panels will be bolted directly to 4 - 85' Long W36x232 Beams

 

[05dtaylor]

Sorry for the back to back to back... posts (keep thinking of things to add after I post)

One more note. They want 40 bar diameters for Lap-Slices

 

[05dtaylor]

Oh and originally the plans called for 17 - 5' wide panels with 9.5" wide panel on each end (hoops would be in 9.5" wide panels). We asked the engineer to modify the design so instead there would be 15 - 5' long panels and a 5'-9.5" panel on each end. We were afraid the 9.5"long x 15'wide x 8"thick would easily crack from tension loads and initial impact loads of traffic.

 

[hokie66]

OK, now I get the picture. They are transverse bars in relation to the panels, but longitudinal to the bridge.

You can't lap post-tensioning rods. I worry about this job, as sounds like the design engineer has little experience or common sense.

 

[rapt]

If the bursting reinforcing is "inside" the bearing plate dimensions, as it is in this case, it is useless! Tell the designer to provide details!

If the panels are spanning transversely 15' and are 8" thick and it is RC in that direction, I hope it is only a pedestrian bridge!

I agree with Hokie!

 

[05dtaylor]

Lap splice spec was just for the reinforcing bar, not for the post-tensioning

Since i'm home now, off the top of my head, I know the engineer's plans stated that the bridge is designed to support H-25 loading and a 65 Ton crane...

 

[05dtaylor]

Another render

 

[dik]

Hokie... sorry, they're horizontal... still not sure how useful they are...

Dik

 

[hokie66]

rapt,
He has 4 longitudinal beams, so the 8" slab will only span a bit less than 5'.

Dik and rapt,
Now that we know these are transverse rods with end plates, I doubt that these bars which the OP was asking about do much. I suppose there could be some concern about splitting stress in the panel, especially with such a big conduit.

 

[05dtaylor]

The post-tensioning rods run longitudinally. Two - 85ft long rods run the entire bridge length inside the PVC ducts.

Now, I was thinking about why such a large duct would be needed. We fabricate our own steel forms and use a (+/-)1/16" tolerance, so misalignment of the ducts wouldn't be an issue. The only reason I can think of is that the rods will have couplers, thus an increased diameter. I did find it odd that stranded steel tendons weren't being used and I would assume it'd be pretty tough to transport two one piece 90ft rods. Thus, I'm assuming the 3" PVC is used due to the fact that the rods are delivered in shorter length and coupled together.

Another interesting note. The bridge is being built to cross a river at a paper mill (Sappi Paper) and is replacing and existing bridge.

 

[hokie66]

I would still call the rods transverse, as they are transverse to the panels. The purpose of the rods is to make the panels act together.

You are doubtless correct about the reason for the large size duct. Couplers would take most of that space.

The problem is, the conduits will cause cracking along the entire length of the bridge. With an 8" slab, you only have 4.5" total of concrete, so 2.25" each top and bottom. It will crack! I know it's not your design, but it doesn't work.

Bridge slabs which are posttensioned together in this manner are normally at least 16" thick.

 

[05dtaylor]

If anyone is interested, here's a picture of one of the panels just before concrete is to be poured. The panels are match casted by the way.