hollow core plank repair

[johndeng]

I have a project with planks bearing on both ends. However, both ends (within 4 feet each end of the 30 feet span) appear to be in bad shape. First I put in small supplemental beams at 4' from each end (I can't depend on the beams since it will over load the tie beams and girders). I am think of removing the top part of the plank, only leaving bottom concrete with strands. Then I want to grout surface and place 3" composite deck on top of bottom concrete, then place a bit slab reinforcement and pour slab (slab is 5 feet long from end, 1 feet pass the bad area).

I am still depending on the plank bottom strands. The new end will be slab on top of bottom plank, a composite slab. I know bottom plank will support all loads above, but since it's near end, only shear matters, so the composite slab will be ok for shear. The bottom plank may be thin, but the moment will be only due to the 4 feet long slab above, and I have small supplemental beam.

Is there anything else I should think of?

Thanks

 

[msquared48]

Posting sketch of the problem and your solution?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[johndeng]

OK please see attached sections.

 

[msquared48]

Didn't post. Try again...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[johndeng]

let me try again

 

[johndeng]

another one

 

[BAretired]

Why are the tops of the hollow core planks in bad shape? Are you able to determine the cause of the problem?

Are you planning to rely on the beams you installed four feet from each end or will you shore the planks at, say five or six feet from each end?

I am still depending on the plank bottom strands. The new end will be slab on top of bottom plank, a composite slab. I know bottom plank will support all loads above, but since it's near end, only shear matters, so the composite slab will be ok for shear. The bottom plank may be thin, but the moment will be only due to the 4 feet long slab above, and I have small supplemental beam.

You are pouring new slab to five feet from each end. That is L/6. The moment at L/6 for a uniform load is wl²/12 which is 2/3 of the maximum simple span moment, so I would not say "only shear matters".

Are you going to be able to get adequate bond between the fresh concrete and the bottom plank to develop horizontal shear? How will the bottom plank behave under prestress after you have removed the damaged material.

Since hollow core planks are not reinforced for diagonal tension, will your repaired section be adequate to carry diagonal tension?

I wonder if it would be more prudent to simply replace the damaged planks.


BA

 

[BAretired]

I wrote my last post before seeing the sections. Do you really want to remove the existing ribs? If you use 3" steel deck between the upper and lower slabs, how will you get any bond?

BA

 

[johndeng]

Thanks BA.
I don't feel I can bond them, so there will be two slabs to hold the shear load.

This is a parking deck, water penetrate thru, and ends of plank are in bad shape. I would shore slab, say five or six feet from each end. New slab will bear on the bottom plank and also both ends.

I really should remove the whole ends if plank bottom also gets bad. But I cant cut the plank since no way to support it. So I am trying to fix the plank, keeping any good part of bottom plank.


I feel the moment is wl²/8/3=wl²/24.

 

[johndeng]

Just my thought, any suggestion would be greatly appreciated.
We would repair the cracked/spalled planks as follows:
1. Install new supplemental structural beams beneath plank.
2. Shore plank and chip out the loose, cracked, spalled top plank concrete;
3. Remove loose middle plank web concrete;
4. Keep original bottom plank concrete and strands as possible, except bottom concrete no good;
5. If both top and bottom plank concrete is solid strong, keep plank in original shape and adding rebar into the open cores in two pieces - inserting each direction, and splicing in the middle per detail; Wetting the cores if required; Pouring concrete into the appropriate cores of the plank.
6. If top plank concrete is chipped out, keep bottom plank concrete and strands in original shape and grout surface flat. Place new deck on top of bottom plank, then place new reinforcement steel per detail;
7. In places where bottom plank concrete to be chipped out, report to engineer first. If only one strand or less than half width of bottom plank, then place new deck between beam supports, place slab reinforcement and pour new slab.

 

[BAretired]

What supports the new supplemental structural beams?

If you are going to shore the planks, do you need supplemental structural beams?

I'm not sure I understand your proposal completely, but if the damage is extensive, I would be in favor of replacing the damaged hollow core units rather than attempting to fix them.

Secondly, it would be desirable to prevent similar damage from recurring in the future.



BA

 

[johndeng]

New supplemental structural beam goes between small tie beams. Say column grids are 30'x30', Plank bears on girders both ends, there are small tie beams between columns in the other direction.
It would need a big crane to replace the plank, owner only want to replace with concrete slab. I can't do it unless I put in beam gird (cut area to 10'x10'), also I need to reinforce the girder since original uniform load turns to be concentrated load.

The plank got water penetration since there is no water proof under the topping. Since I filled plank ends with concrete and add water membrane on top, there would be no further water damage to the "new" plank.

 

[BAretired]

There are several options.

1. Fixing the ends of the existing hollow core planks may not be feasible. Depending on the amount of damage to the existing planks, it may not be possible to repair them so that they may be relied upon to perform satisfactorily in the future.

2. Replacing the damaged units entails use of a large crane which is expensive. Perhaps the plank width could be reduced from eight feet to four feet or less to permit the use of a smaller crane or alternative lifting equipment.

3. Replacing the damaged units with alternate construction may be feasible. Beams at 10' centers spanning between girders could be used in conjunction with lighter slabs. Girder reinforcement may not be required at all or, if required, may not be unduly expensive.

BA