Coring Precast Floor Planks

[patswfc]

Does anyone know of any guidance/rules of thumb for creating small service penetrations in existing precast concrete floor planks.

I'm working on a small renovation job of a 1970's building in the UK. The client wants to create a number of 150mm dia. holes through the 1st floor, which is supported on load bearing masonry.
The position of the holes (1 per plank) will generally be 0.75m from the end of the 7m span. The planks are 1.2m wide and the position of the holes varies across the width at each location.

I can't seem to find much guidance on this issue.
Some of the current manufacturers will allow smaller (up to 75mm dia) holes to be site drilled as long they pass between pre-stressing cables.
The problem that I have is that they are bound to core through a pre-stressing cable or 2 on site with a 150 dia hole. We have no info on who supplied the planks originally so cant get any input from the manufacturer.

My opinion is that given the holes are relatively small and their position in the span they shouldn't adversely effect the slab, but i dont want to proceed based on my opinion with nothing to back it up.

Does anyone have any guidance on the above. Thanks.

 

[concretemasonry]

The best thing is to try to find our what the core configuration is. You goal is obviously to limit the coring to the areas with horizontal cores.

The size of the plant would limit the possible manufacturers at that time (1970). The 1.2 m width is still a viable product, so some contacts to prestressed concrete manufacturers may yield a likely supplier of some insight to the product you may have. The name of the manufacturer may have changed, but due to the investment in facilities, the basic operation may still be operating. I would guess the plank is extruded and sawed to length. At that time, the extrusion concepts are for different core configuration - larger core that may be supported by lightweight aggregate and a method with a drier mix and more, but smaller cores. In either case, the cores are usually consistent in horizontal orientation. Most of the wet cast systems had narrower widths and length limited to 20m or so. The wet cast systems generally have a much smoother and tighter bottom surface.

If you can find a way to access the end of a installed plank, this will give a positive determination of the core orientation and target coring

good luck in your research.

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[ishvaaag]

Installers themselves (sometimes quite closely related to precast fabricators) use to cut quite liberally the planks to adapt to columns, in sizes even more important than the 150 mm. So as a generic opinion I concurr with you that for most cases the planned holes should have small effect in the behaviour. It is clear that normally adding the lost prestress is not available. At the position you quote cut wires will be still helping at midspan, and the general actual capacity in shear -except if extremely critical, that uses not to be the case with presstressed planks- should be enough to pass the forces to support surrounding the hole.

You still have options for reinforcement, actual service (adding topping, maybe lightweight... would help?) and at ultimate (carbon fiber flexural reinforcement).

 

[dcarr82775]

Ground Penetrating Radar is relatively cheap and could help find clear areas to locate the cores.

Are the planks solid or hollow? If hollow the strand is normally located between the cores.

 

[paddingtongreen]

With the problem as stated, I would just go ahead. If you cut a rebar, it has enough length to develop on by the time it is needed. However, if it was post tensioned....



Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[dik]

The holes are close to the end of the span and the HC slabs have keyways that help transfer the load to adjacent panels.

As long as you only core one plank out of three, I wouldn't be too concerned. 8"HC can be used to about 30', so shear shouldn't be a problem and because you are near the end of the panel, you may be terminating 1 of 7 strands (check the number of strands). First, try to avoid a strand. The problem with post-tensioned strand is that the development length is greater than re-bar. To be thorough, you can check development length for bar termination...

Just a few thoughts...

Dik

 

[hokie66]

The OP said the planks are precast, not post-tensioned.

 

[msquared48]

Hokie, he did allude to PS cables...

"The problem that I have is that they are bound to core through a pre-stressing cable or 2 on site with a 150 dia hole. We have no info on who supplied the planks originally so cant get any input from the manufacturer."

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[paddingtongreen]

At1.2M, it sounds like a 4'-0" plank.

This might work if you can see the end of the plank. Calculate the maximum loading effects from the code that was current at the time the building was built. Back fit a minimum concrete strength for those load effects, calculate the rebar tension and divide by the number of rods. You know that the rods were good for at least that load. You can figure the required development length for the rods to the latest code and probably allow a higher concrete strength for continued curing. Check the modified plank.



Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

Mike,
I was just trying to keep the language straight. Precast planks are pretensioned, not post-tensioned.

 

[msquared48]

Sorry Hokie - I missed that.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[dik]

Sorry Hokie... I meant pre-tensioned strand... and the development length for the strand is still greater...

Dik

 

[hokie66]

dik,
I think Paddo threw you off. Both of you knew better, but I was just being my usual pedantic self for any newbies reading along.

 

[dik]

Nope... an outright error... I knew they were pre-tensioned... wrong word came out...

Dik

 

[concretemasonry]

Those planks were probably made in a factory within 30-50 miles. Installations like that do not disappear because of the value of an operating license, permits and land values. The name of the company ownership undoubtedly changed, but records are kept for liability reasons. The manufacturing equipment can be changed, but the size matches a very common prestressed plank size (1200 mm).

The manufacturing facility can be found if it important enough, otherwise just use the visual, probing and non-destructive methods.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[nuche1973]

Go out and "Pach" it. Here in the States, a good field tech can tell you the location and depth of any steel, up to 12" (300mm) with one of the new pachometers.