precast prestressed plank modification 2

[amyrice72]

I have a n existing precast prestressed plank space that a new user wants to modify, requiring greatly increased overall loads as well as new significant partition loads. Any ideas on how to go about strengthening this stuff? Can I create new bearing points to reduce span length/increase capacity? Or d I have o essentially frrame a new floor above the existing?

 

[jike]

I would probably look into adding additional supports. Work with the precast supplier for advice. You probably want to sawcut the plank over the new beams to prevent cracking and also to prevent a crack from wondering off the bearing.

 

[msquared48]

If you know the prestressed plank manufacturer, I would suggest going to them first as they should have a record of the PS used and the implications relating to the changes.

Additional bearing (support) points to increase the capacity would not be recommended as this may cause the plank to fail - literally explode. Not good.

Have you though about the foundation impliations yet?

Mike McCann
MMC Engineering

 

[msquared48]

I was thinking of the extra supports more, and was wondering of adding a 4" topping slab to mimic the probable existing 50 psf live load to the plank, and adding in mild steel over the supports to take the negative moments. The extra supports would be inserted after the new slab had cured. I mention this option as you had mentioned adding a new floor structure above the plank.

Jike: If the PT is straight and centered, that would probably work. I would defer to the manufacturer though.

Mike McCann
MMC Engineering

 

[JAE]

This is why I really don't like hollow core planks. They have so many inherent difficulties for the structural engineer.

 

[amyrice72]

Not sure if I will be able to find out who mannufacturer is. i was thinking to shore existing plank, break open cells at new bearing line, fill and then install steel support below. Breaking cells sounds awful to me, and I worry about the strands in there. Msquared what you are suggesting sounds similar, but the topping slab would provide strngthening at support line?
impact on foundations will be analyzed, but as a preminary the additioanl floor load is not large compared to overall total floor loads
The new floor I am considering is a steel platform structure above existing floor. Of course this reduces headroom. And of course I am limited by what can be carried up in a freight elevator.

 

[jike]

mssquared:

Most plank that I am familiar with has straight tendons in the bottom. If fire rated, they might have increased cover but generally not centered.

 

[ishvaaag]

1. Maybe it is also worth examining the use of some kind of exterior prestress, always with careful examination of the effect of the introduced elements on what existing, case by case.

The scheme uses a cable, one or two posts. The cable pushes up the posts against the floor, so you may make it a participant from the start even for the already existing load, something you won't achieve with passive methods. You may also use an active poncelet truss, essentially a number of the named system for 1 post, with active cables, also.

Do not forget to examine the long term relaxation of the cables in its amount, and the deflections under exploitation of the renewed structure, for if not it could lead to a serviceability problem midterm, specially with partitions above.

This way yo may use the existint support or strong points, but will require additional structural floor thickess. If subsumed in the required ceiling and no conflict with equipment, could be a solution.

2. It may be simpler and factible depending upon the case adding bottom steel or carbon fiber reinforcement and top steel (or concrete), made to work conjointly.

I don't see much feasible posttensioning an infilled core without too much trickery within the hollow cores and above the structural floor, weight could be unsurmountable problem.

 

[patswfc]

I agree with jike.

Assuming the planks are pre-tensioned prestressed, then the prestress is transferred via the bond between the concrete and reinforcement.
The planks are probably designed as simple spans and are designed for sagging moments only.

If you add an intermediate support, you will get a hogging moment that the plank will not have been designed for, and it will snap.

However if you saw cut the plank at the intermediate support it then becomes 2 simple spans, with a reduced span length and thus should be able to support more load. (If the planks are post-tensioned then you could not do this)

Being able to calculate what load the reduced spans will support will be the tricky part, if you have no info regarding the design of the planks.

 

[InDepth]

How about Fiber wrap or

OR

external post tensioning with a quenn post in the middle of the span? This would greatly increase your strength and stiffness. Could also work around existing utilities.

Call Structural Group (Structural Preservation Systems)

 

[Lion06]

What about adding mild steel to the cores and grouting them? I did this with great success on a job some time ago. The deflections can be an issue, but for strength it is an easy solution.

 

[KootK]

Structural EIT,

How did you get the rebar/grout assembly in the cores to behave compositely with the plank? Some kind of special non-shrink bonding grout?

 

[asixth]

I agree with Mike that the planks will fail. I was speaking with a builder who was storing precast prestressed girders onsite when the bearing points started sinking into the ground, causing midpsan to bear and then fail.

 

[hokie66]

asixth,

That would be right, but if the support is provided and the top is sawn immediately, the condition you described will not occur. You would just have two simple spans. Jike's solution will work, provided the centre support is allowed.