PT effects on shoring 3

[ReginaldBoigard]

In a two way PT floor the distributed tendons are usually stressed (100%) first. This would create a line of uplift at midspan and a line of downward force along the column lines. This increase in force is not accounted for in the shoring designs of most contractors. Although the loading is only temporary (until the banded tendons are stressed) is it something I should be concerned about? Has anyone considered or heard of contractors considering this subject? Thank you.

 

[swivel63]

i've never heard of this being considered. if you cut a free body, the load applied hasn't changed.

 

[ReginaldBoigard]

I am unclear as to how the applied load hasnt changed. The horizontal force in the tendon is creating upward and downward forces in the slab, basically redistributing the slab weight towards the tendon high points.

 

[swivel63]

so? if you take bandline with it's tributary, it'll still the same load is it was prior to stressing.

 

[ReginaldBoigard]

I am concerned about the loads on the shores. If the shores are placed at every 4 feet they are designed to take a 4' x 4' tributary weight of the slab. The shores and slab below are designed to distribute that weight out. If only the distributed tendons are stressed it will decrease the shore loads in the middle of the span and increase the shore loads near the column line. The shores and slab below near the column line were designed to distribute a 4' x 4' weight of slab, but are now seeing a larger force due to the redistribution of slab weight...

 

[JKStruct]

There's a flaw in your understanding:


The stressed tendons do not induce load... they precompress parts of the slab to increase the flexural capacity. Concrete still weighs 150pcf.

You might be confused if you're using the load balance method and draw a "reverse loading" diagram on a slab strip. That's just something to make the math work.

Think about it... if you have a simple-span PT beam, stress the tendons to balance 100% of the dead load, then remove the supports, would the beam float?

The concrete still has the same dead load.

 

[slickdeals]

JKStruct is right. You only induce a lateral load from prestressing, not a vertical load.

 

[haynewp]

I think what he is asking is if the entire slab is supported on shoring and the distributed tendons are stressed, part of the slab between the banded region is no longer fully supported by the shores since the prestressing is now taking some of the weight. This would therefore place an additional downward vertical reaction on the banded areas where the shores are still firmly in place below, therefore these shores are now receiving more load than what they were before.

 

[collartie]

yes there will be an increase in force in the shoring under transverse beam lines, but I've never heard of this being accounted for.
(analogous to a precast bridge girder with camber, only supported at the ends since the center has lifted off the ground)

 

[hokie66]

swivel, JKStruct, and slickdeals are incorrect. The prestressing does change the loading distribution on the props. The props should be designed with adequate overload capacity to account for more load being taken along the column lines before the slab carries the load itself.

 

[swivel63]

has anyone ever seen this taken into account in shoring shops or discussions with the shoring engineer? i see where he's going with it, but i still don't think it really matters.

 

[LobstaEata]

I'm with Hokie on this one.

I've designed AASHTO I-beams and Bulb-Tees using strand layouts to produce an upward camber to counter the dead load and superimpose dead load effects of a CIP slab.

Think of the strands as an eccentric axial load on a column. (horizontal in this case) The force of the strands produces internal moment in the concrete member and results in deflection.

Hope this is helpful.

 

[ReginaldBoigard]

I agree, the shores along column lines should be designed to support the overload force from the tendons. Now, what about the slabs below?

The increased shore load along the column lines is accompanied by a decreased shore load at the mid spans. The column lines below (through the loading path) would see the majority of the slab weight above regardless of where the force is placed, (say 80%/20% for column line load/midspan line load). This would mean that the uneven distribution of load from the shores above would not affect the slab design below. Is this an accurate way of thinking?

 

[BAretired]

When all of the tendons have been stressed, the slab actually lifts off the form. If (a) 100% of the distributed tendons are stressed before any of the banded tendons and (b) the slab is supported by closely spaced shores, then the shores near the column lines (high point of the drape) will gradually take more load as stress is applied.

This is usually not the case, however. Normally, the tendons are stressed sequentially in both orthogonal directions until all tendons are fully stressed.

In all PT slabs I have seen, the forms have the capability to span from column line to column line without intermediate supports. Thus the shores supporting the forms are capable of carrying virtually all of the load at or near the column lines.

Best regards,

BA

 

[ron9876]

I also agree with hokie66. I have had slabs lift enough that the shores below fall out. I don't know of anyone ever taking this into account. Concrete has the ability to redistribute loads so the overload shouldn't be a problem.

 

[strguy11]

Do you have a copy of the California Shoring Manual? It is more for bridge structures, but there is a section relating to this. I think it can be found on the California DOT's webiste for free download.

It will depend on the situation of the floor (continuous, or simple span) and whether there is stage construction (continuous spans with hinges). In the latter, it defintely needs to be accounted for. In your case, with a 2 way floor, it should be accounted for, because I think your stressing sequence will cause some dead load redistribution.

 

[hokie66]

I agree with BAretired that the strands in both directions should be stressed gradually. However they are stressed, the loading on the props around the columns will get progressively greater. You want the props to take the load at this stage, not the column, as early age punching shear problems are to be avoided at all costs.

 

[swivel63]

i've always seen distribtuted first then bands.

i see what happens, but on the jobs i've been, tables have run both in the banded direction on some jobs and the uniform directions on others.

can we get someone who works on shoring and table design in here? because a lot of folks who are in here probably do the slab design and not the shoring.

 

[rapt]

hokie66,

I agree that the stressing order needs to be considered. If the distributed tendons are stressed first then the band zone needs to be fully supported by the props. Preferrably there shopuld be partial stressing in each direction to reduce the effects, and the banded direction should be partially stressed first, before it is loaded by the distributed direction. This would remove the prop problem! But to get aprtial stressing done properly on site is probably wishful thinking.

RE the punching shear comment, to get the load of the slab itself to the props instead of the column itself after the stressing is completed in both directions would be impossible. For the load to transfer to the props around the column there would have to be separation at the column (so punching shear has already failed).

Another one you you banded/distributed designers.
PTI and associated parties suggest that the banded tendons should be in the long span direction. This is absolutely incorrect and completely illogical structurally. The banded direction should be the short direction. This is the way the slab wants to act. Doing it the other way is going against nature!!

 

[hokie66]

rapt,

We are in agreement as to everything except my "punching shear comment." My reasoning is that the prestressing applies a gradually increasing downward load to the slab, formwork, and props in the vicinity of the column prior to any release of the forms. This load does not transfer completely to the column until the props are loosened. Depending on the time of stripping and backpropping (if any), there can be maturity issues with the concrete.