Shoring and Reshoring - What is correct 4

[Robert216]

Here is the big question: In a multi-story reinforced concrete building, do shores need to be released and "snugged up" before shoring levels above? This is what makes sense to me. This allows the beams to deflect and "carry their on load".

Let's begin with what most of us agree on. Starting from the ground, and assuming that all beams in the bay are the same section, the lowest level will most always be the stiffest (ie: ground is stiffer than 2nd floor beam, 2nd floor beam will be stiffer than 3rd, etc) , based on age of concrete.

Now lets get into the "gray area". Let's look at a scenario where shores are never released. Lets say 2nd, 3rd, and 4th floors are shored and placed (never released) and then shores were removed from beneath the 2nd floor.It seems that before the shores were removed between the ground and the 2nd floor, all of the load would travel through the shores (beams would be unable to deflect, therefore would be unable to have a moment, therefore would be unable to carry a load. My first thought is that this would cause post shores to be overloaded (assuming that the post shores were not designed to carry 3 levels of elevated structure). Now my thought is since the 2nd level is at the bottom, unshored, and stiffest (relatve to the above), it will now carry three floors of load. Have I missed something here? Did we not learn in school that if there is not deflection, a beam cannot work? How in the world can a beam carry a load if it cannot deflect? Some insight on this would be greatly appreciated. It seems like some engineers swear up and down that re-shoring is not neccessary, while others swear up and down that it is. If anyone knows of any reliable sources that are available or even if there are any articles documenting failures or problems caused by one or the other, that would be very helpful as well. Thanks, Rob.

 

[Robert216]

I just read that last paragraph again. I jumped from the issue of shores being overloaded to the issue with the lowest beam being overloaded, then back to the "no deflection, no load" issue. Sorry for the confusion. Those are my two issues in question. Thanks again. Rob

 

[Lion06]

I agree with the idea that if you're shored to ground that you're not loading the beams (assuming, of course that the shores are in line with each other going up the building).

If you remove the lowest level of shoring, I kind of see your point about the 2nd floor beam being slightly more stiff because of the time dependent modulus of elasticity, but I don't think that it will make THAT much of a difference and I'd be surprised if anyone looked at it that way. They way I would look at it is each beam is doing it's own work by assuming that each beam, with the same section, concrete, steel, and loading will deflect equally (again, your point is well taken about the time dependent nature of E). When you add another floor, the loading from that floor will be shared by all of the beams below (down to where the shorting stops).

 

[Robert216]

Thanks Lion. So would the 1st floor shores be overloaded or do you think that people design these to carry 3 floors?

Regarding the general question, do you believe that loosening shores and re-shoring is necessary?

 

[a2mfk]

What are the chances of shoring posts aligning floor to floor? If they don't, you could quickly have some nice point loads into the beams...

 

[MiketheEngineer]

My recommendation has been - shore and pour the first floor. Release the shores and hand tighten. Now that floor is carrying its own weight.

Install second floor. Pour and release.

Keep going.

IMHO - if you don't do this - the first floor will be carrying ALL the loads and would probably fail.

 

[BAretired]

The attached article by ACI 347 addresses some of the issues raised in this thread:

http://www.gamcoform.com/dropshore/documents/ACI Reshore.pdf

BA

 

[Robert216]

Thanks everyone for the input.

 

[dik]

Thanks, BA... Dik

 

[Lion06]

Robert,
I didn't have a chance to read the articaly that BA posted, but if I know the shoring is supporting three floors of load, then I'm going to make sure it's designed for it.

We wouldn't typically design shoring (that's more of a contractor responsbility that he would hire an engineer to do), but I would check the calcs and send them back Revise and Resubmit if they are not designed properly.

 

[RHTPE]

Robert216

In a multi-story reinforced concrete building, do shores need to be released and "snugged up" before shoring levels above?

Simply to clarify, the next level of formwork & shoring can be installed, but concrete placement should not proceed. See below for more on this.

This allows the beams to deflect and "carry their on load".

Correct.

... the lowest level will most always be the stiffest ..., based on age of concrete.

Basically correct. Floor stiffness is related to the percentage of design strength at the critical point in time. However, shear strength does not increase at the same rate as bending strength as related to % of f'c.

Let's say 2nd, 3rd, and 4th floors are shored and placed (never released) and then shores were removed from beneath the 2nd floor. It seems that before the shores were removed between the ground and the 2nd floor, all of the load would travel through the shores. My first thought is that this would cause post shores to be overloaded (assuming that the post shores were not designed to carry 3 levels of elevated structure). Now my thought is since the 2nd level is at the bottom, unshored, and stiffest (relatve to the above), it will now carry three floors of load. Have I missed something here?

Yes, you have missed something. Once the 2nd floor is no longer supported by shoring, it, and ALL of the completed floors above it are now free to deflect. Since the shores/reshores are generally considered to be infinitely stiff, all connected floors must deflect the same amount. Thus, each carries a portion of the total load (included themselves), distributed based on the ratio of the individual stiffness to the total stiffness of all connected floors.

Lion06

I agree with the idea that if you're shored to ground that you're not loading the beams (assuming, of course that the shores are in line with each other going up the building).

Alignment of shore directly over reshore is important when backshoring (installing reshores before the reshored floor can carry itself), especially for shallow members and short spans. However, in MHO, completely stripping the formwork and shoring and then installing reshores is the most reliable approach. Stripping must occur over more than just one bay keeping in mind the effects of structural continuity. a2mfk note: Reshores need not line up with the shores above, or the reshores below using this methodolgy. If you develop the moment diagram for the floor structure carrying its own weight, apply the point loads from the shores above, and limit the deflection of the floor at each reshore location, you will find that the basic shape of the moment diagram does not change significantly. There will be jogs at each shore or reshore location, but there will be no change in sign. The only exception that I have found is for shallow members with a short span adjacent to much larger spans.

I have also found that the old "rule of thumb" for percentage of cure needed to strip shoring (I typically have seen 75%) does not always apply and can be dangerous. Floors with a low live load to dead load ratio tend to calculate higher required stripping strngth. Conversely, floors with high live load to dead load ratios often fall well below 75%.

Also, the policy of leaving the formwork & shoring in place, but loosening the shore before pouring the next level can not always be reliable. I require that ALL shores in a bay be loosened before ANY one is resnugged. If not, then the floor truly has not been forced to carry its own weight.

Ralph
Structures Consulting
Northeast USA

 

[DTGT2002]

Ralph, can you clue me in to how to quote?



Robert, in your example, if the shores are not designed to carry three levels, they are already overloaded (assuming the same system at each level) as they were carrying all applied load to grade.

When the lowest level of shoring is removed, we would go from a situation where we may have an overloaded shoring system to a situation where we may have an overloaded shoring system AND an overloaded structure.

As previously stated, a shored structure must be allowed some finite deflection to carry its own weight and the weight of applied loads.

The design methods identified by ACI committee 347 (Formwork for Concrete) are based upon assupmtions of stiff reshoring members relative to the spanning structure and assume loads are distributed to elements based on stiffness and deflections are assumed to be equal.

All fine and good except the construction loading accumulated during construction of three slabs would seldom be within the design loading of a single level of the structure.

So while removal of the bottom level of shores would allow SOME deflection and SOME ability for the levels above to carry their own loads, in all likelihood, the deflection is insufficient to allow the necessary load transfer to the building elements. As such some load is still carried in the shoring system and is imparted to the levels below, leading to an overloaded structural system, carrying more than the design loads.

Here, folks get angry as in addition to violating the project specifications to follow loading requirements spelled out in ACI 301 and 318, we have now applied some unknown amount of load to our structure.

So, unless we specifically designed to accommodate multiple slab placements, like Ralph, I require the release of the shoring system in contact with the freshly placed concrete such that the structure may deflect and become a part of the reshoring system.


There are systems on the market supported by engineers that claim the process of reshoring can be avoided through specific form panel types and so on, but the only ones I have investigated or considered for use still involve some sort of loosening mechanism that releases the load across a number of shores/bays prior to re-snugging.

Perhaps I am a small minded, but I cannot envision a satisfactory process that does not allow the slab to deflect to its natural shape prior to being used for carrying loads beyond nominal construction activities.

And while we are talking, with any certainty, only the Design Professional of Record can give a good slab strength for a minimum slab strength for stripping or shoring system release.

There are reasonable approaches one may take to account for reduced strengeth demand due to dead or live loads not in place, but only the designer is aware of the particulars of demand and capacity in the structural model.

So folks, do the contractors a favor and work to include a value we can all depend on in your specs for stripping requirements.

By the way, the "article" linked to is actually a copyrighted publication of the American Concrete Institute. It is the Reshoring Guide prepared by ACI 347. Good reading. Something to purchase and add to your library, no doubt.


Daniel Toon

 

[RHTPE]

Daniel,

Quoted text is done similar to bold or italics. Use "quote" surround by "[]" to start and "/quote" to end. All with the quotation marks of course.

Ralph
Structures Consulting
Northeast USA

 

[RHTPE]

Daniel,

2nd sentence:

Use "quote" surround by "[]" to start and "/quote" to end.

should have read

Use "quote" surrounded by "[]" to start and "/quote" surrounded by "[]" to end.

Ralph
Structures Consulting
Northeast USA