Re-shoring / backpropping (again)

[demayeng]

Another question about backpropping of a suspended slab (not in a multistorey building)

It appears to be common practise around here to strip formwork after a number days and replace with backprops. I have seen backprops at much fewer (at greater centres) than the props that were holding up the formwork during the pour. The backprops are of the same capacity as the formwork props.. approximately 20 kN acrow or similar.

Our formwork code AS3610 says you can strip and backprop after 7 days as long as you comply with the following:
"The ratio of span between supports to the overall depth of the member is less than 280/sqrt(D+100), where D is the overall depth of the section in millimetres"
I assume this ratio is to do with creep and deflection limitation.

The props you would need to use at wide centres like this would need to be high capacity than the normal formwork props.
Is this correct or am I missing something?

Thanks

 

[Lemony]

Not sure if this will answer your question, but load on reshores is based on the distribution of subsequent construction loading by stiffness analysis of the structure.

You could check out ACI SP-4: Formwork for Concrete for an in depth discussion.

 

[slickdeals]

It all depends on the superimposed load the floors are designed for. A residential flat plate is designed for a superimposded load of 40 LL plus about 20 DL or 60 PSF. If the slab is 7" thick it weighs 85 psf plus the weight of shoring and formwork and the work crew. So you need at least two floors to support the weight of one floor being constructed. Also, the age of the concrete for the floor directly below the floor being cast is less than a week old. Flat plates can be constructed one floor every 3 days.

Also, the length of time that a slab is reshored can affect long term creep deflections. So the reshores may not necessarily be there to support the weight of the next floor being cast. For example, the roof may require shoring for one to two weeks, but the formwork can be remove after three days. So reshore are placed to prevent creep deflection of the "green" concrete. The concrete mix design can also control the length of time that the reshores or reshore must remain in place.

The shores needed to support the floor being cast is usually based on the span length of the timbers supporting the shores and not the shore capacity. 100% reshores is the amount of shores necessary to support the weight of the slab and not necessarily the amount of shores needed to support the formwork. Reshores to support 100% of the slab may only be needed at mid span, or for longer spans, at the 1/3 points. They do not need to be uniforming spaced over the area of the slab.

 

[slickdeals]

The above is based on a discussion with a senior engineer I had on this topic.

 

[demayeng]

Yes that all makes sense. In regards to your last paragraph, I understand that the amount of reshores maybe less because you're not having to work with timber spans. However in the case where you only had reshores at midspan, each reshore would need to have a much greater capacity than the original formwork props.

What are normal times for both removing forms and removing backprops (reshores)?

I am guessing it is safest to leave reshores in place for the full 28 days..

 

[slickdeals]

There was some good information on Reshoring Design in March 2003 edition of Concrete International. I tried on their website, but they don't have that discussion available (only articles).

Does anyone have a hard copy of Concrete International from March 2003 and could they scan and post the discussion on "RESHORING DESIGN" starting on Page 20. I am missing Page 22 and beyond.