composite action between precast and cast in situ slabs 1

[oneintheeye]

I have a project with a precast slab placed and a concrete topping cast above. The precast acts as permanent formwork and also acts compositly with the topping slab (RC in both). The bottom precast element has a lattice of steel bars sticking out which bonds the topping slab to it. My question is on how to confirm the capacity of the slabs to act compositly before they fail at the boundary and so the load is taken by the precast (which will then fail I suppose). You might now this type of precast as omnia or omnidec. This is an assessment job of existing so I can't just ask the manufacturer to do it for me!

 

[asixth]

This is called "horizontal shear" or "longitudinal shear". Not using ACI regularly I will reference the approach using AS3600 (Australian Concrete Code).

The longitudinal shear capacity is broken into two components, a concrete capacity at the interface of the two materials and a steel shear capacity.

Both components are a function of the roughness that you have at the interface (greater capacity for a roughened or monolithic construction and smaller capacity for a smooth surface). You should call up the surface finish of the precast member to be rough to increase the horizontal shear capacity. Our precast finish note is "where shown on the drawings, the surface of girders at the interface area with any cross girders to be constructed after erection shall be roughened by scabbling, sand blasting, or use of a surface retarder".

The horizontal shear force at the section follows an equation similar to V*Q/(I*t) however a few assumptions are made to account for the non-linear behavior of reinforced concrete.

Another thing which may put you off and I still have trouble understanding the concept is that the shear stress is a stress whereas the capacity which is calculated in a code is given as a force.

I know the MacGregor textbook has a good section towards the end of the book (Ch 17 maybe) relating to the shear capacity across interfaces.

Let us know if you are having trouble and I can dig out my copy of ACI.

 

[oneintheeye]

sorry i;m UK not US but I understand the thoory of the longitudinal shear. My question really was how is this dealt with in concrete i suppose as you stated its not a linear material.

 

[asixth]

I found this photo that I took when I was visiting a stressing plant earlier this year.

AS3600 Sect 8.4.3 Design Longitudinal Shear Strength

Vuf=?4*As*fsy*d/s+?5*bf*d*f'ct

where ?4 and ?5 are functions that depend on the roughness at the interface (0.9 and 0.4 respectively if deliberately roughened); As is the area of steel anchored each side of the shear plane; fsy is the yield strength of the reinforcement across the shear plane; d is the effective area of the composite section; s is the spacing of reinforcement across the shear plane; bf is the width of the shear plane and f'ct is the characteristic tensile strength of the concrete.

I am still looking for a good published reference material on this topic.

 

[oneintheeye]

yes I've been looking to. Plenty for uniform material - steel girders, wood etc but nothing for concrete (well my textbook has a disclaimer about reinforced concrete). guess its typical that all the theory texts give the easy examples.

 

[asixth]

I have attached the relevent notes from the MacGregor text, Australian Concrete Code and Australian Concrete Code Commentary that I hope will shed some light on how to calculate the longitudinal shear forces for concrete. It is something that I have been meaning to study for a while now however I simply have not had the time to sit down and go through the calculations in any detail.

The design procedure in our office is to say that the maximum longitudinal shear force for a section is equal to the vertical shear force and we design for that action.

Please inform us if you find any good information on the topic.

 

[oneintheeye]

cheers for that. I have given you a star. I have also attached a section from the eurocodes stating how this is calculated there. I didn't knwo about it before as we are still using british standards.

 

[rowingengineer]

herewegothen,
your pdf didn't work.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

 

[oneintheeye]

sorry, try this. Computers argghhhhhh.

 

[oneintheeye]

sorry again you'll notice page 93 is before 92. Its upside down for asixth down under