ACI 318-8 requirement for 2 way slab openings

[Oversee]

“ACI 318: 13.4.2.1 – Openings of any size shall be permitted in the area common to intersecting middle strips, provided total amount of reinforcement required for the panel without the opening is maintained.”

I totally agree with the first part of the sentence. My interpretation about the latter is:

a) The amount of reinforcement equivalent to that interrupted by the opening will be added on the sides of the openings
b) Reinforcing bars affected will be cut right by the opening. The total number (not weight) of reinforcing bars is maintained.

Interpretation a) appears to be more appropriate after reading this sentence for a good few times. But it will be inapplicable for the project I am working on as there will be no room to host the ‘total amount of reinforcement’ within column strips for some big penetrations in the middle.

If interpretation b) is correct, shall slabs by penetrations be treated as free-stand for some oversized holes? And how to quantify the latter?

I wish to thank you all in advance.

 

[Oversee]

Just to give you an example of my query: a 2-way slab (without beams) but needs a penetration for a stair case. Cheers.

 

[hokie66]

Interpretation a) is correct, but the provisions of 13.4.2, including this one, are deemed to comply provisions. It sounds like you should use the provision of 13.4.1.

 

[Oversee]

Thanks Hokie66

I agree with you.

But we have hundreds of elongated openings (typically >2m long) for air services and risers. This means 100% of the reinforcing bars will have to be packed within 2/3~3/4 of the span which varies between 6m and 8m.

I plan to run a ‘global’ analysis to slabs using FEA. Special attentions will go to those fail to achieve clause 13.4.2 requirements, i.e. where it is too cosy to fit in all rebars.

 

[rscassar]

Which FEA program are you using to design a reinforced concrete slab? Would this be a matter of distributing a greater portion of the load to the column strips adjacent to the large opening and designing these column strips for the greater actions.

 

[Oversee]

Thanks Kikflip

I plan to determine the reaction forces in slabs (by taking into consideration the penetration effects) rather than sizing rebars. The latter has to be done manually, or using a spreadsheet. This can be done by using nearly all 3D structure analysis software packages. The drawback is pattern loading has to be considered by applying imposed loads on even and/or odd bays to find out the maximum reactions.

Or alternatively, it will be a conservative method to design a slab strip. One meter/foot wide strip with a free-stand edge by the penetrations. This can be done using Australian software called RAPT. Not only rebars but also serviceability limit state can be checked. But I remember slabs can only be designed to ACI 318-05 in its current version (needs to check it out).

 

[ishvaaag]

I see pertinent as hokie66 to use a general method of design as per 13.4.1, i.e., a FEM program with the openings modeled.