Installing Column Rebar at <90 angle?

[TarikHKJ]

Hi,

I work with the consultant in a reinforced concrete building project, the column sections for the next story will be resized from 100x50 to become 80x40 cm, the contractor suggested that he wanted to do this : image:

http://s19.postimg.org/h519or3ib/screenshot_44.jpg

The contractor does not want to change the reinforcement spacings through the slab as normal, because these rebars are 25mm and it's not possible for them to bend or reuse elsewhere in the project, when all 25mm bars stockpile are used, they can order bent 25mm bars.

is this practice permitted? are there any provisions in ACI codes that regulate this or bans it?

Many Thanks in advance!

 

[TarikHKJ]

I've been searching through the codes, could not find anything..

 

[BAretired]

The ties would have to be bent progressively smaller sizes in order to match the position of the main reinforcement. That would be labor intensive and prone to error.

BA

 

[TarikHKJ]

Assuming that the stirrups being bent progressively, is the main reinforcement being inclined like this break any code provisions?

 

[BAretired]

Not so far as I know.

BA

 

[Klitor]

Possibly more than maximum concrete cover? If your code has that provision?

 

[hokie66]

Codes cannot provide for everything. With this scheme, you will have an extra 100 mm of unreinforced concrete all around the column, and that cover concrete is where the slab bears first. Not on my project. I would tell them to run the bars straight, cut them off, and provide starter dowels to suit the column above.

 

[KootK]

The tie issue might be partially resolved by using lapped U-bars for some ties but even that is going to be messy. Additionally:

1) At the top and bottom of the column with tapered reinforcement, you'll be changing the direction of the force in the rebar from sloped to vertical which may require some consideration. Granted, however, your change of direction is relatively minor, particularly compared to the 1:6 ratio that we're allowed to use for cranked bars.

2) You'll have dowels from the column above and below lapping with the sloped vertical column bars. That might result in some congestion issues where you've got bars coming in at varying angles.

3) While I know of no code prohibition against what your contractor has proposed, I can think of no benefit to doing it that way (and several drawbacks as stated). You should be able to keep the column rebar vertical, as Hokie suggested, and achieve whatever measure of column continuity is required between levels by installing appropriate length dowels into the column from above and below to create non-contact rebar splices.

It's gracious of your contractor to entertain this solution but, in my opinion, you should be steering him or her to something simpler to build and likely to produce a higher quality outcome.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAretired]

It may be simpler to (a) change one dimension only, say 1000 to 800 but retain 500 for the upper column width, (b) terminate four corner bars in lower column within floor thickness (c) move four bars inward to align with column above (d) extend vertical bars to provide a lap in upper column.

In that way, the upper column would have a cross section of 800 by 500 with four less bars than the lower column.


BA

 

[TarikHKJ]

@kootk @hokie66

If we decide to go with adding dowels for the new column section, can the same 25mm deformed rebars be used as dowels and what length should be sufficient through the lower floor column?

Thanks!

 

[hokie66]

No reason you can't use the same size bars for dowels as for the column bars. As for length and quantity of dowels, the design engineer needs to determine tha.

 

[KootK]

can the same 25mm deformed rebars be used as dowels and what length should be sufficient through the lower floor column?

I imagine that the process would go like this:

1) Figure out how much rebar tension needs to be transferred across the joint. Or assume that all of it does based on the more lightly reinforced column.

2) If possible, use ACI non-contact lap splices and an appropriate dowel quantity to accomplish #1.

3) If #2 is not possible, use strut and tie methodology with additional column ties and an appropriate dowel quantity to accomplish #1.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.