column reinforcement (bundled)

[greedypumpkin]

good day!

i recently started working in structural designing. i have little actual experience so there are a few things that i am unsure of.

right now i am designing a 2-storey residential building. due to the architectural requirement, my columns are limited to 200mm width. however, some are unable to resist moment unless i place more bars on the base. but if i put more, it wouldnt comply with the minimum spacing required. i would like to ask if it would be okay to place 3 pcs bundled 16mm rebars on the each corner? these columns are placed in the middle and 450mm length. i have 2 which wouldnt pass the required capacity. and by doing this would result to total of 14 bars. is it okay?

thank you very much

 

[BAretired]

It would be preferable to use a larger bar rather than bundling. Three bundled bars at each corner will be six bars at the bottom of the column where the dowels overlap the main column steel.

BA

 

[greedypumpkin]

hmm i see. right. i would opt to use larger bars. but it was also specified using 16mm dia bars on reinforcements. is it not possible?

 

[BAretired]

Who specified that the reinforcement should be 16mm bars? That seems to be an unusual limitation on the design engineer's freedom to exercise judgment.

It is possible to use bundled bars but you should pay strict attention to code requirements regarding development of bundled bars at a tension splice.



BA

 

[KootK]

I agree with BA. You'll have to deal with the bundles at the footing dowels, at the splice / offset at the second floor level, and at the development condition at the roof slab where you'd normally want a hook. All Trouble. People like little bars because they're easier to handle. I use 30M column bars routinely however -- no big deal. The builder will likely prefer heavy bars to dealing with all of the issues just mentioned.

Are you sure that your column is large/strong enough? Keep in mind that 8" column capacities need to be reduced in most jurisdictions in order to maintain fire ratings.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[jayrod12]

It would depend on your clear cover, tie diameter and max aggregate size.

For Canadian code assuming not exposed the clear cover is either 20mm or the equivalent bar diameter of your bundle, or your max aggregate size (typically 20mm). For 3-16m bars an equivalent diameter is 28mm.

Assuming 10m ties then your actual clear space between bars is 200-2*28-2*10-2*28 = 68mm. Your min spacing is the max of 30mm or 1.4*equivalent bar diameter = 40mm. Therefore you are technically ok but that's going to seem like a ton of reinforcing in a small area.

Although it's only slightly less reinforcing to go with 25m bars than the 3-16m bars, it looks cleaner and is easier to review. And as noted by BA when this vertical reinforcing is spliced it is much nicer to only have 2 bars at each splice instead of 6. And correct me if I'm wrong, but isn't the splice lengths for bundled steel increased?

 

[greedypumpkin]

yes. been having trouble for days now. and considering it is only a 2-storey bldg. am i right in assuming that the 2nd floor which is loaded heabily with CHB partitions for walk-closets and T&B that contributed to this?

and also another thing. a few of my beams require having 8-9 rebars. and are 200x450mm in dimensions. i am thinking that this is a bit too big. but i cant find any other way. i really wanted to increase my rebar diameter but was advised to use only 16. im not sure if it was as per clients request or by the architect

anyway thank you. i'll check on that splicing requirements

 

[greedypumpkin]

and because of the 200mm width, the most is i can place 3-16mm on the short side.

ah. but i would only need the bundled bars on the ground floor. on the 2nd floor, 175x300 would be adequate

 

[CANPRO]

I can't imagine the architect cares what diameter rebar is used. Who told you to use 16mm? Your superviser I assume? Can you ask them for a reason why and try to work around it?

I don't know if you've done this yet, but you should draw this cross-section out to scale at the rebar splice location to get an idea of what this will look like. Remember, someone eventually has to take your drawing and make it real.

What are your floor to floor heights?

 

[KootK]

What country is your project in? Is a 7" column even street legal? At that point, you might as well just have one 35M bar in the middle of the darn thing.

My sense here is that you may be pushing too hard to achieve small member sizes. For a two story column, it's usually cheaper to keep the column size the same on both floors so that the formwork can be reused. This document is an excellent resource for practical proportioning of concrete things: Link. It's even got a table for the maximum number of bars in one layer of a beam.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[BAretired]

You don't say where you are; you're not in Canada as we do not have 16mm bars. Maybe you are in a location where larger bars are difficult to obtain. If that is the case, you will have to make do; otherwise, it is foolish to use such small bars.

Do not reduce the second floor columns to 175x300; stay with the same column size through two floors. There was a time not too long ago in Canada where 10" (250 mm) was the minimum column dimension permitted. That was dropped for some reason (probably to satisfy architects) but I would never reduce the minimum column dimension below 200 mm.

BA

 

[hokie66]

Follow the good advice above. 200 or 175 columns are very skinny, and hard to construct. And you mentioned 200x450 BEAMS. You can't get both the columns and beam bars through that joint. Either the column or beam, maybe both, needs to be wider, and preferably not the same width.