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Column strip bottom bars (Two Way Slab)

Column strip bottom bars (Two Way Slab)

Column strip bottom bars (Two Way Slab)

(OP)
13.3.8.5 — All bottom bars or wires within the
column strip, in each direction, shall be continuous or
spliced with Class B tension lap splices or with
mechanical or welded splices satisfying 12.14.3

Refer to Figure 13.3.8 of ACI 318.

I don't quite understand the intent of this requirement. Assume I have a slab with #4@12" continuous bottom mat, spliced at support and require 4#5 bars in the mid-span of a longer span. Is the code saying that the 4#5 bars column strip bars that I added for strength requirement on a longer span should now be continuous across the entire length of the slab?

Doesn't make sense to me.

RE: Column strip bottom bars (Two Way Slab)

(OP)
I must mention that this slab is not part of the lateral system, i.e., not used as a slab-column frame.

RE: Column strip bottom bars (Two Way Slab)

The intention of the clause is to provide for continuity and some residual strength in the case of damage or removal of a column. I don't know the history of the provision, but it is probably based on life safety provisions in bombings, vehicular impact, etc.

RE: Column strip bottom bars (Two Way Slab)

(OP)
Agree, that's what the commentary says also. But it just doesn't seem right that if I need 4 extra bars in a single column strip then I have to extend it across the entire building width.

RE: Column strip bottom bars (Two Way Slab)

Agree with hokie66 (of course wink).

I think that provision was added in response to the Oklahoma City bombing. At least that's what my very faulty memory suggests. I have an 89 version of the code handy at it doesn't have it in there so the timing seems roughly correct for that to be the case. I do remember that in the aftermath the discussion related to stress reversals from the initial upward blast, breaking the beam where there was no "positive" (no negative) reinforcement and then, once the initial upward impulse ended, the downward force acted on broken beams that then had insufficient shear capacity. Or something to that effect.

Anyway, my interpretation of code is that yes, in your situation those 4-#5 bars would need to be continuous. (Sorry.)

RE: Column strip bottom bars (Two Way Slab)

The above should read "...no "positive" (now negative) reinforcement..."

Is there any way to edit posts?

RE: Column strip bottom bars (Two Way Slab)

It was Class A splice through 318-05, B after that.

I do not continue the add bars beyond the required spans and I've seen the typical details of other firms that also do not. Add bars are in the required span only. I'm not sure if that meets the letter of the code but I think it meets the spirit. Seems like the intent is to provide for some catenary action in the event of a loss or partial loss of support. In that case there is no argument for continuing the bars across the entire building - at the most I guess you could argue for continuing them to the next adjacent column and developing there, although I don't see anyone even doing that.

RE: Column strip bottom bars (Two Way Slab)

I'd agree that you don't add bars all across the building width based on the longest span (and most bottom bars there).

If you simply extend the bottom bars a class B length past the ends of the opposite span bottom bars - or for a bit more conservatism - extend them past the face of support so you have at least developed your bottom bars past the column face by 1.3 x ld.

RE: Column strip bottom bars (Two Way Slab)

I guess I missed the "building width" part of it my earlier response. It doesn't seem to me that the code requires each bar to be developed the full width of the building nor is that it's intent. It seems to me that the requirement and the intent is that the bars that are required should be continuous over the supports in the region in which they are required. In other words they should not be discontinuous over the support as was previously allowed.

Or to put it another way, bottom steel should be continuous over the supports, not cut off in regions of negative moment as was previously allowed. That does not mean that every bar that is required must be continuously developed from one side of the building to the other. Were that the case there would be no need to design more than one bay in any given floor level.

I'm sure the above is poorly written and I can seem to word it clearly. But, I think the point is that there be bottom steel over the columns, not that any bottom steel that is required run from one side of the building to the other.

RE: Column strip bottom bars (Two Way Slab)

(OP)
In most of the cases, there is a bottom mat that would usually provide for 50-75% of the flexural capacity of any bay and which extends continuous end to end.
However, there may be additional bars that are only added per span, which don't extend through the building width.

It appears that the code doesn't permit it which is where the code is poorly written. The commentary for this section talks about progressive collapse resistance, but apart from the minimum 2 bars through the column core, it does not say if only a minimum bottom steel needs to be continuous in the column strip.

RE: Column strip bottom bars (Two Way Slab)

slickdeals,

Although I was not familiar with this requirement, not having used ACI318 for slab design in some years, I think the intent is quite clear.

My interpretation is that wherever you have columns supporting a slab, all the bottom reinforcement in the column strips must extend continuously to the columns either side, with splices permitted in accordance with the Figure. It does leave the question of, in the case of unequal reinforcement requirements, which is to control, but I think the lesser amount would be acceptable in lieu of better definition.

The two bars through the column is a separate requirement, for punching shear.

RE: Column strip bottom bars (Two Way Slab)

(OP)
Thanks. It seems like everyone else is interpreting the requirements in similar fashion and not have the "extra" bottom reinforcement on a few spans continue across all other spans.

I would hope ACI would clarify the requirement in a better way.

RE: Column strip bottom bars (Two Way Slab)

The Code doesn't contemplate "extra" bars. But it does require all the bottom column strip reinforcement, extra or otherwise, to be continuous over the columns.

RE: Column strip bottom bars (Two Way Slab)

(OP)
Dr. Ghosh interprets this requirement as developing the additional bottom bars on either side of the longer span's supports, consistent with JAE's second option.

RE: Column strip bottom bars (Two Way Slab)

I'm not sure what you mean, or what JAE meant. Any chance of a sketch?

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