Tie Spacing for lightly loaded columns

[tclat]

Hi,

My office is adopting ACI318 overthe British Standard due to the fact that we are in a high seismic region and the seismic detailing in ACI is more thorough.

Every now and then we have very lightly loaded r/c columns which are 8x8 and the columns are not part of the lateral load system. What we typically did prior to ACI-318 was space the ties the same as the least column dimension

Based on ACI318-08 Cl 21.13.3.1, the tie spacing should not exceed d/2. For a 8x8 column this works to be less than 4".

The clause seems overkill to me. Any thoughts?

 

[JAE]

If you are in a low to moderate seismic condition, then Chapter 21 doesn't apply and an 8" tie spacing would be acceptable. But in a high seismic condition, even though the columns aren't part of the lateral system, they still carry gravity loads, still must have a good level of deformation compatibility, still are essential to life safety (someone under the column) and so the closer tie spacing is needed in my view.

 

[JedClampett]

I know for CMU that 8 inch columns are discouraged. After you figure clear distance for reinforcing, tie diameter, reinforcing diameter, you'll be left with a small space to get the mud in and vibrate it.
I suspect your loads are very small and it seems like overkill, but unless there are serious space issues, I would go with 10 or 12 inch columns.

 

[BAretired]

At one time, the NBC required minimum column of 10"x10" but that was dropped a few years ago. It still seems like a reasonable minimum size in order to get the steel and concrete in place.

BA

 

[hokie66]

Agree that 8" x 8" columns are too small to cast vertically. I would only use them if precast.

 

[weab]

tclat,

Remember that the idea behind seismic detailing is based on an assumption that the member will be overstressed at some point. Like JAE implies, extra shear ties provides more ductility, that is, the column will bend more before being destroyed. It's a relatively small premium to pay to provide a more stable failure. That is the premise behind much of seismic detailing for concrete as well as steel.

After you do a more of this, it will begin to appear normal.

 

[tclat]

Thanks for all the comments.

8x .... columns are very common where I am. We normally detail them embedded in load bearing masonry walls as opposed to stand alone. We then tie the columns to the walls with horizontal steel every second course. Would you say the spacing requirements still hold?

There are however cases such as small decks with very small loads where we have used 8x8 columns.

Is there anyone else doing this?

 

[vandede427]

I have found that the column tie spacing requirements sometimes "get in the way" especially when expanding on existing structures.

For example, my firm does most of the work for this one particular hospital. A two story moment-frame building was designed by us 20+ years ago for 7 future floors. Last year while doing a study on going up with those floors, we found that because we were required by code to be at least an intermediate moment frame we had to meet the prescriptive column tie requirements given in chapter 21. I think the existing columns had #4 @ 12" and the code required #4 @ 10". We could prove that #4 @ 12" had plenty of capacity for the new loads, but that they technically didn't meet the new code. We're working on getting the city to issue an exemption for this. Being deficiant by 2" on a tie spacing shouldn't keep a whole building from going up especially when the column shear strength is there.

On another project, we were brought in as a third party to investigate the feasibility of adding onto an existing high rise building. It too was built for future. This bldg was a shear-wall bldg, not a moment frame so ties weren't needed for column shear. The problem was that the new engineer claimed the existing structure didn't even meet the code at the time the bldg was built; especially in regards to column tie spacing. He was right, it technically didn't meet the letter of the law for tie spacing, but the strength was there. In my research, I found a very interesting statement in ACI commentary.

Section R7.10.5-Ties

"Limited tests on full-size, axially-loaded, tied columns containing full-length bars (without splices) showed no appreciable difference between ultimate strengths of columns will full tie requirements and no ties at all."

 

[BAretired]

vandede,

I don't think we can take your last statement to the bank. I recall that tied column tests showed that failure was accompanied by bars buckling between ties. There were extensive tests done around 1920, I believe. Can't remember the name of the guy who did them.

BA

 

[JAE]

Yes I was going to pile on a bit too. vandede427, I agree that for a moment frame column you should be able to check shear capacity and that is certainly a valid limit state to check.

But aren't the prescriptive tie spacings in Chapter 21 NOT about shear capacity and ALL about fatigue-like durability in a seismic event?

Seems that you were justifying #4@12 against shear but made no mention about column ductility. They are two completely different limit states.

 

[vandede427]

I didn't write that statement, ACI did. But remember, they are talking about axially capacity only in that case. Don't confuse it with lateral tests done on columns. In that case, there's no way a column without ties holds up like one with ties.

In my example with the moment frame, the difference between code required tie spacing and in-place tie spacing was 2". Are you saying that 2" is going to make a difference in the ductility in a building that is 100+ feet tall. Let's keep everything in perspective here and use engineering reason/judgement.

 

[JAE]

Are you saying that 2" is going to make a difference in the ductility in a building that is 100+ feet tall

Yes.

If a single column (somewhere in a 100+ ft building) is put under repeated high-end deformations under a seismic event I would say that 2" could make a world of difference. If a lower column collapses, that wreaks havoc on everything above...usually.

 

[vandede427]

don't think in absolutes: binary 0 or 1, non-ductile or ductile, failure or safe.

think in terms of percentages. tie spacing @ 12" o.c. has ~90% the ductility of ties @ 10" o.c.

we may just have to agree to disagree on this one.

 

[hokie66]

I agree with JAE. The maximum spacing provisions are for axial load on the column reinforcing. Intuitively, the performance of the compression reinforcing would be inversely proportional to the square of the spacing. So 12" spacing would be only 70% as good as 10".