Is the 2% rule for bracing valid for concrete columns? 2

[HanStrulo]

Hi Everyone.

In Appendix 6 of the AISC, the strength and stiffness requirements for bracing are presented with 2% being the rule of thumb.

Is the 2% of vertical load bracing requirement also valid for concrete columns?

Do concrete columns have any special bracing requirements.

Thanks alot!

 

[Enable]

You wouldn't happen to be shoring columns as you take out an existing slab would you?

I'm quite interested to hear what the true designers on this board have to say about this (of which I am not one). But for shoring purposes I have always been taught to design the temporary lateral resisting structure for 5% of the vertical load when we do demo. Thus far, no accidents!

If you look at CL 9.2.6.2 of S16 (not concrete I know) they give you an equation for Pb based on number of brace points / tolerance of material dimension.

As a first-order analysis, I've previously taken delta_0 = delta_b = L/100 (columns shouldn't be out more than 1/4" over 10ft but i've seen up to 1" / 8ft so I like the L/100). Take Beta = 2.0 per table prescription for single brace point. This gives,

Pb = Beta*(Delta_0 + Delta_b)*Cf / L = Cf*4/100 = 4% of Cf

So the 5% we take for shoring is inline with this equation, which provides some additional confidence in the number.

 

[HanStrulo]

@ Enable

That is right. I am shoring columns as we take an existing slab and excavate down.

I think as a start i will use the 5% you suggested until I check out the references.

Thanks alot!

Edit: just to pick up your brain. what kind of connections have you used between the brace and the concrete column? I am thinking wedge anchors since i see them in reference drawings but i would like to find some more design requirements about them.

 

[KootK]

1) Enable's recommendation is consistent with what I'd do in a similar situation.

2) My only concern with the 2% rule is that the most important thing in these situations is the stiffness of the brace, not the strength. So, when you're using the 2% strength rule of thumb, you are getting an implicit stiffness that depends on the material chosen for the brace. So, if your brace would be made of something other than steel (unlikely, I know), I would do one of the following:

a) Ratio up the 2% based on the ratio of the elastic modulus of the brace material relative to steel or;

b) Just use the AISC appendix 6 recommendations for brace stiffness.

My theory on this is kind of torpedoed by the fact that the 2% rule is also often applied in wood design. So maybe my concern is purely pedantic, it' hard to say. Just don't brace your columns with really strong licorice sticks or anything like that.

3) You see a lot of wallumns out in the wild these days. Wallumns = columns with high h/b on the plan dimensions. If you've got one of those, give some consideration to a potential need to rotationally restrain the column in addition to laterally restraining it.

4) I struggle to know what to make of tolerances in this situation. Most requirements pertain to column variations within a single column between floors. Going from floor to floor, however, I believe that it's somewhat common for contractors to make small corrections in column plumbness and position. As such, one might see greater variation in a newly made two story column than one might find in a column that was cast a two story column from the get go. Yet another good reason for 5% rather than 2% in my opinion.

 

[KootK]

I am thinking wedge anchors since i see them in reference drawings but i would like to find some more design requirements about them.

The manufactures data and ESR reports will provide you with gobs of design information on any wedge anchors that you choose. Wedge anchors would probably be my first choice as well. One thing to keep in mind is that you want to avoid damaging the existing rebar in the columns. In some situations, it might be prudent to clamp some kind of collar onto the columns and attach the braces to that.

What will you be bracing the columns back to? Other columns? Floor decks?

Will your braces need to act in tension as well as compression? Or would compression only suffice?

 

[Enable]

Here's a picture of a garage project we are still working on. We use screw anchors for the garage columns (Hilti HUS-EZ Screw 3/4") and 5/8" HSLs for underneath the building. You dont need wedge anchors to meet the demand of simple garage columns, and as a bonus the HUS anchors are reusable(ish). Loads taken to grade or else roof deck with overburden acting as the counter-weight.

EDIT - Make sure you verify your grade conditions underneath the building. Get a proper geotech out to confirm capacity. On some projects we have had to add in footings because the soil was brutal underneath the SOG!

Rakers are simple steel HSS sections with 1/2" baseplates either end (we use a combo of fixed rakers and those with swivel pins, either will work). The props are MP350 series on angles. They are fine for garage loads but wouldn't use them underneath the building.

 

[HanStrulo]

@ KootK

The column will be braced to a supporting wall behind it and to bedrock in from of it.

The brace to the wall will be horizontal and the one to bedrock will be inclined as the bedrock is lower than the storey level.

For the wedge anchors, I am not sure if we will get a company to supply them or we will make them in-house. I need to find that out.

I am using 5% just because it's my first time. i found a great AISC webinar about chapter 6 applications and i will do a detailed check based on this. The axial load on the brace is about 300 KN so it's on the heavier side but at least it's safe.

Thanks alot!

 

[KootK]

I don't feel that you're allowed to make your own wedge anchors unless you test the bejeezus out of them before using them. Perhaps we're thinking of different products...

 

[HanStrulo]

@ Enable

I am also doing a garage column with only two parking levels above it. it looks identical to yours

The load on the brace with 5% is about 300 KN. This was decided based on a column capacity of 7500KN. The owner did not have detailed drawings since it's an old building so i decided to estimate the load based on the measured column dimensions and a 4% steel ratio just to be safe. is 300 KN seem normal to you?

also would it be possible if i asked about any technical brochures about the Hilti HUS-EZ Screw 3/4"?
Thanks alot!

 

[HanStrulo]

@ KootK

you are right. we are only planning on making a pipe brace with base plates and connecting rods.

 

[KootK]

Where you're bracing back to a wall, I'm guessing that you'll only have bracing on one side of the columns and thus will require your fasteners to serve in tension to brace the column. There, be good and sure that you have robust tension capacity coming in from the anchors. The braces would be for naught if the anchors yank out owing to an edge distance problem or something. If the concrete is deteriorated at all, I'd not mind having some basic load testing done on the anchors on site.

I've done some exploration of slender columns using true second order analyses rather than the code moment amplification procedures. I've found that the code amplification procedures tend to be very conservative and that it's actually pretty darn hard to make a truly slender column. Don't rely on this without doing the analysis, of course, but do let it facilitate easier sleep. For the kind of columns shown in Enable's photos, I'd be astonished if the bracing were truly even required. I'd still do the bracing in most cases though, purely as a matter of good practice & judgment.

 

[Enable]

Sure ask any questions you want!

300kN is in the ballpark for design but on the high side I would say. With a typical 3/4" diameter, 102mm embedment, and assumed 27.6 mPa existing concrete, you would need ~ 9 HUS KH-EZ anchors for an inclined brace (for pipe bracing you only need anchors to secure the ends). Which is more than we would typically use for a 2-story garage (EDIT - to be clear I do not think your garage column will have 7500kN on it. That is nearly 1,700,000 lbs! But you typically design braces for 50 kips so they can be reused in higher load applications).

We would use 6 - 8 / column connection. If you factor in an 80% utilization ratio, you get a bit closer to what we typically use. But trust your own math and since it's your first time I'd run with the higher numbers. Perhaps instead of the HUS KH-EZ anchors use Hilti HCA Coil Anchors (3/4") which have a higher capacity (but are still cheaper than the HSL anchors). They are also reusable but make sure you account for the reduced strength if they are reused.

For the kind of columns shown in Enable's photos, I'd be astonished if the bracing were truly even required.

I agree, assuming that the columns were built close(ish) to as intended. However, many columns are not. Cages are not centered. Steel is missing. Etc. Also, deterioration over time. We are repairing these structures after all! This kind of shoring is inherently conservative because of large risk factors / unknowns that one typically never gets to investigate fully.

 

[KootK]

There's another thing that can be quite beneficial for parade columns that carry through to the upper floors of taller buildings: by the time that you get to the lower levels, axial loads dominate so heavily relative to floor level moments that the applied moments barely even matter any longer. You start to close in on a true "post" situation but with a healthy amount of column rotational restraint top and bottom.

 

[phamENG]

I don't have anything productive to add, but just want to say...I love the "Caution: Proceed Slowly" sign.

 

[KootK]

However, many columns are not. Cages are not centered. Steel is missing. Etc. Also, deterioration over time.

None of that would change my answer:

1) If one cannot assume existing structures to be built substantially as detailed, our hands are pretty much tied and we might as well all just put our heads down for naps.

2) Of course apparent deterioration and deficiencies need to be accounted for. I said so myself in the discussion of the anchors.

3) At no point did I actually recommend a relaxation of the 5% or any other conservative aspect of the design of the bracing. I was simply, in my opinion, offering more considerations that one might factor into their honest appraisal of risk in a situation such as this. And I stand by them.

 

[KootK]

First off, I'm not for a second claiming that what follows constitutes a serious concern for me. Rather, just some Devil's advocacy and interesting discussion.

In evaluating the effective stiffness of the brace, the stiffness of the slab would factor in. And, unless I'm mistaken, that wouldn't normally be accounted for in design. The presumed smallness of the bracing load obviously helps.

 

[Enable]

I don't have anything productive to add, but just want to say...I love the "Caution: Proceed Slowly" sign.

In my defense...if someone was to drive to the edge of my repair I'd want them to be driving real slow!

First off, I'm not for a second claiming that what follows constitutes a serious concern for me. Rather, just some Devil's advocacy and interesting discussion.

I cant help but feel like I've rubbed you the wrong way again. My apologies if so, I didn't mean imply that you were being unconservative in your thinking or anything of the sort!

As for your sketch, I agree that the rotation of the slab due to column buckling is of some cause for concern. Also you are correct in that we don't account for it explicitly in design (or at least I dont mostly because I have no idea how one would attempt that accurately enough to make it worthwhile). But if we strut up to the roof deck we typically have overburden that mitigates the rotation and where we dont have large amounts of overburden (16" or deeper of soil), such as in paved drive aisles, we add additional surcharge to help counteract the vertical force in the brace.

But decidedly yes, as this thing wants to move all of the components we are using to resist the movement derive their stiffness from the surrounding elements that are, well, moving! Pipe braces to the foundation walls alleviate this issue to some degree (and used to be how everyone did restoration jobs).

BTW: loads are a bit off in the sketch. The OP has assumed 7500kN column load of which 300kN is his design force for the brace. We typically design our braces for 220kN but that's because that tends to represent the worst condition we'll use them in (read: underneath buildings at shear walls). For garage columns, we would not see anywhere near that force.

 

[KootK]

Upon further reflection, I can do a bit better job of expressing myself here.

It seams to me that a conventional brace to a suspended slab really just encourages the column and slab to buckle together, as a frame, enjoying the additional capacity that comes along with that relative to a pin-pin assumption. But, then, this leads me to then wonder if the beam to column joint might actually do a pretty decent job of that even without the braces.

 

[KootK]

I cant help but feel like I've rubbed you the wrong way again.

1) I appreciate the apology but, seriously, there is/was nothing whatsoever to apologize for. You're pretty new here; in time, you'll come to realize that this is just how I am. I love it when people question my opinions and I regularly go to the mat debating even relatively trivial points of disagreement. That's how I hone my own technical expertise and it reflects how intensely interesting I find this stuff. I know of no way to do it without some measure of confrontation however.

2) If you ever need tips on how to deal with me without going insane, hit phamENG up for advice. He's one of the finest KootK-handlers here. He probably doesn't think that I realize when I'm being "handled" but, often, I do. I like it, it smooths conversations nicely. I sense regional politics in his future...

3) I'll not claim that I'm somehow so evolved that I enjoy being wrong. In the heat of the moment, someone challenging my opinions does get my hackles up a bit. That said, it's minor I wear my big girl skirt most of the time. When the dust settles, I almost always look back on the debates that I have here fondly. Ditto for the folks that I debate with.

 

[KootK]

But if we strut up to the roof deck we typically have overburden that mitigates the rotation and where we don't have large amounts of overburden (16" or deeper of soil), such as in paved drive aisles, we add additional surcharge to help counteract the vertical force in the brace.

The overburden doesn't make sense to me:

1) If it's unbalanced, it basically encourages the column to buckle in one direction or the other.

2) If it's balanced, it does nothing since you'd have buckling encouragement and restraint in equal measure on opposite sides of the column.

3) Either way, I don't feel that the overburden would affect the stiffness of the system which is, of course, paramount.

If anything, the overburden puts more load into the column and, depending on when they're installed, potentially the braces.

BTW: loads are a bit off in the sketch. The OP has assumed 7500kN column load of which 300kN is his design force for the brace.

Wowzer! At that level of load, even punching shear probably warrants a cursory check. A 70 kip vertical load on slab is certainly enough to induce a little movement.