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]

The slab has to rotate in order for the column to buckle and this would induce a vertical displacement in the roof deck. In order to achieve such displacement, the force needs to be greater than the mass * gravity of material over whatever area is being displaced. I see the overburden as helping to resist this and at 16" you'll need quite a bit of force to move a bays worth of soil vertically.

The overburden is not an added load to the column via the brace either (which is what I think you were saying in one of your comments). The load is already resolved via the roof deck (in case of existing overburden) and so no surcharge is going through the brace. Though, if things did buckle maybe the momentum of the opposing side might cause an additional force to go through the brace where it nets out. Not sure on that one.

Shoring to grade is also the preferred option, as it eliminates some of these unquantifiable issues. Just sometimes we cant do that for whatever reason (e.g. bad soil, access constraints, etc)

 

[KootK]

The slab has to rotate in order for the column to buckle and this would induce a vertical displacement in the roof deck. In order to achieve such displacement, the force needs to be greater than the mass * gravity of material over whatever area is being displaced. I see the overburden as helping to resist this and at 16" you'll need quite a bit of force to move a bays worth of soil vertically.

I feel that statement is fundamentally in error. It neglects the fact that the overburden will already be in equilibrium with the structure prior to buckling. The lateral brace demand comes later, as an additional force added to the system. You can't add a new force to a system and then hope to resist it with an external load that was already in equilibrium with the structure before the new load was introduced.

The overburden is not an added load to the column via the brace either (which is what I think you were saying in one of your comments). The load is already resolved via the roof deck (in case of existing overburden) and so no surcharge is going through the brace.

I may need a sketch to understand what you're getting at there but I have a hard time imagining a situation in which you add load to the framing bays surrounding a column and somehow none of that load actually winds up on the column.

Egos aside, you should be rooting for me to be correct about this. For, If I am correct about this, then the overburden is a wasteful exercise that you can discontinue. You can then either pocket any associated savings or pass them along to your clients.

I could see the additional mass helping with a dynamic load of course. But column buckling is quasi-static enough to preclude that kind of treatment I feel. The added mass may just slow down the rate at which your column buckles.

 

[Agent666]

For concrete I've always used 5%, for no other reason that this is what is explicitly required by our local concrete code.

https://engineervsheep.com

 

[human909]

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.

Yep that is the kootk that some love and possibly more hate! In fact I'm normally that person in other non engineering forums and probably end up with even more haters because enthusiastic and rational debate is often less accepted in non professional forums. Being an echo chamber wins more friends than taking contrarian views.

just some Devil's advocacy and interesting discussion.

Note that he did capitalise Devil. I suspect sometimes he might even be on a first name basis with him.

As far as my input goes. I agree with kootk the stiffness of the slab absolutely should factor in. By the sounds of thing Enable has done this dozens of times so likely knows what he/she is doing. But that doesn't negate the fact that the stiffness of the slab matters.

 

[WARose]

I'm late to the party.....and it may have already been said....but if you have Yura's lecture notes on bracing, they go through the background on a lot of what you see for AISC's bracing requirements. You can back out of that for whatever member we are talking about.

For their part, ACI says this (in 318-11, Sect. 10.10.1):

Code:

It shall be permitted to consider compression
members braced against sidesway when bracing
elements have a total stiffness, resisting lateral
movement of that story, of at least 12 times the gross
stiffness of the columns within the story.

Commentary:

The Commentary used to state that a compression member
may be assumed braced if located in a story in which the
bracing elements have a total stiffness, resisting lateral
movement of the story, at least six times the sum of the stiffnesses
of all the columns in the story. In ACI 318-95, the
language was changed to: “… the bracing elements have
such substantial lateral stiffness to resist the lateral deflections
of the story that any resulting lateral deflection is not large
enough to affect the column strength substantially.” The
change was made because of some concern that the
multiplier of six might not be conservative enough. For the
2008 Code, a more conservative multiplier of 12 was
chosen. The stiffness of the lateral bracing is considered
based on the principal directions of the framing system.
Bracing elements in typical building structures consist of
shear walls or lateral braces. Torsional eccentricity of the
structural system can increase second-order effects and
should be considered.

 

[Enable]

Note that he did capitalise Devil. I suspect sometimes he might even be on a first name basis with him.

As far as my input goes. I agree with kootk the stiffness of the slab absolutely should factor in. By the sounds of thing Enable has done this dozens of times so likely knows what he/she is doing. But that doesn't negate the fact that the stiffness of the slab matters.

In my view one of the unfortunate problems in this profession (for the practitioner) is the lack of ability to test theories of structures in the same way that a scientist tests a theory of the state of nature. They can, at least in some domains, subject their theories to risky tests (in the Popperian sense of the term) where modus tollens can greatly influence their understanding and avenues of inquiry; also, if the tests are sufficiently risky - where based on the spielraum, the consequent is unlikely sans the theory - they can provide great corroboration of a theory even though their inferences are based on an invalid figure in the syllogism.

Of course, we don’t have that in structures. Our test is almost universally: “I think things work this way, and if this is so, the structure will stand”. But the consequent of “this structure will stand” is consistent with a multiverse of correct and incorrect beliefs alike whereby achievement of it is not such a great corroborator of one’s view. Indeed, safety factors see to it that this is so.

I have been restoring buildings for a long time. But that doesn’t mean my views, on any of it, are necessarily any good. It’s part of why I joined the board. I would rather look like a fool than continue to think like one. Maybe KootK will help discover some foolish thinking I have had on this. That said, thus far all I can say is that I agree that the stiffness matters but disagree about his view on overburden; because regardless of the mechanism that supplies the work, the same work needs to be performed to deform the structure to a shape. He seems to think that you get that for free with buckling, but you don’t.

I am busy for the next couple days and will write something up dealing with potential energy and all that jazz after the weekend. EDIT - clearly I am wrong in a big way on this as shown by human909 so I will go drink some scotch and reflect on my sins instead

 

[human909]

I like computers. That help flesh out my thoughts without having to do the hard mathematics. Anyway. Here is the results.

THE UNMODIFIED CASE:
Column Height: 3m
Calculated effective length: 2.443
Calculated implied k factor: 0.8
Code implied effective length: 0.7-0.85 depending on assumptions (conservative)

FLOOR REMOVED NO BRACING
Column Height: 6m
Calculated effective length: 3.382
Calculated implied k factor: 0.56
Code implied effective length: 0.7-0.85 depending on assumptions (conservative)

FLOOR REMOVED BRACING MID HEIGHT
Column Height: 3m (If bracing is considered as lateral restraint)
Calculated effective length: 2.864
Calculated implied k factor: 0.95
Code implied effective length: 0.7-0.85 depending on assumptions ([highlight #F57900]UNCONSERVATIVE[/highlight]!)
[highlight #FCE94F]A 18% improvement in effective length when one might incorrectly assume a 100% improvement![/highlight]

FLOOR REMOVED BRACING MID HEIGHT BRACED TO EXTREMELY STIFF SLAB/BEAM ABOVE
Column Height: 3m (If bracing is considered as lateral restraint)
Calculated effective length: 2.084
Calculated implied k factor: 0.69
Code implied effective length: 0.7-0.85 depending on assumptions (Satisfactory, bracing performs as expected)

 

[Enable]

human909 can you kick the braces up to the roof deck and run two cases: one with a surcharge load and one without? That would be the typical setup.

 

[human909]

human909 can you kick the braces up to the roof deck and run two cases: one with a surcharge load and one without? That would be the typical setup.

Sure. A UDL surcharge?

Note I've edited some aspects above to make things clearer.

 

[Enable]

Yeah like this. BTW what program are you using?

 

[human909]

Ok. A quick comment. If I was designing the above bracing I'd do the second level brace also diagonal to the to left and top right if the structure is laterally braced. Doing it vertically is far less effective.

I am using SpaceGass. Please note that the calculated effective lengths from these buckling analysis is sometimes a little on the high side unless you up the load.

 

[Enable]

Yes, sometimes we go to vertical elements via pipe braces if space and economy allows other times it's vertical. I meant to draw the exterior elements as columns like a section through a continuous garage span but it wont matter for our purposes here.

 

[human909]

FULLY BRACED
2.489m Effective length k=0.8

So all good.

Here is the requested model. Note that the foundations are now considered pinned not fixed.
2.899m Effective length k=0.97
Column Height: 3m (If bracing is considered as lateral restraint)
Calculated effective length: 2.899m
Calculated implied k factor: 0.97
Code implied effective length: 1.0 (Satisfactory, bracing performs as expected)

(note fixing the bottom foundation gives 2.55 which is only slightly more than the corner braced system than my fully braced approach)

Surcharge makes precisely zero effect as it does not change the stiffness of the bracing system. Effectively length is still 2.899. (I can certainly understand how intuitively it should make a difference but when you think of it terms of the stiffness of the bracing/beam/slab then surcharge shouldn't change anything.)

 

[human909]

It is worth noting that the assumed beams stiffness to column stiffness makes a big difference. The proportions I've assumed here are closer to a 10-20 story building. If the structure is only 2-4 stories high then the beam to column stiffness is much higher and the bracing is certainly stiff enough to reduce the effective length.

I chose the higher column loads as I presumed we are talking about larger structures here as the above photo shows some beefy columns.

 

[Enable]

Well that's enlightening (EDIT - To be crystal clear I was wrong as fuck it would appear but glad yall helped me out with that!). Much appreciated

That's right about the structure in this case. Typical repair projects we deal with are apartments / condos of that size. I think the one in the picture was 22 floors.

 

[Settingsun]

Does UDL on only one side make a difference?

 

[human909]

Well that's enlightening (EDIT - To be crystal clear I was wrong as fuck it would appear but glad yall helped me out with that!). Much appreciated

Glad to 'help'. I usually learn just as much through such exercises as the person asking the question or being that I'm 'challenging'.

Yes, I'd conclude that if you were assuming your bracing was providing sufficient lateral restraint to be equivalent to the slab you were removing then yes I would politely agree that your assumption was incorrect.

Deciphering all the information I present above is a little hard given that we have continuity of beam/slab and column. If we assume pinned connections then the interpretation becomes clear. [highlight #FCE94F]Where you might expect the effective length to be the same due to the bracing, you are actually only getting a 50% increase in effective length rather than 100%[/highlight]. Naturally this can vary alot depending on the assumptions. Bracing the the floor above helps significantly on my modelling.

Does UDL on only one side make a difference?

No the stiffness doesn't change. And depending on the order which the surcharge is place vs the bracing place, tightened and secured it could worsen things.

 

[Settingsun]

That's what I wanted to check. But it would take buckling load ratios rather than effective length to check because the load in the column changes, and staging analysis as you mentioned. These things have a funny way of cancelling mathematically.

 

[Retrograde]

The stiffness of the restraint needs to be greater than 2P/L

P = axial load in column
L = distance between restraints

Have a read of this:

https://www.aisc.org/globalassets/aisc/research-library/bracing-for-stability.pdf

 

[Agent666]

Star for human909 here. What he has adequately demonstrated is that stiffness matters. Merrily adding braces does nothing if there isn't sufficient stiffness.

For the original problem, your best structural option would be to brace back to the floor, mobilising the larger in plane stiffness of the floor.

https://engineervsheep.com