Sway or nonsway RC columns for my space frame
Sway or nonsway RC columns for my space frame
(OP)
Hello,
I'd like to check the stability of RC columns against the reactions of a space frame. The columns' height is 12 m and their diameter is 60 cm. The issue is to decide the effective length factor value to adopt for this case to determine the critical buckling load Pc.
At first sight, the structure is sway as no bracing elements are present.
The question is: can I consider my structure as nonsway if it complies with the following condition (10-10) of ACI 318-11 and adopt an effective length factor of 1 instead of 2, or is this conditon only available for the determination of the Moment magnification procedure ?
Thank you.

I'd like to check the stability of RC columns against the reactions of a space frame. The columns' height is 12 m and their diameter is 60 cm. The issue is to decide the effective length factor value to adopt for this case to determine the critical buckling load Pc.
At first sight, the structure is sway as no bracing elements are present.
The question is: can I consider my structure as nonsway if it complies with the following condition (10-10) of ACI 318-11 and adopt an effective length factor of 1 instead of 2, or is this conditon only available for the determination of the Moment magnification procedure ?
Thank you.







RE: Sway or nonsway RC columns for my space frame
The difference in state a magnification by sway or nonsway remains in the consideration of P-Delta Effects, besides the P-deflection ones.
RE: Sway or nonsway RC columns for my space frame
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Sway or nonsway RC columns for my space frame
RE: Sway or nonsway RC columns for my space frame
RE: Sway or nonsway RC columns for my space frame
Yes Ingenuity, it's the same structure. The concrete tests reveals high level of chloride content, it seems to be the principal cause of steel corrosion that has led to concrete cracking.
The columns are always slender even with a k factor of 1 and I will need a magnification factor in all cases. By considering a factor equal to 2 the design load exceed the critical buckling load. I thought the columns stiffness would provide lateral resistance, in view of the non significance of lateral loads.
RE: Sway or nonsway RC columns for my space frame
RE: Sway or nonsway RC columns for my space frame
You have stated that there are no bracing elements, so it is a sway frame.
And stop trying to use "interpretations" of unrelated code clauses to get around engineering logic.
RE: Sway or nonsway RC columns for my space frame
Double check your calculation of the critical buckling load, Pc:
What is the magnitude of your factored axial load on the interior columns?
RE: Sway or nonsway RC columns for my space frame
As per ACI 318-11 Eq. (10-15) EI = 0.4EcIg / 1 + βdns ≅ 30 x 1012 N.mm²
lu = 12.5 m taken from the top of foundation
Pc = 480 KN and my factored axial load is around 800 KN
RE: Sway or nonsway RC columns for my space frame
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Sway or nonsway RC columns for my space frame
"In the calculation of EI, βds will normally be zero for a sway frame because the lateral loads are generally of short
duration. Sway deflections due to short-term loads such as wind or earthquake are a function of the short-term stiffness
of the columns following a period of sustained gravity load. For this case, the definition of βds in 10.10.4.2 gives βds = 0.
In the unusual case of a sway frame where the lateral loads are sustained, βds will not be zero. This might occur if a
building on a sloping site is subjected to earth pressure on one side but not on the other."
In this case, is the thermal load to be considered ? especially with high temperature differences and long structure.
RE: Sway or nonsway RC columns for my space frame
May be the space frame is not enough stiff to constrain the columns to buckle together.
Do you mean that I don't have to check each column's loads against its critical load ?
RE: Sway or nonsway RC columns for my space frame
Based on your comment about the stiffness of the space frame, it sounds as though you've got the idea. I wholly expect that the columns will buckle as a system, with more lightly loaded columns bracing heavier loaded columns for a time, rather than as individual columns somehow plowing through the space frame independently. Some strength and stiffness would be required of the space frame but I don't see that being much of a problem.
As a high level check, I would sum the axial capacities of all the columns and compare that to the total axial load on all of the columns. If that gets you into the right ballpark, dig a bit deeper.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Sway or nonsway RC columns for my space frame
Hopefully your δs does not magnify too much!
RE: Sway or nonsway RC columns for my space frame
Now you have to design the footings to support the cantilever columns. If the footings can rotate, all bets are off.
RE: Sway or nonsway RC columns for my space frame
Is this, conceptually, much different than the ubiquitous practice of having "gravity columns" that are stabilized laterally by other elements? It takes a village. As evidenced by Ingenuity's latest, the notion of whole level buckling is kinda baked into the ACI cake. And one of the reasons that AISC's modern Direct Design Method was developed was to specifically give account to system behavior of this sort.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Sway or nonsway RC columns for my space frame
The only way to determine the relative buckling effects in this sort of situation is to do a full 2nd order sway analysis. The simplified slenderness rules in codes do not apply in this case. They basically consider the "average" column and do not allow the designer to evaluate the effects on the worst column.
RE: Sway or nonsway RC columns for my space frame
Assuming 800 kN is for your INTERIOR column factored load, and then assuming that the EDGE column factored loads are 400 kN (50% trib area) and CORNER column factored loads are 200 kN (25% trib area) then:
Further assuming all 15 columns have the same rebar size and concrete strength:
So 'plug and chug' into equation (10-21):
So basically doubling your sway moments.
At 1,200 kN factored axial load your corresponding moment capacity is about 500 kN.m (assuming you have 35 MPa concrete and 1.5% reinforcement).
RE: Sway or nonsway RC columns for my space frame
The space frame supplier provided reactions on supposed rigid supports. When I designed the structure on supports of equivalent columns' stiffness, the horizontal reactions have been reduced to some negligebale values (max Fx = 0.6 KN). It should be closer to the reality than rigid support.
RE: Sway or nonsway RC columns for my space frame
RE: Sway or nonsway RC columns for my space frame
I will check reactions from temperature load too. It shall be limited to almost 30 % of vertical reactions by the adoption of sliding supports.
RE: Sway or nonsway RC columns for my space frame
I think we don't need sliding supports, it'd be useless.