another ACI slender column question
another ACI slender column question
(OP)
I am having a bit of trouble checking some slender columns. ACI really only gives guidance for columns that are a part of frames. How do you approach a column that is truly a gravity only column? I have a P.C. column that is supporting a WF that bears on top of the P.C. column and is not rigidly attached. The only end moments are due to the minimum e of 1" and I can't imagine this qulaifies as a frame. I was thinking I could just treat it as a non-sway frame anyway (since it is not a lateral column), but the problem I am having is that my kl/r>100 (107), which means I need a true second order analysis (I can't use the ACI method), but this column is not actually part of a frame so I am having trouble figuring out how to do a true second order analysis.
Any guidance here would be greatly appreciated!!
Any guidance here would be greatly appreciated!!






RE: another ACI slender column question
RE: another ACI slender column question
MUCH APPRECIATED. I see the reason for the 1" movement of the top of the column (Is this a seemingly arbitrary, assigned story drift?). What is the reason for breaking the column into 4 or 5 parts (wouldn't the P-small delta effects be taken care of with the second order analysis?)?
RE: another ACI slender column question
RE: another ACI slender column question
I have tried modelling the column in 4 pieces and running a second order analysis on it with equal end moments in single curvature and I am getting smaller moments in the middle of the column - that is completely baffling me.
I am using RAM Advanse - it allows me to input a cracked factor for the concrete.
RE: another ACI slender column question
RE: another ACI slender column question
You do have to assume the e at the bottom as well, right?
RE: another ACI slender column question
To get P-(LittleDelta) effects, give the volumn some sweep also.
RE: another ACI slender column question
RE: another ACI slender column question
One question. KL/r=107. What L did you use? I don't have my Code in front of me and it's been a while, but can't you use the clear distance for L? Hopefully you used the centerline dimension and KL/r<100.
Reading back through the thread, I don't agree with just moving the top of the column over 1". You need to use a 1" long rigid link at the top and bottom. Use a pin restraint at the bottom one and apply the vertical load at the end of the top one, so you have a constant 1" eccentricity over the entire column. Break up your column into several segments, using trial and error to see how many are required, probably 5-6. Then run your 2nd-order analysis and get your design forces.
Check to make sure that no part of the section has a stress exceeding the modulus of rupture (shouldn't be a problem). If there's cracking, then change Ig to Icr and re-run the 2nd-order analysis. The next step up in rigor is an event-to-event analysis changing Ig to Icr after each load step that results in a little part of the column cracking. I can't imagine anybody doing this, though, LOL.
RE: another ACI slender column question
I was accounting for this by using a cracked factor that incorporated both factors into it.
271828-
This Precast column sits directly on a footing and has a WF riding over top of it. I took the unbraced length as the bottom of column to the top of column - I don't think I can reduce this at all.
Do you see a problem in modelling it with axial loads and end moments?
General-
My intent was to get the moment from RAMAdvanse (using the second order analysis), and design the reinforcing in PCA Column as a short column with the second order moments already added in. Does this seem reasonable?
RE: another ACI slender column question
Your last paragraph makes perfect sense to me.
RE: another ACI slender column question
RE: another ACI slender column question
I appreciate both of your help. Can either of you tell me the significance of breaking the member into 4-6 pieces? I know both of you said that this would capture the slenderness effects, but if these 4-6 pieces are rigidly connected how is it different from using one piece? Also, if a program offers a second order analysis wouldn't this take the slenderness into account? What kind of second order anaylsis could it do that wouldn't take slenderness into account?
I was applying a modified cracked factor to a (roughly) equivalent square section (RamAdvanse doesn't offer round sections for RC columns). This cracked factor was the 0.25 shown in ACI and modified to get the square section down the the circular section. I assumed this would also take care of the Ec modification (RamAdvanse doesn't let you play with that either).
RE: another ACI slender column question
RE: another ACI slender column question
"What kind of second order anaylsis could it do that wouldn't take slenderness into account? "
Hmmm, I can't think of one. There are two basic types of second-order analyses: geometric nonlinear analyses and material nonlinear analyses, although they might go by different names than that in some circles. Geometric nonlinear analyses take into account nodes moving by considering equilibrium of the deformed geometry. Material nonlinear usually consider geometry effects, but also consider plastic hinge formation, changes in MOI (like Ig -> Icr) as load increases, etc.
It would be possible to consider material effects, but still consider equilibrium of the undeformed geometry, which would do what you asked about. I've never seen this done, though.
Usually the geometric nonlinearities are accounted for using geometric stiffness matrices. Some use these on the element level, but some do it on the system level. Some iterate and some don't need to.
Material nonlinear analyses can be done different ways, but the easiest to explain is event-to-event. Add load until properties need to change, then re-formulate the stiffness matrix, add more load, etc. until all the load is there.
Large displacement analyses, like the ones for cables, are another step up from these. These are really hard to understand, IMO, and use various schemes.
Probably a lot more than you wanted to know! It's a really interesting subject, usually covered in a second semester matrix analysis class.
RE: another ACI slender column question
second star from me. That clears it up - Thank you!!