RAM SS Concrete Column Slenderness

Allen Adams, our chief structural engineer, recently presented a webinar on slenderness requirements for concrete structures. If you are interested in watching a recording of this video (its free), click on the link below and filter "RAM" in the "Select Product" menu. The seminar was titled "Code Requirements for the Analysis of Concrete Structures."

http://pages.info.bentley.com/videos/

The web page covering concrete column slenderness in RAM Concrete will also be useful:

http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/27705.ram-concrete-column-aci-slenderness-design

To answer your questions:

1. Yes, this provision was removed in ACI 318-08. However, it is still enforced when ACI 318-08 or ACI 318-11 is selected as the design code. This is a conservative defect. It is logged as Defect #215695 in our database.

2. ACI 318-08 and later permit the following to evaluate slenderness: moment magnification, elastic second-order, and non-linear second order. These are discussed on the ACI slenderness wiki above. RAM Concrete uses moment magnification for the design of gravity columns and uses an elastic second-order to design frame columns. The sway moment magnification procedure is not implemented in the program.

Thank you very much for your quick response. So basically as of now, RAM's only way of designing for column slenderness is utilizing the requirements from ACI 318-05, which in all cases is a conservative approach. I just want to make sure that if I design for the kl/r values and limits it notes, I will not be underdesigning for the current ACI code.

Are there any future plans to implement any of the three aforementioned slenderness analysis techniques into the program?

The ACI 318-05 slenderness provisions are very similar to those in ACI 318-11 and later. The section titled "Other Notes" on the ACI slenderness design wiki shared earlier will outline the omissions or defects that you should be aware when using the ACI design code for slenderness design in RAM Concrete.

Of the 3 slenderness methods, the program includes 2 of them: moment magnification and elastic second-order. The non-linear, second order analysis procedure is not implemented and there are no immediate plans to add it to the program. The implemented moment magnification procedure is for non-sway columns only. Sway columns should be modeled as frame members and analyzed using the P-Delta method in RAM Frame. Implementing the sway moment magnification procedure is a logged enhancement (see "Other Notes" section in wiki) and something we would like to add in the future.

Thank you so much for this information. I think I understand now how the program deals with slenderness. I do have another question. Based on the wiki link that you sent, there are four different type of columns. In the program, how would I specify a column as either "braced" or "sway" for gravity and lateral columns? I know how to specify a column as either gravity or lateral, and I know how to specify a column as braced, but what would I have to do to specify a column as sway? Is that an inherent characteristic once I have called out a column as lateral? I want to ensure that my lateral columns are sway columns and my gravity columns are braced.

Non-sway and sway correspond to the Sidesway assignments made in RAM Concrete Analysis - Criteria - Sidesway. See below. This global setting can be overridden on a column-by-column basis using  RAM Concrete Analysis - Assign - Columns - Sidesway.

Braced = Non-Sway and Unbraced = Sway

Karl -

In response to your comment: "Sway columns should be modeled as frame members and analyzed using the P-Delta method in RAM Frame," can RAM Concrete or RAM Frame design sway frame columns at all? Or do I have to use the loads provided in RAM and design them by hand or with other software?

Thanks,
Kyle Poklar

Karl -

In response to your comment: "Sway columns should be modeled as frame members and analyzed using the P-Delta method in RAM Frame," can RAM Concrete or RAM Frame design sway frame columns at all? Or do I have to use the loads provided in RAM and design them by hand or with other software?

Thanks,
Kyle Poklar

Kyle,

Yes, RAM Structural System can design sway, frame columns. The forces used to design these columns in RAM Concrete Column are taken from the RAM Frame Analysis (there is an option to use gravity forces from RAM Concrete Analysis). The design follows the elastic, second order analysis (ACI 318-11 10.10.4) and P-Delta should be included in the RAM Frame Analysis. The program does not calculate sway moment magnifiers and design these columns using the moment magnification procedure (ACI 318-11 10.10.5). As noted above, this is a future enhancement that we will consider.

Other notes and limitations are discussed on this web page:

http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/27705.ram-concrete-column-aci-slenderness-design

Karl,

Can you explain how that relates to this paragraph at the bottom of the link you sent me? It seems to say that the P-delta effects aren't included, and therefore these columns can't be designed.

"ACI 318-08 and 318-11 10.10.2.2 requires second-order effects along the length of the member to be considered when the member is slender. Since P-delta effects are not included in RAM Frame, the program should apply 10.10.6 to slender, sway columns. This is not currently done by the program. This is logged as Defect #215527 in our database."

Thank you for the quick response,

Kyle

Kyle,

There are second order effects: (1) secondary moment associated with axial load and lateral displacement at the end of a member (P-Delta) and (2) secondary moment associated with axial load and displacement between member ends (P-delta).

RAM Frame accounts for (1) but not (2). ACI 318-11 requires (2) to be included in the design, however, and identifies the moment magnification procedure as one way to account for it. If the P-delta effects are significant, than the column may be under designed in RAM Concrete.

The Defect that is noted in the slenderness tech note has been flagged as a critical known issue. It is something that we are working to correct. The web page below discusses the known issue. We will update the status of the defect on this page as we work on the fix:

http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/31240.aci-318-column-slenderness-second-order-effects-along-member-length

Karl,

Now that the slenderness moment amplifier issue is fixed, I assume the only difference between assigning concrete columns "Lateral-Braced" (non-sway) and "Lateral-Unbraced" (sway) is how RAM handles calculating "k" using nomograph charts.

If I am designing a multi-story, two-way slab structure, it is my understanding that I must override the "k" value because the program is unable to accurately calculate the effective length of columns in the absence of beams.  If an effective length override is required, then it makes no difference whether the columns are assigned "braced" or "unbraced."  Please confirm.

Thanks!

Yes, RAM Structural System does not account for flexural stiffness of two-way slabs when determining K. If the columns supporting the two-way slab are defined as non-sway, I recommend using K=1. If the columns supporting the two-way are defined as sway, I would determine K accounting for an appropriate slab flexural stiffness and assign that value manually.

There are different slenderness limts for non-sway and sway columns below which slenderness effects can be neglected. These are outlined in ACI 318-14 6.2.5. Those limits can result in different designs for a given K if the column assignment is switched from braced to unbraced.