RAM SS Concrete Column Slenderness

RE: RAM SS Concrete Column Slenderness

Alejandro Alvergue — Wed, 18 Nov 2020 23:46:08 GMT

Thank you.

So what does the program do in these cases where the two-way slab is the only element which braces the column? It still needs to calculate a k value for slenderness checks.

RE: RAM SS Concrete Column Slenderness

Karl G — Wed, 18 Nov 2020 18:37:44 GMT

RAM Structural System calculates k using the stiffness of the column and the stiffness of beams framing into the column only. The calculation excludes the stiffness of two-way decks that are connected to the column. If you deem it acceptable to exclude the flexural stiffness of the two-way slab in the situation you describe, then you would not need to manually assign the k. If you want to include the slab stiffness, however, you would still need to manually calculate k.

RE: RAM SS Concrete Column Slenderness

Alejandro Alvergue — Wed, 18 Nov 2020 17:30:33 GMT

In a scenario where a slab is defined as two-way (to enable out of plane stiffness for separate reasons) but there are beams framing into a column (in x direction but not in Y). Is it still necessary to manually assign the k values, or will RAM Concrete be able to calculate k based on the ratio of column to beam stiffness (per ACI), for that direction where beams frame the column?

RE: RAM SS Concrete Column Slenderness

Karl G — Wed, 18 Nov 2020 15:45:58 GMT

Yes, that is correct. A manually calculated k that is greater than 1 should be assigned so that the slenderness thresholds in ACI 318-14 6.2.5 are calculated correctly. When calculating the non-sway moment magnifier to account for small p-delta effects for that same column, k = 1 would now be used instead of the larger k as noted in the release notes.

RE: RAM SS Concrete Column Slenderness

Alejandro Alvergue — Wed, 18 Nov 2020 15:39:56 GMT

Thanks!

I've understood the changes but want to clarify one last thing:

The k value for sway/non-braced columns supporting two-way slabs will still need to be calculated (and user assigned) with the ACI provided nomographs, in order for RAM Concrete to check for slenderness effects appropriately (per ACI 318-14 6.2.5).

RE: RAM SS Concrete Column Slenderness

Karl G — Wed, 18 Nov 2020 15:14:09 GMT

The fix was released in v17.01.00 and is included in the version that you are running. The change is documented in the v17.01.00 release notes, which can be reviewed using the link below. See "ACI 318 Moment Magnification" in the New Features and Enhancements section.

https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/48807/ram-ss-v17-01-release-notes

RE: RAM SS Concrete Column Slenderness

Alejandro Alvergue — Wed, 18 Nov 2020 15:01:13 GMT

Karl,

I'm running RAM SS V17.01.01. Has this issue been addressed? I was unable to find any documentation by Bentley on this. Thanks!

RE: RAM SS Concrete Column Slenderness

Stephen Richards — Wed, 27 May 2020 17:31:00 GMT

Thank you both. I'll keep an eye out for 17.01 to drop and hopefully clear this up. Thanks again.

RE: RAM SS Concrete Column Slenderness

Karl G — Wed, 27 May 2020 17:14:58 GMT

CEI Engineer is correct. For sway columns supporting a two-way slab with no beams, a K-value greater than 1 should be assigned to check the slenderness limit of ACI 318-14 6.2.5. When accounting for the second order effects between member ends for the same column using the non-way sway moment magnification procedure, K=1 should be used. However, RAM Concrete Column is incorrectly using the larger K instead. This was logged as Defect #1028340, has now be fixed, and will be included in v17.01.00, which will be released within the next few weeks.

RE: RAM SS Concrete Column Slenderness

CEI Engineer — Wed, 27 May 2020 17:02:21 GMT

I do not want to speak for Karl / Bentley, but I agree with your assessment that k=1.0 is appropriate for calculating the moment magnification factor associated with Sections 6.7.1.2 and 6.6.4.5. I also agree that the P-Delta analysis performed in RAM Frame satisfies the elastic second-order analysis permitted by Section 6.6.4.6.2. The selected k-value should have no impact on the RAM Frame analysis.

However, an accurate k-value is needed to properly check the slenderness limits of 6.2.5. The problem I see is that RAM does not apply different k-values for the slenderness checks of 6.2.5 and the critical buckling load (Pc) in section 6.6.4.4.2. The user must either apply k=1.0 and risk incorrectly ignoring slenderness effects, or apply k>1.0 and deal with a conservatively large magnification factor which, in some cases, could result in a very conservative column design.

Karl / Bentley - Can you comment on this assessment? Much appreciated!

RE: RAM SS Concrete Column Slenderness

Stephen Richards — Tue, 26 May 2020 20:38:32 GMT

Karl,

Your last reply has me questioning this, so I'm hoping you can either confirm and/or clarify. For sway columns in a two-way slabs without beams, my understanding is that k only needs to be manually calculated for sway columns if a first order analysis with moment magnification is used. If an elastic second order analysis was performed (P-Delta on in RAM Frame), then k should still be defined as 1 since ACI 318-14 6.7.1.2 directs to ACI 318-14 6.6.4.5 for moment amplification due to deflection along the member length of non-sway members. ACI 318-14 6.6.4.4.3 then states that k may be taken as 1 for non-sway members. Thank you for your help.

RE: RAM SS Concrete Column Slenderness

Karl G — Thu, 16 Apr 2020 12:41:13 GMT

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.

RE: RAM SS Concrete Column Slenderness

CEI Engineer — Wed, 15 Apr 2020 21:36:10 GMT

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!

RE: RAM SS Concrete Column Slenderness

Karl G — Tue, 10 Jan 2017 18:18:44 GMT

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

RE: RAM SS Concrete Column Slenderness

Kyle Poklar — Tue, 10 Jan 2017 16:43:46 GMT

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

RE: RAM SS Concrete Column Slenderness

Karl G — Tue, 10 Jan 2017 16:30:09 GMT

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

RE: RAM SS Concrete Column Slenderness

Kyle Poklar — Tue, 10 Jan 2017 16:10:03 GMT

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

RE: RAM SS Concrete Column Slenderness

Karl G — Wed, 16 Nov 2016 19:24:46 GMT

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

RE: RAM SS Concrete Column Slenderness

Beth Feero — Tue, 15 Nov 2016 22:32:13 GMT

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.

RE: RAM SS Concrete Column Slenderness

Karl G — Tue, 15 Nov 2016 19:41:43 GMT

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.

RE: RAM SS Concrete Column Slenderness

Beth Feero — Tue, 15 Nov 2016 19:33:17 GMT

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?

RE: RAM SS Concrete Column Slenderness

Karl G — Tue, 15 Nov 2016 19:14:56 GMT

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.