Hello, I wanted to reach out just to make sure that I am understanding the thought process behind designing sway frame columns (assigned as lateral) in RAM Structural System. I have a few questions outlined below.
From what I've gathered from other forums/links is that RAM does not use moment magnification for sway columns (which is why the report shows a moment magnifier of 1 in the RAM Concrete output). Instead, you should design these columns in RAM Concrete Column using the forces taken from either RAM Concrete or RAM Frame analysis (including P-Delta effects). The only difference in these two options is the effect on the non-sway moments, correct? For instance:
1. Gravity loads from RAM Concrete = Non sway moments are gravity moments (does not account for P-Delta effects) from RAM Concrete, with magnified sway moments (Delta_Sway*Ms) from RAM Frame
2. Gravity loads from RAM Frame = Non sway moments are gravity moments (with P-Delta effects) from RAM Frame, with magnified sway moments (Delta_Sway*Ms) from RAM Frame
Also, version 15.09 and newer of RAM now incorporates P-delta effects in addition to P-Delta effects in RAM Frame, correct?
My last question is, what is the easiest way to look at your individual components of the total column moment? Is there a way to look at what your nonsway and sway moments are individually?
Thank you in advance for your time in reading through all of this. I always appreciate your feedback!
Start with this wiki: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/27705/ram-concrete-column---aci-slenderness-design It answers some of your questions.
I don't believe with have any solution for the local, or small p-delta in Ram Frame yet.
There is no easy way to isolate the effects of P-Delta other than running the model twice in Ram Frame and comparing results. To check the raw forces in the columns prior to any moment magnification use the Ram Concrete - Column Design Forces report. The magnified results are in the column design report.
Answer Verified By: Andrea Armacost
Thank you Seth. In regards to p-delta, can you elaborate a little on the information presented in this link? https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/31240/aci-318-column-slenderness-second-order-effects-along-member-length
It's a little confusing, because it notes that the "problem" is the lack of consideration of second order effects along the length of the member. It then seems to describe two solutions to fix the problem under "reason". Those two options are listed as either incorporating the elastic second order analysis to include small p-delta effects OR accounting for the effects using moment magnification. Under "solution" it says that the issue was fixed. I am wondering what is meant by that, were small p-delta effects or moment magnification incorporated to resolve the issue? Or is there still an outstanding issue here?
RAM Frame considers P-Delta effects (displacement at member ends) but not P-delta effects (displacement along member length). However, ACI 318-14 6.6.4.6.4 requires the P-small delta effect to be considered in the design of sway frames. That same provision permits us to account for that P-delta effect (displacement along member length) using the non-sway moment magnification procedure (6.6.4.5). In RAM Structural System, this is accomplished by taking the second-order moments from the P-Delta analysis in RAM Frame and multiplying them by the non-sway moment magnification factor.
Prior to v15.09, the non-sway moment magnification factor was not being applied, which means that the P-delta effect (displacement along member length) was not accounted for since it is not included in RAM Frame analysis. In v15.09 and later that non-sway moment magnifier is now applied so there is not an outstanding issue.
Thank you, Karl. This applies to sway frame columns, correct? So where can I see what the program is using for the non-sway moment magnification factor? When I am reviewing the results in RAM Concrete Column, am I looking at the non-sway or sway moment magnifier?
From the article Seth sent above: "In previous versions, ACI 318 permitted magnified sway moments (Delta_Sway*Ms) to be determined from an elastic, second order analysis (see ACI 318-05 10.13.4.1). This is the assumption that is used in RAM Concrete, and a sway moment magnifier of 1 is assumed."
Yes, we are discussing sway, frame columns.
RAM Structural System does not calculate sway moment magnifiers per ACI 318-14 6.6.4.6. Instead, the program uses a second-order, elastic analysis for frame columns, which is permitted by 6.2.3. If a second-order analysis is used, then the sway moment magnifer does not apply, which is why Seth (and the manual) notes that a value of 1 is applied. Also, it is worth noting that P-Delta should always be turned on in RAM Frame when designing sway, frame columns. If it is not, then the analysis is first-order.
As discussed previously, the second-order analysis in RAM Frame accounts for P-Delta but not P-delta. The P-delta effect must be included and is included in RAM Structural System by applying the non-sway moment magnifier, which is permitted by ACI 318-14 6.6.4.6.4.
The moment magnifier is output in the Concrete Column Design Report in RAM Concrete Column.