The purpose of this tech note is to outline the implementation of the ACI slenderness methods for the design of frame and gravity columns in RAM Concrete.
Slenderness requirements for concrete columns are addressed in Section 10.10 of ACI 318-08 and ACI 318-11. ACI 318 outlines three methods to account for slenderness effects:
A nonlinear second order analysis must consider geometric and material nonlinearities. This type of analysis cannot be performed in RAM Structural System.
The P-Delta method in RAM Frame is an elastic second-order analysis. However, the RAM Frame results can only be used to design frame members; these results are not available to design gravity members. There is an option in RAM Frame to include gravity members under two-way decks in the analysis; however, there is not a way to use these forces for design in RAM Concrete. Also, the P-Delta method in RAM Frame does not include P-delta effects (second-order effects associated with deflections along the member length), which are required by ACI 318-11 10.10.2.2.
The moment magnification procedure is used to design gravity members. The forces used to design gravity columns are taken from RAM Concrete, which does not include P-Delta effects. ACI 318-11 10.10.7 outlines a moment magnification procedure for sway members. An important program defect associated with this procedure was resolved in v15.09.00.
More information on the analysis assumptions in each module can be found here.
There are four possible design categories for concrete columns:
Analysis and design notes for each of these groups are summarized in the following sections.
These columns are designed using RAM Concrete Analysis forces. The design forces are calculated using the non-sway moment magnification procedure. The forces reported in the Column Design Report are the magnified forces and not the forces from the analysis.
These columns are designed using RAM Concrete Analysis forces. The program assumes that the forces for sway columns are from an elastic second-order analysis. The program does not adjust these using the moment magnification procedure for sway frames. This follows from earlier editions of the ACI code, which allowed for the magnified sway moment to be taken from an elastic second order analysis (see ACI 318-05 10.3.4.1). This is a problem since a P-Delta analysis is not completed in RAM Concrete and RAM Frame results are not available for these columns.
Gravity concrete columns should never be designated as sway columns in RAM Concrete. Instead, these columns should be converted to frame members so that they are included in the RAM Frame Analysis. ACI 318-11 10.10.5.1 permits columns to be assumed non-sway if the increase in column end moment due to second-order effects does not exceed 5%. Gravity, sway column would only occur in structures with significant overturning under gravity load, which would be rare. In future versions of the program, it will not be possible to designate gravity columns as sway columns.
These columns are designed using lateral forces from the RAM Frame Analysis and gravity forces from either RAM Concrete or RAM Frame (see option in RAM Concrete Column – Criteria – Column Design – Design Checks/Forces tab). The design forces are calculated using the non-sway moment magnification procedure. The forces reported in the Column Design Report are the magnified forces and not the forces from the analysis. Since the moment magnification accounts for both P-Delta and P-delta effects, P-Delta can be turned off in RAM Frame.
These columns are designed using lateral forces from the RAM Frame Analysis and gravity forces from either RAM Concrete or RAM Frame (see option in RAM Concrete Column – Criteria – Column Design – Design Checks/Forces tab).
The design procedure follows the moment magnification procedure for sway frames in ACI 318-05 10.13. The equation for the magnified end moments is M = Mns + Delta_Sway*Ms, where
M = total magnified end moment
Mns = non-sway moment
Delta_Sway = sway moment magnifier
Ms = sway moment
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. Due to this assumption, P-Delta should always be included in the RAM Frame Analysis.
The non-sway moments are the gravity moments, which are taken from RAM Frame or RAM Concrete. In the ACI equation above, the non-sway moments are not magnified. If the option to use gravity forces from RAM Frame is used, the analyzed gravity load cases would include second-order effects from the P-Delta analysis. In most cases, the story displacements associated with the gravity load forces are small, so the increase in forces due to P-Delta effects should also be small. If there is a concern about using an amplified non-sway moment due to P-Delta effects, then gravity forces from the RAM Concrete analysis should be used to design the columns.
For slender columns with high axial loads, the maximum moment may be between the ends of the column and P-delta effects can be important. For this reason, the ACI 318-05 and earlier separated columns for which the slenderness (lu/r) was greater than 35/sqrt(Pu/f’c/Ag). When this threshold was exceeded, the program calculated from the following equation (see ACI 318-05 10.13.5)
M = Delta_NonSway*(Mns + Delta_sway*Ms)
This provision was removed in ACI 318-08.
In ACI 318-08 and later, the sway moments are required to be determined from one of the three slenderness methods (nonlinear second-order, elastic second-order, or moment magnification) for any slenderness ratio. As noted previously, the program is not equipped to run a nonlinear second-order analysis, the elastic second-order analysis does not include P-delta effects, and the sway moment magnification procedure is not implemented. The best the program can do is to combine the elastic second-order analysis with the moment magnification procedure using the same design procedure that is used when the ACI 318-05 is selected.
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.
ACI 318-05 limited kl/r to 100 unless a non-linear, second-order analysis was used. This provision was removed in ACI 318-08 but was still enforced in RAM Concrete when ACI 318-08 or ACI 318-11 is used. Defect #215695 was filed for this issue, and it was corrected in v15.09.00.
RAM Concrete does not check ACI 318-11 10.10.2.1: total moment including second-order effects shall not exceed 1.4 times the moment due to first-order effects. Defect #215527 has been filed with the development team.
RAM Concrete does not calculate Q ACI 318-11 10.10.5.2 (Eq 10-10). Enhancement #275537 has been filed .with the development team.
The Column Design Report still references ACI 318-05 10.3.5 when the slenderness ratio is high. This is a reporting error only. This does not affect the design. Defect #215175 has been filed with the development team.
RAM Concrete does not include an option for using the moment magnification procedure for sway columns. Enhancement #215521 has been filed with the development team.
ACI 318-05 10.12.3.1 included a lower bound of 0.4 for the parameter Cm. This lower bound was removed in ACI 318-08 but was still enforced in RAM Concrete when ACI 318-08 or 318-11 was used. Deflect #214767 filed for this issue, and it was corrected in v15.09.00.
Slenderness effects are not included in RAM Concrete Shear Wall.
Concrete Column Minimum Eccentric Moment
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