Note: This is a companion to the article, ASCE 7, AISC 360, and the Direct Analysis Method in the RAM Structural System.
The RAM Structural System contains several powerful features to assist in the analysis and design of steel members in conformance with the International Building Code. That code specifies that designs conform to the requirements of ASCE 7 and AISC 360. This document provides a detailed outline of the steps to take to obtain valid designs. The references in the document are based on the requirements of ASCE 7-16 and AISC 360-16 which are referenced by IBC 2018 and IBC 2021.
ASCE 7 gives requirements for the determination of loads and load combinations, and limits on the resulting drifts and stability. AISC 360 gives requirements for the analysis and design of steel structures. Two commonly used methods are the Direct Analysis Method given in Chapter C Design for Stability, and the Effective Length Method given in Appendix 7.2 Effective Length Method. In the RAM Structural System, the Direct Analysis Method should be used for moment frames, while the Effective Length method is recommended for braced frames.
For braced frames it is easier to use the Effective Length Method because doing so eliminates some steps in the process because it isn’t necessary to perform analyses with and without the stiffness reduction required for the Direct Analysis method. However, AISC 360 Appendix 7 Section 7.2.1(b) explicitly forbids the use of the Effective Length Method when the ratio of second-order drift to first-order drift is greater than 1.5, so if that is the case the Direct Analysis method should be used for that structure, although this is very unusual for a braced frame. For moment frames it is necessary to use the Direct Analysis Method because in RAM Frame it is assumed that that method will be used for moment frames, and it always assigns K=1 consistent with that method; the Direct Analysis Method is more accurate than the Effective Length Method for moment frames.
This article addresses the use of the Effective Length Method for braced frames. It should not be used for moment frames (unless the Exception given in AISC 360 Section 7.2.3(b) is satisfied; if the ratio of second-order drift to first-order drift is less than 1.1, it is permitted to use K=1.0 for such moment frames, so those frames could be analyzed and designed in the RAM Structural System using the Effective Length Method). For the analysis and design of moment frames and of braced frames for which the ratio of second-order drift to first-order drift is greater than 1.5, see this article:
The general steps outlined below are not unique to RAM Frame but would be required in order to obtain valid designs with any software.
For brevity, this document is based on LRFD requirements. Additional adjustments (e.g., determination of the ratio of 2nd-order drift to 1st-order drift, notional loads, etc.) may be necessary if ASD is used for design; the engineer should be aware of those requirements and apply them as necessary.
This document is not intended to be a comprehensive outline of all necessary actions and criteria settings, such as diaphragm settings, flange bracing, reduced beam sections (RBS) if applicable, joints, etc. Its purpose is to outline one possible workflow, highlighting productivity-enhancing features available to aid in producing designs that conform to the requirements of the code.
After the model has been created and the gravity designs performed in RAM Steel, perform the following steps in RAM Frame.
Step 1 Create the Wind and Seismic Load Cases for Drift
Create the Wind and Seismic Loads for the drift analysis using the Loads – Load Cases command. It is recommended that the label given to the seismic load cases clearly identifies that these are the drift load cases.
For the Seismic load cases when using the seismic load generator:
For the Wind load cases, select Use Calculated n for Natural Frequency and at this time do not select the option Exempted from Torsional Cases 2 and 4 per Appendix D (unless you are certain that it is; it will be determined in a later step):
It is also recommended, although not necessary, to create an Eigen Solution Dynamic load case so that the building mode shapes are available for viewing. If not explicitly created the program will internally create the eigen solution dynamic load case in order to calculate the building periods, necessary for the generation of the wind and seismic story forces, but the mode shapes will not be available for viewing. Therefore, it is recommended that the eigen solution load case be explicitly created.
Step 2 Specify the Criteria for Analysis
Select the P-Delta criteria using the Criteria – General command. Generally, the most preferred option is the Use Gravity Loads option. The scale factors should be those associated with the load combination most likely to govern for the lateral columns. For example, since the seismic or wind loads are likely to control the designs, the strength design combination 4 of Section 2.3.1 or combination 6 of Section 2.3.6 are likely to control. In those combinations the factor on Dead Load is 1.2 and the factor on Live Load is either 0.5 or 1.0, as specified by Exception 1. These factors on P-Delta will be conservative for the uplift combinations 5 and 7, but P-Delta isn’t an issue for those combinations anyway. Conservatively, the factors of 1.2 and 1.6 per combination 2 could be used, guaranteeing that the worst P-Delta condition is covered for all combinations. Note that these values should be ultimate factors even if ASD will be used in design of the members so that the P-Delta analysis will be performed at an ultimate level, which is necessary for the principle of superposition of load cases to be valid. Also note that these are not the factors that will be used in the load combinations for design, these are merely the factors used to calculate the ultimate gravity loads used in the P-Delta analysis technique used by the program.
Do not select the Use Reduced Stiffness for Steel Members option for AISC 360 Direct Analysis Method. This option should only be used when the Direct Analysis Method is being utilized, and is not appropriate for the Effective Length Method, as it will have a detrimental impact on the building periods, the story drifts, and the generated story forces.
As necessary, specify all other pertinent criteria items in the Criteria menu, assign pertinent properties using the commands in the Assign menu, and verify and specify the appropriate values and options for loads and masses in the Loads menu.
Step 3 Analyze
Analyze the structure using the Process – Analyze command, selecting the gravity-, seismic drift-, wind-, and eigen solution load cases.
Note that if the initial sizes assigned to the frames are too small the structure will be unstable, and the P-Delta analysis will fail and/or the eigen solution analysis will not converge. Make sure that reasonable initial sizes have been assigned before the analysis is performed. Note that if sizes are not assigned to the frame members in the Modeler before the beams and columns are designed in the Steel Beam and Steel Column modules, the program will automatically select and assign beam and column sizes for the frame members that satisfy only the gravity load combinations when designs are performed in the Steel Beam and Steel Column modules; these sizes may be too small for either the P-delta analysis or the eigen analysis to successfully complete.
Review the Loads and Applied Forces report for accuracy and reasonableness. Verify that the specified criteria and input values are correct.
It is highly recommended that you view the deflected shapes and mode shapes. This will help identify some modeling errors or indicate a structure that is not well-defined.
View the deflected shapes using the Process – Results – Deflected Shapes command. Select the option to Animate.
View the mode shapes using the Process – Results – Mode Shapes command. Select the option to Animate. With Mode 1 selected, begin the animation by selecting the Apply button. To view each of the other mode shapes select the Mode Number and select the Apply button. In a regular, well-proportioned structure with orthogonal frames the first mode shape will usually be a translational mode, in either the X- or Y-direction, the second mode will usually be a translational mode in the orthogonal direction to the first mode, and the third mode will usually be a rotational mode. That is not always the case, so if not, be sure you understand why not (e.g., the frames are very significantly stiffer in one axis than the other).
If any diaphragms have been defined as Semi-rigid, and if extreme out-of-plane displacements of the diaphragm appear when the mode shapes are displayed, this probably indicates that the diaphragm properties or options need to be modified to eliminate these diaphragm modes (that almost certainly don’t exist in reality); the discussion of this problem is beyond the scope of this wiki.
Review the Periods and Modes report. If the %Mass values listed under Modal Effective Mass Factors for Mode 1 for X-Dir, Y-Dir, and Rotation are all 0.00, this indicates that some individual member/element (such as an individual beam, an out-of-plane wall or column, or a diaphragm), rather than the structural frames, is producing the first mode results. If not corrected, the program will erroneously use this period in the generation of wind and seismic forces. The exception to this is that if all diaphragms are Semirigid the Torsion values in that report will be 0.0; this doesn't indicate an error if the X- and Y-direction values are non-zero.
Make any necessary model changes (e.g., fixities, diaphragm thickness and properties, etc.) to correct the error conditions that may have been exposed by looking at these results, and reanalyze if necessary.
If it has not already been done, assign Frame Numbers to the various frames using the Assign – Frame Numbers command. This will be helpful when viewing some reports.
Step 4 Check for Structural Irregularities
Horizontal Structural Irregularities are defined in Table 12.3-1 and Vertical Structural Irregularities are defined in Table 12.3-2 of ASCE 7. Horizontal Structural Irregularities 2 through 5 can be determined by inspection or with simple calculations. Horizontal Structural Irregularities 1a and 1b are more complicated and will be discussed below. Vertical Structural Irregularities generally require some computation. Several reports are available that can assist in these calculations.
In some cases the existence of an Irregularity requires an increase in the member design forces, and some Irregularities are prohibited in structures assigned to Seismic Design Categories C, D, E, and F, requiring a change in the configuration of the structural framing. A detailed discussion of Irregularities is beyond the scope of this wiki, but two webinars on the topic are available on-demand: Identifying Structural Irregularities (Part 1) and Dealing with Structural Irregularities (Part 2). These webinars show how the RAM Structural System analysis and reports can be used to identify structural irregularities, and how to satisfy the requirements when they exist:
Identifying and Dealing with Structural Irregularities: Part 1 and Part 2
One type of structural irregularity, Torsional Irregularity, has particular impact on the analysis and design process, and will be discussed in more detail here. ASCE 7 Table 12.3-1 defines Torsional Irregularity to exist when the drift at one end of the structure is greater than 1.2 times the average drift of the two ends, and it defines Extreme Torsional Irregularity to exist when the drift at one end of the structure is greater than 1.4 times the average drift of the two ends. A report for this can be obtained using the Process – Results – Drift – At Control Points command by inputting the coordinates for key locations on the plan, typically at the center of mass (those coordinates, Xm and Ym, can be found in the Story Mass Data section of the Criteria, Mass and Exposure Data report) and at the four corners. At the end of the Drift report there is a section titled Torsional Irregularity Data; the last column lists the worst case at each level, in each axis, of the maximum drift divided by the average drift (the second to last column lists the ratio of the drifts at each end; that value is used by some international codes, but can be ignored here).
If at any level the value is greater than 1.2, the structure is Torsionally Irregular (Horizontal Irregularity Type 1a); if at any level the value is greater than 1.4, the structure is Extreme Torsionally Irregular (Horizontal Irregularity Type 1b). This has an impact on what you need to do next, as given in Section 220.127.116.11 Prohibited Horizontal and Vertical Irregularities for Seismic Design Categories D through F, and Section 18.104.22.168 Accidental Torsion:
Note: it is possible that the structure is torsionally irregular in one axis but not the other; the above checks are performed for each axis independently.
Step 5 Check Drift
Note: Drift rarely controls the design of braced frames, but should be investigated.
ASCE 7 Section 12.12.1 Story Drift Limit limits the seismic design story drift, Δ, to the appropriate Δa value listed in Table 12.12-1. Note that the Δa values listed in the table are story drifts (e.g., 0.020hsx), and that the coefficients (e.g., 0.020) are the story drift ratios (which is simply the story drift divided by the story height, hsx). ASCE 7 Section 12.8.6 indicates that for structures assigned to Seismic Design Category C, D, E, or F, if the structure has either torsional irregularity Type 1a or Type 1b, the limits apply to drift at any point around the perimeter of the structure; elsewise the limits apply to the drift at the point of center of mass and it is not necessary to investigate drift at extreme points. Drift values can be obtained using the Process – Results – Drift at a Point command and clicking on any point on a floor plan or the Process – Results – Drift at Control Points command by inputting the coordinates of key locations on the plan, typically the centers of mass or the four corners, as explained. The resulting Drift report lists the displacements, story drifts, and drift ratios. These deflections are the elastic deflections, or δxe defined in Section 12.8.6. The design deflections are then given by Eq. (12.8-15):
Cd is the Deflection Amplification Factor given in Table 12.2-1 and Ie is the seismic Importance Factor given in Section 11.5.1.
Rather than factoring the elastic deflections in this way, calculating story drifts, and then comparing against the allowable story drift values in Table 12.12-1, a practical approach is to take the applicable coefficient (the story drift ratio) value from Table 12.12-1 and modify it so that it can be compared directly with the drift ratio values listed in the Drift report:
Maximum Allowable Elastic Drift Ratio = (Coefficient)(Ie)/Cd
For example, a building four stories tall or less in Occupancy Category II has an Allowable Story Drift of 0.025hsx, which means that the allowable story drift ratio is 0.025, Cd = 5.0 for a steel special concentrically braced frame, and Ie = 1.00 for Occupancy Category II. The Maximum Allowable Drift Ratio can then be computed as:
= 0.025(1.00)/5.0 = 0.0050 for this example
This limiting value can then be compared directly with the Drift Ratio values listed for the seismic cases in the Drift report:
Note: If the option to include P-Delta in the Criteria – General command was not selected for the analysis, the reported drifts won’t include any contribution due to P-Delta, so you must amplify the drift values by the factor of 1.0 / (1.0 - θ) using the value θ calculated for each story unless θ is less than 0.1, as indicated in Section 12.8.7 P-Delta Effects in ASCE 7. Again, to simplify the process you could calculate a modified (reduced) Maximum Allowable Elastic Drift Ratio for each story using θ calculated for each story:
Maximum Allowable Elastic Drift Ratio = (Coefficient)(Ie)(1.0 – θ)/Cd
These limits can then be compared directly against the elastic drift values listed in the report, using the limit appropriate for a given story. For simplicity it is recommended to use the P-Delta option in the program rather than using this approximate method.
Limits on Wind drift can similarly be compared against the values listed for the Wind load cases (no explicit limits for wind drift are given in ASCE 7).
If the drift ratios exceed the allowable, the member sizes should be adjusted or new frames added as necessary and Steps 3, 4, and 5 repeated until satisfactory drift ratios are obtained. Note that if these changes might change the status of the Torsional Irregularity or the amplifier on the accidental torsion, you will need to reset the Load Cases to generate all of the seismic and wind load cases (i.e., consider +/- Ecc for seismic and not exempt from Torsional Cases for wind), and you will need to reset the Diaphragm Masses to use the 5% Eccentricity.
Step 6 Stability Coefficient
Calculate and determine the acceptability of the Stability Coefficient. Section 12.8.7 specifies a maximum allowable stability coefficient, θmax, given by Eq. (12.8-17). To determine conformance to this requirement, the ASCE 7 Stability Coefficients report lists the values of θ and θmax at each story for each seismic load case. This report is available using the Reports – ASCE 7 Stability Coefficients command. Specify Cd (given in Table 12.2-1) for each direction; the ratio of shear demand to shear capacity for the story, β, which can be conservatively taken as 1.0; and the Seismic Importance Factor, Ie, given in Section 11.5.1. The Factors for Px are those from the seismic design combination, except no factor need exceed 1.0:
Section 12.8.7 indicates that when the analysis includes the P-Delta effects, the value of θ/(1+θ), rather than θ, may be compared against θmax. The report gives both values, but if P-Delta was included in the analysis use the θ/(1+θ) values:
If the Stability Coefficient exceeds the maximum allowable, θmax, at any level for any seismic load combination, the member sizes must be adjusted or new frames added as necessary, and Steps 3 through 6 repeated until satisfactory stability coefficient values are obtained. Note: if β was conservatively assumed to be 1.0, it may be worth the effort to calculate a more precise value of β in order to get a more correct (larger) value of θmax. The shear demand (the story shears) can be obtained from the Building Story Shear report. The Commentary to ASCE 7 defines the shear capacity as the "shear in the story that occurs simultaneously with the attainment of the development of first significant yield of the overall structure." As explained in the Commentary this can be determined by iteratively increasing the applied story forces until the first significant yield occurs at a given story (i.e., the demand on any frame member exceeds its capacity). The story shear on this story is then the value to be used in the calculation of β. This process would need to be continued until the story shear was similarly determined when first yield occurs for each story. The Commentary provides a simplified alternative: determine the worst interaction equation value for any frame member at a given level and use that interaction value for β. This can be done by looking at the Member Code Check reports for steel members or the Concrete Column Design or Concrete Beam Design reports for concrete members. Note that if the shear interaction is larger than the Axial/Moment interaction, the larger value should be used.
Section 12.8.7 also indicates that when the stability coefficient, θ, is less than or equal to 0.10, it is not necessary to include P-Delta effects in the analysis. Note that this is only true for the analysis used to calculate drifts; the Effective Length Method requires (despite what ASCE 7 says) that P-Delta effects be included in the analysis if the design equations of AISC 360 are to be used. Therefore, for simplicity it is suggested that P-Delta always be included in the analysis (in braced frames, the inclusion of the P-Delta effects will generally have an insignificant impact).
Step 7 Create Load Cases and Perform Analysis for Member Design
The seismic load cases created previously were for the purpose of checking drifts and stability. It is now necessary to create new seismic load cases that can be used for member design.
In the Loads – Load Cases command, add a new set of Seismic load cases. Select “Member Forces” for the Provisions for option. If the existence of a Torsional Irregularity requires that both plus- and minus eccentricity be considered as discussed in Step 4, select the + And – option for Eccentricity, otherwise select None:
[Note: For Seismic Design Category A it is not necessary to create this Seismic Load case. For Seismic Design Category A the Provisions for Member Forces will give the same results as the Provisions for Drift (the base shear is not a function of the period, T), so the results from that analysis can be used in the subsequent steps.]
It is not necessary to create new Wind load cases, the cases used for the investigation of drift can be used for the design forces used in member design. [Note: Some engineers use wind load cases with a shorter mean recurrence interval when checking drift (for example, see the Commentary to ASCE 7 Appendix C; if so, it is necessary to create new wind load cases for strength design.]
In the Loads – Load Cases command create the AISC 360 Notional Load cases. Specify 0.002 for the Fraction of Gravity Loads:
Perform the Analysis. Select the Dead, Live, Roof, and Wind load cases and the new Seismic and Notional load cases, but do not select the original seismic drift load cases.
Caveat: AISC 360-16 Section C2.1(b) makes a distinction between including P-Δ and P-δ effects in the analysis and including them in the member forces. The purpose of including them in the analysis is to capture any system or local instabilities caused by them, exacerbated by the interplay between them. The purpose of including them in the member forces is so that the members are designed for the higher forces created by those effects. The Specification always requires that members are designed for the higher forces; this is correctly accomplished in the program by selecting the P-Delta option or by selecting the B2 option for the P-Δ effects, and by selecting the B1 option for the P-δ effects. The Specification also always requires that P-Δ effects be included in the analysis, but only sometimes requires that P-δ be included in the analysis (when the conditions described in Section C2.1(b) are not satisfied). Most structures satisfy those conditions, so most structures don’t require the inclusion of P-δ effects in the analysis. The RAM Structural System includes the P-Δ effects but does not include the P-δ effects in the analysis, it does not perform the iterative analysis required for P-δ. This is only a potential problem if the structure does not satisfy the conditions of Section C2.1(b). You can get a sense of the magnitude of the problem by looking at the values calculated for B1 for the various members (performed in Step 9); these values are often 1.0, and if so it is unlikely that the inclusion of the P-δ effects in the analysis would have more than minimal impact on the stability of the structure or member. If both B1 and B2 are reasonable values it is highly unlikely that there are instabilities in the structure, and it is unlikely that a structure that satisfies ASCE 7’s Stability Coefficient limits and drift limits will have large values of B1 and B2. In short, the lack of consideration of the P-δ effects in the analysis performed by RAM Structural System is acceptable per the Specification for most structures, and is only likely to be a problem in the most extreme case of a structure that is unstable or nearly so, but you need to use your engineering judgement on this.
Step 8 Specify Code, Load Combinations and Criteria for Design
Go to the Steel – Standard Provisions module.
Select the desired AISC 360 steel design code.
In the Load Combination Generation dialog select the Code for Combinations. Generally, the IBC 2018 / ASCE 7-16 ASD or LRFD selection is appropriate.
If Seismic drift load cases were analyzed, they should be deselected before the load combinations are generated to avoid generating unnecessary combinations.
Generate the combinations.
Select the Criteria – B1 and B2 Factors command. To account for the P-δ effects, select the Apply B1 Factors option. To account for the P-Δ effects, the B2 factors can be used in lieu of the P-Delta analysis (the value of RM is 1.0 for braced frames and calculated from Eq. (A-8-8) for moment frames), but if the P-Delta option has been selected for the analysis it is not necessary to also apply the B2 factors (it is not necessary to select the Apply B2 Factors option):
There may be specific requirements for your structure not discussed here; specify all necessary criteria items in the Criteria menu, and override the criteria on a member-by-member basis if necessary, using the assign commands in the Assign menu. It is not necessary to specify or assign K-factors. An effective length factor, K, of 1.0 is used automatically by RAM Frame when AISC 360 is selected as the design code, and need not be specified in the Criteria.
Step 9 Perform AISC 360 Standard Provisions Member and Joint Checks
Perform a Code Check on the members using the Process – Member Code Check command.
When the Direct Analysis method is being used there is a report, AISC 360 Direct Analysis Validation Report, that specifically identifies the validity of the design in accordance with the requirements of that method. There is not a corresponding report for the Effective Length Method. However, some information given in the AISC 360 Direct Analysis Validation Report may be useful in verifying that P-Δ and P-δ were appropriately considered and that Notional loads were analyzed and handled properly in the load combinations. Any information in the REDUCED STIFFNESS section of that report can be ignored, except the statements indicating whether or not the stiffnesses were reduced. While applying reduced stiffness is required for the Direct Analysis Method, they should not be applied when using the Effective Length Method; doing so is an error, and should be corrected by unselecting the option to Use Reduced Stiffness for Steel Members in Criteria – General.
In particular, note the Maximum B2 value listed in that report; if it is greater than 1.5, the Effective Length Method is not allowed, and you must use the Direct Analysis Method.
In the report, text in blue indicates unnecessary (conservative and/or redundant) selections, such as “Both P-Delta and B2 factors were applied. Only one or the other is required” which would appear in the SECOND-ORDER ANALYSIS section of the report if both the P-Delta option and the B2 factors option were selected:
Text in red indicates erroneous selections, such as “B1 factors were not applied” which would appear if the B1 option was not selected:
Since this report is intended for validation of the Direct Analysis Method, not the Effective Length Method, there may be some messages that do not apply. In order to have a valid design based on the AISC 360 Effective Length Method it is necessary to:
Once the analysis and design options have been validated, verify the acceptability of the member sizes by looking at the on-screen code check results (failing members are shown in red) or the Member Code Checks Summary report, and change the sizes as necessary. The Process – Member View/Update command is very helpful in investigating and modifying sizes.
It is common practice to designate the beam-to-column connection in braced frames as “Pinned” in the analytical model. In that case the program does not perform any checks on the joints to determine the necessity of web plates or stiffener plates. If the beam is designated as “Fixed”, the Process – Joint Code Check command will check the column for the need for web plates and stiffener plates. The joint symbols can be made more visible using the Increase symbol sizes button on the toolbar. A Green ball indicates the joint is acceptable. A Red ball indicates that the joint fails, requiring a change of member sizes. A Blue ball indicates the joint requires a web plate. Blue plates indicate that the joint requires web stiffeners. Change the sizes as necessary to obtain acceptable joint designs or to eliminate doublers and stiffeners if desired. The Process – Joint View/Update command is very helpful in investigating and modifying column sizes when trying to eliminate the need for web plates and stiffener plates.
Step 10 Perform AISC 341 Seismic Provisions Member Checks
Go to the Steel – Seismic Provisions module.
Select and specify the code settings and load combination options and values, consistent with those selected in Steel – Standard Provisions mode. It is not necessary to include the Notional Loads in any combinations.
Assign the frame type (e.g., Special Concentric Braced Frame - Chevron) to the frames using the Assign – Frame Type command.
Specify all necessary criteria items in the Criteria menu and override the criteria on a member-by-member basis, if necessary, using the assign commands in the Assign menu. It is not necessary to specify or assign K-factors.
Perform a member code check using the Process – Member Code Check command. Modify the sizes as necessary to satisfy the seismic requirements. The Process – Member View/Update command is very helpful in investigating and modifying sizes.
Although in a few cases there are some joint checks required by AISC 341 for braced frames (for example, Section F2.6b gives some requirements if the beam-to-column connection is incapable of acting as a pinned connection), they are not implemented. No seismic joint checks are performed on braced frame joints using the Process – Joint Code Check command and the Process – Joint View/Update command.
Step 11 Iterate
Repeat the above steps until acceptable designs are obtained. When sizes are changed, the stiffness of the frames change, which impacts drifts, distribution of forces, P-delta, etc. Since the selection of proper sizes is an iterative process with trial member sizes increasing and decreasing, it may require that some or all of the above steps be repeated. To work more productively, it may be best not to spend too much time trying to refine the sizes too much for each individual step, since a subsequent step may require significant size changes. Rather, at each step select sizes that are close but not necessarily “perfect”, especially in the early stages. The refinement of sizes should occur as the steps are iterated.
Contact Allen Adams at firstname.lastname@example.org for comments or if you want to discuss this further.